Hypothetical types of biochemistry

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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^[1]

Hypothetical types of biochemistry are forms of biochemistry speculated to be scientifically viable but not proven to exist at this time.^[2] 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; it is also possible that there are organisms with quite different chemistries^[3]—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 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, although the silicon analogs of organic compounds are generally less stable. 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.

Shadow biosphere

The Arecibo message (1974) sent information into space about basic chemistry of Earth life.

A shadow biosphere is a hypothetical microbial biosphere of Earth that uses radically different biochemical and molecular processes than currently known life.^[4][5] 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 of opposite chirality have identical chemical properties to their mirrored forms, so life that used 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.^[6]

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,^{[citation needed]} 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.^[7] Sagan used the term "carbon chauvinism" for such an assumption.^[8] He regarded silicon and germanium as conceivable alternatives to carbon;^[8] but, on the other hand, he noted that carbon does seem more chemically versatile and is more abundant in the cosmos.^[9]

Silicon biochemistry

Structure of silane, analog of methane.

 

Structure of the silicone polydimethylsiloxane (PDMS).

 

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.^[10]

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. 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.^[10] 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".^[10] 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 bio-organic chemistry).

Silanes, which are chemical compounds of hydrogen and silicon that are analogous to the alkane hydrocarbons, 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.^[11]

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.^[12] Moreover, of those 8 compounds, four 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 which 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.^[13]

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.^[14][15]

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.^[16]

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.^[17][18]

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".

Other exotic element-based biochemistries

• Boranes are dangerously explosive in Earth's atmosphere, but would be more stable in a reducing environment. 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;^{[citation needed]} 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.^[19] 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.^[19]
• 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.^[20] Sulfur-reducing bacteria can utilize elemental sulfur instead of oxygen, reducing sulfur to hydrogen sulfide.)

Arsenic as an alternative to phosphorus

Arsenic, which is chemically similar to phosphorus, while poisonous for most life forms on Earth, is incorporated into the biochemistry of some organisms.^[21] 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).^[22] 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 in place of phosphorus in the structure of their DNA.^[23] 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.^[24]

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.^[25][26] 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.^[26] This claim was heavily criticized almost immediately after publication for the perceived lack of appropriate controls.^[27][28] 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".^[29] 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.^[30] Alternatively, it was suggested that GFAJ-1 cells grow by recycling phosphate from degraded ribosomes, rather than by replacing it with arsenate.^[31]

Non-water solvents

Carl Sagan speculated alien life might use ammonia, hydrocarbons or hydrogen fluoride instead of water.

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,^[32] and by the astrobiological committee chaired by John A. Baross.^[33] Solvents discussed by the Baross committee include ammonia,^[34] sulfuric acid,^[35] formamide,^[36] hydrocarbons,^[36] and (at temperatures much lower than Earth's) liquid nitrogen, or hydrogen in the form of a supercritical fluid.^[37]

Carl Sagan once described himself as both a carbon chauvinist and a water chauvinist;^[38] however on another occasion he said he was a carbon chauvinist but "not that much of a water chauvinist".^[39] He speculated on hydrocarbons,^[39]:11 hydrofluoric acid,^[40] and ammonia^[39][40] as possible alternatives to water.

Some of the properties of water that are important for life processes include a large temperature range over which it is liquid, a high heat capacity (useful for temperature regulation), a large heat of vaporization, and the ability to dissolve a wide variety of compounds. Water is also amphoteric, meaning it can donate and accept an H⁺ ion, allowing it to act as an acid or a base. This property is crucial in many organic and biochemical reactions, where water serves as a solvent, a reactant, or a product. There are other chemicals with similar properties that have sometimes been proposed as alternatives. Additionally, water has the unusual property of being less dense as a solid (ice) than as a 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.^[41] For instance, water ice has a high albedo,^[41] 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.^[41]

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.^[42] 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.^[43]

Numerous chemical reactions are possible in an ammonia solution, and liquid ammonia has chemical similarities with water.^[42][44] 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).^[44]

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⁻).^[34] Compared to water, however, ammonia is more inclined to accept an H⁺ ion, and less inclined to donate one; it is a stronger nucleophile.^[34] 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.^[44] 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.^[34]

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.^[45] Ammonia is also flammable in oxygen, and could not exist sustainably in an environment suitable for aerobic metabolism.^[46]

Titan's theorized internal structure, subsurface ocean shown 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.^[46] 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.^[42]

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).^[34]

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.^[47]

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.^[42] 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.^[42] 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.^[48][49][50] Water is a stronger solvent than the hydrocarbons, enabling easier transport of substances in a cell.^[51] However, water is also more chemically reactive, and can break down large organic molecules through hydrolysis.^[48] A life-form whose solvent was a hydrocarbon would not face the threat of its biomolecules being destroyed in this way.^[48] Also, the water molecule's tendency to form strong hydrogen bonds can interfere with internal hydrogen bonding in complex organic molecules.^[41] Life with a hydrocarbon solvent could make more use of hydrogen bonds within its biomolecules.^[48] Moreover, the strength of hydrogen bonds within biomolecules would be appropriate to a low-temperature biochemistry.^[48]

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.^[52] 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.^[51][52][53] 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.^[51] 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.^[54] He noted that even a non-biological catalyst effective at 95 K would in itself be a startling discovery.^[54]

Azotosome

A hypothetical cell membrane termed an azotosome capable of functioning in liquid methane in Titan conditions was computer-modeled in a paper 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.^[55][56] 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.^[57][58]

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^[59] and Carl Sagan.^[40]

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.^[40] 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.^[60]

However, hydrogen fluoride is cosmically rare, unlike water, ammonia, and methane.^[61]

Hydrogen sulfide

Hydrogen sulfide is the closest chemical analog to water,^[62] but is less polar and a weaker inorganic solvent.^[63] Hydrogen sulfide is quite plentiful on Jupiter's moon Io, and may be in liquid form a short distance below the surface; and astrobiologist Dirk Schulze-Makuch has suggested it as a possible solvent for life there.^[64] 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 off of 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 glass, silica, or 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 could have lower boiling points than silica. 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.^[65]

Other solvents or cosolvents

Sulfuric acid (H₂SO₄).

Other solvents sometimes proposed:

• Supercritical fluids: supercritical carbon dioxide and supercritical hydrogen^[66]
• Simple hydrogen compounds: hydrogen chloride^[67]
• More complex compounds: sulfuric acid,^[35] formamide,^[36] methanol^[67]
• Very-low-temperature fluids: liquid nitrogen,^[37] and hydrogen^[37]
• High-temperature liquids: sodium chloride^[68]

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 will slowly decompose. 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.^[35]

A proposal has been made that life on Mars may exist and be using a mixture of water and hydrogen peroxide as its solvent.^[69] A 61.2% (by weight) mix of water and hydrogen peroxide has a freezing point of −56.5 °C, and also tends to super-cool rather than crystallize. It is also hygroscopic, an advantage in a water-scarce environment.^[70][71]

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.^[66]

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.^[72][73] These studies indicate that, although blue photosynthetic plants would be less likely, yellow or red plants are plausible.^[73]

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.^[74] 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,^[74] 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.^[75] 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.

Nonplanetary life

Dust and plasma-based

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.^[76][77] 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 the helical grain structure reproduction".

