Top row, left to right: Earth, Mars, Venus, Jupiter; bottom row, left to right: Europa, Io, Titan, Triton. (Not in scale.)

Also see: Habitable solar systems, Alien planets

Most dissertations on the subject of life in the Universe assume that life-bearing planets should be very similar to Earth in aspects such as size, temperature, chemistry, etc. According to Peter Ward's Rare Earth hypothesis, the emergence of life, or at least complex plant-like and animal-like life requires even more factors such as a right-sized moon, the right percentage of metals in the core, and so on.

In their book Cosmic Biology: How Life Could Evolve on Other Worlds, Louis Neal Irwin and Dirk Schulze-Makuch argue that at least simple life could in fact be common even inside the Solar System. A number of very different planets and moons are presented as objects of speculation about what sort of life they could host, and as models for more favourable world in the rest of the Universe. These are Earth, Mars, Venus, Jupiter, Europa and Io (moons of Jupiter), Titan (moon of Saturn) and Triton (moon of Neptune).

Model 1: Earth

Geologically complex, water-rich world, with an oxidizing atmosphere
  • Orbital radius: 1 AU from the Sun
  • Relative radius, mass, density: 1.0, 1.0, 1.0
  • Average surface temperature: 287 K (14°C)
  • Surface atmospheric pressure: 1 atm
  • Atmospheric composition: nitrogen (78%), oxygen (21%), argon (1%), traces of carbon dioxide and water vapor
  • Surface gravity: 1 g

Planetary description: Earth has a thin solid crust, mostly composed of silicate rocks, spread over a fluid mantle kept warm by the radioactive decay in the core. The mantle constantly cracks and reshapes the crust, creating basins, mountain ranges and continents. Most of the crust is covered by a layer of liquid water rich in dissolved ions; the atmosphere is mainly nitrogen and oxygen, and it's thick enough to maintain Earth's surface temperature relatively constant, and usually above the freezing point of water. It also shields the surface from most ionizing radiations, and creats a dynamic weather. A large moon keeps the axis stable and produces cyclic tides.

Conjectures on life: Endowed with strong tectonic activity, plentiful liquid water, a great topographic diversity and bathed in sunlight, Earth and Earth-like planets are prime candidates for the origin and development of life. At Earth's surface temperatures, carbon is the ideal building block for life, as silicon, though more abundant, does not form stable polymers. Hydrocarbons (carbon and hydrogen molecules) and carbohydrates (carbon, hydrogen and oxygen) will feature heavily in Earth biochemistry.

Sunlight is an efficient and reliable energy source for all surface life: Earth's continent are expected to be covered in static photoautotrophs, and in motile heterotrophs that feed on them. In caves and seafloors, chemotrophy will be dominant instead, thanks to the amount of minerals emitted from the mantle; methanogenesis and the oxidation of iron and sulfur will be especially common (see here). The abundant free oxygen will be an excellent oxidizing agent for most life.

All this energy will allow life on Earth to grow abundant and complex, with highly stratified food chains, both motile and static organisms and possibly advanced intelligence; consumers will include filter-feeders, grazers and browsers, active predators, scavengers and detritivores, etc. It is even conceivable that a civilization or social systems that pass on culture and learning may install themselves on the planet given time for intellectual enough animals to evolve.

Model 2: Mars

Cold and dry world, with a thin atmosphere
  • Orbital radius: 1.5 AU from the Sun
  • Relative radius, mass, density: 0.53, 0.11, 0.71
  • Average surface temperature: 210 K (-63°C)
  • Surface atmospheric pressure: 0.63 atm
  • Atmospheric composition: carbon dioxide (95.3%), nitrogen (2.7%), argon (1.6%)
  • Surface gravity: 0.38 g

Planetary description: Mars is a rocky planet much smaller than the Earth, with only a third of its surface gravity. Due to the greater distance from the Sun and the tenuous atmosphere, Mars is much colder than Earth, so that all the water present on its surface is locked in polar icecaps that grow and retreat with the seasons.

