In astronomy, metallicity is the abundance of elements present in an object that are heavier than hydrogen and helium. Most of the normal physical matter in the Universe is either hydrogen or helium, and astronomers use the word "metals" as a convenient short term for "all elements except hydrogen and helium". This word-use is distinct from the conventional chemical or physical definition of a metal as an electrically conducting solid. Stars and nebulae
with relatively high abundances of heavier elements are called
"metal-rich" in astrophysical terms, even though many of those elements
are non-metals in chemistry.
The presence of heavier elements hails from stellar nucleosynthesis, where the majority of elements heavier than hydrogen and helium in the Universe (metals, hereafter) are formed in the cores of stars as they evolve. Over time, stellar winds and supernovae deposit the metals into the surrounding environment, enriching the interstellar medium and providing recycling materials for the birth of new stars. It follows that older generations of stars, which formed in the metal-poor early Universe, generally have lower metallicities than those of younger generations, which formed in a more metal-rich Universe.
Observed changes in the chemical abundances of different types of
stars, based on the spectral peculiarities that were later attributed
to metallicity, led astronomer Walter Baade in 1944 to propose the existence of two different populations of stars.
These became commonly known as Population I (metal-rich) and Population II (metal-poor) stars. A third stellar population was introduced in 1978, known as Population III stars. These extremely metal-poor stars were theorized to have been the "first-born" stars created in the Universe.
Common methods of calculation
Astronomers
use several different methods to describe and approximate metal
abundances, depending on the available tools and the object of interest.
Some methods include determining the fraction of mass that is
attributed to gas versus metals, or measuring the ratios of the number of atoms of two different elements as compared to the ratios found in the Sun.
Mass fraction
Stellar composition is often simply defined by the parameters X, Y and Z. Here X is the mass fraction of hydrogen, Y is the mass fraction of helium, and Z is the mass fraction of all the remaining chemical elements. Thus
In most stars, nebulae, H II regions,
and other astronomical sources, hydrogen and helium are the two
dominant elements. The hydrogen mass fraction is generally expressed as , where is the total mass of the system, and is the mass of the hydrogen it contains. Similarly, the helium mass fraction is denoted as .
The remainder of the elements are collectively referred to as "metals",
and the metallicity—the mass fraction of elements heavier than
helium—can be calculated as
For the surface of the Sun, these parameters are measured to have the following values:
Description
Solar value
Hydrogen mass fraction
Helium mass fraction
Metallicity
Due to the effects of stellar evolution,
neither the initial composition nor the present day bulk composition of
the Sun is the same as its present-day surface composition.
Chemical abundance ratios
The overall stellar metallicity is conventionally defined using the total hydrogen content, since its abundance is considered to be relatively constant in the Universe, or the iron content of the star, which has an abundance that is generally linearly increasing in the Universe. Iron is also relatively easy to measure with spectral observations in the star's spectrum given the large number of iron lines in the star's spectra (even though oxygen is the most abundant heavy element – see metallicities in HII regions below). The abundance ratio is the common logarithm of the ratio of a star's iron abundance compared to that of the Sun and is calculated thus:
where and are the number of iron and hydrogen atoms per unit of volume respectively. The unit often used for metallicity is the dex, contraction of "decimal exponent". By this formulation, stars with a higher metallicity than the Sun have a positive common logarithm,
whereas those more dominated by hydrogen have a corresponding negative
value. For example, stars with a [Fe/H] value of +1 have 10 times the
metallicity of the Sun (101); conversely, those with a [Fe/H] value of −1 have 1⁄10, while those with a [Fe/H] value of 0 have the same metallicity as the Sun, and so on. Young Population I stars have significantly higher iron-to-hydrogen ratios than older Population II stars. Primordial Population III stars are estimated to have metallicity less than −6, a millionth of the abundance of iron in the Sun.
The same notation is used to express variations in abundances between
other individual elements as compared to solar proportions. For example,
the notation "[O/Fe]" represents the difference in the logarithm of the
star's oxygen abundance versus its iron content compared to that of the
Sun. In general, a given stellar nucleosynthetic
process alters the proportions of only a few elements or isotopes, so a
star or gas sample with certain [/Fe] values may well be indicative of
an associated, studied nuclear process.
Photometric colors
Astronomers can estimate metallicities through measured and calibrated systems that correlate photometric measurements and spectroscopic measurements (see also Spectrophotometry). For example, the Johnson UVB filters can be used to detect an ultraviolet (UV) excess in stars, where a smaller UV excess indicates a larger presence of metals that absorb the UV radiation, thereby making the star appear "redder". The UV excess, δ(U−B), is defined as the difference between a star's U and B band magnitudes, compared to the difference between U and B band magnitudes of metal-rich stars in the Hyades cluster. Unfortunately, δ(U−B) is sensitive to both metallicity and temperature: if two stars are equally metal-rich, but one is cooler than the other, they will likely have different δ(U−B) values (see also Blanketing effect). To help mitigate this degeneracy, a star's B−V color
can be used as an indicator for temperature. Furthermore, the UV excess
and B−V color can be corrected to relate the δ(U−B) value to iron
abundances.
