Habitats for Life in the Solar System

The features of some major bodied of our Solar System, in tabular form, when considered as potential abodes for hypothetical extra-terrestrial life. The list includes all the eight planets (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune); Earth's satellite, the Moon; Jupiter's satellites Io, Europa, Ganymede, and Callisto; Saturn's satellites Enceladus, Titan, and Iapetus; Uranus' satellite Titania; Neptune's satellite Triton; the twin dwarf planets Pluto and Charon; and comets as general category.
Mostly based on Dirk Schulze-Makuch and Louis N. Irwin's book Life in the Universe: Expectation and Constraints (2004, Springer; highly recommended if you like this sort of things). Schulze-Makuch and Irwin tend to be really optimistic about the chances of life in the universe, and I've tried to follow their spirit. Our knowledge of the Solar System changes rapidly (for example, most of what I write here about Pluto and Charon was unknown, or suspected at best, until the New Horizon mission reached them in 2015); additional references are for the material missing from my main book sources.

Column 1: the name of the object under examination and a picture of its surface.

Column 2: overall rating of the object  as potential abode for life, modified from the scale used by Schulze-Makuch and Irwin (2004). Ranking:
I. ("Excellent") Demonstrable presence of liquid water, available energy, & organic compounds (Earth, duh).
II. ("Good") Evidence of past or present liquid water, available energy, & inference of organic compounds (Mars, Europa, Ganymede, Titan).
III. ("Moderate") Physically extreme conditions, but with a liquid medium other than water, available energy and complex chemistry, possibly suitable for non-Earth-like life (Venus, Io, Callisto, Enceladus, Triton, Pluto, Charon).
IV. ("Questionable") Plausible presence of a liquid medium and available energy at least in isolated regions, conditions unsuitable for Earth-like life or local origin of life (Jupiter, Saturn, Iapetus, Uranus, Titania, Neptune).
V. ("Unlikely") No plausible persistent presence of a liquid medium or available energy (Mercury, Moon, comets).

Column 3: general physical features of the object, namely the orbital radius (average distance from the Sun, given in Astronomical Units, defined as Earth being 1); the mass and radius relative to Earth's; the average surface temperature, given in Kelvin (or the temperature at the depth at which pressure is 1 atm for gas giants); the average surface pressure, given in atmosphere (thus with Earth's surface having 1); and the surface gravity at the equator, given in g (thus with Earth, once again, having 1).