The surface is covered in iron-rich red dust and shows traces of ancient tectonic activity: the austral hemisphere is noticeably higher than the other, with two large basins and the kilometres-high Tharsis Rise on the Equator; the boreal hemisphere is mostly occupied by a flat plain, the Vastitas Borealis. Some geologic features suggest that the Vastitas was once occupied by a large water ocean, and the rest of the surface also shows outflow channels. Mars' weaker gravity allows mountains to be much higher than on Earth (up to Olympus Mons' 26 km).

Conjectures on life: Soon after his formation, Mars was very similar to contemporary Earth, with a substantial tectonic activity and at least a large northern ocean. Given its small size, the internal heating disappeared soon, and with that the plate movement and the magnetic field that helped retain its atmosphere: the planet became cold and dry. Liquid water lasted probably about half a billion years, enough to allow the origin of life; it's possible that the rapid changes and the high habitat fragmentation have induced a faster evolution, perhaps up to a simple multicellular biota even before it appeared on Earth.

After the disappearance of liquid water, life might have survived retreating beneath the ground, between the grains of dust or within the ancient lava tubes that surround the Olympus Mons. Hygroscopic substances such as hydrogen peroxide, ethanol or methanol, sucrose etc. might have been employed to retain moisture. Multicellular tissues, if they exist, would be thick and hard.

Life on Mars, or on Mars-like exoplanets, will probably have slower biological processes (both metabolism and reproduction) and a smaller biomass mainly composed by autotrophic producers, with at most one layer of consumers, since there's much less energy to sustain life and oxidizing agents are lacking; where light is not available, the most likely source of energy is the chemotrophic reduction of iron. A current underground Martian biosphere would probably be limited to xerophytic "plants" and small invertebrate-like organisms.

Model 3: Venus

Sulfur-rich world with a thick, acidic atmosphere and a runaway greenhouse effect
  • Orbital radius: 0.72 AU from the Sun
  • Relative radius, mass, density: 0.95, 0.82, 0.95
  • Average surface temperature: 735 K (462°C)
  • Surface atmospheric pressure: 92 atm
  • Atmospheric composition: carbon dioxide (96.5%), nitrogen (3.5%)
  • Surface gravity: 0.9 g

Planetary description: From a purely geological point of view, Venus is the planet most similar to Earth: it has roughly the same mass, the same internal composition, the same gravity. Despite this, its surface is nothing like Earth's: water evaporation, through greenhouse effect, increased itself creating an extremely dense and highly acidic atmosphere, and raising the temperature well beyond 400°C, even higher than the temperatures on Mercury.

Venus' atmosphere is composed mostly of carbon dioxide and rises to 70-80 km above the surface; at 45-65 km there's a layer of clouds of sulfuric acid, which reflect 80% of the incoming sunlight, though the air still manages to retain a lot of heat. As on Mars, the surface shows channels probably produced by an ancient flow of water, and most of it is covered in lava plains and dormant volcanoes. Though tectonic activity has long stopped, the internal heat is still discharged through periodic global eruption, the last of which happened 500-700 million years ago.

Conjectures on life: Like Mars, Venus was initially very Earth-like, with several small basins full of liquid water, constant volcanic activity and abundant sunlight (roughly twice the amount of light that reaches Earth) that would have fed a large biomass of photoautotrophs. Then, water evaporated, and the vapour leaked into space or combined with volcanic sulfur dioxide to form sulfuric acid clouds; today the surface of Venus experiences such pressure, temperatures and acidity that any life on it is probably impossible.

Early evolution on Venus had the advantages of both highly fractionated and quickly changing habitats and more available energy, so it should be expected to have been faster than on either Mars or Earth, but it'd still had to come to an end when liquid water disappeared (even at 92 atm pressures, water stays liquid only up to 306°C). Silicon-based life would be stable at these temperatures, albeit not much dynamic, but it still lacks any suitable solvent (the only common liquid on Venus' surface is lava, which is not suitable to life at all).