Other photometric systems that can be used to determine metallicities of certain astrophysical objects include the Strӧmgren system, the Geneva system, the Washington system, and the DDO system.
Metallicities in various astrophysical objects
Stars
At a given mass and age, a metal-poor star will be slightly warmer. Population II stars' metallicities are roughly 1/1000 to 1/10 of the Sun's ([Z/H] = −3.0 to −1.0), but the group appears cooler than Population I overall, as heavy Population II stars have long since died. Above 40 solar masses, metallicity influences how a star will die: outside the pair-instability window, lower metallicity stars will collapse directly to a black hole, while higher metallicity stars undergo a Type Ib/c supernova and may leave a neutron star.
Relationship between stellar metallicity and planets
A star's metallicity measurement is one parameter that helps determine whether a star may have a giant planet,
as there is a direct correlation between metallicity and the presence
of a giant planet. Measurements have demonstrated the connection between
a star's metallicity and gas giant planets, like Jupiter and Saturn. The more metals in a star and thus its planetary system and proplyd,
the more likely the system may have gas giant planets. Current models
show that the metallicity along with the correct planetary system
temperature and distance from the star are key to planet and planetesimal formation. For two stars that have equal age and mass but different metallicity, the less metallic star is bluer. Among stars of the same color, less metallic stars emit more ultraviolet radiation. The Sun, with 8 planets and 5 known dwarf planets, is used as the reference, with a [Fe/H] of 0.00.
HII regions
Young, massive and hot stars (typically of spectral types O and B) in H II regions emit UV photons that ionize ground-statehydrogen atoms, knocking electrons and protons free; this process is known as photoionization. The free electrons can strike other atoms nearby, exciting bound metallic electrons into a metastable state, which eventually decay back into a ground state, emitting photons with energies that correspond to forbidden lines.
Through these transitions, astronomers have developed several
observational methods to estimate metal abundances in HII regions, where
the stronger the forbidden lines in spectroscopic observations, the
higher the metallicity.
These methods are dependent on one or more of the following: the
variety of asymmetrical densities inside HII regions, the varied
temperatures of the embedded stars, and/or the electron density within
the ionized region.
Theoretically, to determine the total abundance of a single
element in an HII region, all transition lines should be observed and
summed. However, this can be observationally difficult due to variation
in line strength. Some of the most common forbidden lines used to determine metal abundances in HII regions are from oxygen (e.g. [O II] λ = (3727, 7318, 7324) Å, and [O III] λ = (4363, 4959, 5007) Å), nitrogen (e.g. [NII] λ = (5755, 6548, 6584) Å), and sulfur (e.g. [SII] λ = (6717,6731) Å and [SIII] λ = (6312, 9069, 9531) Å) in the optical spectrum, and the [OIII] λ = (52, 88) μm and [NIII] λ = 57 μm lines in the infrared spectrum. Oxygen
has some of the stronger, more abundant lines in HII regions, making it
a main target for metallicity estimates within these objects. To
calculate metal abundances in HII regions using oxygen flux measurements, astronomers often use the R23 method, in which
where is the sum of the fluxes from oxygen emission lines measured at the rest frame λ = (3727, 4959 and 5007) Å wavelengths, divided by the flux from the Hβ emission line at the rest frame λ = 4861 Å wavelength. This ratio is well defined through models and observational studies,
but caution should be taken, as the ratio is often degenerate,
providing both a low and high metallicity solution, which can be broken
with additional line measurements. Similarly, other strong forbidden line ratios can be used, e.g. for sulfur, where
Metal abundances within HII regions are typically less than 1%, with
the percentage decreasing on average with distance from the Galactic Center.
False-color Cassini radar mosaic of Titan's north polar region; the blue areas are lakes of liquid hydrocarbons.
"The
existence of lakes of liquid hydrocarbons on Titan opens up the
possibility for solvents and energy sources that are alternatives to
those in our biosphere and that might support novel life forms
altogether different from those on Earth."—NASA Astrobiology Roadmap
2008
Hypothetical types of biochemistry are forms of biochemistry agreed to be scientifically viable but not proven to exist at this time. The kinds of living organisms currently known on Earth all use carbon compounds for basic structural and metabolic functions, water as a solvent, and DNA or RNA to define and control their form. If life exists on other planets or moons it may be chemically similar, though it is also possible that there are organisms with quite different chemistries –
for instance, involving other classes of carbon compounds, compounds
of another element, or another solvent in place of water.
The possibility of life-forms being based on "alternative"
biochemistries is the topic of an ongoing scientific discussion,
informed by what is known about extraterrestrial environments and about
the chemical behaviour of various elements and compounds. It is of
interest in synthetic biology and is also a common subject in science fiction.
The element silicon has been much discussed as a hypothetical alternative to carbon. Silicon is in the same group as carbon on the periodic table and, like carbon, it is tetravalent. Hypothetical alternatives to water include ammonia, which, like water, is a polar molecule, and cosmically abundant; and non-polar hydrocarbon solvents such as methane and ethane, which are known to exist in liquid form on the surface of Titan.