Column 4: physical planetary structures:
* An iron core is formed by heavy metallic elements, of which iron is the most abundant, sinking at the center of a body. A body with a large iron core is also likely to have enough radioactive elements to keep its interior hot with their decay (radiogenic heat). A rotating layer of molten iron will produce a magnetic field.
* Tectonic activity can be produced from internal radiogenic heat and/or by tidal stress. Plate tectonics (division of a rocky surface into sliding plates that form mountain chains and may subduct into the molten interior) is known only on Earth, where abundant water lowers the melting point of silicates. The ice shields of gas giant's satellites is often broken by tidal stress; ice plate subduction is known on Europa. Molten interiors may relieve pressure through volcanoes, as on Venus or Io, and oceanic vents. These processes recycle minerals and volatiles between the surface and interior, favoring complex chemical developments, and create sharp thermal gradients. On colder worlds, volcanoes can erupt liquid water (as on Enceladus) or nitrogen (as on Triton) ("cryovolcans").
* A solid surface, composed of rock (usually mostly silicates) and/or ice, is found on all bodies of the Solar System except the gas giants (it might exist in their core, but it would be inaccessible in any case). Solid surfaces might be important for the development of life by providing catalytic sites for complex reactions, as well as anchoring points for various autotrophic organisms; subsurface oceans enjoy a solid interface on both sides. A static surface without tectonic activity is revealed by abundant impact craters, as on Mercury and Callisto.
* A significant atmosphere, as on Earth, Mars, Venus, Titan, and the gas giants, protects the surface and potential life from damage by solar wind (the stream of electrically charged particles emitted by the Sun), or its equivalent from a nearby gas giant, and cosmic radiation, as well as destroying incoming meteorites, while circulating important reactive chemicals such as oxygen. On Earth and Mars, it's also clear enough to allow sunlight to reach the ground, while on the atmosphere of Titan and the gas giants, and to a lesser degree that of Venus, is opaque. It also allows the existence of liquids on surface, which requires a minimal external pressure. A thick atmosphere can contain clouds composed by suspended liquid droplets (e.g. water and sulfuric acid on Venus) in which microscopic life could exist, although they are not known as an important environment on Earth.
* A liquid ocean is found at the surface only on Earth, as other bodies are too hot or cold. However, radioactive decay and tidal stress might warm the interior enough to allow liquid pockets, e.g. on Mars; subsurface oceans are likely to exist under the ice crusts of Europa, Ganymede, Callisto, Enceladus, and Triton, and perhaps Titan and Pluto. Liquid methane is likely to form lakes on Titan's surface. A liquid medium is probably ideal for the formation and development of life, allowing for free reaction without the disperseness of gases. A substance can only exist liquid above a minimal pressure (e.g. 0.006 atm for water), else it changes directly between the solid and gaseous state. A substance can also stay liquid under its usual freezing point by forming an eutectic mixture with another (e.g. with ammonia or hydrogen peroxide for water), and above its ebullition point by applying higher pressure (e.g. the extremely hot and high-pressure water ocean deep within Neptune). In the interior of gas giants, extreme pressure produces a "sea" of liquid metallic hydrogen, which could host only highly exotic lifeforms, if any.
* An ice shield is found on most solid bodies beyond the orbit of Mars, mostly composed of water ice, though in cold bodies like Triton and Pluto (<60 K) it includes frozen ammonia and nitrogen. It's made possible by the fact that ice (except for some high-pressure forms, and unlike most solids) is less dense than liquid water. It can be tens or hundreds of kilometers deep, and so completely opaque: it shields internal pockets or subsurface oceans from solar wind and cosmic radiation, but it also blocks all incoming sunlight. When it lies above a subsurface ocean, it also provides a second series of solid substrates for chemical catalysis.
* A magnetic field is generated by the movement of electrical charges; the circulation of a conductive material, such as molten iron in Earth's core, produces a planet-wide magnetic field that protects the surface by deflecting solar wind and cosmic radiation. Mercury, Earth, all gas giants, and Ganymede generate their own magnetic field (Mars and maybe Venus could have had one in the past); Europa and Callisto generate induced fields in response to Jupiter's because of the circulation of conductive salty water.
* Organic compounds, i.e. carbon-based compounds as those fundamental to life on Earth, have been detected in the atmosphere of Jupiter and Titan, and on the ice shield of Ganymede, Triton, and Pluto, and are likely to exist on Europa, Callisto, Iapetus, Titania, and Charon. They are also found in large amounts on comets and meteorites. Such compounds may form when sunlight or radiations transfer energy to methane or carbon dioxide; icy satellites probably gather the organic debris suspended in the magnetic field of gas giants. Complex molecules based on elements other than carbon (e.g. silicon, boron & nitrogen, sulfur) may be imagined, but they are not known in significant amount, and their potential for life is hard to determine.

Column 5: general remarks and observations on the object in question, with some speculations about the nature of hypothetical life.

Column 6: major components of the object's atmosphere, if any is present. Lighter molecules escape more easily from an object's gravity, especially at high temperature. The stronger a body's gravity is, and the lower its temperature, the lighter are the molecules that can be retained. Thus, Mercury cannot hold carbon dioxide as Mars does, despite having the same gravity, on account of its higher temperature; the large and cold gas giants can retain great amounts of hydrogen and helium that easily escape all smaller bodies. The presence of a magnetic field also helps retaining an atmosphere, as it would otherwise be eroded by the solar wind.