The environment on Venus most useful for life is the layer of clouds, where the temperature is low enough for water to exist as micrometre-sized droplets of mist mixed with sulfuric acid. The pH close to 0, while challenging, is survivable for several acidophilic bacteria on Earth. In the atmosphere, sunlight is plentiful, and sulfur can be both oxidized into sulfur dioxide and reduced into hydrogen sulfide, fueling a variety of chemotrophic reactions. Strong winds, that range from 50 to 200 m/s in the clouds, could also be exploited by kinetotrophs.

Anyway, the conditions on Venus are at the very boundary of survival for carbon-based life: while the amount of energy could feed an extensive biomass, individual organisms larger than bacteria are very unlikely to be found and even then they would need to be thermophiles (heat-resistant extremophiles) to some extent, likely beyond that of any known species.

Model 4: Jupiter

World with a thick and volatile atmosphere, lacking a solid surface
  • Orbital radius: 5.2 AU from the Sun
  • Relative radius, mass, density: 11.2, 318, 0.24
  • Average temperature: 165 K (-108°C) at 1 atm depth, 112 K (-161°C) at 0.1 atm depth
  • Atmospheric pressure: 0.2-2 atm (in the cloud layer)
  • Atmospheric composition: hydrogen (89.8%), helium (10.2%), methane (0.3%)
  • Gravity in the cloud layer: 2.4 g

Planetary description: Over 300 times more massive than earth, Jupiter is the largest planet. It's considered a gas giant, as it lacks a solid surface but has an extremely dense atmosphere that gradually increases in temperature and pressure: the hydrogen behaves at first as a gas, then as a liquid, then as a metal in which electrons flow free from the nuclei.

Jupiter's atmosphere has a barotropic circulation: it's divided in parallel low-pressure zones and high-pressure belts, that flow in opposite directions, creating storms and hurricanes at their interface, such as the Great Red Spot. Above the thick, hydrocarbon-rich clouds there are thinner clouds of ammonia; below, the main layer of hydrogen that goes all the way down to the rocky core. At the Equator, the winds can reach 110 m/s.

Conjectures on life: Given the lack of a solid substrate, the dispersion of chemicals and the low stability of the environment, a local origin of life on any gas giant seems very unlikely. It's possible though that life arose on a moon such as Europa or Io (see below), and later was brought on the gas giant through panspermia; organisms suited to the dense atmosphere of Titan should be at ease on Saturn. In the deep, liquid and metallic layers of the atmosphere any kind of biological chemistry is most probably impossible, while the upper layers could conceivably be inhabited by phototrophic aeroplankton or similar organisms. The temperature falls in the range needed for liquid water at a region with pressure between 0.1 and 10 atm.

At greater depths (but still in gaseous clouds) life could be powered by chemotrophy and maybe kinetotrophy. The relative lack of water and the high presence of ionizing radiation will be a challenge for every lifeform; microbes might be present as hygroscopic floating colonies. Large floating organisms, perhaps covered by a photosynthetic film, could exist, but they couldn't have evolved by smaller floaters, as floating is not efficient at small sizes (see here). Near the poles, the strong magnetic field could power magnetotrophy.

Alternatives in the Solar System: Besides hydrogen and helium, Uranus and Neptune have a substantial amount of methane (2.3% and 1.5%, respectively) and frozen ammonia and ammonium hydrosulfide, which would provide more carbon and nitrogen to sustain a biochemistry. Neptune also has the fastest wind in the Solar System (over 600 m/s) and possibly an inner ocean of liquid water mixed with silicates, ammonia and methane: it would be extremely hot, though, and liquid only thanks to pressure, so it's unlikely to host life.