Perhaps the least unusual alternative biochemistry would be one with differing chirality of its biomolecules. In known Earth-based life, amino acids are almost universally of the L form and sugars are of the D form. Molecules using D amino acids or L sugars are possible, though would be incompatible with organisms using the opposing chirality molecules.
Ammonia biochemistry
Non-water solvents
Ammonia-based life
Ammonia is relatively abundant in the universe and has chemical similarities to water. The possible role of liquid ammonia as an alternative solvent for life is an idea that goes back at least to 1954, when J. B. S. Haldane raised the topic at a symposium about life's origin.
Arsenic, which is chemically similar to phosphorus, while poisonous for most life forms on Earth, is incorporated into the biochemistry of some organisms.
Boranes
are dangerously explosive in Earth's atmosphere, but would be more
stable in a reducing environment. Boron, however, is exceedingly rare
in the universe in comparison to its neighbours carbon, nitrogen, and
oxygen. On the other hand, structures containing alternating boron and
nitrogen atoms share some properties with hydrocarbons.
Cosmic necklace-based biology
Nonplanetary life
Non-chemical life
In 2020, Luis A. Anchordoqu and Eugene M. Chudnovsky hypothesized that life composed of magnetic semipoles connected by cosmic strings could evolve inside stars
In 2007, Vadim N. Tsytovich and colleagues proposed that lifelike behaviors could be exhibited by dust particles suspended in a plasma, under conditions that might exist in space.
Methane (CH4) is relatively abundant in the solar system and the universe, and is known to exist in liquid form on Titan, the largest moon of Saturn.
Non-green photosynthesizers
Other speculations
Alternate plant life
Physicists have noted that, although photosynthesis on Earth
generally involves green plants, a variety of other-colored plants could
also support photosynthesis, essential for most life on Earth, and that
other colors might be preferred in places that receive a different mix
of stellar radiation than Earth. In particular, retinal is capable of, and has been observed to, perform photosynthesis. Bacteria capable of photosynthesis are known as microbial rhodopsins. A plant or creature that uses retinal photosynthesis is always purple.
A shadow biosphere is a hypothetical microbialbiosphere of Earth that uses radically different biochemical and molecular processes than currently known life.
Like carbon, silicon can create molecules that are sufficiently
large to carry biological information; however, the scope of possible
silicon chemistry is far more limited than that of carbon.
Gerald Feinberg and Robert Shapiro
have suggested that molten silicate rock could serve as a liquid medium
for organisms with a chemistry based on silicon, oxygen, and other
elements such as aluminium.
Sulfur biochemistry
Alternative biochemistry
Sulfur-based life
The biological use of sulfur as an alternative to carbon is purely
hypothetical, especially because sulfur usually forms only linear chains
rather than branched ones.
Alternative nucleic acids
Alternative biochemistry
Different genetic storage
Xeno nucleic acids
(XNA) may possibly be used in place of RNA or DNA. XNA is the general
term for a nucleic acid with an altered sugar backbone. Examples of XNA
include TNA, which uses threose, HNA, which uses 1,5-anhydrohexitol, GNA, which uses glycol, CeNA, which uses cyclohexene, LNA, which utilizes a form of ribose that contains an extra linkage between its 4' carbon and 2' oxygen, FANA, which uses arabinose
but with a single fluorine atom attached to its 2' carbon, and PNA,
which uses, in place of sugar and phosphate, N-(2-aminoethyl)-glycine
units connected by peptide bonds. In comparison, Hachimoji DNA changes the base pairs instead of the backbone. These new base pairs are P (2-Aminoimidazo[1,2a][1,3,5]triazin-4(1H)-one), Z (6-Amino-5-nitropyridin-2-one), B (Isoguanine), and S (rS = Isocytosine for RNA, dS = 1-Methylcytosine for DNA).
The Arecibo message (1974) sent information into space about basic chemistry of Earth life.
A shadow biosphere is a hypothetical microbialbiosphere of Earth that uses radically different biochemical and molecular processes than currently known life.
Although life on Earth is relatively well-studied, the shadow biosphere
may still remain unnoticed because the exploration of the microbial
world targets primarily the biochemistry of the macro-organisms.
Alternative-chirality biomolecules
Perhaps the least unusual alternative biochemistry would be one with differing chirality of its biomolecules. In known Earth-based life, amino acids are almost universally of the L form and sugars are of the D form. Molecules using D amino acids or L
sugars may be possible; molecules of such a chirality, however, would
be incompatible with organisms using the opposing chirality molecules.
Amino acids whose chirality is opposite to the norm are found on Earth,
and these substances are generally thought to result from decay of
organisms of normal chirality. However, physicist Paul Davies speculates that some of them might be products of "anti-chiral" life.