Scientists who have published on this topic

Scientists who have considered possible alternatives to carbon-water biochemistry include:

• Isaac Asimov (1920–1992), biochemist and science fiction writer.^[42]
• William Bains, Cambridge biologist, a contributor to the journal Astrobiology.^[78]
• John Baross, oceanographer and astrobiologist, who chaired a committee of scientists under the United States National Research Council that published a report on life's limiting conditions in 2007.^[79] The report addresses the concern that a space agency might conduct a well-resourced search for life on other worlds "and then fail to recognize it if it is encountered".^[80]
• Gerald Feinberg (1933–1992), physicist and Robert Shapiro (1935–2011), chemist, co-authors of the book Life Beyond Earth.^[81][82]
• V.Axel Firsoff (1910–1981), British astronomer.^[83]
• Robert A. Freitas Jr. (1952–present), specialist in nano-technology and nano-medicine; author of the book Xenology.^[84][85]
• J. B. S. Haldane (1892–1964), a geneticist noted for his work on abiogenesis.^[43]
• George Pimentel (1922–1989), American chemist, University of California, Berkeley.^[86]
• Carl Sagan (1934–1996), astronomer,^[86] science popularizer, and SETI proponent.
• Peter Sneath (1923–2011), microbiologist, author of the book Planets and Life.^[59]
• Jonathan Lunine, (b. 1959) American planetary scientist and physicist

In fiction

• Alternate chirality: In Arthur C. Clarke's short story "Technical Error", there is an example of differing chirality.
• The concept of reversed chirality also figured prominently in the plot of James Blish's Star Trek novel Spock Must Die!, where a transporter experiment gone awry ends up creating a duplicate Spock who turns out to be a perfect mirror-image of the original all the way down to the atomic level.
• The eponymous organism in Michael Crichton's The Andromeda Strain is described as reproducing via the direct conversion of energy into matter.
• Silicoids: John Clark, in the introduction to the 1952 shared-world anthology The Petrified Planet, outlined the biologies of the planet Uller, with a mixture of siloxane and silicone life, and of Niflheim, where metabolism is based on hydrofluoric acid and carbon tetrafluoride.
• In the original Star Trek episode "The Devil in the Dark", a highly intelligent silicon-based creature called Horta, made almost entirely of pure rock, with eggs which take the form of silicon nodules scattered throughout the caverns and tunnels of its home planet. Subsequently, in the non-canonical Star Trek book 'The Romulan Way', another Horta is a junior officer in Starfleet.
• In Star Trek: The Next Generation, the Crystalline Entity appeared in two episodes, "Datalore" and "Silicon Avatar". This was an enormous spacefaring crystal lattice that had taken thousands of lives in its quest for energy. It was destroyed before communications could be established.
• In the Star Trek: The Next Generation episode "Home Soil" the Enterprise investigates the sabotage of a planetary terraforming station and the death of one of its members; these events are finally attributed to a completely non-organic, solar powered, saline thriving sentient life form.
• In the 1994 The X-Files episode "Firewalker", Mulder and Scully investigate a death in a remote research base and discover that a new silicon-based fungus found in the area may be affecting and killing the researchers.
• The Orion's Arm Universe Project, an online collaborative science-fiction project, includes a number of extraterrestrial species with exotic biochemistries, including organisms based on low-temperature carbohydrate chemistry, organisms that consume and live within sulfuric acid, and organisms composed of structured magnetic flux tubes within neutron stars or gas giant cores.

Life on Mars

From Wikipedia, the free encyclopedia

The possibility of life on the planet Mars is a subject of significant interest to astrobiology due to its proximity and similarities to Earth. To date no proof has been found of past or present life on Mars. However, cumulative evidence is now clear that during the ancient Noachian time period, the surface environment of Mars had liquid water and may have been habitable for microorganisms. The existence of habitable conditions does not necessarily indicate the presence of life.

Scientific searches for evidence of life began in the 19th century, and they continue today via telescopic investigations and deployed probes. While early work focused on phenomenology and bordered on fantasy, modern scientific inquiry has emphasized the search for water, chemical biosignatures in the soil and rocks at the planet's surface, and biomarker gases in the atmosphere.^[1][2] On November 22, 2016, NASA reported finding a large amount of underground ice in the Utopia Planitia region of Mars. The volume of water detected has been estimated to be equivalent to the volume of water in Lake Superior.^[3][4][5]

Mars is of particular interest for the study of the origins of life because of its similarity to the early Earth. This is especially so since Mars has a cold climate and lacks plate tectonics or continental drift, so it has remained almost unchanged since the end of the Hesperian period. At least two thirds of Mars's surface is more than 3.5 billion years old, and Mars may thus hold the best record of the prebiotic conditions leading to abiogenesis, even if life does not or has never existed there.^[6][7] In May 2017, evidence of the earliest known life on land on Earth may have been found in 3.48-billion-year-old geyserite and other related mineral deposits (often found around hot springs and geysers) uncovered in the Pilbara Craton of Western Australia.^[8][9] These findings may be helpful in deciding where best to search for early signs of life on the planet Mars.^[8][9]

On January 24, 2014, NASA reported that the Curiosity and Opportunity rovers started searching for evidence of past life, including a biosphere based on autotrophic, chemotrophic, or chemolithoautotrophic microorganisms, as well as ancient water, including fluvio-lacustrine environments (plains related to ancient rivers or lakes) that may have been habitable.^{[10][11][12][13]} The search for evidence of habitability, taphonomy (related to fossils), and organic carbon on the planet Mars is now a primary NASA objective.^[10]

In July 2017, researchers reported that the surface on the planet Mars may be more toxic to microorganisms, especially a common terrestrial type, Bacillus subtilis, than thought earlier. This is based on studies with perchlorates, common on Mars, in a simulated Martian ultraviolet atmosphere.^[14][15]

On September 5, 2017, scientists reported that the Curiosity rover on Mars detected boron, an essential ingredient for life on Earth. Such a finding, along with previous discoveries that liquid water was clearly present on ancient Mars, further supports the possible early habitability of Gale Crater on Mars.^[16][17]

Early speculation

Historical map of Mars from Giovanni Schiaparelli

Mars canals illustrated by astronomer Percival Lowell, 1898

Mars' polar ice caps were discovered in the mid-17th century. In the latter part of the 18th century, William Herschel proved they grow and shrink alternately, in the summer and winter of each hemisphere. By the mid-19th century, astronomers knew that Mars had certain other similarities to Earth, for example that the length of a day on Mars was almost the same as a day on Earth. They also knew that its axial tilt was similar to Earth's, which meant it experienced seasons just as Earth does — but of nearly double the length owing to its much longer year. These observations led to the increase in speculation that the darker albedo features were water, and brighter ones were land. It was therefore natural to suppose that Mars may be inhabited by some form of life.^{[citation needed][original research?]}

In 1854, William Whewell, a fellow of Trinity College, Cambridge, who popularized the word scientist, theorized that Mars had seas, land and possibly life forms.^[18] Speculation about life on Mars exploded in the late 19th century, following telescopic observation by some observers of apparent Martian canals — which were later found to be optical illusions. Despite this, in 1895, American astronomer Percival Lowell published his book Mars, followed by Mars and its Canals in 1906,^[19] proposing that the canals were the work of a long-gone civilization.^[20] This idea led British writer H. G. Wells to write The War of the Worlds in 1897, telling of an invasion by aliens from Mars who were fleeing the planet's desiccation.

