Space manufacturing or In-space manufacturing (ISM in short) is the fabrication, assembly or integration of tangible goods
beyond Earth's atmosphere (or more generally, outside a planetary
atmosphere), involving the transformation of raw or recycled materials
into components, products, or infrastructure in space, where the
manufacturing process is executed either by humans or automated systems
by taking advantage of the unique characteristics of space. Synonyms of Space/In-space manufacturing are In-orbit manufacturing (since most production capabilities are limited to low Earth orbit), Off-Earth manufacturing, Space-based manufacturing, Orbital manufacturing, In-situ manufacturing, In-space fabrication, In-space production, etc. In-space manufacturing is a part of the broader activity of in-space servicing, assembly and manufacturing (ISAM) and is related to in situ resource utilization (ISRU).
Three major domains of In-space manufacturing are ISM for space
(space-for-space) where products remain in space, ISM for Earth
(space-for-Earth) where goods with improved properties produced in
outer-space microgravity are transported back to Earth, and ISM for
surface where goods are produced on or sent to surfaces of celestial
bodies like the Moon, Mars, and asteroids.
In-space manufacturing uses processes such as additive manufacturing (printing a 3D object in successive layers), subtractive manufacturing
(making 3D objects by successively removing material from a solid),
hybrid manufacturing (usually combining additive
manufacturing and subtractive manufacturing) and welding (joining pieces
of material by melting or plasticizing along a joint line).
In-space manufacturing removes spacecraft design limitations due
to launch parameters (mass, vibration, structural load, etc.) and volume
limitations imposed by payload size. It allows for recycling of
launched materials, utilization space-mined resources and on-demand
spare parts production, which enables on-site repair of critical parts
(increasing reliability and redundancy) and infrastructure development.
It takes advantage of unique space features such as microgravity,
ultra-vacuum and containerless processing, which are difficult to do on
Earth.
Areas
In-space manufacturing (ISM) can be categorized into three different areas according to the end use of manufactured products. In-space manufacturing for space (space-for-space) involves activities
focused on in-orbit construction intended for use in space. ISM for
Earth (space-for-Earth) is the production of new materials and products
that exhibit enhanced properties when manufactured in microgravity,
subsequently transported back to Earth. Lastly, ISM for surface extends
to surface operations on celestial bodies such as the Moon, Mars, and
asteroids.
Rationale
There are several motivating factors behind in-space manufacturing. The space environment, in particular the effects of microgravity and vacuum,
enable the research of and production of goods that could otherwise not
be manufactured on Earth. Secondly, the extraction and processing of
raw materials from other astronomical bodies, also called In-Situ Resource Utilisation (ISRU),
could enable more sustainable space exploration missions at reduced
cost compared to launching all required resources from Earth.
Furthermore, raw materials could be transported to low Earth orbit where
they could be processed into goods that are shipped to Earth. By
replacing terrestrial production on Earth, this seeks to preserve the
Earth. Moreover, raw materials of very high value, for example gold,
silver, or platinum, could be transported to low Earth orbit for
processing or transfer to Earth which is thought to have the potential
to become economically viable. In-space manufacturing supports
long-duration space missions and colonization by enabling on-site repair
and infrastructure development beyond Earth. Additionally, in the area
of spaceflight technology, space manufacturing enhances mission safety
by decentralizing manufacturing activities and establishing redundancy
in critical systems, allows for customized production tailored to
specific mission requirements, fostering rapid iteration and adaptation
of designs, drives technological innovation in materials science,
robotics, and additive manufacturing, with applications extending beyond
space exploration, and lays the foundation for space-based
infrastructure development, supporting a wide range of commercial
activities and scientific research.
History
During the Soyuz 6 mission of 1969, Russian
cosmonauts performed the first welding experiments in space. Three
different welding processes were tested using a hardware unit called
Vulkan. The tests included welding aluminum, titanium, and stainless steel.
The Skylab
mission, launched in May 1973, served as a laboratory to perform
various space manufacturing experiments. The station was equipped with a
materials processing facility that included a multi-purpose electric furnace, a crystal growth chamber, and an electron
beam gun. Among the experiments to be performed was research on molten
metal processing; photographing the behavior of ignited materials in
zero-gravity; crystal growth; processing of immiscible alloys; brazing of stainless steel tubes, electron beam welding, and the formation of spheres from molten metal. The crew spent a total of 32 man-hours on materials science and space manufacturing investigation during the mission.
The Space Studies Institute began hosting a bi-annual Space Manufacturing Conference in 1977.
Microgravity research in materials processing continued in 1983 using the Spacelab facility. This module has been carried into orbit 26 times aboard the Space Shuttle, as of 2002. In this role the shuttle served as an interim, short-duration research platform before the completion of the International Space Station.
The Wake Shield Facility is deployed by the Space Shuttle's robotic arm. NASA image
In February 1994 and September 1995, the Wake Shield Facility was carried into orbit by the Space Shuttle. This demonstration platform used the vacuum created in the orbital wake to manufacture thin films of gallium arsenide and aluminum gallium arsenide.
On May 31, 2005, the recoverable, uncrewed Foton-M2
laboratory was launched into orbit. Among the experiments were crystal
growth and the behavior of molten-metal in weightlessness.
The completion of the International Space Station
has provided expanded and improved facilities for performing industrial
research. These have and will continue to lead to improvements in our
knowledge of materials sciences, new manufacturing techniques on Earth,
and potentially some important discoveries in space manufacturing
methods. NASA and Tethers Unlimited will test the Refabricator aboard the ISS, which is intended to recycle plastic for use in space additive manufacturing.
The Material Science Laboratory Electromagnetic Levitator (MSL-EML) on board the Columbus Laboratory is a science facility that can be used to study the melting and solidification properties of various materials. The Fluid Science Laboratory (FSL) is used to study the behavior of liquids in microgravity.
Material properties in the space environment
There are several unique differences between the properties of
materials in space compared to the same materials on the Earth. These
differences can be exploited to produce unique or improved manufacturing
techniques.
The microgravity environment allows control of convection in
liquids or gasses, and the elimination of sedimentation. Diffusion
becomes the primary means of material mixing, allowing otherwise
immiscible materials to be intermixed.
The environment allows enhanced growth of larger, higher-quality crystals in solution.
The ultraclean vacuum of space allows the creation of very pure
materials and objects. The use of vapor deposition can be used to build
up materials layer by layer, free from defects.
Surface tension
causes liquids in microgravity to form perfectly round spheres. This
can cause problems when trying to pump liquids through a conduit, but it
is very useful when perfect spheres of consistent size are needed for
an application.
Space can provide readily available extremes of heat and cold.
Sunlight can be focused to concentrate enough heat to melt the
materials, while objects kept in perpetual shade are exposed to
temperatures close to absolute zero. The temperature gradient can be
exploited to produce strong, glassy materials.
Material processing
For most manufacturing applications, specific material requirements must be satisfied. Mineralores need to be refined to extract specific metals, and volatile organic compounds
will need to be purified. Ideally these raw materials are delivered to
the processing site in an economical manner, where time to arrival, propulsionenergy expenditure, and extraction costs are factored into the planning process. Minerals can be obtained from asteroids, the lunar surface, or a planetary body. Volatiles could potentially be obtained from a comet, carbonaceous chondrite or "C-Type" asteroids, or the moons of Mars or other planets. It may also prove possible to extract hydrogen in the form of water ice or hydrated minerals from cold traps on the poles of the Moon.
Unless the materials processing and the manufacturing sites are
co-located with the resource extraction facilities, the raw materials
would need to be moved about the Solar System. There are several proposed means of providing propulsion for this material, including solar sails, electric sails, magnetic sails, electric ion thrusters, microwave electrothermal thrusters, or mass drivers (this last method uses a sequence of electromagnets mounted in a line to accelerate a conducting material).
At the materials processing facility, the incoming materials will
need to be captured by some means. Maneuvering rockets attached to the
load can park the content in a matching orbit. Alternatively, if the
load is moving at a low delta-v relative to the destination, then it can be captured by means of a mass catcher. This could consist of a large, flexible net or inflatable structure that would transfer the momentum
of the mass to the larger facility. Once in place, the materials can be
moved into place by mechanical means or by means of small thrusters.
Materials can be used for manufacturing either in their raw form,
or by processing them to extract the constituent elements. Processing
techniques include various chemical, thermal, electrolytic, and magnetic methods for separation. In the near term, relatively straightforward methods can be used to extract aluminum, iron, oxygen, and silicon
from lunar and asteroidal sources. Less concentrated elements will
likely require more advanced processing facilities, which may have to
wait until a space manufacturing infrastructure is fully developed.
Some of the chemical processes will require a source of hydrogen for the production of water and acid mixtures. Hydrogen gas can also be used to extract oxygen from the lunar regolith, although the process is not very efficient. So a readily available source of useful volatiles is a positive factor
in the development of space manufacturing. Alternatively, oxygen can be
liberated from the lunar regolith without reusing any imported
materials by heating the regolith to 4,530 °F (2,500 °C) in a vacuum.
This was tested on Earth with lunar simulant in a vacuum chamber. As
much as 20% of the sample was released as free oxygen. Eric Cardiff
calls the remainder slag. This process is highly efficient in terms of
imported materials used up per batch, but is not the most efficient
process in energy per kilogram of oxygen.
One proposed method of purifying asteroid materials is through the use of carbon monoxide (CO). Heating the material to 500 °F (260 °C) and exposing it to CO causes the metals to form gaseous carbonyls. This vapor can then be distilled to separate out the metal
components, and the CO can then be recovered by another heating cycle.
Thus an automated ship can scrape up loose surface materials
from, say, the relatively nearby 4660 Nereus
(in delta-v terms), process the ore using solar heating and CO, and
eventually return with a load of almost pure metal. The economics of
this process can potentially allow the material to be extracted at
one-twentieth the cost of launching from Earth, but it would require a
two-year round trip to return any mined ore.
Manufacturing
Due to speed of light
constraints on communication, manufacturing in space at a distant point
of resource acquisition will either require completely autonomous
robotics to perform the labor, or a human crew with all the accompanying
habitat and safety requirements. If the plant is built in orbit around
the Earth, or near a crewed space habitat, however, telerobotic devices can be used for certain tasks that require human intelligence and flexibility.
Solar power
provides a readily available power source for thermal processing. Even
with heat alone, simple thermally-fused materials can be used for basic
construction of stable structures. Bulk soil from the Moon or asteroids
has a very low water content, and when melted to form glassy materials
is very durable. These simple, glassy
solids can be used for the assembly of habitats on the surface of the
Moon or elsewhere. The solar energy can be concentrated in the
manufacturing area using an array of steerable mirrors.