It is questionable, however, whether such a biochemistry would be truly alien. Although it would certainly be an alternative stereochemistry, molecules that are overwhelmingly found in one enantiomer throughout the vast majority of organisms can nonetheless often be found in another enantiomer in different (often basal) organisms such as in comparisons between members of Archaea and other domains, making it an open topic whether an alternative stereochemistry is truly novel.
Non-carbon-based biochemistries
On
Earth, all known living things have a carbon-based structure and
system. Scientists have speculated about the pros and cons of using atoms
other than carbon to form the molecular structures necessary for life,
but no one has proposed a theory employing such atoms to form all the
necessary structures. However, as Carl Sagan
argued, it is very difficult to be certain whether a statement that
applies to all life on Earth will turn out to apply to all life
throughout the universe. Sagan used the term "carbon chauvinism" for such an assumption. He regarded silicon and germanium as conceivable alternatives to carbon (other plausible elements include but are not limited to palladium and titanium); but, on the other hand, he noted that carbon does seem more chemically versatile and is more abundant in the cosmos). Norman Horowitz devised the experiments to determine whether life might exist on Mars that were carried out by the Viking Lander of 1976,
the first U.S. mission to successfully land an unmanned probe on the
surface of Mars. Horowitz argued that the great versatility of the
carbon atom makes it the element most likely to provide solutions, even
exotic solutions, to the problems of survival on other planets.
He considered that there was only a remote possibility that non-carbon
life forms could exist with genetic information systems capable of
self-replication and the ability to evolve and adapt.
Marine diatoms –
carbon-based organisms that extract silicon from sea water, in the
form of its oxide (silica) and incorporate it into their cell walls
The silicon atom has been much discussed as the basis for an alternative biochemical system, because silicon has many chemical properties similar to those of carbon and is in the same group of the periodic table, the carbon group. Like carbon, silicon can create molecules that are sufficiently large to carry biological information.
However, silicon has several drawbacks as an alternative to
carbon. Silicon, unlike carbon, lacks the ability to form chemical bonds
with diverse types of atoms as is necessary for the chemical
versatility required for metabolism, and yet this precise inability is
what makes silicon less susceptible to bond with all sorts of impurities
from which carbon, in comparison, is not shielded. Elements creating
organic functional groups with carbon include hydrogen, oxygen,
nitrogen, phosphorus, sulfur, and metals such as iron, magnesium, and
zinc. Silicon, on the other hand, interacts with very few other types of
atoms.
Moreover, where it does interact with other atoms, silicon creates
molecules that have been described as "monotonous compared with the
combinatorial universe of organic macromolecules". This is because silicon atoms are much bigger, having a larger mass and atomic radius, and so have difficulty forming double bonds (the double-bonded carbon is part of the carbonyl group, a fundamental motif of carbon-based bio-organic chemistry).
Silanes, which are chemical compounds of hydrogen and silicon that are analogous to the alkanehydrocarbons, are highly reactive with water, and long-chain silanes spontaneously decompose. Molecules incorporating polymers of alternating silicon and oxygen atoms instead of direct bonds between silicon, known collectively as silicones,
are much more stable. It has been suggested that silicone-based
chemicals would be more stable than equivalent hydrocarbons in a
sulfuric-acid-rich environment, as is found in some extraterrestrial
locations.
Of the varieties of molecules identified in the interstellar medium as of 1998, 84 are based on carbon, while only 8 are based on silicon. Moreover, of those 8 compounds, 4 also include carbon within them. The cosmic abundance
of carbon to silicon is roughly 10 to 1. This may suggest a greater
variety of complex carbon compounds throughout the cosmos, providing
less of a foundation on which to build silicon-based biologies, at least
under the conditions prevalent on the surface of planets. Also, even
though Earth and other terrestrial planets
are exceptionally silicon-rich and carbon-poor (the relative abundance
of silicon to carbon in Earth's crust is roughly 925:1), terrestrial
life is carbon-based. The fact that carbon is used instead of silicon
may be evidence that silicon is poorly suited for biochemistry on
Earth-like planets. Reasons for this may be that silicon is less
versatile than carbon in forming compounds, that the compounds formed by
silicon are unstable, and that it blocks the flow of heat.
Even so, biogenic silica is used by some Earth life, such as the silicate skeletal structure of diatoms. According to the clay hypothesis of A. G. Cairns-Smith, silicate minerals in water played a crucial role in abiogenesis: they replicated their crystal structures, interacted with carbon compounds, and were the precursors of carbon-based life.
Although not observed in nature, carbon–silicon bonds have been
added to biochemistry by using directed evolution (artificial
selection). A heme containing cytochrome c protein from Rhodothermus marinus has been engineered using directed evolution to catalyze the formation of new carbon–silicon bonds between hydrosilanes and diazo compounds.
Silicon compounds may possibly be biologically useful under
temperatures or pressures different from the surface of a terrestrial
planet, either in conjunction with or in a role less directly analogous
to carbon. Polysilanols, the silicon compounds corresponding to sugars, are soluble in liquid nitrogen, suggesting that they could play a role in very-low-temperature biochemistry.