Spectroscopic analysis of Mars' atmosphere began in earnest in 1894, when U.S. astronomer William Wallace Campbell showed that neither water nor oxygen were present in the Martian atmosphere.^[21] By 1909, better telescopes and the best perihelic opposition of Mars since 1877 conclusively put an end to the canal hypothesis.

Habitability

Chemical, physical, geological, and geographic attributes shape the environments on Mars. Isolated measurements of these factors may be insufficient to deem an environment habitable, but the sum of measurements can help predict locations with greater or lesser habitability potential.^[22] The two current ecological approaches for predicting the potential habitability of the Martian surface use 19 or 20 environmental factors, with emphasis on water availability, temperature, presence of nutrients, an energy source, and protection from solar ultraviolet and galactic cosmic radiation.^[23][24]

Scientists do not know the minimum number of parameters for determination of habitability potential, but they are certain it is greater than one or two of the factors in the table below.^[22] Similarly, for each group of parameters, the habitability threshold for each is to be determined.^[22] Laboratory simulations show that whenever multiple lethal factors are combined, the survival rates plummet quickly.^[25] There are no full-Mars simulations published yet that include all of the biocidal factors combined.^[25]

Habitability factors[24]
Water	· liquid water activity (aw)  · Past/future liquid (ice) inventories  · Salinity, pH, and Eh of available water
Chemical environment	Nutrients:  · C, H, N, O, P, S, essential metals, essential micronutrients  · Fixed nitrogen  · Availability/mineralogy Toxin abundances and lethality:  · Heavy metals (e.g., Zn, Ni, Cu, Cr, As, Cd, etc., some essential, but toxic at high levels)  · Globally distributed oxidizing soils
Energy for metabolism	Solar (surface and near-surface only) Geochemical (subsurface)  · Oxidants  · Reductants  · Redox gradients
Conducive physical conditions	· Temperature  · Extreme diurnal temperature fluctuations  · Low pressure (Is there a low-pressure threshold for terrestrial anaerobes?)  · Strong ultraviolet germicidal irradiation  · Galactic cosmic radiation and solar particle events (long-term accumulated effects)  · Solar UV-induced volatile oxidants, e.g., O2−, O−, H2O2, O3  · Climate/variability (geography, seasons, diurnal, and eventually, obliquity variations)  · Substrate (soil processes, rock microenvironments, dust composition, shielding)  · High CO2 concentrations in the global atmosphere  · Transport (aeolian, ground water flow, surface water, glacial)

Past

Recent models have shown that, even with a dense CO₂ atmosphere, early Mars was colder than Earth has ever been.^[26] However, transiently warm conditions related to impacts or volcanism could have produced conditions favoring the formation of the late Noachian valley networks, even though the mid–late Noachian global conditions were probably icy. Local warming of the environment by volcanism and impacts would have been sporadic, but there should have been many events of water flowing at the surface of Mars.^[26] Both the mineralogical and the morphological evidence indicates a degradation of habitability from the mid Hesperian onward. The exact causes are not well understood but may be related to a combination of processes including loss of early atmosphere, or impact erosion, or both.^[26]

Alga crater is thought to have deposits of impact glass that may have preserved ancient biosignatures, if present during the impact.^[27]

The loss of the Martian magnetic field strongly affected surface environments through atmospheric loss and increased radiation; this change significantly degraded surface habitability.^[28] When there was a magnetic field, the atmosphere would have been protected from erosion by solar wind, which would ensure the maintenance of a dense atmosphere, necessary for liquid water to exist on the surface of Mars.^[29] The loss of the atmosphere was accompanied by decreasing temperatures. A part of the liquid water inventory sublimed and was transported to the poles, while the rest became trapped in permafrost, a subsurface ice layer.^[26]

Observations on Earth and numerical modeling have shown that a crater-forming impact can result in the creation of a long lasting hydrothermal system when ice is present in the crust. For example, a 130 km large crater could sustain an active hydrothermal system for up to 2 million years, that is, long enough for microscopic life to emerge,^[26] but unlikely to have progressed any further down the evolutionary path.^[30]

Soil and rock samples studied in 2013 by NASA's Curiosity rover's onboard instruments brought about additional information on several habitability factors.^[31] The rover team identified some of the key chemical ingredients for life in this soil, including sulfur, nitrogen, hydrogen, oxygen, phosphorus and possibly carbon, as well as clay minerals, suggesting a long-ago aqueous environment — perhaps a lake or an ancient streambed — that was neutral and not too salty.^[31] On December 9, 2013, NASA reported that, based on evidence from Curiosity studying Aeolis Palus, Gale Crater contained an ancient freshwater lake which could have been a hospitable environment for microbial life.^[32][33] The confirmation that liquid water once flowed on Mars, the existence of nutrients, and the previous discovery of a past magnetic field that protected the planet from cosmic and Solar radiation,^[34][35] together strongly suggest that Mars could have had the environmental factors to support life.^[36][37] However, the assessment of past habitability is not in itself evidence that Martian life has ever actually existed. If it did, it was probably microbial, existing communally in fluids or on sediments, either free-living or as biofilms, respectively.^[28]

Impactite, shown to preserve signs of life on Earth, was discovered on Mars and could contain signs of ancient life, if life ever existed on the planet.^[38]

Present

Conceivably, if life exists (or existed) on Mars, evidence of that life could be found, or is best preserved, in the subsurface, away from present-day harsh surface conditions.^[39] Present-day life on Mars, or its biosignatures, could occur kilometers below the surface, or in subsurface geothermal hot spots, or it could occur a few meters below the surface. The permafrost layer on Mars is only a couple of centimeters below the surface, and salty brines can be liquid a few centimeters below that but not far down. Water is close to its boiling point even at the deepest points in the Hellas basin, and so cannot remain liquid for long on the surface of Mars in its present state, except when covered in ice or after a sudden release of underground water.

So far, NASA has pursued a "follow the water" strategy on Mars and has not searched for biosignatures for life there directly since the Viking mission. As of 2017, the consensus by astrobiologists at NASA is that it may be necessary to access the Martian subsurface to find currently habitable environments.^[39]

Dormant subsurface life

Curiosity measured ionizing radiation levels of 76 mGy a year.^[40] This level of ionizing radiation is sterilizing for dormant life on the surface of Mars. However, it varies considerably in habitability depending on its orbital eccentricity and the tilt of its axis. If the surface life has been reanimated as recently as 450,000 years ago, then rovers on Mars could find dormant but still viable life at a depth of one meter below the surface, according to an estimate.^[41]

Cosmic radiation

In 1965, the Mariner 4 probe discovered that Mars had no global magnetic field that would protect the planet from potentially life-threatening cosmic radiation and solar radiation; observations made in the late 1990s by the Mars Global Surveyor confirmed this discovery.^[42] Scientists speculate that the lack of magnetic shielding helped the solar wind blow away much of Mars's atmosphere over the course of several billion years.^[43] As a result, the planet has been vulnerable to radiation from space for about 4 billion years.^[44]

However, recent in-situ data from Curiosity rover indicates that ionizing radiation from galactic cosmic rays (GCR) and solar particle events (SPE) may not be a limiting factor in habitability assessments for present-day surface life on Mars. The level of 76 mGy per year measured by Curiosity is similar to levels inside the ISS.^[45] In the 2014 Findings of the Second MEPAG Special Regions Science Analysis Group, their conclusion was:^[46]

"From MSL RAD measurements, ionizing radiation from galactic cosmic rays (GCR) at Mars is so low as to be negligible. Intermittent Solar particle events (SPE) can increase the atmospheric ionization down to ground level and increase the total dose, but these events are sporadic and last at most a few (2–5) days. These facts are not used to distinguish Special Regions on Mars." A Special Region is defined as a region on the Mars surface where Earth life could potentially survive.