Column 7: substances present or plausibly present on the body at the liquid state, if any, that could serve as solvents for life. Most processes of known life occur in a liquid state, and more specifically in water, which forms most of the volume of both intracellular and extracellular fluids. While water remains by far the most common chemical compound in the universe, other substances have been proposed as replacements, such as ammonia and hydrogen sulfide. These substances share with water (though to a lesser degree) the property of being polar: that is, one side of the molecule is positively charged, and another is negatively charged. This has important consequences for the reactions taking place within the liquid environment. Another class of potential solvents, which include liquid methane and nitrogen, are apolar: all the outer atoms of the molecule share the same electric charge. This would imply very different sets of possible reactions with complex molecules, of which relatively little is known.

Column 8: sources of energy. In general, energy can be extracted from an environmental disequilibrium (gradient or difference) by moving it towards a lower-energy equilibrium; energy ill be released in some form, and some can be captured into a biological system, while the rest will be dispersed as waste heat. Sources:
* Visible light is one of the two sources of energy used by life on Earth. Photons emitted by the Sun carry energy that can excite an electron from specific molecules, such as chlorophyll, allowing a series of chemical reactions. It can also provide energy indirectly, creating by photolysis reactive molecules such as the oxygen (on Venus) or acetylene (on Jupiter). It probably becomes negligible at or beyond Jupiter's orbit. (Phototrophy)
* Redox chemistry is the process of transferring electrons from some ions or molecules to lower energy states. The extracted energy can be transferred to storage or structural molecules within organisms. This is the second source of energy used by life on Earth, for example when a cell uses oxygen to remove bond electrons from the organic molecules in their food. Since life itself is a chemical process, this is very convenient. Most planets and large moons in the Solar System have enough chemical diversity to support some form of redox chemistry; Mercury and our Moon may be an exception. Geochemical reactions and radioactive decay often produce hydrogen that can be made to react with carbon dioxide to yield both energy and organic molecules. (Chemotrophy)
* Thermal gradients (differences in temperature) are a common, if inefficient, source of energy. Many bodies have interiors hotter than the surface, either because of the decay of radioactive elements (e.g. Venus, Earth), or because of the gravitational stress from other bodies (e.g. Io, Triton); the bodies closest to the Sun have their surface differentially heated by sunlight, especially Mercury and the Moon. An organism could move between hotter and cooler regions, thereby causing vesicles in their body to expand and contract, harvesting energy from conformational changes while transferring heat from the hotter to the cooler region. Geothermal energy, in which a body's internal heat leaks to the surface through vents or volcanoes (as on Earth, Jupiter's moons, Enceladus, or Triton), gives the sharpest and most useful gradients. Only a small fraction of thermal energy can be directly captured into chemical bonds, however. (Thermotrophy)
* Kinetic energy can be extracted from the motion of fluids, especially from the convective motions that are produced by temperature differences; e.g., the wind in thick atmosphere as on Venus, Earth, Titan, or the gas giants. The masses of water that exist on Europa, Ganymede, Callisto, and probably Enceladus and Triton are likely to have currents. An organism might anchor itself to a substrate (harder on gas giants) and use fluid motion to bend piezoelectric materials that generate an electric current when deformed, or to act upon membrane channels. (Kinetotrophy)
* Tidal energy is a particular form of gravitational energy in which a body is periodically deformed by the gravitational field of another. The tides on Earth are a weak example. On the scale of life, gravitational energy is far too weak to be useful. However, satellites close to a gas giant, such as Io, Europa, Enceladus, or Triton, as well as Pluto and Charon, have their solid surface deformed or fractured by tidal stress. This could be exploited as kinetic energy (e.g. changing tides of liquid masses, descending & ascending fluid in convective motion), or, by heating the interior of a body, it might create a geothermal gradient. Tidal tectonic stress, such as the fracturing of Europa's ice, could be harvested directly with piezoelectric filaments. (Gravitotrophy)
* Pressure gradients occur where a thick layer of gas, such as the atmospheres of Venus and the gas giants, is unequally compressed by gravity. Pressure gradients tend to exist on very large scales, so they would be hard to exploit, especially when not anchored to a stable substrate. (Barotrophy)
* Osmotic gradients are spatial differences in the concentration of a solute, e.g. a salt, in a medium. They could be very significant where an ocean is enriched in minerals by a rocky floor on one side, and diluted by melting ice on the other, as on Europa, and probably Callisto, Enceladus, and Triton. An organism could move between layers of water that would enter through its membranes where the environmental concentration is lowest, and seep out where it's highest, allowing it to harvest energy from the flow. The flow of water could be replaced by an opposite one of ions. Oceans entirely encased in ice as on Ganymede and Titan would not have such a strong gradient. On Earth, it might be found at the mouth of rivers reaching the sea. (Osmotrophy/Ionotrophy)
* Electromagnetic fields are produced by the rotation of a conductive material -- such as the molten iron layer of Earth's core. Mercury and Ganymede also have much weaker magnetic fields, and all the gas giants have a much stronger one, which also envelops their inner moons. In a conducting medium (e.g. salt water), a magnetic field can be exploited to drive apart protons and electrons with the Lorentz force, building up a gradient. A rotating subsurface salty ocean on a giant's moon, such as Europa, can interact with the giant's field creating a secondary one. (Magnetotrophy)
* Ionizing radiations, including both high-energy particles (protons, helium nuclei) and photons with much higher frequency than visible light (X-rays, gamma rays), are emitted in large amounts by the gas giants, reaching their inner moon. Like sunlight, they can create high-energy compounds that can be made to react to release this energy, such as the radiolytic oxygen around Europa. These radiations are generally very damaging to organic molecules, though some fungi on Earth can extract energy from gamma rays. (Radiotrophy)
* Spin configurations offer another way to extract energy from molecular hydrogen, the most common molecule in the cosmos. In one form, orthohydrogen, the two atomic nuclei spin in the same direction, whereas in the other, parahydrogen, they spin in opposite directions. Orthohydrogen is the higher-energy state, but it's also very stable; a mechanism to convert it into lower-energy parahydrogen would release plentiful energy, especially at extremely low temperatures (say, <20 K). Molecular hydrogen is found in large amounts only in gas giants, as it easily escapes the lower gravity of solid bodies. However, this effect can be exploited in other molecules with two hydrogen atoms (e.g. water) as well as in molecular oxygen. (Idiostrotrophy)