The availability and favorable physical properties of metals will
make them a major component of space manufacturing. Most of the metal
handling techniques used on Earth can also be adopted for space
manufacturing. A few of these techniques will need significant
modifications due to the microgravity environment.
The production of hardened steel in space will introduce some new factors. Carbon
only appears in small proportions in lunar surface materials and will
need to be delivered from elsewhere. Waste materials carried by humans
from the Earth is one possible source, as are comets. The water normally
used to quench steel will also be in short supply, and require strong
agitation.
Casting
steel can be a difficult process in microgravity, requiring special
heating and injection processes, or spin forming. Heating can be
performed using sunlight combined with electrical heaters. The casting
process would also need to be managed to avoid the formation of voids as
the steel cools and shrinks.
Various metal-working techniques can be used to shape the metal into the desired form. The standard methods are casting, drawing, forging, machining, rolling, and welding.
Both rolling and drawing metals require heating and subsequent cooling.
Forging and extrusion can require powered presses, as gravity is not
available. Electron beam welding has already been demonstrated on board
the Skylab,
and will probably be the method of choice in space. Machining
operations can require precision tools which will need to be imported
from the Earth for some duration.
New space manufacturing technologies are being studied at places such as Marshall's National Center for Advanced Manufacturing.
The methods being investigated include coatings that can be sprayed on
surfaces in space using a combination of heat and kinetic energy, and
electron beam free form fabrication of parts. Approaches such as these, as well as examination of material
properties that can be investigated in an orbiting laboratory, will be
studied on the International Space Station by NASA and Made In Space, Inc.
3D-printing in space
Additive manufacturing in space has progressed from initial research (2014–2016) to early development (2016–2017), with patent activity since early 2020 indicating accelerated advancements.
The option of 3D printing
items in space holds many advantages over manufacturing situated on
Earth. With 3D printing technologies, rather than exporting tools and
equipment from Earth into space, astronauts have the option to
manufacture needed items directly. On-demand patterns of manufacturing
make long-distance space travel more feasible and self-sufficient as
space excursions require less cargo. Mission safety is also improved.
The Made In Space, Inc.3D printers, which launched in 2014 to the International Space Station,
are designed specifically for a zero-gravity or micro-gravity
environment. The effort was awarded the Phase III Small Business
Innovation and Research Contract. The Additive Manufacturing Facility will be used by NASA to carry out repairs (including during emergency situations), upgrades, and installation. Made In Space lists the advantages of 3D printing as easy
customization, minimal raw material waste, optimized parts, faster
production time, integrated electronics, limited human interaction, and
option to modify the printing process.
The Refabricator experiment, under development by Firmamentum, a division of Tethers Unlimited, Inc.
under a NASA Phase III Small Business Innovation Research contract,
combines a recycling system and a 3D printer to perform demonstration of
closed-cycle in-space manufacturing on the International Space Station
(ISS). The Refabricator experiment, which was delivered to the ISS aboard Cygnus NG-10 on November 19, 2018, processes plastic feedstock through multiple printing and recycling
cycles to evaluate how many times the plastic materials can be re-used
in the microgravity environment before their polymers degrade to
unacceptable levels.
Additionally, 3D printing in space can also account for the printing of meals. NASA's
Advanced Food Technology program is currently investigating the
possibility of printing food items in order to improve food quality,
nutrient content, and variety.
Airbus is developing and planning with the European Space Agency
to send and test the first 3D-printer printing metals in space at the
ISS in a year from 2022, and establishing space manufacturing in three
to four years from 2022.
Mitsubishi Electric Research Laboratories (MERL)
has developed a UV-curable resin that is stable in the LEO environment,
as well as an extruder and positioner to fabricate structures in orbit.
Products
There are thought to be a number of useful products that can
potentially be manufactured in space and result in an economic benefit.
Research and development is required to determine the best commodities
to be produced, and to find efficient production methods. The following
products are considered prospective early candidates:
As the infrastructure is developed and the cost of assembly drops,
some of the manufacturing capacity can be directed toward the
development of expanded facilities in space, including larger scale
manufacturing plants. These will likely require the use of lunar and
asteroid materials, and so follow the development of mining bases.
Rock is the simplest product, and at minimum is useful for
radiation shielding. It can also be subsequently processed to extract
elements for various uses.
Water from lunar sources, Near Earth Asteroids or Martian moons
is thought to be relatively cheap and simple to extract, and gives
adequate performance for many manufacturing and material shipping
purposes. Separation of water into hydrogen and oxygen can be easily
performed in small scale, but some scientists believe that this will not be performed on any large scale initially
due to the large quantity of equipment and electrical energy needed to
split water and liquify the resultant gases. Water used in steam rockets
gives a specific impulse of about 190 seconds;less than half that of hydrogen/oxygen, but this is adequate for delta-v's that are found between Mars and Earth. Water is useful as a radiation shield and in many chemical processes.
Ceramics made from lunar or asteroid soil can be employed for a variety of manufacturing purposes. These uses include various thermal and electrical insulators, such as
heat shields for payloads being delivered to the Earth's surface.
Metals can be used to assemble a variety of useful products,
including sealed containers (such as tanks and pipes), mirrors for
focusing sunlight, and thermal radiators. The use of metals for
electrical devices would require insulators for the wires, so a flexible
insulating material such as plastic or fiberglass will be needed.
A notable output of space manufacturing is expected to be solar
panels. Expansive solar energy arrays can be constructed and assembled
in space. As the structure does not need to support the loads that would
be experienced on Earth, huge arrays can be assembled out of
proportionately smaller amounts of material. The generated energy can
then be used to power manufacturing facilities, habitats, spacecraft,
lunar bases, and even beamed down to collectors on the Earth with microwaves.
Other possibilities for space manufacturing include propellants
for spacecraft, some repair parts for spacecraft and space habitats,
and, of course, larger factories. Ultimately, space manufacturing facilities can hypothetically become
nearly self-sustaining, requiring only minimal imports from the Earth.
The microgravity environment allows for new possibilities in
construction on a massive scale, including megascale engineering. These future projects might potentially assemble space elevators,
massive solar array farms, very high capacity spacecraft, and rotating
habitats capable of sustaining populations of tens of thousands of
people in Earth-like conditions.
Challenges
The space environment is expected to be beneficial for production of a
variety of products assuming the obstacles to it can be overcome. The
most significant cost is overcoming the energy hurdle for boosting
materials into orbit. Once this barrier is significantly reduced in cost
per kilogram, the entry price for space manufacturing can make it much more attractive to entrepreneurs. After the heavy capitalization costs of assembling the mining
and manufacturing facilities are paid, the production will need to be
economically profitable in order to become self-sustaining and
beneficial to society.
The economic requirements of space manufacturing imply a need to
collect the requisite raw materials at a minimum energy cost. The cost
of space transport is directly related to the delta-v,
or change in velocity required to move from the mining sites to the
manufacturing plants. Bringing material to Earth orbit from bodies such
as Near-Earth asteroids, Phobos, Deimos or the lunar
surface requires far less delta-v than launching from Earth itself,
despite the greater distances involved. This makes these places
economically attractive as sources of raw materials.
Natural shielding against space weather and solar wind, such as the magnetosphere depicted in this artistic rendition, is required for planets to sustain life for prolonged periods.
A Habitable Zone for Complex Life (HZCL) is a range of distances from a star suitable for complex aerobic life. Different types of limitations preventing complex life give rise to different zones. Conventional habitable zones are based on compatibility with water. Most zones start at a distance from the host star and then end at a distance farther from the star. A planet would need to orbit
inside the boundaries of this zone. With multiple zonal constraints,
the zones would need to overlap for the planet to support complex life.
The requirements for bacterial life produce much larger zones than those for complex life, which requires a very narrow zone.
Unstable stars are young and old stars, or very large or small stars. Unstable stars have changing solar luminosity that changes the size of the life habitable zones. Unstable stars also produce extreme solar flares and coronal mass ejections. Solar flares and coronal mass ejections can strip away a planet's atmosphere that is not replaceable. Thus life habitable zones require and very stable star like the Sun, at ±0.1% solar luminosity change.Finding a stable star, like the Sun, is the search for a solar twin, with solar analogs that have been found. Star metallicity, mass, age, color, and temperature all effect luminosity variations. The Sun, a G2V star, has a mid-range metallicity optimal for the formation of rocky planets. Dwarf stars (red dwarf/orange dwarf/brown dwarf/subdwarf) are not only unstable, but also emit low energy, so the habitable zone is very close to the star and planets become tidally locked on the timescales needed for the development of life. Giant stars (subgiant/giant star/red giant/red supergiant) are unstable and emit high energy, so the habitable zone is very far from the star. Multiple-star systems
are also very common and are not suitable for complex life, as the
planet orbit would be unstable due to multiple gravitational forces and
solar radiation. Liquid water is possible in Multiple-star systems.
A conventional habitable zone is defined by liquid water.
Habitable zone
(HZ) (also called the circumstellar habitable zone), the orbit around a
star that would allow liquid water to remain for a short period of time
(a given period of time) on at least a small part of the planet's surface. Thus within the HZ, water, (H2O) is between 0 °C (32 °F; 273 K) and 100 °C (212 °F; 373 K) temperature. This zone is a temperature zone, set by the star's radiation and distance from the star. In the Solar System the planet Mars is just at the outer boundary of the habitable zone. The planet Venus is at the inner edge of the habitable zone, but due to its thick atmosphere it has no water. The HZ includes planets with elliptic orbits;
such planets might orbit into and out of the HZ. When a planet moves
out of the HZ, all its water would freeze to ice on the outside of the
HZ, and/or all water would become steam on the inner side. The HZ could be defined as the region where bacteria, a form of life, could possibly survive for a short period of time. The HZ is also sometimes called the "Goldilocks" zone.
Optimistic habitable zone (OHZ): a zone where liquid surface
water could have been on a planet at some time in its past history. This
zone would be larger than the HZ. Mars is an example of a planet in the
OHZ.: it is just beyond the HZ today, but had liquid water for a short
time span before the Mars carbonate catastrophe, some 4 billion years ago.
Continuously habitable zone (CHZ): a zone where liquid water
persists on the surface of a planet for years. This requires a
near-circular planetary orbit and a stable star. The zone may be much
smaller than the habitable zone.
Conservative habitable zone: a zone where liquid surface water remains on a planet over a long time span, as on Earth. This might also need a greenhouse effect provided by gases such as CO2 and water vapor to maintain the correct temperature. Rayleigh scattering would also be needed.