Model 5: Europa

World with water oceans covered by an opaque ice crust
  • Orbital radius: 5.2 AU from the Sun, 671 Mm from Jupiter
  • Relative radius, mass, density: 0.25, 0.008, 0.55
  • Average surface temperature: 102 K (-171°C)
  • Surface atmospheric pressure: 10-12 atm
  • Atmospheric composition: mostly oxygen
  • Surface gravity: 0.13 g

Planetary description: Europa, the second large moon of Jupiter, is extraordinarily rich in water: the whole surface is covered by a thick crust of ice, and the distortions it creates in Jupiter's magnetic field suggest a large metallic core, a large salty water ocean, or both. That means that Europa is very likely to have a huge volume of liquid water fed by minerals that seep from the mantle.

The atmosphere is almost negligible: a thin cover of oxygen created by radiolysis of the water molecules. The ice crust is covered in cracks and fissures probably produced by Jupiter's tidal friction (see here) through the upheaval of the liquid mass below; the reddish streaks could be produced by salts such as magnesium sulfate, or other sulfur compounds. The ice crust should be 10-30 km thick, and the ocean could be as much as 100 km deep.

Conjectures on life: Europa's ocean is an ideal setting for the origin of life, which then would have to evolve in complete darkness. It receives a good amount of energy as tidal flexing from Jupiter's gravity, and probably from radioactive decay from the core as well, in the form of hydrothermal vents. The two liquid-solid interfaces (water-ice and water-rock) could help to catalyze chemical reactions and offer stable habitats.

The ice crust is completely opaque, so the only phototrophs would be limited as algae on the external icy surface or in tiny hypersaline water pockets a few cm below (which would protect them from the radiations). The oxygen from the atmosphere can be combined with hydrogen to produce water, with a great energy gain; hydrogen could be used to reduce Fe3+ and carbon dioxide into Fe2+ and methane, continuously reoxidized at the surface.

If hydrothermal vents are available on the seafloor, the flux of heat towards the cold water can be exploited by thermotrophs that cyclically rise and fall or that pump water through an elongated body; the difference in salinity between the water enriched by the mineral in the seafloor below and the water diluted by the melting ice above could also power mobile osmotrophs and ionotrophs.

The seafloor of Europa could thus be covered by masses of methanogens and iron reducers, fixed thermotrophs and kinetotrophic "reeds"; swarms of mobile thermotrophs, ionotrophs and osmotrophs would travel on the convective currents above them (perhaps hunted by predatory "fish" and "jellyfish"?); ciliate kinetotrophs and aerobic heterotrophs would cling at the icy ceiling, and a thin mass of algae and microscopic aerobic grazers would inhabit the surface. After Earth, Europa might host the greatest biodiversity in the Solar System.

Alternatives in the Solar System: Enceladus, moon of Saturn, is also covered in a homogeneous icy crust. While fissures are less obvious, they do show traces of tectonic activity, and geyser-like plumes of water have been observed near to its south pole. It's far smaller than Europa (0.04 Earth radii, about the size of Kansas or Romania) and far colder (-190°C, while the hottest spots reach -100°C). It should have a similar ocean, and the geyser are suspected to contain also nitrogen and carbon oxides; thus, Enceladus might be as able to develop life as Europa is, if less energy-rich.

Model 6: Io

Cold and sulfur-rich world with a strong volcanic activity
  • Orbital radius: 5.2 AU from the Sun, 422 Mm from Jupiter
  • Relative radius, mass, density: 0.29, 0.015, 0.64
  • Average surface temperature: 130 K (-143°C)
  • Surface atmospheric pressure: negligible
  • Atmospheric composition: sulfur dioxide (90%)
  • Surface gravity: 0.18 g

Planetary description: Io is the closest large moon of Jupiter, and the body with the strongest volcanic activity in the whole Solar System: the surface is covered by eruptions of sulfur and sulfur dioxide that create a yellow tinge. This activity is mostly due to Jupiter's strong tidal flexing that constantly breaks open the crust. Io is also the densest moon in the Solar System, which suggests that it has a large liquid interior.