In cinematic and literary science fiction, at a moment when
man-made machines cross from nonliving to living, it is often posited, this new form would be the first example of non-carbon-based life. Since the advent of the microprocessor in the late 1960s, these machines are often classed as computers (or computer-guided robots)
and filed under "silicon-based life", even though the silicon backing
matrix of these processors is not nearly as fundamental to their
operation as carbon is for "wet life".
Boranes are dangerously explosive in Earth's atmosphere, but would be more stable in a reducing atmosphere. However, boron's low cosmic abundance makes it less likely as a base for life than carbon.
Various metals, together with oxygen, can form very complex and thermally stable structures rivaling those of organic compounds; the heteropoly acids
are one such family. Some metal oxides are also similar to carbon in
their ability to form both nanotube structures and diamond-like crystals
(such as cubic zirconia). Titanium, aluminium, magnesium, and iron
are all more abundant in the Earth's crust than carbon.
Metal-oxide-based life could therefore be a possibility under certain
conditions, including those (such as high temperatures) at which
carbon-based life would be unlikely. The Cronin group at Glasgow
University reported self-assembly of tungsten polyoxometalates into cell-like spheres.
By modifying their metal oxide content, the spheres can acquire holes
that act as porous membrane, selectively allowing chemicals in and out
of the sphere according to size.
Sulfur
is also able to form long-chain molecules, but suffers from the same
high-reactivity problems as phosphorus and silanes. The biological use
of sulfur as an alternative to carbon is purely hypothetical, especially
because sulfur usually forms only linear chains rather than branched
ones. (The biological use of sulfur as an electron acceptor is
widespread and can be traced back 3.5 billion years on Earth, thus
predating the use of molecular oxygen. Sulfur-reducing bacteria can utilize elemental sulfur instead of oxygen, reducing sulfur to hydrogen sulfide.)
Arsenic, which is chemically similar to phosphorus, while poisonous for most life forms on Earth, is incorporated into the biochemistry of some organisms. Some marine algae incorporate arsenic into complex organic molecules such as arsenosugars and arsenobetaines. Fungi and bacteria can produce volatile methylated arsenic compounds. Arsenate reduction and arsenite oxidation have been observed in microbes (Chrysiogenes arsenatis). Additionally, some prokaryotes
can use arsenate as a terminal electron acceptor during anaerobic
growth and some can utilize arsenite as an electron donor to generate
energy.
It has been speculated that the earliest life forms on Earth may have used arsenic biochemistry in place of phosphorus in the structure of their DNA. A common objection to this scenario is that arsenate esters are so much less stable to hydrolysis than corresponding phosphate esters that arsenic is poorly suited for this function.
The authors of a 2010 geomicrobiology study, supported in part by NASA, have postulated that a bacterium, named GFAJ-1, collected in the sediments of Mono Lake in eastern California, can employ such 'arsenic DNA' when cultured without phosphorus. They proposed that the bacterium may employ high levels of poly-β-hydroxybutyrate or other means to reduce the effective concentration of water and stabilize its arsenate esters. This claim was heavily criticized almost immediately after publication for the perceived lack of appropriate controls. Science writer Carl Zimmer
contacted several scientists for an assessment: "I reached out to a
dozen experts ... Almost unanimously, they think the NASA scientists
have failed to make their case".
Other authors were unable to reproduce their results and showed that the
study had issues with phosphate contamination, suggesting that the low
amounts present could sustain extremophile lifeforms.
Alternatively, it was suggested that GFAJ-1 cells grow by recycling
phosphate from degraded ribosomes, rather than by replacing it with
arsenate.
Non-water solvents
In
addition to carbon compounds, all currently known terrestrial life also
requires water as a solvent. This has led to discussions about whether
water is the only liquid capable of filling that role. The idea that an
extraterrestrial life-form might be based on a solvent other than water
has been taken seriously in recent scientific literature by the
biochemist Steven Benner, and by the astrobiological committee chaired by John A. Baross. Solvents discussed by the Baross committee include ammonia, sulfuric acid, formamide, hydrocarbons, and (at temperatures much lower than Earth's) liquid nitrogen, or hydrogen in the form of a supercritical fluid.
Carl Sagan once described himself as both a carbon chauvinist and a water chauvinist; however, on another occasion he said that he was a carbon chauvinist but "not that much of a water chauvinist".
He speculated on hydrocarbons,hydrofluoric acid, and ammonia as possible alternatives to water.
Some of the properties of water that are important for life processes include:
A complexity which leads to a large number of permutations of possible reaction paths including acid–base chemistry, H+ cations, OH−
anions, hydrogen bonding, van der Waals bonding, dipole–dipole and
other polar interactions, aqueous solvent cages, and hydrolysis. This
complexity offers a large number of pathways for evolution to produce
life, many other solvents have dramatically fewer possible reactions, which severely limits evolution.
Thermodynamic stability: the free energy of formation of liquid
water is low enough (−237.24 kJ/mol) that water undergoes few reactions.
Other solvents are highly reactive, particularly with oxygen.