Named habitable zones for complex life
Over time and with more research, astronomers, cosmologists and astrobiologist
have discovered more parameters needed for life. Each parameter could
have a corresponding zone. Some of the named zones include:
Ultraviolet habitable zone: a zone where the ultraviolet (UV) radiation from a star is neither too weak nor too strong for life to exist. Life needs the correct amount of ultraviolet for synthesis of biochemicals. The extent of the zone depends on the amount of ultraviolet radiation from the star, the range of UV wavelengths, the age of the star, and the atmosphere of the planet. In humans UV is used to produce vitamin D. Extreme ultraviolet (EUV) can cause atmospheric loss.
Photosynthetic habitable zone: a zone where both long-term liquid water and oxygenic photosynthesis can occur.
Tropospheric habitable zone, or ozone habitable zone: a zone where the planet would have the correct amount of ozone needed for life. Inhaling too much ozone causes inflammation and irritation, whereas too little troposphere ozone would produce biochemical smog. On Earth, the troposphere ozone is part of the ground-level ozone protection. Tropospheric ozone is formed by the interaction of ultraviolet light with hydrocarbons and nitrogen oxides.
Planet rotation rate habitable zone: the zone where a planet's rotation
rate is best for life. If the rotation is too slow, the day/night
temperature difference is too great. The rotation rate also changes the
planet's reflectivity and thus temperature, rotation as altered cloud distributions, cloud altitudes, and cloud opacities. A fast rotation rate increases wind speed on the planet. The rotation rate affects the planet's clouds and their reflectivity. Slowing the rotation rate changes cloud distributions, cloud altitudes, and cloud opacities.
These changes in the clouds changes the temperature of the planet. A
high rotation rate also can cause continuous, very fast winds on the
surface. High rotation rates, coupled with strong Coriolis effects, generates continuous, high-speed winds by enabling the formation of persistent, fast-moving jet streams and atmospheric belts.
Planet rotation axis tilt habitable zone, or obliquity habitable zone: the region where a stable axial tilt for a planet's rotation is maintained. Earth's axis is tilted 23.5°; this gives seasons, providing snow and ice that can melt to provide water run off in the summer. Obliquity has a major impact on a planet's temperature, thus its habitable zone.
Tidal habitable zone. Planets too close to the star become tidally locked.
The mass of the star and the distance from the star set the tidal
habitable zone. A planet tidally locked has one side of the planet
facing the star, this side would be very hot. The face away from the
star would be well below freezing. A planet too close to the star will
also have tidal heating
from the star. Tidal heating can vary the planet's orbital
eccentricity. Too far from the star and the planet will not receive
enough solar heat.
Atmosphere electric field habitable zone: the place in which the ambipolar electric field is correct for the planet's electric field to help ions overcome gravity. The planet's ionosphere
must be correct to protect against the loss of the atmosphere. This is
addition to a strong magnetic field to protect against the solar wind
stripping away the atmosphere and water into outer space.
Orbital eccentricity habitable zone: the zone in which
planets maintain a nearly circular orbit. As orbits with eccentricity
have the planets move in and out of the habitable zones. In the Solar System, the grand tack hypothesis proposes the theory of the unique placement of the gas giants, the Solar System belts and the planets near circular orbits.[
Coupled planet-moon - Magnetosphere habitable zone: the zone that planet's moon and the planet's core produce a strong magnetosphere,
magnetic field to protect against the solar wind stripping away the
planet's atmosphere and water into outer space. Just as Mars had a magnetic field for a short time. Earth's Moon had a large magnetosphere for several hundred million years after its formation, as proposed in a 2020 study by Saied Mighani. The Moon's magnetosphere would have given added protection of Earth's atmosphere
as the early Sun was not as stable as it today. In 2020, James Green
modeled the coupled planet-moon-magnetosphere habitable zone. The
modeling showed a coupled planet–moon magnetosphere that would give
planet the protection from stellar wind in the early Solar System. In
the case of Earth, the Moon was closer to Earth in the early formation
of the Solar System, giving added protection. This protection was needed
then as the Sun was less stable.
Pressure-dependent habitable zone: the zone in which planets may have the correct atmospheric pressure
to have liquid surface water. With a low atmospheric pressure, the
temperature at which water boils is much lower, and at pressures below
that of the triple point, liquid water cannot exist. The average surface pressure on Mars today is close to that of the
triple point of water; thus, liquid water cannot exist there. Planets with high-pressure atmospheres may have liquid surface water, but life forms may experience difficulty with respiratory systems
in high-pressure atmospheres. As pressure increases, gas becomes
denser, making it more difficult to move in and out of the lungs, also
airway resistance is increased as the viscosity and density of the air
mixture is greater thus the higher resistance within the airways.This
greatly increases the effort required for breathing.
Galactic habitable zone (GHZ): The GHZ, also called the Galactic Goldilocks zone, is the place in a galaxy in which heavy elements needed for a rocky planet and life are present, but also a place where strong cosmic rays will not kill life and strip the atmosphere off the planet. The term Goldilocks zone
is used, as it is a fine balance between the two sites (heavy elements
and strong cosmic rays). Not all galaxies are able to support life. In many galaxies, life-killing events such as gamma-ray bursts can occur. About 90% of galaxies have long and frequent gamma ray bursts, thus no life. Cosmic rays pose a threat to life. Galaxies with many stars too close together or without any dust protection also are not hospitable for life. Irregular galaxies and other small galaxies do not have enough heavy elements. Elliptical galaxies are full of lethal radiation and lack heavy elements. Large spiral galaxies, like the Milky Way, have the heavy element needed for life at the center and out to about half distance from the center bar. Not all large spiral galaxies are the same, as spiral galaxies with too much active star formation can be deadly to life.Too little star formation and the spiral arms will collapse. Not all spiral galaxies have the correct galactic ram pressure stripping parameters; too much ram pressure can deplete the galaxy of gas and thus end star formation. The Milky Way is a barred spiral galaxy,
the bar is important to star formation and metallicity of the galaxy's
stars and planets. A barred spiral galaxy must have stable arms with the
just right star formation. Barred spiral galaxies make up about 65% of
spiral galaxies, but most have too much star formation. Peculiar galaxies lack stable spiral arms, while irregular galaxies contain too many new stars and lack heavy elements. Unbarred spiral galaxies do not correct star formation and metallicity for a galactic Goldilocks zone. For long term life on a planet, the spiral arms must be stable for a
long period of time, as in the Milky Way. The spiral arms must not be
too close to each other, or there will be too much ultraviolet
radiation. If the planet moves into or across a spiral arm, the orbits
of the planets could change from gravitational disturbances. Movement
across a spiral arms also would cause deadly asteroid impacts and high radiation. The planet must be in the correct place in the spiral galaxy: near the
galactic center, radiation and gravitational forces are too great for
life, whereas the outskirts of a spiral galaxy are metal-poor. The Sun
in 28,000 light years from the center bar, in the galactic Goldilocks
zone. At this distance, the Sun revolves in the galaxy at the same rate
as the spiral-arm rotation, thus minimizing arm crossings.
Supergalactic habitable zone: a place in a supercluster of galaxies
that can provide for habitability of planets. The supergalactic
habitable zone takes into account events in galaxies that can end
habitability not only in a galaxy, but all galaxies nearby, such as galaxies merging, active galactic nucleus, starburst galaxy, supermassive black holes and merging black holes,
all which output intense radiation. The supergalactic habitable zone
also takes into account the abundance of various chemical elements in
the galaxy, as not all galaxies or regions within have all the needed
elements for life.
Habitable zone for complex life (HZCL): the place that all the life habitable zones overlap for a long period of time, as in the Solar System. The list of habitable zones for complex life has grown longer with
increasing understanding of the Universe, galaxies, and the Solar
System. Complex life is normally defined as eukaryote life forms, including all animals, plants, fungi, and most unicellular organisms. Simple life forms are normally defined as prokaryotes.
Other orbital-distance related factors
Some factors that depend on planetary distance and may limit complex aerobic life have not been given zone names. These include:
Milankovitch cycle The Milankovitch cycle and ice age have been key is shaping Earth.Life on Earth today is using water melting from the last ice age. The
ice ages cannot be too long or too cold for life to survive.
Milankovitch cycle has an impact on the planet's obliquity also.
Nanoinformatics is the application of informatics to nanotechnology.
It is an interdisciplinary field that develops methods and software
tools for understanding nanomaterials, their properties, and their
interactions with biological entities, and using that information more
efficiently. It differs from cheminformatics in that nanomaterials usually involve nonuniform
collections of particles that have distributions of physical properties
that must be specified. The nanoinformatics infrastructure includes ontologies for nanomaterials, file formats, and data repositories.
Nanoinformatics has applications for improving workflows in fundamental research, manufacturing, and environmental health, allowing the use of high-throughput data-driven methods to analyze broad sets of experimental results. Nanomedicine applications include analysis of nanoparticle-based pharmaceuticals for structure–activity relationships in a similar manner to bioinformatics.
Background
Context
of nanoinformatics as a convergence of science and practice at the
nexus of safety, health, well-being, and productivity; risk management;
and emerging nanotechnology.
While conventional chemicals are specified by their chemical composition, and concentration, nanoparticles have other physical properties that must be measured for a complete description, such as size, shape, surface properties, crystallinity, and dispersion state. In addition, preparations of nanoparticles are often non-uniform,
having distributions of these properties that must also be specified.
These molecular-scale properties influence their macroscopic chemical
and physical properties, as well as their biological effects. They are
important in both the experimental characterization of nanoparticles and their representation in an informatics system. The context of nanoinformatics is that effective development and
implementation of potential applications of nanotechnology requires the
harnessing of information at the intersection of safety, health,
well-being, and productivity; risk management; and emerging nanotechnology.
A graphical representation of a working definition of nanoinformatics as a life-cycle process
One working definition of nanoinformatics developed through the community-based Nanoinformatics 2020 Roadmap and subsequently expanded is:
Determining which information is relevant to meeting the safety,
health, well-being, and productivity objectives of the nanoscale
science, engineering, and technology community;
Developing and implementing effective mechanisms for collecting,
validating, storing, sharing, analyzing, modeling, and applying the
information;
Confirming that appropriate decisions were made and that desired
mission outcomes were achieved as a result of that information; and
finally
Conveying experience to the broader community, contributing to generalized knowledge, and updating standards and training.
Data representations
Although nanotechnology is the subject of significant
experimentation, much of the data are not stored in standardized formats
or broadly accessible. Nanoinformatics initiatives seek to coordinate
developments of data standards and informatics methods.