The sulfur compounds raise for many kilometres, being degraded by Jupiter's strong magnetic field into sulfur and oxygen ions that make an extremely thin atmosphere. The eruptions also release hot (up to 1700°C) molten silicates, which produce vast lava fields; the coldest parts of the crust (down to -183°C) are instead covered in ice that should contain mostly sulfur dioxide, with a small content of water and hydrogen sulfide. At the largest volcanic depression, Loki Patera, temperatures that allow for liquid water are found at a few tens of kilometres from the crater.

Conjectures on life: With the violence of its volcanic manifestations, Io is not very conductive to the origin of life, though it may have been more stable in the past. While usually there are no liquids on its surface, the eruptions can locally melt the frozen sulfur compounds and create ephemeral ponds at the interface of cold and heat. Microscopic organisms could survive as dormant spores that survive without activity for several years between eruptions.

Sulfur dioxide (-75° to -10°C) seems to be the most abundant and likely solvent on Io, though it lacks the ability of forming hydrogen bonds, an important feature in complex biomolecules; hydrogen sulfide (-85°C to -60°C) does not, and while it's less common it might be employed as intracellular solvent in the narrow range of temperatures (-75°C to -60°C) where both compounds are liquids. It's possible, however, that longer-lasting pools of sulfur dioxide exist beneath the surface at higher pressure.

Life inside Io's pools isn't likely to be based on carbon: its biomolecules might be based on rings and polymers of sulfur, or on phosphorus-nitrogen chains. As for energy, the presence of very strong and concentrated sources of heat surrounded by freezing cold should make thermotrophy really useful. As said above, sulfur can both be oxidized into sulfur dioxide and reduced into hydrogen sulfide; chemotrophy would also benefit from the highly oxidizing ions (O+, O2+, S+, S2+)  in the upper atmosphere. Even there, sunlight is very weak, but since Jupiter's magnetic field in Io's orbit is 12 times stronger than Earth's, magnetotrophy might arise.

Io's biosphere should thrive locally in brief and sparse bursts: the organisms will probably be small, short-living, highly prolific and of low trophic level, but with a good number of motile forms; in deep liquid pools, they could grow larger and longer-living, but food chains would still be simple and short.

Model 7: Titan

World rich in hydrocarbons and other organic compounds, with a thick atmosphere
  • Orbital radius: 9.5 AU from the Sun, 1220 Mm from Saturn
  • Relative radius, mass, density: 0.40, 0.023, 0.34
  • Average surface temperature: 94 K (-179°C)
  • Surface atmospheric pressure: 1.5 atm
  • Atmospheric composition: nitrogen (95%), methane (4.9%)
  • Surface gravity: 0.14 g

Planetary description: Titan is Saturn's largest moon, being 24 times heavier than all the other moons together. It's likely differentiated in a crust rich in water ice and methane clathrate, and in a hot silicate core. It's also the only moon in the Solar System with a sizeable atmosphere, mostly composed by nitrogen, but also rich in methane, and contains a great amount of organic compounds, such as hydrocarbons, hydrogen cyanide, nitriles, acetylene and cyanoacetilene, etc.

The frozen ground hosts two different liquids: large pools of hydrocarbons, mostly methane and ethane have been observed on the surface (liposphere), and probably deep reservoirs or water mixed with ammonia beneath (hydrosphere). Methane clouds form at 30 and 120 km of altitude; in the middle absorbed radiations form tholins, nitrogen-rich organic compounds that don't exist on Earth. The thick atmosphere helps keeping the climate stable, shields the surface from radiations, and creates seasonal weather patterns, with ethane rain and snow.

Conjectures on life: The large amount of organic molecules, and a dynamic but not chaotic surface environment, make Titan a prime candidate for the origin of life in the Solar System. Hydrocarbon lakes can grow quite large in the polar regions, and they could serve as a solvent for life, but since they're not polar (differently from water), lipids cannot build cellular membranes there as they do on Earth; they could be replaced by tholin sheaths and maybe silicon polymers such as polysilanes. Carbon-nitrogen chains might take the role of DNA-like information-carrying biomolecules.