Cumulative effects

Even the hardiest cells known could not possibly survive the cosmic radiation near the surface of Mars since Mars lost its protective magnetosphere and atmosphere.^[47][48] After mapping cosmic radiation levels at various depths on Mars, researchers have concluded that over time, any life within the first several meters of the planet's surface would be killed by lethal doses of cosmic radiation.^[47][49][50] The team calculated that the cumulative damage to DNA and RNA by cosmic radiation would limit retrieving viable dormant cells on Mars to depths greater than 7.5 metres below the planet's surface.^[49] Even the most radiation-tolerant Earthly bacteria would survive in dormant spore state only 18,000 years at the surface; at 2 meters —the greatest depth at which the ExoMars rover will be capable of reaching— survival time would be 90,000 to half a million years, depending on the type of rock.^[51]

Data collected by the Radiation assessment detector (RAD) instrument on board the Curiosity rover revealed that the actual absorbed dose measured is 76 mGy/year at the surface,^[52] and that "ionizing radiation strongly influences chemical compositions and structures, especially for water, salts, and redox-sensitive components such as organic matter."^[52] Regardless of the source of Martian organic compounds (meteoric, geological, or biological), its carbon bonds are susceptible to breaking and reconfigurating with surrounding elements by ionizing charged particle radiation.^[52] These improved subsurface radiation estimates give insight into the potential for the preservation of possible organic biosignatures as a function of depth as well as survival times of possible microbial or bacterial life forms left dormant beneath the surface.^[52] The report concludes that the in situ "surface measurements —and subsurface estimates— constrain the preservation window for Martian organic matter following exhumation and exposure to ionizing radiation in the top few meters of the Martian surface."^[52]

In September 2017, NASA reported radiation levels on the surface of the planet Mars were temporarily doubled, and were associated with an aurora 25-times brighter than any observed earlier, due to a massive, and unexpected, solar storm in the middle of the month.^[53]

UV radiation

On UV radiation, a 2014 report concludes ^[46] that "[T]he Martian UV radiation environment is rapidly lethal to unshielded microbes but can be attenuated by global dust storms and shielded completely by < 1 mm of regolith or by other organisms." In addition, laboratory research published in July 2017 demonstrated that when irradiated with a simulated Martian UV flux, perchlorates become bacteriocidal.^[54]

Penetration depth of UV radiation into soils is in the sub-millimetre to millimetre range and depends on the properties of the soil.^[55]

Perchlorates

The Martian regolith is known to contain a maximum of 0.5% (w/v) perchlorate (ClO₄⁻) that is toxic for most living organisms,^[56] but since they drastically lower the freezing point of water and few extremophiles can use it as an energy source (see Perchlorates - Biology), it has prompted speculation of what their influence would be on habitability.^[54][57][58]

In July 2017 it was shown that when irradiated with a simulated Martian UV flux, perchlorates become bacteriocidal. Even dormant spores lost viability within minutes.^[54] In addition, two other compounds of the Martian surface, iron oxides and hydrogen peroxide, act in synergy with irradiated perchlorates to cause a 10.8-fold increase in cell death when compared to cells exposed to UV radiation after 60 seconds of exposure.^[54][55] It was also found that abraded silicates (quartz and basalt) lead to the formation of toxic reactive oxygen species.^[59] The researchers concluded that "the surface of Mars is lethal to vegetative cells and renders much of the surface and near-surface regions uninhabitable."^[60] This research demonstrates that the present-day surface is more uninhabitable than previously thought,^[54][61] and reinforces the notion to inspect at least a few meters into the ground to ensure the levels of radiation would be relatively low.^[61][62]

Recurrent slope lineae

Recurrent slope lineae (RSL) features form on Sun-facing slopes at times of the year when the local temperatures reach above the melting point for ice. The streaks grow in spring, widen in late summer and then fade away in autumn. This is hard to model in any other way except as involving liquid water in some form, though the streaks themselves are thought to be a secondary effect and not a direct indication of dampness of the regolith. Although these features are now confirmed to involve liquid water in some form, the water could be either too cold or too salty for life. At present they are treated as potentially habitable, as "Uncertain Regions, to be treated as Special Regions".

The 'Special Regions assessment' says of them:^[46] "Although no single model currently proposed for the origin of RSL adequately explains all observations, they are currently best interpreted as being due to the seepage of water at > 250 K, with ${\displaystyle a_{w}}$ (water activity) unknown and perhaps variable. As such they meet the criteria for Uncertain Regions, to be treated as Special Regions. There are other features on Mars with characteristics similar to RSL, but their relationship to possible liquid water is much less likely." They were first reported in 2011.^[63] They were already suspected as involving flowing brines back then, as all the other models available involved liquid water in some form.^{[64][65][66][67]}

The thermodynamic availability of water (water activity) strictly limits microbial propagation on Earth, particularly in hypersaline environments, and there are indications that the brine ionic strength is a barrier to the habitability of Mars. Experiments show that high ionic strength, driven to extremes on Mars by the ubiquitous occurrence of divalent ions, "renders these environments uninhabitable despite the presence of biologically available water."^[68]

Nitrogen fixation

After carbon, nitrogen is arguably the most important element needed for life. Thus, measurements of nitrate over the range of 0.1% to 5% are required to address the question of its occurrence and distribution. There is nitrogen (as N₂) in the atmosphere at low levels, but this is not adequate to support nitrogen fixation for biological incorporation.^[69] Nitrogen in the form of nitrate could be a resource for human exploration both as a nutrient for plant growth and for use in chemical processes. On Earth, nitrates correlate with perchlorates in desert environments, and this may also be true on Mars. Nitrate is expected to be stable on Mars and to have formed by thermal shock from impact or volcanic plume lightning on ancient Mars.^[70]

On 24 March 2015, NASA reported that the SAM instrument on the Curiosity rover detected nitrates by heating surface sediments. The nitrogen in nitrate is in a "fixed" state, meaning that it is in an oxidized form that can be used by living organisms. The discovery supports the notion that ancient Mars may have been hospitable for life.^[70][71][72] It is suspected that all nitrate on Mars is a relic, with no modern contribution.^[73] Nitrate abundance ranges from non-detection to 681 ± 304 mg/kg in the samples examined until autumn 2017.^[73] Modeling indicates that the transient condensed water films on the surface should be transported to lower depths (≈10 m) potentially transporting nitrates, where subsurface microorganisms could thrive.^[74]

In contrast, phosphate, one of the chemical nutrients thought to be essential for life, is readily available on Mars.^[75]