Ontologies
An overview of the eNanoMapper nanomaterial ontology
In the context of information science, an ontology is a formal representation of knowledge within a domain,
using hierarchies of terms including their definitions, attributes, and
relations. Ontologies provide a common terminology in a
machine-readable framework that facilitates sharing and discovery of data. Having an established ontology for nanoparticles is important for cancer nanomedicine due to the need of researchers to search, access, and analyze large amounts of data.
The NanoParticle Ontology is an ontology for the preparation,
chemical composition, and characterization of nanomaterials involved in
cancer research. It uses the Basic Formal Ontology framework and is implemented in the Web Ontology Language. It is hosted by the National Center for Biomedical Ontology and maintained on GitHub. The eNanoMapper Ontology is more recent and reuses wherever possible
already existing domain ontologies. As such, it reuses and extends the
NanoParticle Ontology, but also the BioAssay Ontology, Experimental Factor Ontology, Unit Ontology, and ChEBI.
File formats
Flowchart
depicting the ways to identify different components of a material
sample to guide the creation of an ISA-TAB-Nano Material file
ISA-TAB-Nano is a set of four spreadsheet-based file formats for representing and sharing nanomaterial data based on the ISA-TAB metadata standard. In Europe, other templates have been adopted that were developed by the Institute of Occupational Medicine, and by the Joint Research Centre for the NANoREG project.
Tools
Nanoinformatics is not limited to aggregating and sharing information
about nanotechnologies, but has many complementary tools, some
originating from chemoinformatics and bioinformatics.
Databases and repositories
Over the past decase, various publicly available nanomaterials
databases and repositories have been constructed to support
nanoinformatics and toxicology modelling. These databases often store standardised physicochemical, biological,
and toxicological data on engineered nanomaterials and offer model-ready
datasets to the scientific community to enable data reuse. Given the
limitation of data availability in nanoinformatics tasks, the curation
of large datasets and storage into accessible repositories is
prioritised to support computational modelling, regulatory assessment,
and data-driven research.
caNanoLab, developed by the U.S. National Cancer Institute, focuses on nanotechnologies related to biomedicine. The NanoMaterials Registry, maintained by RTI International, is a curated database of nanomaterials, and includes data from caNanoLab.
The eNanoMapper database, a project of the EU NanoSafety Cluster,
is a deployment of the database software developed in the eNanoMapper
project. It has since been used in other settings, such as the EU Observatory for NanoMaterials (EUON).
Other databases include the Center for the Environmental Implications of NanoTechnology's NanoInformatics Knowledge Commons (NIKC) and NanoDatabank, PEROSH's Nano Exposure & Contextual Information Database (NECID), Data and Knowledge on Nanomaterials (DaNa), NanoPharos and Springer Nature's Nano database.
Applications
Nanoinformatics has applications for improving workflows in fundamental research, manufacturing, and environmental health, allowing the use of high-throughput data-driven methods to analyze broad sets of experimental results.
Nanoinformatics is especially useful in nanoparticle-based cancer
diagnostics and therapeutics. They are very diverse in nature due to
the combinatorially large numbers of chemical and physical modifications
that can be made to them, which can cause drastic changes in their
functional properties. This leads to a combinatorial complexity that
far exceeds, for example, genomic data. Nanoinformatics can enable structure–activity relationship modelling for nanoparticle-based drugs. Nanoinformatics and biomolecular nanomodeling provide a route for effective cancer treatment. Nanoinformatics also enables a data-driven approach to the design of materials to meet health and environmental needs.
Modeling and NanoQSAR
Viewed as a workflow process, nanoinformatics deconstructs experimental studies using data, metadata, controlled vocabularies and ontologies
to populate databases so that trends, regularities and theories will be
uncovered for use as predictive computational tools. Models are
involved at each stage, some material (experiments, reference materials, model organisms)
and some abstract (ontology, mathematical formulae), and all intended
as a representation of the target system. Models can be used in
experimental design, may substitute for experiment or may simulate how a
complex system changes over time.
At present, nanoinformatics is an extension of bioinformatics
due to the great opportunities for nanotechnology in medical
applications, as well as to the importance of regulatory approvals to
product commercialization. In these cases, the models target, their
purposes, may be physico-chemical, estimating a property based on
structure (quantitative structure–property relationship, QSPR); or
biological, predicting biological activity based on molecular structure (quantitative structure–activity relationship, QSAR) or the time-course development of a simulation (physiologically based toxicokinetics, PBTK). Each of these has been explored for small moleculedrug development with a supporting body of literature.
Particles differ from molecular entities, especially in having
surfaces that challenge nomenclature system and QSAR/PBTK model
development. For example, particles do not exhibit an octanol–water partition coefficient, which acts as a motive force in QSAR/PBTK models; and they may dissolve in vivo or have band gaps. Illustrative of current QSAR and PBTK models are those of Puzyn et al. and Bachler et al. The OECD has codified regulatory acceptance criteria, and there are guidance roadmaps with supporting workshops to coordinate international efforts.
Communities
Communities active in nanoinformatics include the European UnionNanoSafety Cluster, The U.S. National Cancer Institute National Cancer Informatics Program's Nanotechnology Working Group, and the US–EU Nanotechnology Communities of Research.
Nanoinformatics roles, responsibilities, and communication interfaces
Individuals who engage in nanoinformatics can be viewed as fitting
across four categories of roles and responsibilities for nanoinformatics
methods and data:
Customers, who need either the methods to create the data, the
data itself, or both, and who specify the scientific applications and
characterization methods and data needs for their intended purposes;
Creators, who develop relevant and reliable methods and data to meet the needs of customers in the nanotechnology community;
Curators, who maintain and ensure the quality of the methods and associated data; and
Analysts, who develop and apply methods and models for data analysis
and interpretation that are consistent with the quality and quantity of
the data and that meet customers' needs.
In some instances, the same individuals perform all four roles. More
often, many individuals must interact, with their roles and
responsibilities extending over significant distances, organizations,
and time. Effective communication is important across each of the twelve
links (in both directions across each of the six pairwise interactions)
that exist among the various customers, creators, curators, and
analysts.
History
One of the first mentions of nanoinformatics was in the context of handling information about nanotechnology.
An early international workshop with substantial discussion of
the need for sharing all types of information on nanotechnology and
nanomaterials was the First International Symposium on Occupational
Health Implications of Nanomaterials held 12–14 October 2004 at the
Palace Hotel, Buxton, Derbyshire, UK. The workshop report included a presentation on Information Management for Nanotechnology Safety and Health that described the development of a Nanoparticle Information Library
(NIL) and noted that efforts to ensure the health and safety of
nanotechnology workers and members of the public could be substantially
enhanced by a coordinated approach to information management. The NIL
subsequently served as an example for web-based sharing of
characterization data for nanomaterials.
The National Cancer Institute prepared in 2009 a rough vision of, what was then still called, nanotechnology informatics, outlining various aspects of what nanoinformatics should comprise. This
was later followed by two roadmaps, detailing existing solutions,
needs, and ideas on how the field should further develop: the Nanoinformatics 2020 Roadmap and the EU US Roadmap Nanoinformatics 2030.
A 2013 workshop on nanoinformatics described current resources,
community needs and the proposal of a collaborative framework for data
sharing and information integration.
Stages in the origin of life process range from the well understood, such as the habitable Earth and the abiotic synthesis of simple molecules, to the largely unknown, like the derivation of the last universal common ancestor (LUCA) with its complex molecular functionalities.
Abiogenesis or the origin of life (sometimes called biopoesis) is the natural process by which life arises from non-living matter, such as simple organic compounds. The prevailing scientific hypothesis is that the transition from non-living to living entities on Earth was not a single event, but a process of increasing complexity involving the formation of a habitable planet, the prebiotic synthesis of organic molecules, molecular self-replication, self-assembly, autocatalysis, and the emergence of cell membranes.
The transition from non-life to life has not been observed
experimentally, but many proposals have been made for different stages
of the process.
The study of abiogenesis aims to determine how pre-life chemical reactions gave rise to life under conditions strikingly different from those on Earth today. It uses tools from biology and chemistry, attempting a synthesis of many sciences. Life functions through the chemistry of carbon and water, and builds on four chemical families: lipids for cell membranes, carbohydrates such as sugars, amino acids for protein metabolism, and the nucleic acidsDNA and RNA for heredity. A theory of abiogenesis must explain the origins and interactions of these classes of molecules.
Many approaches investigate how self-replicating molecules came into existence. Researchers think that life descends from an RNA world, although other self-replicating and self-catalyzing molecules may have preceded RNA. Other approaches ("metabolism-first" hypotheses) focus on how catalysis on the early Earth might have provided the precursor molecules for self-replication. The 1952 Miller–Urey experiment demonstrated that amino acids can be synthesized from inorganic compounds under conditions like early Earth's. Subsequently, amino acids have been found in meteorites, comets, asteroids, and star-forming regions of space.
While the last universal common ancestor
of all modern organisms (LUCA) existed millions of years after the
origin of life, its study can guide research into early universal
characteristics. A genomics approach has sought to characterize LUCA by identifying the genes shared by Archaea and Bacteria, major branches of life. It appears there are 60 proteins common to all life and 355 prokaryotic genes that trace to LUCA; their functions imply that LUCA was anaerobic with the Wood–Ljungdahl pathway, deriving energy by chemiosmosis, and used DNA, the genetic code, and ribosomes. Earlier cells might have had a leaky membrane and been powered by a naturally occurring proton gradient near a deep-sea white smoker hydrothermal vent; or, life may have originated inside the continental crust or in water at Earth's surface.
Although Earth is the only place known to harbor life, astrobiologists assume that life exists and came into being by similar processes on other planets. Geochemical and fossil evidence informs most studies. The Earth was formed at 4.54 Gya, and the earliest evidence of life on Earth dates from 3.8 Gya from Western Australia. Fossil micro-organisms may have lived in hydrothermal vent precipitates from Quebec, soon after ocean formation during the Hadean, so the process appears to have been relatively rapid in terms of geological time.
NASA's 2015 strategy for astrobiology
aimed to solve the puzzle of the origin of life – how a fully
functioning living system could emerge from non-living components –
through research on the prebiotic origin of life's chemicals, both in space and on planets, as well as the functioning of early biomolecules to catalyse reactions and support inheritance.
The challenge for origin of life researchers is to explain how such a complex and tightly interlinked
system could develop by evolutionary steps, as at first sight all its parts are necessary
to enable it to function. For example, a cell, whether the LUCA or in a
modern organism, copies its DNA with the DNA polymerase enzyme, which
is itself produced by translating the DNA polymerase gene in the DNA.