While the abundance of organic molecules is useful to sustain quick reproduction in a large biomass, being already highly reduced molecules in an environment poor in oxidizers they wouldn't be as useful as energy sources. However, organisms such as the bacterium Bacillus cereus and the fungus Fusarium alkanophyllum are known to degrade complex hydrocarbons; if free hydrogen is available, acetylene (C2H2) could be reduced into methane, breaking a carbon-carbon triple bond (C≡C) extremely rich in energy.

If there really is a subsurface ocean (an aqueous solution of ammonia can stay liquid down to -98°C at Titan's surface pressure), we'd expect its life to be chemically and morphologically similar to that on Europa. The liposphere life wouldn't probably need much activity: it'd include static autotrophs and filter-feeders, perhaps amoeboid floaters, biofilm grazers and burrowing dwellers of the tar-like tholin deposits.

Model 8: Triton

Very small and extremely cold world, with a thin atmosphere
  • Orbital radius: 30 AU from the Sun, 355 Mm from Neptune
  • Relative radius, mass, density: 0.21, 0.0036, 0.30
  • Average surface temperature: 38 K (-235°C)
  • Surface atmospheric pressure: 10-5 atm
  • Atmospheric composition: nitrogen (99.9%), methane traces
  • Surface gravity: 0.08 g

Planetary description: Early Triton was probably formed as a comet-like object in the Kuiper Belt, and later was captured by Neptune's gravity, becoming its largest moon. As typical of that sort of bodies, 1/4 to 1/3 of its mass is water ice and other frozen gas. Triton's surface is the coldest known in the Solar System, and entirely covered in ice, mostly nitrogen, water and carbon dioxide, with traces of methane, carbon monoxide and maybe ammonia; the different colours on its surface appear to suggest organic compounds (yellow and pink) and frozen nitrogen and nitriles (blue-green).

Triton's atmosphere, also composed mostly of nitrogen, is very thin, but shows traces of wind. Particles emitted from Neptune's magnetic field combine nitrogen and methane into organic compounds, that then fall to the surface, where they probably decompose quickly. The peculiar "cantaloupe terrain", found nowhere else in the Solar System, is formed by many basins 30-40 km in diameter and several straight ridges (sulci), and is probably caused by weak tectonic activity and cryovolcanism; in fact, there is enough rock in Triton's core to produce heat via radioactive decay.

Conjectures on life: While Triton's surface is far too cold for any plausible solvent to exist in liquid form, liquid ammonia or nitrogen might exist in deep reservoirs, if there really is a molten core. The surface is also vulnerable to radiations, and the atmosphere is both thin and poorly reactive - if there's any life, it's bound to be hidden deep in the crust.

Biochemistry might be based on polysilanes, which are expected to be stable at low temperatures and high pressures, without liquid water or strong oxidizers; they could be employed in non-polar solvents such as methane or methanol (if there's enough of them) or, more likely, nitrogen. Since there's so much water ice, it's possible that closer to the core there's also a Europa-like ocean, perhaps with carbon-based life; above it, microbial life could inhabit micro-environments such as saline water pockets in the ice and thin films of liquid nitrogen (which can exist only at a very narrow range of temperatures: -210°C to -196°C).

Neptune's magnetic field is not very strong; sunlight intensity is 900 times lower than on Earth, and the surface is unsuitable for life anyway. Underwater life might resort to osmotrophy and ionotrophy; the exceptional cold outside means that heat leaking from the core could be efficiently exploited by thermotrophs. Given the overall scarcity of energy, life on Triton would be uncommon, small and mainly static, at most forming fungal or moss-like growths on the water-rock and water-ice interfaces.

Alternatives in the Solar System: Pluto and Charon, twin dwarf planets orbiting each other, are very similar to Triton in history and composition. They should be composed for one third of water ice, and have an extremely thin nitrogen atmosphere with traces of methane and carbon monoxide; not much else is known about them, but most of the thoughts about Triton should be expected to apply to them too.


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