Low pressure

Further complicating estimates of the habitability of the Martian surface is the fact that very little is known on the growth of microorganisms at pressures close to the conditions found on the surface of Mars. Some teams determined that some bacteria may be capable of cellular replication down to 25 mbar, but that is still above the atmospheric pressures found on Mars (range 1–14 mbar).^[76] In another study, twenty-six strains of bacteria were chosen based on their recovery from spacecraft assembly facilities, and only Serratia liquefaciens strain ATCC 27592 exhibited growth at 7 mbar, 0 °C, and CO₂-enriched anoxic atmospheres.^[76]

Liquid water

Liquid water is a necessary but not sufficient condition for life as we know it, as habitability is a function of a multitude of environmental parameters.^[77] Liquid water cannot exist on the surface of Mars except at the lowest elevations for minutes or hours.^[78][79] Liquid water does not appear at the surface itself,^[80] but it could form in minuscule amounts around dust particles in snow heated by the Sun.^[81][82] Also, the ancient equatorial ice sheets beneath the ground may slowly sublimate or melt, accessible from the surface via caves.^{[83][84][85][86]}

Water on Mars exists almost exclusively as water ice, located in the Martian polar ice caps and under the shallow Martian surface even at more temperate latitudes.^[87][88] A small amount of water vapor is present in the atmosphere.^[89] There are no bodies of liquid water on the Martian surface because its atmospheric pressure at the surface averages 600 pascals (0.087 psi)—about 0.6% of Earth's mean sea level pressure—and because the temperature is far too low, (210 K (−63 °C)) leading to immediate freezing. Despite this, about 3.8 billion years ago,^[90] there was a denser atmosphere, higher temperature, and vast amounts of liquid water flowed on the surface,^{[91][92][93][94]} including large oceans.^{[95][96][97][98][99]}

A series of artist's conceptions of past water coverage on Mars

It has been estimated that the primordial oceans on Mars would have covered between 36%^[100] and 75% of the planet.^[101] On November 22, 2016, NASA reported finding a large amount of underground ice in the Utopia Planitia region of Mars. The volume of water detected has been estimated to be equivalent to the volume of water in Lake Superior.^[3][4][5] Analysis of Martian sandstones, using data obtained from orbital spectrometry, suggests that the waters that previously existed on the surface of Mars would have had too high a salinity to support most Earth-like life. Tosca et al. found that the Martian water in the locations they studied all had water activity, a_w ≤ 0.78 to 0.86—a level fatal to most Terrestrial life.^[102] Haloarchaea, however, are able to live in hypersaline solutions, up to the saturation point.^[103]

In June 2000, possible evidence for current liquid water flowing at the surface of Mars was discovered in the form of flood-like gullies.^[104][105] Additional similar images were published in 2006, taken by the Mars Global Surveyor, that suggested that water occasionally flows on the surface of Mars. The images did not actually show flowing water. Rather, they showed changes in steep crater walls and sediment deposits, providing the strongest evidence yet that water coursed through them as recently as several years ago.

There is disagreement in the scientific community as to whether or not the recent gully streaks were formed by liquid water. Some suggest the flows were merely dry sand flows.^{[106][107][108][109]} Others suggest it may be liquid brine near the surface,^{[110][111][112]} but the exact source of the water and the mechanism behind its motion are not understood.^[113]

Silica

The silica-rich patch discovered by Spirit rover

In May 2007, the Spirit rover disturbed a patch of ground with its inoperative wheel, uncovering an area 90% rich in silica.^[114] The feature is reminiscent of the effect of hot spring water or steam coming into contact with volcanic rocks. Scientists consider this as evidence of a past environment that may have been favorable for microbial life, and theorize that one possible origin for the silica may have been produced by the interaction of soil with acid vapors produced by volcanic activity in the presence of water.^[115]

Based on Earth analogs, hydrothermal systems on Mars would be highly attractive for their potential for preserving organic and inorganic biosignatures.^{[116][117][118]} For this reason, hydrothermal deposits are regarded as important targets in the exploration for fossil evidence of ancient Martian life.^{[119][120][121]}

Possible biosignatures

Methane

Possible trace amounts of methane in the atmosphere of Mars were first discovered in 2003 with earth based telescopes and fully verified in 2004 by the ESA Mars Express spacecraft in orbit around Mars.^{[122][123][124][125][126][127]} As methane is an unstable gas, its presence indicates that there must be an active source on the planet in order to keep such levels in the atmosphere. It is estimated that Mars must produce 270 ton/year of methane,^[128][129] but asteroid impacts account for only 0.8% of the total methane production. Although geologic sources of methane such as serpentinization are possible, the lack of current volcanism, hydrothermal activity or hotspots^[130] are not favorable for geologic methane. It has been suggested that the methane was produced by chemical reactions in meteorites, driven by the intense heat during entry through the atmosphere. Although research published in December 2009 ruled out this possibility,^[131] research published in 2012 suggest that a source may be organic compounds on meteorites that are converted to methane by ultraviolet radiation.^[132]

Distribution of methane in the atmosphere of Mars in the Northern Hemisphere during summer

The existence of life in the form of microorganisms such as methanogens is among possible, but as yet unproven sources. Methanogens do not require oxygen or organic nutrients, are non-photosynthetic, use hydrogen as their energy source and carbon dioxide (CO₂) as their carbon source, so they could exist in subsurface environments on Mars.^[133] If microscopic Martian life is producing the methane, it probably resides far below the surface, where it is still warm enough for liquid water to exist.^[134]

Since the 2003 discovery of methane in the atmosphere, some scientists have been designing models and in vitro experiments testing growth of methanogenic bacteria on simulated Martian soil, where all four methanogen strains tested produced substantial levels of methane, even in the presence of 1.0wt% perchlorate salt.^[135] The results reported indicate that the perchlorates discovered by the Phoenix Lander would not rule out the possible presence of methanogens on Mars.^[135][136]

A team led by Levin suggested that both phenomena—methane production and degradation—could be accounted for by an ecology of methane-producing and methane-consuming microorganisms.^[136][137]

Research at the University of Arkansas presented in June 2015 suggested that some methanogens could survive on Mars's low pressure. Rebecca Mickol found that in her laboratory, four species of methanogens survived low-pressure conditions that were similar to a subsurface liquid aquifer on Mars. The four species that she tested were Methanothermobacter wolfeii, Methanosarcina barkeri, Methanobacterium formicicum, and Methanococcus maripaludis.^[133] In June 2012, scientists reported that measuring the ratio of hydrogen and methane levels on Mars may help determine the likelihood of life on Mars.^[138][139] According to the scientists, "...low H₂/CH₄ ratios (less than approximately 40) indicate that life is likely present and active."^[138] Other scientists have recently reported methods of detecting hydrogen and methane in extraterrestrial atmospheres.^[140][141]

The Curiosity rover, which landed on Mars in August 2012, is able to make measurements that distinguish between different isotopologues of methane, and in 2014, Curiosity detected a "tenfold spike" in the level of methane in the Martian atmosphere compared to the usual background readings.^{[142][143][144]} However, even if the mission is to determine that microscopic Martian life is the seasonal source of the methane, the life forms probably reside far below the surface, outside the rover's reach.^[145] The first measurements with the Tunable Laser Spectrometer (TLS) in the Curiosity rover indicated that there is less than 5 ppb of methane at the landing site at the point of the measurement.^{[146][147][148][149]} On July 19, 2013, NASA scientists published the results of a new analysis of the atmosphere of Mars, reporting a lack of methane around the landing site of the Curiosity rover.^{[150][151][152]} On September 19, 2013, NASA again reported no detection of atmospheric methane with a measured value of 0.18±0.67 ppbv corresponding to an upper limit of only 1.3 ppbv (95% confidence limit) and, as a result, concluded that the probability of current methanogenic microbial activity on Mars is reduced.^{[153][154][155]} On 16 December 2014, NASA reported that Curiosity had detected a tenfold increase ('spike') in methane in the atmosphere around it in late 2013 and early 2014. Four measurements taken over two months in this period averaged 7 ppb, suggesting that methane is released at intervals.^{[142][143][156]}

India's Mars Orbiter Mission, launched on November 5, 2013, is searching for methane in the atmosphere of Mars using its Methane Sensor for Mars (MSM). The orbiter has been orbiting Mars since September 24, 2014. The ExoMars Trace Gas Orbiter planned to launch in 2016 would further study the methane,^[157][158] as well as its decomposition products such as formaldehyde and methanol.