Neither the enzyme nor the DNA can be produced without the other. The evolutionary process could have started with molecular self-replication, self-assembly such as of cell membranes, and autocatalysis via RNA ribozymes in an RNA world environment. The transition of non-life to life has not been observed experimentally. Some scientists see both life and the origin of life as aspects of the same process.
The preconditions to the development of a living cell like the
LUCA are known, though disputed in detail: a habitable world is formed
with a supply of minerals and liquid water. Prebiotic synthesis creates a
range of simple organic compounds, which are assembled into polymers
such as proteins and RNA. On the other side, the process after the LUCA
is readily understood: biological evolution caused the development of a
wide range of species with varied forms and biochemical capabilities.
However, the derivation of the LUCA from simple components is far from
understood.
Although Earth remains the only place where life is known, the science of astrobiology
seeks evidence of life on other planets. The 2015 NASA strategy on the
origin of life aimed to solve the puzzle by identifying interactions,
intermediary structures and functions, energy sources, and environmental
factors that contributed to evolvable macromolecular systems, and mapping the chemical landscape of potential primordial informational polymers. The advent of such polymers was most likely a critical step in prebiotic chemical evolution. Those polymers derived, in turn, from simple organic compounds such as nucleobases, amino acids, and sugars, likely formed by reactions in the environment. A successful theory of the origin of life must explain how all these chemicals came into being.
The Miller–Urey experiment
was a synthesis of small organic molecules in a mixture of simple gases
in a thermal gradient created by heating (right) and cooling (left) the
mixture at the same time, with electrical discharges.
One ancient view of the origin of life, from Aristotle until the 19th century, was of spontaneous generation. This held that "lower" animals such as insects were generated by decaying organic substances, and that life arose by chance. This was questioned from the 17th century, in works like Thomas Browne's Pseudodoxia Epidemica. In 1665, Robert Hooke published the first drawings of a microorganism. In 1676, Antonie van Leeuwenhoek drew and described microorganisms, probably protozoa and bacteria. Van Leeuwenhoek disagreed with spontaneous generation, and by the 1680s
convinced himself, using experiments ranging from sealed and open meat
incubation and the close study of insect reproduction, that the theory
was incorrect. In 1668 Francesco Redi showed that no maggots appeared in meat when flies were prevented from laying eggs. By the middle of the 19th century, spontaneous generation was considered disproven.
Dating back to Anaxagoras in the 5th century BC, panspermia is the idea that life originated elsewhere in the universe and came to Earth. The modern version of panspermia holds that life may have been distributed to Earth by meteoroids, asteroids, comets or planetoids. This shifts the origin of life to another heavenly body. The advantage
is that life is not required to have formed on each planet it occurs on,
but in a more limited set of locations, and then spread about the galaxy to other star systems. There is some interest in the possibility that life originated on Mars and later transferred to Earth.
The idea that life originated from non-living matter in slow stages appeared in Herbert Spencer's 1864–1867 book Principles of Biology, and in William Turner Thiselton-Dyer's 1879 paper "On spontaneous generation and evolution". On 1 February 1871 Charles Darwin wrote about these publications to Joseph Hooker, and set out his own speculation that the original spark of life may have been in a "warm little pond, with all sorts of ammonia and phosphoric salts,—light,
heat, electricity &c present, that a protein compound was
chemically formed". Darwin explained that "at the present day such
matter would be instantly devoured or absorbed, which would not have
been the case before living creatures were formed."
Alexander Oparin in 1924 and J. B. S. Haldane in 1929 proposed that the earliest cells slowly self-organized from a primordial soup, the Oparin–Haldane hypothesis. Haldane suggested that the Earth's prebiotic oceans consisted of a "hot
dilute soup" in which organic compounds could have formed. J. D. Bernal showed that such mechanisms could form most of the necessary molecules for life from inorganic precursors. In 1967, he suggested three "stages": the origin of biological monomers; the origin of biological polymers; and the evolution from molecules to cells.
In 1952, Stanley Miller and Harold Urey
carried out a chemical experiment to demonstrate how organic molecules
could have formed spontaneously from inorganic precursors under prebiotic conditions like those posited by the Oparin–Haldane hypothesis. It used a highly reducing (lacking oxygen) mixture of gases—methane, ammonia, and hydrogen, with water vapor—to form organic monomers such as amino acids. Bernal said of the Miller–Urey experiment that "it is not enough to
explain the formation of such molecules, what is necessary, is a
physical-chemical explanation of the origins of these molecules that
suggests the presence of suitable sources and sinks for free energy." However, current scientific consensus describes the primitive atmosphere as weakly reducing or neutral, diminishing the amount and variety of amino acids that could be produced. The addition of iron and carbonate minerals, present in early oceans, produces a diverse array of amino acids. Later work has focused on two other potential reducing environments: outer space and deep-sea hydrothermal vents.
Soon after the Big Bang, roughly 14 Gya, the only chemical elements present in the universe were hydrogen, helium, and lithium,
the three lightest atoms in the periodic table. These elements
gradually accreted and began orbiting in disks of gas and dust.
Gravitational accretion of material at the hot and dense centers of
these protoplanetary disks formed stars by the fusion of hydrogen. Early stars were massive and short-lived, producing all the heavier elements by stellar nucleosynthesis. Such element formation proceeds to its most stable element Iron-56. Heavier elements were formed during supernovae at the end of a star's lifecycle. Carbon, currently the fourth most abundant element in the universe, was formed mainly in white dwarf stars. As these stars reached the end of their lifecycles,
they ejected heavier elements, including carbon and oxygen, throughout
the universe. These allowed for the formation of rocky planets. According to the nebular hypothesis, the Solar System began to form 4.6 Gya with the gravitational collapse of part of a giant molecular cloud. Most of the collapsing mass collected in the center, forming the Sun, while the rest flattened into a protoplanetary disk out of which the planets formed.
The age of the Earth is 4.54 Gya as found by radiometric dating of calcium-aluminium-rich inclusions in carbonaceous chrondrite meteorites, the oldest material in the Solar System. Earth, during the Hadean
eon (from its formation until 4.031 Gya,) was at first inhospitable to
life. During its formation, the Earth lost much of its initial mass, and
so lacked the gravity to hold molecular hydrogen and the bulk of the original inert gases. Soon after initial accretion of Earth at 4.48 Gya, its collision with Theia, a hypothesised impactor, is thought to have created the ejected debris that eventually formed the Moon. This impact removed the Earth's primary atmosphere, leaving behind
clouds of viscous silicates and carbon dioxide. This unstable atmosphere
was short-lived, soon condensing to form the bulk silicate Earth,
leaving behind an atmosphere largely consisting of water vapor, nitrogen, and carbon dioxide, with smaller amounts of carbon monoxide, hydrogen, and sulfur compounds. The solution of carbon dioxide in water is thought to have made the seas slightly acidic, with a pH of about 5.5.
Condensation to form liquid oceans is theorised to have occurred as early as the Moon-forming impact. This scenario is supported by the dating of 4.404 Gya zircon crystals with high δ18O values from metamorphosed quartzite of Mount Narryer in Western Australia. The Hadean atmosphere has been characterized as a "gigantic, productive
outdoor chemical laboratory," similar to volcanic gases today which
still support some abiotic chemistry. Despite the likely increased
volcanism from early plate tectonics, the Earth may have been a
predominantly water world between 4.4 and 4.3 Gya. It is debated whether
crust was exposed above this ocean. Immediately after the Moon-forming impact, Earth likely had little if any continental crust, a turbulent atmosphere, and a hydrosphere subject to intense ultraviolet light from a T Tauri stage Sun. It was also affected by cosmic radiation, and continued asteroid and comet impacts.
The Late Heavy Bombardment hypothesis posits that a period of intense impact occurred at 4.1 to 3.8 Gya during the Hadean and early Archean eons. Originally it was thought that the Late Heavy Bombardment was a single
cataclysmic impact event occurring at 3.9 Gya; this would have had the
potential to sterilize Earth by volatilizing liquid oceans and blocking
sunlight needed for photosynthesis, delaying the earliest possible
emergence of life. More recent research questioned the intensity of the Late Heavy
Bombardment and its potential for sterilisation. If it was not one giant
impact but a period of raised impact rate, it would have had much less
destructive power. The 3.9 Gya date arose from dating of Apollo mission sample returns collected mostly near the Imbrium Basin, biasing the age of recorded impacts. Impact modelling of the lunar surface reveals that rather than a
cataclysmic event at 3.9 Gya, multiple small-scale, short-lived periods
of bombardment likely occurred. Terrestrial data backs this idea by showing multiple periods of ejecta
in the rock record both before and after the 3.9 Gya marker, suggesting
that the early Earth was subject to continuous impacts with less impact
on extinction.
If life evolved in the ocean at depths of more than ten meters,
it would have been shielded both from late impacts and the then high
levels of ultraviolet radiation from the sun. The available energy is
maximized at 100–150 °C, the temperatures at which hyperthermophilic bacteria and thermoacidophilicarchaea live.
Based on evidence from the geologic record,
life most likely emerged on Earth between 4.32 and 3.48 Gya. In 2017,
the earliest physical evidence of life was reported to consist of microbialites in the Nuvvuagittuq Greenstone Belt of Northern Quebec, in banded iron formation
rocks at least 3.77 and possibly as old as 4.32 Gya. The
micro-organisms could have lived within hydrothermal vent precipitates,
soon after the 4.4 Gya formation of oceans
during the Hadean. The microbes resemble modern hydrothermal vent
bacteria, supporting the view that abiogenesis began in such an
environment. Later research disputed this interpretation of the data, stating that
the observations may be better explained by abiotic processes in
silica-rich waters, "chemical gardens," circulating hydrothermal fluids, or volcanic ejecta.
Biogenic graphite has been found in 3.7 Gya metasedimentary rocks from southwestern Greenland and in microbial mat fossils from 3.49 Gya cherts in the Pilbara region of Western Australia. Evidence of early life in rocks from Akilia Island, near the Isua supracrustal belt in southwestern Greenland, dating to 3.7 Gya, have shown biogenic carbon isotopes. In other parts of the Isua supracrustal belt, graphite inclusions trapped within garnet crystals are connected to the other elements of life: oxygen, nitrogen, and possibly phosphorus in the form of phosphate, providing further evidence for life 3.7 ;Gya. In the Pilbara region of Western Australia, compelling evidence of early life was found in pyrite-bearing
sandstone in a fossilized beach, with rounded tubular cells that
oxidized sulfur by photosynthesis in the absence of oxygen. Carbon isotope ratios on graphite inclusions from the Jack Hills
zircons suggest that life could have existed on Earth from 4.1 Gya. A 2024 study inferred LUCA's age as around 4.2 Gya (4.09–4.33 Gya) by
analysing pre-LUCA gene duplicates, with calibration from fossil
micro-organisms, much sooner after the origin of life than previously
thought.