Water does not combust in oxygen because it is already the
combustion product of hydrogen with oxygen. Most alternative solvents
are not stable in an oxygen-rich atmosphere, so it is highly unlikely
that those liquids could support aerobic life.
A large temperature range over which it is liquid.
High solubility of oxygen and carbon dioxide at room temperature
supporting the evolution of aerobic aquatic plant and animal life.
A high heat capacity (leading to higher environmental temperature stability).
Water is a room-temperature liquid leading to a large population of
quantum transition states required to overcome reaction barriers.
Cryogenic liquids (such as liquid methane) have exponentially lower
transition state populations which are needed for life based on chemical
reactions. This leads to chemical reaction rates which may be so slow
as to preclude the development of any life based on chemical reactions.
Spectroscopic transparency allowing solar radiation to penetrate
several meters into the liquid (or solid), greatly aiding the evolution
of aquatic life.
The ability to dissolve a wide variety of compounds.
The solid (ice) has lower density than the liquid, so ice floats on
the liquid. This is why bodies of water freeze over but do not freeze
solid (from the bottom up). If ice were denser than liquid water (as is
true for nearly all other compounds), then large bodies of liquid would
slowly freeze solid, which would not be conducive to the formation of
life.
Water as a compound is cosmically abundant, although much of it is in
the form of vapour or ice. Subsurface liquid water is considered likely
or possible on several of the outer moons: Enceladus (where geysers have been observed), Europa, Titan, and Ganymede. Earth and Titan are the only worlds currently known to have stable bodies of liquid on their surfaces.
Not all properties of water are necessarily advantageous for life, however. For instance, water ice has a high albedo, meaning that it reflects a significant quantity of light and heat from the Sun. During ice ages, as reflective ice builds up over the surface of the water, the effects of global cooling are increased.
There are some properties that make certain compounds and
elements much more favorable than others as solvents in a successful
biosphere. The solvent must be able to exist in liquid equilibrium over a
range of temperatures the planetary object would normally encounter.
Because boiling points vary with the pressure, the question tends not to
be does the prospective solvent remain liquid, but at what pressure. For example, hydrogen cyanide has a narrow liquid-phase temperature range at 1 atmosphere, but in an atmosphere with the pressure of Venus, with 92 bars (91 atm) of pressure, it can indeed exist in liquid form over a wide temperature range.
Ammonia
Artist's conception of how a planet with ammonia-based life might look
The ammonia molecule (NH3),
like the water molecule, is abundant in the universe, being a compound
of hydrogen (the simplest and most common element) with another very
common element, nitrogen. The possible role of liquid ammonia as an alternative solvent for life is an idea that goes back at least to 1954, when J. B. S. Haldane raised the topic at a symposium about life's origin.
Numerous chemical reactions are possible in an ammonia solution, and liquid ammonia has chemical similarities with water.Ammonia can dissolve most organic molecules at least as well as water
does and, in addition, it is capable of dissolving many elemental
metals. Haldane made the point that various common water-related organic
compounds have ammonia-related analogs; for instance the
ammonia-related amine group (−NH2) is analogous to the water-related hydroxyl group (−OH).
Ammonia, like water, can either accept or donate an H+ ion. When ammonia accepts an H+, it forms the ammonium cation (NH4+), analogous to hydronium (H3O+). When it donates an H+ ion, it forms the amide anion (NH2−), analogous to the hydroxide anion (OH−). Compared to water, however, ammonia is more inclined to accept an H+ ion, and less inclined to donate one; it is a stronger nucleophile. Ammonia added to water functions as Arrhenius base: it increases the concentration of the anion hydroxide. Conversely, using a solvent system definition
of acidity and basicity, water added to liquid ammonia functions as an
acid, because it increases the concentration of the cation ammonium.
The carbonyl group (C=O), which is much used in terrestrial
biochemistry, would not be stable in ammonia solution, but the analogous
imine group (C=NH) could be used instead.
However, ammonia has some problems as a basis for life. The hydrogen bonds between ammonia molecules are weaker than those in water, causing ammonia's heat of vaporization to be half that of water, its surface tension to be a third, and reducing its ability to concentrate non-polar molecules through a hydrophobic
effect. Gerald Feinberg and Robert Shapiro have questioned whether
ammonia could hold prebiotic molecules together well enough to allow the
emergence of a self-reproducing system. Ammonia is also flammable in oxygen and could not exist sustainably in an environment suitable for aerobic metabolism.
Titan's theorized internal structure, subsurface ocean shown in blue
A biosphere
based on ammonia would likely exist at temperatures or air pressures
that are extremely unusual in relation to life on Earth. Life on Earth
usually exists within the melting point and boiling point of water at normal pressure, between 0 °C (273 K)
and 100 °C (373 K); at normal pressure ammonia's melting and boiling
points are between −78 °C (195 K) and −33 °C (240 K). Chemical reactions
generally proceed more slowly at a lower temperature. Therefore,
ammonia-based life, if it exists, might metabolize more slowly and
evolve more slowly than life on Earth.
On the other hand, lower temperatures could also enable living systems
to use chemical species that would be too unstable at Earth temperatures
to be useful.