Sources:
Schulze-Makuch D, Irwin LN (2004). Life in the Universe: Expectations and Constraints, Springer
Schulze-Makuch D, Irwin LN (2011). Cosmic Biology: How Life Could Evolve on Other Worlds, Springer
McFadden LA, Weissman PR, Johnson TV (ed.) (2007). Encyclopedia of the Solar System (2nd edition), Elsevier
en.wikipedia.org/wiki/List_of_…

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Image sources (public domain or Creative Commons license):
Mercury (link) (NASA/JPL, PD); Venus (link) (NASA/JPL-Caltech, PD); Earth (link) (NASA/Apollo 17, PD); Moon (link) (Gregory H. Revera, CC BY-SA 3.0); Mars (link) (ESA,  CC BY-SA 3.0); Jupiter (link) (NASA, ESA, and A. Simon, PD); Io (link) (NASA/JPL/University of Arizona, PD); Europa (link) (NASA/JPL/DLR, PD); Ganymede (link) (NASA/JPL, PD); Callisto (link) (NASA/JPL/DLR, PD); Saturn (link) (NASA / JPL / Space Science Institute, PD); Enceladus (link) (NASA/JPL, PD); Titan (link) (NASA, PD); Iapetus (link) (NASA/JPL/Space Science Institute, PD); Uranus (link) (NASA/JPL-Caltech, PD); Titania (link) (NASA/JPL, PD); Neptune (link) (NASA/Justin Cowart, CC BY 2.0); Triton (link) (NASA/JPL/U.S. Geological Survey, PD); Pluto (link) (NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute/Alex Parker, PD); Charon (link) (NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute/Alex Parker, PD); Comet (link) (NASA/W. Liller, PD)