The Pilbara region of Western Australia contains the Dresser Formation with rocks 3.48 Gya, including layered structures called stromatolites. Their modern counterparts are created by photosynthetic micro-organisms including cyanobacteria. These lie within undeformed hydrothermal-sedimentary strata; their
texture indicates a biogenic origin. Parts of the Dresser formation
preserve hot springs on land, but other regions seem to have been shallow seas. A molecular clock analysis suggests the LUCA emerged prior to 3.9 Gya.
All chemical elements derive from stellar nucleosynthesis except for hydrogen and some helium and lithium. Basic chemical ingredients of life – the carbon-hydrogen molecule (CH), the carbon-hydrogen positive ion (CH+) and the carbon ion (C+) – can be produced by ultraviolet light from stars. Complex molecules, including organic molecules, form naturally both in space and on planets. Organic molecules on the early Earth could have had either terrestrial
origins, with organic molecule synthesis driven by impact shocks or by
other energy sources, such as ultraviolet light, redox coupling, or electrical discharges; or extraterrestrial origins (pseudo-panspermia), with organic molecules formed in interstellar dust clouds raining down on to the planet.
An organic compound is a chemical whose molecules contain carbon. Carbon is abundant in the Sun, stars, comets, and in the atmospheres of most planets of the Solar System. Organic compounds are relatively common in space, formed by "factories
of complex molecular synthesis" which occur in molecular clouds and circumstellar envelopes, and chemically evolve after reactions are initiated mostly by ionizing radiation. Purine and pyrimidine nucleobases including guanine, adenine, cytosine, uracil, and thymine, as well as sugars, have been found in meteorites. These could have provided the materials for DNA and RNA to form on the early Earth. The amino acid glycine was found in material ejected from comet Wild 2; it had earlier been detected in meteorites. Comets are encrusted with dark material, thought to be a tar-like
organic substance formed from simple carbon compounds under ionizing
radiation. A rain of material from comets could have brought such
complex organic molecules to Earth. During the Late Heavy Bombardment, meteorites may have delivered up to five million tons of organic prebiotic elements to Earth per year. Currently 40,000 tons of cosmic dust falls to Earth each year.
Polycyclic aromatic hydrocarbons (PAH) are the most common and abundant polyatomic molecules in the observable universe, and are a major store of carbon. They seem to have formed shortly after the Big Bang, and are associated with new stars and exoplanets. They are a likely constituent of Earth's primordial sea. PAHs have been detected in nebulae, and in the interstellar medium, in comets, and in meteorites.
A star, HH 46-IR, resembling the sun early in its life, is
surrounded by a disk of material which contains molecules including
cyanide compounds, hydrocarbons, and carbon monoxide. PAHs in the interstellar medium can be transformed through hydrogenation, oxygenation, and hydroxylation to more complex organic compounds used in living cells.
Organic compounds introduced on Earth by interstellar dust particles can help to form complex molecules, thanks to their peculiar surface-catalytic activities. The RNA component uracil and related molecules, including xanthine, in the Murchison meteorite were likely formed extraterrestrially, as suggested by studies of 12C/13C isotopic ratios. NASA studies of meteorites suggest that all four DNA nucleobases
(adenine, guanine and related organic molecules) have been formed in
outer space. The cosmic dust permeating the universe contains complex organics ("amorphous organic solids with a mixed aromatic–aliphatic structure") that could be created rapidly by stars. Glycolaldehyde, a sugar molecule and RNA precursor, has been detected in regions of space including around protostars and on meteorites.
Laboratory synthesis
As early as the 1860s, experiments demonstrated that biologically
relevant molecules can be produced from interaction of simple carbon
sources with abundant inorganic catalysts. The spontaneous formation of
complex polymers from abiotically generated monomers under the
conditions posited by the "soup" theory is not straightforward. Besides
the necessary basic organic monomers, compounds that would have
prohibited the formation of polymers were also formed in high
concentration during the Miller–Urey experiment and Joan Oró experiments. Biology uses essentially 20 amino acids for its coded protein enzymes,
representing a very small subset of the structurally possible products.
Since life tends to use whatever is available, an explanation is needed
for why the set used is so small. Formamide is attractive as a medium that potentially provided a source
of amino acid derivatives from simple aldehyde and nitrile feedstocks.
The Breslow catalytic cycle for formaldehyde dimerization and C2-C6 sugar formation
Alexander Butlerov showed in 1861 that the formose reaction created sugars including tetroses, pentoses, and hexoses when formaldehyde
is heated under basic conditions with divalent metal ions like calcium.
R. Breslow proposed that the reaction was autocatalytic in 1959.
Nucleobases
Nucleobases, such as guanine and adenine, can be synthesized from simple carbon and nitrogen sources, such as hydrogen cyanide (HCN) and ammonia. On early Earth, HCN has been shown in modelling experiments to have
likely been supplied via photochemical production in transient, highly
reducing atmospheres (see Prebiotic atmosphere) following major impacts. Formamide,
produced by the reaction of water and HCN, is ubiquitous and produces
all four ribonucleotides when warmed with terrestrial minerals. It can
be concentrated by the evaporation of water. HCN is poisonous only to aerobic organisms,
which did not exist during the earliest phases of life's origin. It can
contribute to chemical processes such as the synthesis of the amino
acid glycine.
DNA and RNA components including uracil, cytosine and thymine can
be synthesized under outer space conditions, using starting chemicals
such as pyrimidine found in meteorites. Pyrimidine may have been formed
in red giant stars, in interstellar dust and gas clouds, or may have been synthesized on Earth via precursors such as cyanoacetylene and other intermediates made available following early asteroid impacts. All four RNA-bases may be synthesized from formamide in high-energy density events like extraterrestrial impacts. Several ribonucleotides for RNA formation have been synthesized in a laboratory environment which replicates prebiotic conditions via autocatalytic formose reaction.
Other pathways for synthesizing bases from inorganic materials have been reported. Freezing temperatures assist the synthesis of purines, by concentrating key precursors such as HCN. However, while adenine and guanine require freezing conditions, cytosine and uracil may require boiling temperatures. Seven amino acids and eleven types of nucleobases formed in ice when ammonia and cyanide were left in a freezer for 25 years. S-triazines
(alternative nucleobases), pyrimidines including cytosine and uracil,
and adenine can be synthesized by subjecting a urea solution to
freeze-thaw cycles under a reductive atmosphere with spark discharges. The unusual speed of these low-temperature reactions is due to eutectic freezing, which crowds impurities in microscopic pockets of liquid within the ice.
Peptides
Prebiotic peptide synthesis could have occurred by several routes.
Some center on high temperature/concentration conditions in which
condensation becomes energetically favorable, while others use plausible
prebiotic condensing agents.
Experimental evidence for the formation of peptides in uniquely
concentrated environments is bolstered by work suggesting that wet-dry
cycles and the presence of specific salts can greatly increase
spontaneous condensation of glycine into poly-glycine chains. Other work suggests that while mineral surfaces, such as those of
pyrite, calcite, and rutile catalyze peptide condensation, they also
catalyze their hydrolysis. The authors suggest that additional chemical
activation or coupling would be necessary to produce peptides at
sufficient concentrations. Thus, mineral surface catalysis, while
important, is not sufficient alone for peptide synthesis.
Many prebiotically plausible condensing/activating agents have
been identified, including the following: cyanamide, dicyanamide,
dicyandiamide, diaminomaleonitrile, urea, trimetaphosphate, NaCl, CuCl2, (Ni,Fe)S, CO, carbonyl sulfide (COS), carbon disulfide (CS2), SO2, and diammonium phosphate (DAP).
A 2024 experiment used a sapphire substrate with a web of thin cracks under a heat flow, mimicking deep-ocean vents,
to concentrate prebiotically-relevant building blocks from a dilute
mixture by up to three orders of magnitude. This could help to create
biopolymers such as peptides. A similar role has been suggested for clays, though this speculation has not been supported through experimental evidence.
The prebiotic synthesis of peptides from simpler molecules such as CO, NH3 and C, skipping the step of amino acid formation, is also very efficient.
The largest unanswered question in evolution is how simple protocells first arose and differed in reproductive contribution to the following generation, thus initiating evolution. The lipid world theory postulates that the first self-replicating object was lipid-like. Phospholipids form lipid bilayers
(as in cell membranes) in water while under agitation. These molecules
were not present on early Earth, but other membrane-forming amphiphilic long-chain molecules were. These bodies may expand by insertion of additional lipids, and may spontaneously split into two offspring
of similar size and composition. Lipid bodies may have provided
sheltering envelopes for information storage, allowing the evolution of
information-storing polymers like RNA. Only one or two types of
vesicle-forming amphiphiles have been studied. There is an enormous number of possible arrangements of lipid bilayer
membranes, and those with the best reproductive characteristics would
have converged toward a hypercycle reaction, a positive feedback
composed of two mutual catalysts represented by a membrane site and a
specific compound trapped in the vesicle. Such site/compound pairs are
transmissible to the daughter vesicles, leading to the emergence of
distinct lineages of vesicles, subject to natural selection.
A protocell is a self-organized, self-ordered, spherical collection of lipids proposed as a stepping-stone to life. A functional protocell has (as of 2014) not yet been achieved in a laboratory setting. Self-assembled vesicles are essential components of primitive cells. The theory of classical irreversible thermodynamics treats
self-assembly under a generalized chemical potential within the
framework of dissipative systems. The second law of thermodynamics requires that overall entropy increases, yet life is distinguished by its great degree of organization. Therefore, a boundary is needed to separate ordered life processes from chaotic non-living matter.
Irene Chen and Jack W. Szostak
suggest that elementary protocells can give rise to cellular behaviors
including primitive forms of differential reproduction, competition, and
energy storage. Competition for membrane molecules would favor stabilized membranes,
suggesting a selective advantage for cross-linked fatty acids and even
modern phospholipids. Such micro-encapsulation
would allow for metabolism within the membrane and the exchange of
small molecules, while retaining large biomolecules inside. Such a
membrane is needed for a cell to create its own electrochemical gradient. Fatty acid vesicles in alkaline hydrothermal vent conditions can be
stabilized by isoprenoids, synthesized by the formose reaction; the
advantages and disadvantages of isoprenoids within the lipid bilayer in
different microenvironments might have led to the divergence of the
membranes of archaea and bacteria.