Ammonia could be a liquid at Earth-like temperatures, but at much higher pressures; for example, at 60 atm, ammonia melts at −77 °C (196 K) and boils at 98 °C (371 K).
Ammonia and ammonia–water mixtures remain liquid at temperatures
far below the freezing point of pure water, so such biochemistries might
be well suited to planets and moons orbiting outside the water-based habitability zone. Such conditions could exist, for example, under the surface of Saturn's largest moon Titan.
Methane and other hydrocarbons
Methane (CH4)
is a simple hydrocarbon: that is, a compound of two of the most common
elements in the cosmos: hydrogen and carbon. It has a cosmic abundance
comparable with ammonia. Hydrocarbons could act as a solvent over a wide range of temperatures, but would lack polarity. Isaac Asimov, the biochemist and science fiction writer, suggested in 1981 that poly-lipids could form a substitute for proteins in a non-polar solvent such as methane. Lakes composed of a mixture of hydrocarbons, including methane and ethane, have been detected on the surface of Titan by the Cassini spacecraft.
There is debate about the effectiveness of methane and other hydrocarbons as a solvent for life compared to water or ammonia. Water is a stronger solvent than the hydrocarbons, enabling easier transport of substances in a cell. However, water is also more chemically reactive and can break down large organic molecules through hydrolysis. A life-form whose solvent was a hydrocarbon would not face the threat of its biomolecules being destroyed in this way.
Also, the water molecule's tendency to form strong hydrogen bonds can
interfere with internal hydrogen bonding in complex organic molecules. Life with a hydrocarbon solvent could make more use of hydrogen bonds within its biomolecules. Moreover, the strength of hydrogen bonds within biomolecules would be appropriate to a low-temperature biochemistry.
Astrobiologist Chris McKay
has argued, on thermodynamic grounds, that if life does exist on
Titan's surface, using hydrocarbons as a solvent, it is likely also to
use the more complex hydrocarbons as an energy source by reacting them
with hydrogen, reducing ethane and acetylene to methane. Possible evidence for this form of life on Titan was identified in 2010 by Darrell Strobel of Johns Hopkins University;
a greater abundance of molecular hydrogen in the upper atmospheric
layers of Titan compared to the lower layers, arguing for a downward
diffusion at a rate of roughly 1025 molecules per second and
disappearance of hydrogen near Titan's surface. As Strobel noted, his
findings were in line with the effects Chris McKay had predicted if methanogenic life-forms were present.
The same year, another study showed low levels of acetylene on Titan's
surface, which were interpreted by Chris McKay as consistent with the
hypothesis of organisms reducing acetylene to methane.
While restating the biological hypothesis, McKay cautioned that other
explanations for the hydrogen and acetylene findings are to be
considered more likely: the possibilities of yet unidentified physical
or chemical processes (e.g. a non-living surface catalyst enabling acetylene to react with hydrogen), or flaws in the current models of material flow. He noted that even a non-biological catalyst effective at 95 K would in itself be a startling discovery.
Azotosome
A hypothetical cell membrane termed an azotosome, capable of functioning in liquid methane in Titan conditions was computer-modeled in an article published in February 2015. Composed of acrylonitrile,
a small molecule containing carbon, hydrogen, and nitrogen, it is
predicted to have stability and flexibility in liquid methane comparable
to that of a phospholipid bilayer (the type of cell membrane possessed by all life on Earth) in liquid water.
An analysis of data obtained using the Atacama Large Millimeter /
submillimeter Array (ALMA), completed in 2017, confirmed substantial
amounts of acrylonitrile in Titan's atmosphere.
Hydrogen fluoride
Hydrogen fluoride
(HF), like water, is a polar molecule, and due to its polarity it can
dissolve many ionic compounds. Its melting point is −84 °C, and its
boiling point is 19.54 °C (at atmospheric pressure);
the difference between the two is a little more than 100 K. HF also
makes hydrogen bonds with its neighbor molecules, as do water and
ammonia. It has been considered as a possible solvent for life by
scientists such as Peter Sneath and Carl Sagan.
HF is dangerous to the systems of molecules that Earth-life is made of, but certain other organic compounds, such as paraffin waxes, are stable with it.
Like water and ammonia, liquid hydrogen fluoride supports an acid–base
chemistry. Using a solvent system definition of acidity and basicity, nitric acid functions as a base when it is added to liquid HF.
However, hydrogen fluoride is cosmically rare, unlike water, ammonia, and methane.
Hydrogen sulfide
Hydrogen sulfide is the closest chemical analog to water, but is less polar and a weaker inorganic solvent. Hydrogen sulfide is quite plentiful on Jupiter's moon Io and may be in liquid form a short distance below the surface; astrobiologist Dirk Schulze-Makuch has suggested it as a possible solvent for life there.
On a planet with hydrogen-sulfide oceans the source of the hydrogen
sulfide could come from volcanos, in which case it could be mixed in
with a bit of hydrogen fluoride,
which could help dissolve minerals. Hydrogen-sulfide life might use a
mixture of carbon monoxide and carbon dioxide as their carbon source.