Formaldehyde

In February 2005, it was announced that the Planetary Fourier Spectrometer (PFS) on the European Space Agency's Mars Express Orbiter had detected traces of formaldehyde in the atmosphere of Mars. Vittorio Formisano, the director of the PFS, has speculated that the formaldehyde could be the byproduct of the oxidation of methane and, according to him, would provide evidence that Mars is either extremely geologically active or harbouring colonies of microbial life.^[159][160] NASA scientists consider the preliminary findings well worth a follow-up, but have also rejected the claims of life.^[161][162]

Viking lander biological experiments

The 1970s Viking program placed two identical landers on the surface of Mars tasked to look for biosignatures of microbial life on the surface. Of the four experiments performed by each Viking lander, only the 'Labeled Release' (LR) experiment gave a positive result for metabolism, while the other three did not detect organic compounds. The LR was a specific experiment designed to test only a narrowly defined critical aspect of the theory concerning the possibility of life on Mars; therefore, the overall results were declared, officially, inconclusive.^[21] Further, no Mars lander mission has found meaningful traces of biomolecules or biosignatures. The claim of extant microbial life on Mars is based on old data collected by the Viking landers, currently reinterpreted as sufficient evidence of life, mainly by Gilbert Levin,^[163] Joseph D. Miller,^[164] Navarro,^[165] Giorgio Bianciardi and Patricia Ann Straat,^[166] that the Viking LR experiments did, indeed, detect extant microbial life on Mars.
Relatively recent assessments published in December 2010 by Rafael Navarro–Gonzáles^{[167][168][169][170]} indicate that organic compounds "could have been present" in the soil analyzed by both Viking 1 and 2. The study determined that perchlorate —discovered in 2008 by Phoenix lander^[171][172]— can destroy organic compounds when heated, and produce chloromethane and dichloromethane as byproduct, the identical chlorine compounds discovered by both Viking landers when they performed the same tests on Mars. Because perchlorate would have broken down any Martian organics, the question of whether or not Viking found organic compounds is still wide open.^[173][174]

The Labeled Release evidence was not generally accepted initially, and, to this day lacks the consensus of the scientific community.^[175]

Meteorites

NASA maintains a catalog of 34 Mars meteorites.^[176] These assets are highly valuable since they are the only physical samples available of Mars. Studies conducted by NASA's Johnson Space Center show that at least three of the meteorites contain potential evidence of past life on Mars, in the form of microscopic structures resembling fossilized bacteria (so-called biomorphs). Although the scientific evidence collected is reliable, its interpretation varies. To date, none of the original lines of scientific evidence for the hypothesis that the biomorphs are of exobiological origin (the so-called biogenic hypothesis) have been either discredited or positively ascribed to non-biological explanations.^[177]

Over the past few decades, seven criteria have been established for the recognition of past life within terrestrial geologic samples. Those criteria are:^[177]

1. Is the geologic context of the sample compatible with past life?
2. Is the age of the sample and its stratigraphic location compatible with possible life?
3. Does the sample contain evidence of cellular morphology and colonies?
4. Is there any evidence of biominerals showing chemical or mineral disequilibria?
5. Is there any evidence of stable isotope patterns unique to biology?
6. Are there any organic biomarkers present?
7. Are the features indigenous to the sample?

For general acceptance of past life in a geologic sample, essentially most or all of these criteria must be met. All seven criteria have not yet been met for any of the Martian samples, but continued investigations are in progress.^[177]

As of 2010, reexaminations of the biomorphs found in the three Martian meteorites are underway with more advanced analytical instruments than previously available.

ALH84001

An electron microscope reveals bacteria-like structures in meteorite fragment ALH84001

In 1996, the Martian meteorite ALH84001, a specimen that is much older than the majority of Martian meteorites that have been recovered so far, received considerable attention when a group of NASA scientists led by David S. McKay reported microscopic features and geochemical anomalies that they considered to be best explained by the rock having hosted Martian bacteria in the distant past. Some of these features resembled terrestrial bacteria, aside from their being much smaller than any known form of life. Much controversy arose over this claim, and ultimately all of the evidence McKay's team cited as evidence of life were found to be explainable by non-biological processes. Although the scientific community has largely rejected the claim ALH 84001 contains evidence of ancient Martian life, the controversy associated with it is now seen as a historically significant moment in the development of exobiology.^[178][179]

Nakhla meteorite

Nakhla

The Nakhla meteorite fell on Earth on June 28, 1911 on the locality of Nakhla, Alexandria, Egypt.^[180][181]

In 1998, a team from NASA's Johnson Space Center obtained a small sample for analysis. Researchers found preterrestrial aqueous alteration phases and objects^[182] of the size and shape consistent with Earthly fossilized nanobacteria. Analysis with gas chromatography and mass spectrometry (GC-MS) studied its high molecular weight polycyclic aromatic hydrocarbons in 2000, and NASA scientists concluded that as much as 75% of the organic matter in Nakhla "may not be recent terrestrial contamination".^[177][183]

This caused additional interest in this meteorite, so in 2006, NASA managed to obtain an additional and larger sample from the London Natural History Museum. On this second sample, a large dendritic carbon content was observed. When the results and evidence were published in 2006, some independent researchers claimed that the carbon deposits are of biologic origin. However, it was remarked that since carbon is the fourth most abundant element in the Universe, finding it in curious patterns is not indicative or suggestive of biological origin.^[184][185]

Shergotty

The Shergotty meteorite, a 4 kg Martian meteorite, fell on Earth on Shergotty, India on August 25, 1865 and was retrieved by witnesses almost immediately.^[186] It is composed mostly of pyroxene and thought to have undergone preterrestrial aqueous alteration for several centuries. Certain features in its interior suggest remnants of a biofilm and its associated microbial communities.^[177] Work is in progress on searching for magnetites within alteration phases.

Yamato 000593

Yamato 000593 is the second largest meteorite from Mars found on Earth. Studies suggest the Martian meteorite was formed about 1.3 billion years ago from a lava flow on Mars. An impact occurred on Mars about 12 million years ago and ejected the meteorite from the Martian surface into space. The meteorite landed on Earth in Antarctica about 50,000 years ago. The mass of the meteorite is 13.7 kg (30 lb) and has been found to contain evidence of past water movement.^{[187][188][189]} At a microscopic level, spheres are found in the meteorite that are rich in carbon compared to surrounding areas that lack such spheres. The carbon-rich spheres may have been formed by biotic activity according to NASA scientists.^{[187][188][189]}

Geysers on Mars

Artist concept showing sand-laden jets erupt from geysers on Mars.