Vesicles can undergo an evolutionary process under pressure cycling conditions. Simulating the systemic environment in tectonic fault zones within the Earth's crust, pressure cycling forms vesicles periodically, as well as random peptide
chains which are selected for ability to integrate into the vesicle
membrane. Further selection of vesicles for stability could lead to
functional peptide structures, increasing vesicle survival rate.
Life requires a loss of entropy, or disorder, as molecules organize
themselves into living matter. At the same time, the emergence of life
is associated with the formation of structures beyond a certain
threshold of complexity. The emergence of life with increasing order and complexity does not
contradict the second law of thermodynamics, which states that overall
entropy never decreases, since a living organism creates order in some
places (e.g. its living body) at the expense of an increase of entropy
elsewhere (e.g. heat and waste production).
Multiple sources of energy were available for chemical reactions on the early Earth. Heat from geothermal processes is a standard energy source for chemistry. Other examples include sunlight, lightning, atmospheric entries of micro-meteorites, and implosion of bubbles in sea and ocean waves. This has been confirmed by experiments and simulations. Unfavorable reactions can be driven by highly favorable ones, as in the
case of iron-sulfur chemistry. For example, this was probably important
for carbon fixation. Carbon fixation by reaction of CO2 with H2S
via iron-sulfur chemistry is favorable, and occurs at neutral pH and
100 °C. Iron-sulfur surfaces, which are abundant near hydrothermal
vents, can drive the production of small amounts of amino acids and
other biomolecules.
In 1961, Peter Mitchell proposed chemiosmosis
as a cell's primary system of energy conversion. The mechanism, now
ubiquitous in living cells, powers energy conversion in micro-organisms
and in the mitochondria of eukaryotes, making it a likely candidate for early life. Mitochondria produce adenosine triphosphate
(ATP), the energy currency of the cell used to drive cellular processes
such as chemical syntheses. The mechanism of ATP synthesis involves a
closed membrane in which the ATP synthase enzyme is embedded. The energy required to release strongly bound ATP has its origin in protons that move across the membrane. In modern cells, those proton movements are caused by the pumping of
ions across the membrane, maintaining an electrochemical gradient. In
the first organisms, the gradient could have been provided by the
difference in chemical composition between the flow from a hydrothermal
vent and the surrounding seawater, or perhaps meteoric quinones that were conducive to the development of
chemiosmotic energy across lipid membranes if at a terrestrial origin.
The RNA world hypothesis proposes that undirected polymerisation led to the emergence of ribozymes, and in turn to an RNA replicase.
The RNA world hypothesis describes an early Earth with self-replicating and catalytic RNA but no DNA or proteins. It was proposed in 1962 by Alexander Rich; the term was coined by Walter Gilbert in 1986. Many researchers concur that an RNA world must have preceded modern DNA-based life. However, it may not have been the first to exist. There may have been over 30 chemical events between pre-RNA world to near-LUCA, just involving RNA.
RNA is central to the translation process. Small RNAs can
catalyze all the chemical groups and information transfers required for
life. RNA both expresses and maintains genetic information in modern
organisms; its components are easily synthesized under early Earth
conditions. The structure of the ribosome has been called the "smoking gun", with a central core of RNA and no amino acid side chains within 18 Å of the active site that catalyzes peptide bond formation.
RNA replicase
can both code and catalyse further RNA replication, i.e. it is
autocatalytic. Some catalytic RNAs can link smaller RNA sequences
together, enabling self-replication.Natural selection would then favor the proliferation of such autocatalytic sets. Self-assembly of RNA may occur spontaneously in hydrothermal vents. A preliminary form of tRNA could have assembled into a replicator molecule. When this began to replicate, it may have had all three mechanisms of Darwinian selection: heritability, variation, and differential reproduction. Its fitness would have depended on its ability to adapt, determined by its nucleotide sequence, and resource availability.
From RNA to directed protein synthesis
In line with the RNA world hypothesis, much of modern biology's
templated protein biosynthesis is done by RNA molecules—namely tRNAs and
the ribosome (consisting of both protein and rRNA). The most central
reaction of peptide bond synthesis is carried out by base catalysis by
the 23S rRNA domain V. Di- and tripeptides can be synthesized with a system consisting of only aminoacyl phosphate adaptors and RNA guides. Aminoacylation ribozymes that can charge tRNAs with their cognate amino acids have been selected in in vitro experimentation.
Early functional peptides
The first proteins had to arise without a fully-fledged system of
protein biosynthesis. Random sequence peptides would not have had
biological function. Thus, significant study has gone into exploring how
early functional proteins could have arisen from random sequences.
Evidence on hydrolysis rates shows that abiotically plausible peptides
likely contained significant "nearest-neighbor" biases. This could have had some effect on early protein sequence diversity. A search found that approximately 1 in 1011 random sequences had ATP binding function.
Starting with the work of Carl Woese from 1977, genomics
studies have placed the last universal common ancestor (LUCA) of all
modern life-forms between Bacteria and a clade formed by Archaea and Eukaryota in the phylogenetic tree of life. It lived over 4 Gya. A minority of studies have placed the LUCA in Bacteria, proposing that
Archaea and Eukaryota are evolutionarily derived from within Eubacteria; Thomas Cavalier-Smith suggested in 2006 that the phenotypically diverse bacterial phylum Chloroflexota contained the LUCA.
In 2016, a set of 355 genes likely present in the LUCA was
identified. A total of 6.1 million prokaryotic genes from Bacteria and
Archaea were sequenced, identifying 355 protein clusters from among
286,514 protein clusters that were probably common to the LUCA. The
results suggest that the LUCA was anaerobic with a Wood–Ljungdahl (reductive Acetyl-CoA) pathway, nitrogen- and carbon-fixing, thermophilic. Its cofactors suggest dependence upon an environment rich in hydrogen, carbon dioxide, iron, and transition metals.
Its genetic material was probably DNA, requiring the 4-nucleotide
genetic code, messenger RNA, transfer RNA, and ribosomes to translate
the code into proteins such as enzymes. LUCA likely inhabited an
anaerobic hydrothermal vent setting in a geochemically active
environment. It was evidently already a complex organism, and must have
had precursors; it was not the first living thing. The physiology of LUCA has been in dispute.Previous research identified 60 proteins common to all life. Metabolic reactions inferred in LUCA are the incomplete reverse Krebs cycle, gluconeogenesis, the pentose phosphate pathway, glycolysis, reductive amination, and transamination.
A variety of geologic and environmental settings have been proposed
for an origin of life. These theories are often in competition with one
another as there are many views of prebiotic compound availability,
geophysical setting, and early life characteristics. The first organism
on Earth likely differed from LUCA.
Between the first appearance of life and where all modern phylogenies
began branching, an unknown amount of time passed, with unknown gene
transfers, extinctions, and adaptation to environmental niches. Modern phylogenies provide more genetic evidence about LUCA than about its precursors.
Early micro-fossils may have come from a hot world of gases such as methane, ammonia, carbon dioxide, and hydrogen sulfide, toxic to much current life. Analysis of the tree of life
places thermophilic and hyperthermophilic bacteria and archaea closest
to the root, suggesting that life may have evolved in a hot environment. The deep sea or alkaline hydrothermal vent theory posits that life began at submarine hydrothermal vents. William Martin and Michael Russell have suggested that this could have been in metal-sulphide-walled compartments acting as precursors for cell walls.
These form where hydrogen-rich fluids emerge from below the sea floor, as a result of serpentinization of ultra-maficolivine
with seawater and a pH interface with carbon dioxide-rich ocean water.
The vents form a sustained chemical energy source derived from redox
reactions, in which electron donors (molecular hydrogen) react with
electron acceptors (carbon dioxide); see iron–sulfur world theory. These are exothermic reactions.
Proposed
model of an early cell powered by external proton gradient near a
deep-sea hydrothermal vent. As long as the membrane (or passive ion
channels within it) is permeable to protons, the mechanism can function
without ion pumps.
Russell demonstrated that alkaline vents create an abiogenic proton motive force chemiosmotic gradient, ideal for abiogenesis. Their microscopic compartments "provide a
natural means of concentrating organic molecules," composed of
iron-sulfur minerals such as mackinawite, endowed these mineral cells with the catalytic properties envisaged by Günter Wächtershäuser. This movement of ions across the membrane depends on two factors:
Diffusion force caused by concentration gradient—all particles including ions diffuse from higher concentration to lower.
Electrostatic force caused by electrical potential gradient—cations like protons H+ diffuse down the electrical potential, anions in the opposite direction.
These two gradients together can be expressed as an electrochemical
gradient, providing energy for abiogenic synthesis. The proton motive
force measures the potential energy stored as proton and voltage
gradients across a membrane (differences in proton concentration and
electrical potential).
The surfaces of mineral particles inside deep-ocean hydrothermal
vents have catalytic properties similar to those of enzymes, and can
create simple organic molecules, such as methanol (CH3OH) and formic, acetic, and pyruvic acids out of the dissolved CO2 in the water, if driven by an applied voltage or by reaction with H2 or H2S.
Starting in 1981, researchers proposed that life might have started at hydrothermal vents, that spontaneous chemistry in the Earth's crust driven by rock–water
interactions at disequilibrium thermodynamically underpinned life's
origin,and that the founding lineages of the archaea and bacteria were H2-dependent autotrophs that used CO2 as their terminal acceptor in energy metabolism. In 2016, Martin suggested that the LUCA "may have depended heavily on the geothermal energy of the vent to survive". That same year, RNA was produced in synthetic alkaline hydrothermal chimneys simulating deep-sea vents. Researchers were able to generate RNA oligomers of up to 4 units in
length. This RNA was synthesized using activated ribonucleotides.
Additionally, these RNA oligomers could only be synthesized under
certain conditions.
Pores at deep sea hydrothermal vents are suggested to have been
occupied by membrane-bound compartments which promoted biochemical
reactions. Metabolic intermediates in the Krebs cycle, gluconeogenesis, amino acid
bio-synthetic pathways, glycolysis, the pentose phosphate pathway, and
including sugars like ribose, and lipid precursors can occur
non-enzymatically at conditions relevant to deep-sea alkaline
hydrothermal vents.
If the deep marine hydrothermal setting was the site, then life could have arisen as early as 4.0–4.2 Gya.
If life evolved in the ocean at depths of more than ten meters, it
would have been shielded both from impacts and the then high levels of
solar ultraviolet radiation. The available energy in hydrothermal vents
is maximized at 100–150 °C, the temperatures at which hyperthermophilic bacteria and thermoacidophilicarchaea live.