They might produce and live on sulfur monoxide, which is analogous to oxygen (O2).
Hydrogen sulfide, like hydrogen cyanide and ammonia, suffers from the
small temperature range where it is liquid, though that, like that of
hydrogen cyanide and ammonia, increases with increasing pressure.
Silicon dioxide and silicates
Silicon dioxide,
also known as silica and quartz, is very abundant in the universe and
has a large temperature range where it is liquid. However, its melting
point is 1,600 to 1,725 °C (2,912 to 3,137 °F), so it would be
impossible to make organic compounds in that temperature, because all of
them would decompose. Silicates are similar to silicon dioxide and some
have lower melting points than silica. Feinberg and Shapiro have
suggested that molten silicate rock could serve as a liquid medium for
organisms with a chemistry based on silicon, oxygen, and other elements
such as aluminium.
Sulfuric acid in liquid form is strongly polar. It remains liquid at
higher temperatures than water, its liquid range being 10 °C to 337 °C
at a pressure of 1 atm, although above 300 °C it slowly decomposes.
Sulfuric acid is known to be abundant in the clouds of Venus, in the form of aerosol droplets. In a biochemistry that used sulfuric acid as a solvent, the alkene
group (C=C), with two carbon atoms joined by a double bond, could
function analogously to the carbonyl group (C=O) in water-based
biochemistry.
A proposal has been made that life on Mars may exist and be using a mixture of water and hydrogen peroxide as its solvent.
A 61.2% (by mass) mix of water and hydrogen peroxide has a freezing point of −56.5 °C and tends to super-cool rather than crystallize. It is also hygroscopic, an advantage in a water-scarce environment.
Supercritical carbon dioxide has been proposed as a candidate for
alternative biochemistry due to its ability to selectively dissolve
organic compounds and assist the functioning of enzymes and because
"super-Earth"- or "super-Venus"-type planets with dense high-pressure
atmospheres may be common.
Other speculations
Non-green photosynthesizers
Physicists
have noted that, although photosynthesis on Earth generally involves
green plants, a variety of other-colored plants could also support
photosynthesis, essential for most life on Earth, and that other colors
might be preferred in places that receive a different mix of stellar
radiation than Earth.
These studies indicate that blue plants would be unlikely; however yellow or red plants may be relatively common.
Variable environments
Many
Earth plants and animals undergo major biochemical changes during their
life cycles as a response to changing environmental conditions, for
example, by having a spore or hibernation state that can be sustained for years or even millennia between more active life stages.
Thus, it would be biochemically possible to sustain life in
environments that are only periodically consistent with life as we know
it.
For example, frogs in cold climates can survive for extended periods of time with most of their body water in a frozen state,
whereas desert frogs in Australia can become inactive and dehydrate in
dry periods, losing up to 75% of their fluids, yet return to life by
rapidly rehydrating in wet periods.
Either type of frog would appear biochemically inactive (i.e. not
living) during dormant periods to anyone lacking a sensitive means of
detecting low levels of metabolism.
Alanine world and hypothetical alternatives
Early stage of the genetic code (GC-Code) with "alanine world" and its possible alternatives.
The genetic code evolved during the transition from the RNA world to a protein world. The Alanine World Hypothesis postulates that the evolution of the genetic code (the so-called GC phase) started with only four basic amino acids: alanine, glycine, proline and ornithine (now arginine). The evolution of the genetic code ended with 20 proteinogenic amino acids. From a chemical point of view, most of them are Alanine-derivatives particularly suitable for the construction of α-helices and β-sheets – basic secondary structural elements of modern proteins. Direct evidence of this is an experimental procedure in molecular biology known as alanine scanning.
The hypothetical "Proline World" would create a possible alternative
life with the genetic code based on the proline chemical scaffold as the
protein backbone. Similarly, "Glycine" and "Ornithine" worlds are also conceivable, but nature has chosen none of them. Evolution of life with Glycine, Proline or Ornithine as the basic structure for protein-like polymers (foldamers) would lead to parallel biological worlds. They would have morphologically radically different body plans and genetics from the living organisms of the known biosphere.
In 2007, Vadim N. Tsytovich and colleagues proposed that lifelike behaviors could be exhibited by dust particles suspended in a plasma, under conditions that might exist in space.
Computer models showed that, when the dust became charged, the
particles could self-organize into microscopic helical structures, and
the authors offer "a rough sketch of a possible model of...helical grain
structure reproduction".
Cosmic necklace-based
In 2020, Luis A. Anchordoqu and Eugene M. Chudnovsky of the City University of New York hypothesized that cosmic necklace-based life composed of magnetic semipoles connected by cosmic strings could evolve inside stars.
Life on a neutron star
Frank Drake suggested in 1973 that intelligent life could inhabit neutron stars. Physical models in 1973 implied that Drake's creatures would be microscopic. In 1980, Robert L Forward wrote the science fiction novel Dragon's Egg using Drake's suggestion as a thesis.
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