Close up of dark dune spots, probably created by cold geyser-like eruptions.

The seasonal frosting and defrosting of the southern ice cap results in the formation of spider-like radial channels carved on 1 meter thick ice by sunlight. Then, sublimed CO₂ – and probably water – increase pressure in their interior producing geyser-like eruptions of cold fluids often mixed with dark basaltic sand or mud.^{[190][191][192][193]} This process is rapid, observed happening in the space of a few days, weeks or months, a growth rate rather unusual in geology – especially for Mars.

A team of Hungarian scientists proposes that the geysers' most visible features, dark dune spots and spider channels, may be colonies of photosynthetic Martian microorganisms, which over-winter beneath the ice cap, and as the sunlight returns to the pole during early spring, light penetrates the ice, the microorganisms photosynthesize and heat their immediate surroundings. A pocket of liquid water, which would normally evaporate instantly in the thin Martian atmosphere, is trapped around them by the overlying ice. As this ice layer thins, the microorganisms show through grey. When the layer has completely melted, the microorganisms rapidly desiccate and turn black, surrounded by a grey aureole.^{[194][195][196]} The Hungarian scientists believe that even a complex sublimation process is insufficient to explain the formation and evolution of the dark dune spots in space and time.^[197][198] Since their discovery, fiction writer Arthur C. Clarke promoted these formations as deserving of study from an astrobiological perspective.^[199]

A multinational European team suggests that if liquid water is present in the spiders' channels during their annual defrost cycle, they might provide a niche where certain microscopic life forms could have retreated and adapted while sheltered from solar radiation.^[200] A British team also considers the possibility that organic matter, microbes, or even simple plants might co-exist with these inorganic formations, especially if the mechanism includes liquid water and a geothermal energy source.^[201] However, they also remark that the majority of geological structures may be accounted for without invoking any organic "life on Mars" hypothesis.^[201] It has been proposed to develop the Mars Geyser Hopper lander to study the geysers up close.^[202]

Forward contamination

Planetary protection of Mars aims to prevent biological contamination of the planet.^[203] A major goal is to preserve the planetary record of natural processes by preventing human-caused microbial introductions, also called forward contamination. There is abundant evidence as to what can happen when organisms from regions on Earth that have been isolated from one another for significant periods of time are introduced into each other's environment. Species that are constrained in one environment can thrive – often out of control – in another environment much to the detriment of the original species that were present. In some ways this problem could be compounded if life forms from one planet were introduced into the totally alien ecology of another world.^[204]

The prime concern of hardware contaminating Mars derives from incomplete spacecraft sterilization of some hardy terrestrial bacteria (extremophiles) despite best efforts.^[24][205] Hardware includes landers, crashed probes, end-of-mission disposal of hardware, and hard landing of entry, descent, and landing systems. This has prompted research on survival rates of radiation-resistant microorganisms including the species Deinococcus radiodurans and genera Brevundimonas, Rhodococcus, and Pseudomonas under simulated Martian conditions.^[206] Results from one of these experimental irradiation experiments, combined with previous radiation modeling, indicate that Brevundimonas sp. MV.7 emplaced only 30 cm deep in Martian dust could survive the cosmic radiation for up to 100,000 years before suffering 10⁶ population reduction.^[206] Surprisingly, the diurnal Mars-like cycles in temperature and relative humidity affected the viability of Deinococcus radiodurans cells quite severely.^[207] In other simulations, Deinococcus radiodurans also failed to grow under low atmospheric pressure, under 0 °C, or in the absence of oxygen.^[208]

Survival under simulated Martian conditions

On 26 April 2012, scientists reported that an extremophile lichen survived and showed remarkable results on the adaptation capacity of photosynthetic activity within the simulation time of 34 days under Martian conditions in the Mars Simulation Laboratory (MSL) maintained by the German Aerospace Center (DLR).^{[209][210][211][212][213][214]} However, the ability to survive in an environment is not the same as the ability to thrive, reproduce, and evolve in that same environment, necessitating further study.

Although numerous studies point to resistance to some of Mars conditions, they do so separately, and none has considered the full range of Martian surface conditions, including temperature, pressure, atmospheric composition, radiation, humidity, oxidizing regolith, and others, all at the same time and in combination.^[215] Laboratory simulations show that whenever multiple lethal factors are combined, the survival rates plummet quickly.^[25]

Missions

Mars-2

Mars-1 was the first spacecraft launched to Mars in 1962,^[216] but communication was lost while en route to Mars. With Mars-2 and Mars-3 in 1971-1972, information was obtained on the nature of the surface rocks and altitude profiles of the surface density of the soil, its thermal conductivity, and thermal anomalies detected on the surface of Mars. The Program found that its northern polar cap has a temperature below -110 °C and that the water vapor content in the atmosphere of Mars is five thousand times less than on Earth. No signs of life were found.^[217]

Mariner 4

Mariner Crater, as seen by Mariner 4 in 1965. Pictures like this suggested that Mars is too dry for any kind of life.

Streamlined Islands seen by Viking orbiter showed that large floods occurred on Mars. Image is located in Lunae Palus quadrangle.

Mariner 4 probe performed the first successful flyby of the planet Mars, returning the first pictures of the Martian surface in 1965. The photographs showed an arid Mars without rivers, oceans, or any signs of life. Further, it revealed that the surface (at least the parts that it photographed) was covered in craters, indicating a lack of plate tectonics and weathering of any kind for the last 4 billion years. The probe also found that Mars has no global magnetic field that would protect the planet from potentially life-threatening cosmic rays. The probe was able to calculate the atmospheric pressure on the planet to be about 0.6 kPa (compared to Earth's 101.3 kPa), meaning that liquid water could not exist on the planet's surface.^[21] After Mariner 4, the search for life on Mars changed to a search for bacteria-like living organisms rather than for multicellular organisms, as the environment was clearly too harsh for these.

Viking orbiters

Liquid water is necessary for known life and metabolism, so if water was present on Mars, the chances of it having supported life may have been determinant. The Viking orbiters found evidence of possible river valleys in many areas, erosion and, in the southern hemisphere, branched streams.^{[218][219][220]}

Viking experiments

Carl Sagan poses next to a replica of the Viking landers.

The primary mission of the Viking probes of the mid-1970s was to carry out experiments designed to detect microorganisms in Martian soil because the favorable conditions for the evolution of multicellular organisms ceased some four billion years ago on Mars.^[221] The tests were formulated to look for microbial life similar to that found on Earth. Of the four experiments, only the Labeled Release (LR) experiment returned a positive result,^{[dubious – discuss]} showing increased ¹⁴CO₂ production on first exposure of soil to water and nutrients. All scientists agree on two points from the Viking missions: that radiolabeled ¹⁴CO₂ was evolved in the Labeled Release experiment, and that the GCMS detected no organic molecules. However, there are vastly different interpretations of what those results imply.