Arguments against a vent setting
Arguments against a hydrothermal origin of life state that hyperthermophily was a result of convergent evolution in bacteria and archaea, and that a mesophilic environment is more likely.
Production of prebiotic organic compounds at hydrothermal vents is estimated to be 108 kg/yr. While a large amount of key prebiotic compounds, such as methane, are
found at vents, they are in far lower concentrations than in a
Miller-Urey Experiment environment. Additionally, some organic compounds
originally thought to have been formed at vents are now understood to
have been formed by other geological processes and later inherited by
vents. Methane at alkaline vents, for example, was once thought to have
been synthesized from catalytic synthesis after serpentinization, but is
now understood to more likely come from leached fluid inclusions formed
deeper in oceanic crust from magmatic carbon. The concentrations of methane the rate is 2–4 orders of magnitude lower than those in Miller-Urey experiments.
Other counter-arguments include the inability to concentrate
prebiotic materials, due to strong dilution by seawater. This open
system cycles compounds through vent minerals, leaving little residence
time to accumulate. All modern cells rely on phosphates and potassium for nucleotide
backbone and protein formation respectively, making it likely that the
first life forms shared these functions. These elements were not
available in high quantities in the Archaean oceans, as both primarily
come from the weathering of continental rocks on land, far from vents,
and phosphate is lost into relatively insoluble apatite (calcium
phosphate). However, phosphate can be concentrated in lakes, and modern
analogs exist, such as the most phosphate-rich natural body of water in
the world, Last Chance Lake, Canada. Submarine hydrothermal vents are not conducive to condensation reactions needed for polymerisation of macromolecules.
An older argument was that key polymers were encapsulated in
vesicles after condensation, which supposedly would not happen in
saltwater. However, while salinity inhibits vesicle formation from
low-diversity mixtures of fatty acids, vesicle formation from a broader, more realistic mix of fatty-acid and 1-alkanol species is more resilient.
Importantly, no studies to date have been able to experimentally
demonstrate synthesis of de novo sugars, amino acids, nucleases,
nucleosides, nucleotides, or membrane-forming fatty acids under
plausible vent conditions.
Surface bodies of water provide environments that dry out and rewet. Wet-dry cycles concentrate prebiotic compounds and enable condensation reactions to polymerise macromolecules. Moreover, lakes and ponds receive detrital input from weathering of continental apatite-containing
rocks, the most common source of phosphates. The amount of exposed
continental crust in the Hadean is unknown, but models of early ocean
depths and rates of ocean island and continental crust growth make it
plausible that there was exposed land. Another line of evidence for a surface start to life is the requirement for Ultraviolet radiation (UV) for organism function. UV is necessary for the formation of the U+C nucleotide base pair by partial hydrolysis and nucleobase loss.[260]
Simultaneously, UV can be harmful and sterilising to life, especially
for simple early lifeforms with little ability to repair radiation
damage. Radiation levels from a young Sun were likely greater, and, with
no ozone layer,
harmful shortwave UV rays would reach the surface of Earth. For life to
begin, a shielded environment with influx from UV-exposed sources is
necessary to both benefit and protect from UV. Shielding under ice,
liquid water, mineral surfaces (e.g. clay) or regolith is possible in a
range of surface water settings.
Most branching phylogenies are thermophilic or hyperthermophilic,
making it possible that LUCA and preceding lifeforms were similarly
thermophilic. Hot springs are formed from the heating of groundwater by
geothermal activity. This intersection allows for influxes of material
from deep penetrating waters and from surface runoff that transports
eroded continental sediments. Interconnected groundwater systems create a
mechanism for distribution of life to wider area.
Mulkidjanian and co-authors argue that marine environments did
not provide the ionic balance and composition universally found in
cells, or the ions required by essential proteins and ribozymes,
especially with respect to high K+/Na+ ratio, Mn2+, Zn2+ and phosphate concentrations. They argue that the only environments that do this are hot springs similar to ones at Kamchatka. Mineral deposits in these environments under an anoxic atmosphere would
have suitable pH, contain precipitates of photocatalytic sulfide
minerals that absorb harmful ultraviolet radiation, and have wet-dry
cycles that concentrate substrate solutions enough for spontaneous
formation of biopolymers created both by chemical reactions in the hydrothermal environment, and by exposure to UV light during transport from vents to adjacent pools. The hypothesized pre-biotic environments are similar to hydrothermal
vents, with additional components that help explain peculiarities of the
LUCA.
A phylogenomic and geochemical analysis of proteins plausibly
traced to the LUCA shows that the ionic composition of its intracellular
fluid is identical to that of hot springs. The LUCA likely was
dependent upon synthesized organic matter for its growth. Experiments show that RNA-like polymers can be synthesized in wet-dry
cycling and UV light exposure. These polymers were encapsulated in
vesicles after condensation. Potential sources of organics at hot springs might have been transport
by interplanetary dust particles, extraterrestrial projectiles, or
atmospheric or geochemical synthesis. Hot springs could have been
abundant in volcanic landmasses during the Hadean.
Temperate surface bodies of water
A mesophilic start in surface bodies of waters hypothesis has evolved from Darwin's concept of a 'warm little pond' and the Oparin-Haldane hypothesis.
Freshwater bodies under temperate climates can accumulate prebiotic
materials while providing suitable environmental conditions conducive to
simple life forms. The Archaean climate is uncertain. Atmospheric
reconstructions from geochemical proxies and models suggest that
sufficient greenhouse gases were present to maintain surface
temperatures between 0–40 °C. If so, the temperature was suitable for
life could begin.
Evidence for mesophily from biomolecular studies includes Galtier's G+C
nucleotide thermometer. G+C are more abundant in thermophiles due to
the added stability of an additional hydrogen bond not present between
A+T nucleotides. rRNA sequencing of modern lifeforms shows that LUCA's reconstructed G+C content was likely representative of moderate temperatures.
The diversity of thermophiles today could be a product of
convergent evolution and horizontal gene transfer rather than an
inherited trait from LUCA.[267] The reverse gyrasetopoisomerase is found exclusively in thermophiles and hyperthermophiles, as it allows for coiling of DNA. This enzyme requires the complex molecule ATP
to function. If an origin of life is hypothesised to involve a simple
organism that had not yet evolved a membrane, let alone ATP, this would
make the existence of reverse gyrase improbable. Moreover, phylogenetic
studies show that reverse gyrase originated in archaea, and transferred
to bacteria by horizontal gene transfer, implying it was not present in
the LUCA.
Icy surface bodies of water
Cold-start theories presuppose large ice-covered regions. Stellar
evolution models predict that the Sun's luminosity was ≈25% weaker than
it is today. Fuelner states that although this significant decrease in
solar energy would have formed an icy planet, there is strong evidence
for the presence of liquid water, possibly driven by a greenhouse
effect. This would mean an early Earth with both liquid oceans and icy
poles.
Ice melts that form from ice sheets or glacier melts create
freshwater pools, another niche capable of wet-dry cycles. While surface
pools would be exposed to intense UV radiation, bodies of water within
and under ice would be shielded, while remaining connected to exposed
areas through ice cracks. Impact melting would allow freshwater and
meteoritic input, creating prebiotic components. Near-seawater levels of sodium chloride destabilize fatty acid membrane
self-assembly, making freshwater settings appealing for early
membranous life.
Icy environments would trade the faster reaction rates that occur
in warm environments for increased stability and accumulation of larger
polymers. Experiments simulating Europa-like conditions of ≈20 °C have
synthesised amino acids and adenine, showing that Miller-Urey type
syntheses can occur at low temperatures. In an RNA world,
the ribozyme would have had even more functions than in a later
DNA-RNA-protein-world. For RNA to function, it must be able to fold, a
process hindered by temperatures above 30 °C. While RNA folding in psychrophilic
organisms is slower, so is hydrolysis, so folding is more successful.
Shorter nucleotides would not suffer from higher temperatures.
Inside the continental crust
An alternative geological environment has been proposed by the
geologist Ulrich Schreiber and the physical chemist Christian Mayer: the
continental crust. Tectonic fault
zones could present a stable and well-protected environment for
long-term prebiotic evolution. Inside these systems of cracks and
cavities, water and carbon dioxide present the bulk solvents. Their
phase state could vary between liquid, gaseous and supercritical,
depending on pressure and temperature. When forming two separate phases
(e.g. liquid water and supercritical carbon dioxide in depths of little
more than 1 km), the system provides optimal conditions for phase transfer reactions.
Concurrently, the contents of the tectonic fault zones are being
supplied by a multitude of inorganic educts (e.g. carbon monoxide,
hydrogen, ammonia, hydrogen cyanide, nitrogen, and even phosphate from
dissolved apatite) and simple organic molecules formed by hydrothermal
chemistry (e.g. amino acids, long-chain amines, fatty acids, long-chain
aldehydes).
Part of the tectonic fault zones is at a depth of around 1000 m.
For the carbon dioxide part of the bulk solvent, it provides temperature
and pressure conditions near the phase transition point between the supercritical and the gaseous state. This allows lipophilic organic molecules that dissolve well in supercritical CO2 to accumulate, but not in its gaseous state, leading to their local precipitation. Periodic pressure variations such as caused by geysers or tidal influences result in periodic phase transitions, keeping the local reaction environment in a constant non-equilibrium state. In presence of amphiphilic compounds (such as the long chain amines and fatty acids), subsequent generations of vesicles are formed that are constantly selected for their stability.
Many biomolecules, such as L-glutamic acid, are asymmetric, and occur in living systems in only one of the two possible forms, in the case of amino acids the left-handed form. Prebiotic chemistry would produce both forms, creating a puzzle for abiogenesis researchers.
Homochirality is the uniformity of materials composed of chiral (non-mirror-symmetric) units. Living organisms use molecules with the same chirality: with almost no exceptions, amino acids are left-handed while nucleotides and sugars
are right-handed. Chiral molecules can be synthesized, but in the
absence of a chiral source or a chiral catalyst, they are formed in a
50/50 (racemic) mixture of both forms. Non-racemic mixtures can arise from racemic materials by asymmetric physical laws such as the electroweak interaction or asymmetric environments such as circularly polarized light.
Once established, chirality would be selected for. A small bias in the population can be amplified by asymmetric autocatalysis, as in the Soai reaction, where a chiral molecule catalyzes its own production.
![LUCA systems and environment included the Wood–Ljungdahl pathway.[10]](https://en.wikipedia.org/wiki/File:LUCA_systems_and_environment.svg)