A 2011 astrobiology textbook notes that the GCMS was the decisive factor due to which "For most of the Viking scientists, the final conclusion was that the Viking missions failed to detect life in the Martian soil."^[222]

One of the designers of the Labeled Release experiment, Gilbert Levin, believes his results are a definitive diagnostic for life on Mars.^[21] Levin's interpretation is disputed by many scientists.^[223] A 2006 astrobiology textbook noted that "With unsterilized Terrestrial samples, though, the addition of more nutrients after the initial incubation would then produce still more radioactive gas as the dormant bacteria sprang into action to consume the new dose of food. This was not true of the Martian soil; on Mars, the second and third nutrient injections did not produce any further release of labeled gas."^[224] Other scientists argue that superoxides in the soil could have produced this effect without life being present.^[225] An almost general consensus discarded the Labeled Release data as evidence of life, because the gas chromatograph & mass spectrometer, designed to identify natural organic matter, did not detect organic molecules.^[163] More recently, high levels of organic chemicals, particularly chlorobenzene, were detected in powder drilled from one of the rocks, named "Cumberland", analyzed by the Curiosity rover.^[142][143] Nonetheless, the results of the Viking mission concerning life are considered by the general expert community, at best, as inconclusive.^{[21][225][226]}

In 2007, during a Seminar of the Geophysical Laboratory of the Carnegie Institution (Washington, D.C., USA), Gilbert Levin's investigation was assessed once more.^[163] Levin still maintains that his original data were correct, as the positive and negative control experiments were in order.^[166] Moreover, Levin's team, on 12 April 2012, reported a statistical speculation, based on old data —reinterpreted mathematically through cluster analysis— of the Labeled Release experiments, that may suggest evidence of "extant microbial life on Mars."^[166][227] Critics counter that the method has not yet been proven effective for differentiating between biological and non-biological processes on Earth so it is premature to draw any conclusions.^[228]

A research team from the National Autonomous University of Mexico headed by Rafael Navarro-González, concluded that the GCMS equipment (TV-GC-MS) used by the Viking program to search for organic molecules, may not be sensitive enough to detect low levels of organics.^[170] Klaus Biemann, the principal investigator of the GCMS experiment on Viking wrote a rebuttal.^[229] Because of the simplicity of sample handling, TV–GC–MS is still considered the standard method for organic detection on future Mars missions, so Navarro-González suggests that the design of future organic instruments for Mars should include other methods of detection.

After the discovery of perchlorates on Mars by the Phoenix lander, practically the same team of Navarro-González published a paper arguing that the Viking GCMS results were compromised by presence of perchlorates.^[230] A 2011 astrobiology textbook notes that "while perchlorate is too poor an oxidizer to reproduce the LR results (under the conditions of that experiment perchlorate does not oxidize organics), it does oxidize, and thus destroy, organics at the higher temperatures used in the Viking GCMS experiment."^[231] Biemann has written a commentary critical of this Navarro-González paper as well,^[232] to which the latter have replied;^[233] the exchange was published in December 2011.

Phoenix lander, 2008

An artist's concept of the Phoenix spacecraft

The Phoenix mission landed a robotic spacecraft in the polar region of Mars on May 25, 2008 and it operated until November 10, 2008. One of the mission's two primary objectives was to search for a "habitable zone" in the Martian regolith where microbial life could exist, the other main goal being to study the geological history of water on Mars. The lander has a 2.5 meter robotic arm that was capable of digging shallow trenches in the regolith. There was an electrochemistry experiment which analysed the ions in the regolith and the amount and type of antioxidants on Mars. The Viking program data indicate that oxidants on Mars may vary with latitude, noting that Viking 2 saw fewer oxidants than Viking 1 in its more northerly position. Phoenix landed further north still.^[234] Phoenix's preliminary data revealed that Mars soil contains perchlorate, and thus may not be as life-friendly as thought earlier.^{[235][236][237]} The pH and salinity level were viewed as benign from the standpoint of biology. The analysers also indicated the presence of bound water and CO₂.^[238] A recent analysis of martian meteorite EETA79001 found 0.6 ppm ClO₄⁻, 1.4 ppm ClO₃⁻, and 16 ppm NO₃⁻, most likely of martian origin. The ClO₃⁻ suggests presence of other highly oxidizing oxychlorines such as ClO₂⁻ or ClO, produced both by UV oxidation of Cl and X-ray radiolysis of ClO₄⁻. Thus only highly refractory and/or well-protected (sub-surface) organics are likely to survive.^[239] In addition, recent analysis of the Phoenix WCL showed that the Ca(ClO₄)₂ in the Phoenix soil has not interacted with liquid water of any form, perhaps for as long as 600 Myr. If it had, the highly soluble Ca(ClO₄)₂ in contact with liquid water would have formed only CaSO₄. This suggests a severely arid environment, with minimal or no liquid water interaction.^[240]

Curiosity rover self-portrait.

Mars Science Laboratory

The Mars Science Laboratory mission is a NASA project that launched on November 26, 2011, the Curiosity rover, a nuclear-powered robotic vehicle, bearing instruments designed to assess past and present habitability conditions on Mars.^[241][242] The Curiosity rover landed on Mars on Aeolis Palus in Gale Crater, near Aeolis Mons (a.k.a. Mount Sharp),^{[243][244][245][246]} on August 6, 2012.^{[247][248][249]}
On 16 December 2014, NASA reported the Curiosity rover detected a "tenfold spike", likely localized, in the amount of methane in the Martian atmosphere. Sample measurements taken "a dozen times over 20 months" showed increases in late 2013 and early 2014, averaging "7 parts of methane per billion in the atmosphere." Before and after that, readings averaged around one-tenth that level.^[142][143] In addition, low levels of chlorobenzene (C
₆H
₅Cl), were detected in powder drilled from one of the rocks, named "Cumberland", analyzed by the Curiosity rover.^[142][143]

Future astrobiology missions

• ExoMars is a European-led multi-spacecraft programme currently under development by the European Space Agency (ESA) and the Russian Federal Space Agency for launch in 2016 and 2020.^[251] Its primary scientific mission will be to search for possible biosignatures on Mars, past or present. A rover with a 2 m (6.6 ft) core drill will be used to sample various depths beneath the surface where liquid water may be found and where microorganisms (or organic biosignatures^{[citation needed]}) might survive cosmic radiation.^[36]
• Mars 2020 – The Mars 2020 rover is a Mars planetary rover mission by NASA with a planned launch in 2020. It is intended to investigate an astrobiologically relevant ancient environment on Mars, investigate its surface geological processes and history, including the assessment of its past habitability and potential for preservation of biosignatures within accessible geological materials.^[252]
• Mars Sample Return Mission — The best life detection experiment proposed is the examination on Earth of a soil sample from Mars. However, the difficulty of providing and maintaining life support over the months of transit from Mars to Earth remains to be solved. Providing for still unknown environmental and nutritional requirements is daunting. Should dead organisms be found in a sample, it would be difficult to conclude that those organisms were alive when obtained.

Human colonization of Mars

Some of the main reasons for colonizing Mars include economic interests, long-term scientific research best carried out by humans as opposed to robotic probes, and sheer curiosity. Surface conditions and the presence of water on Mars make it arguably the most hospitable of the planets in the Solar System, other than Earth. Human colonization of Mars would require in situ resource utilization (ISRU); A NASA report states that "applicable frontier technologies include robotics, machine intelligence, nanotechnology, synthetic biology, 3-D printing/additive manufacturing and autonomy. These technologies combined with the vast natural resources should enable, pre- and post-human arrival ISRU to greatly increase reliability and safety and reduce cost for human colonization of Mars."^{[253][254][255]}