Planetary habitability in the Solar System

Planetary habitability in the Solar System is the study that searches the possible existence of past or present extraterrestrial life in those celestial bodies. As exoplanets are too far away and can only be studied by indirect means, the celestial bodies in the Solar System allow for a much more detailed study: direct telescope observation, space probes, rovers and even human spaceflight.

Outer space
The vacuum of outer space is a harsh environment. Besides the vacuum itself, temperatures are extremely low and there is a high amount of radiation from the Sun. Multicellular life can not endure such conditions. Bacteria can not thrive in the vacuum either, but may be able to survive under special circumstances. An experiment by microbiologist Akihiko Yamagishi held at the International Space Station exposed a group of bacteria to the vacuum, completely unprotected, for three years. The Deinococcus radiodurans survived the exposure. In earlier experiments, it had survived radiation, vacuum, and low temperatures in lab-controlled experiments. The outer cells of the group had died, but their remains shielded the cells on the inside, which were able to survive.

Those studies give credence to the theory of panspermia, which proposes that life may be moved across planets within meteorites. Yamagishi even proposed the term massapanspermia for cells moving across the space in clumps instead of rocks. However, astrobiologist Natalie Grefenstette considers that unprotected cell clumps would have no protection during the ejection from one planet and the re-entry into another one.

Mercury
According to NASA, Mercury is not a suitable planet for Earth-like life. It has a surface boundary exosphere instead of a layered atmosphere, extreme temperatures that range from 800 °F (430 °C) during the day to -290 °F (-180 °C) during the night, and high solar radiation. It is unlikely that any living beings can withstand those conditions. It is unlikely to ever find remains of ancient life, either. If any type of life ever appeared on the planet, it would have suffered an extinction event in a very short time. It is also suspected that most of the planetary surface was stripped away by a large impact, which would have also removed any life on the planet.

The spacecraft MESSENGER found evidence of water ice on Mercury, within permanently shadowed craters not reached by sunlight. As a result of the thin atmosphere, temperatures within them stay cold and there is very little sublimation. There may be scientific support, based on studies reported in March 2020, for considering that parts of the planet Mercury may have hosted sub-surfaced volatiles. The geology of Mercury is considered to be shaped by impact craters and earthquakes caused by a large impact at the Caloris basin. The studies suggest that the required times would not be consistent and that it could be instead that sub-surface volatiles were heated and sublimated, causing the surface to fall apart. Those volatiles may have condensed at craters elsewhere on the planet, or lost to space by solar winds. It is not known which volatiles may have been part of this process.

Venus
The surface of Venus is completely inhospitable for life. As a result of a runaway greenhouse effect Venus has a temperature of 900 degrees Fahrenheit (475 degrees Celsius), hot enough to melt lead. It is the hottest planet in the Solar System, even more than Mercury, despite being farther away from the Sun. Likewise, the atmosphere of Venus is almost completely carbon dioxide, and the atmospheric pressure is 90 times that of Earth. There is no significant temperature change during the night, and the low axial tilt, only 3.39 degrees with respect to the Sun, makes temperatures quite uniform across the planet and without noticeable seasons.

Venus likely had liquid water on its surface for at least a few million years after its formation. The Venus Express detected that Venus loses oxygen and hydrogen to space, and that the escaping hydrogen doubles the oxygen. The source could be Venusian water, that the ultraviolet radiation from the Sun splits into its basic composition. There is also deuterium in the planet's atmosphere, a heavy type of hydrogen that is less capable of escaping the planet's gravity. However, the surface water may have been only atmospheric and not form any oceans. Astrobiologist David Grinspoon considers that although there is no proof of Venus having oceans, it is likely that it had them, as a result of similar processes to those that took place on Earth. He considers that those oceans may have lasted for 600 million years, and were lost 4 billion years ago. The growing scarcity of liquid water altered the carbon cycle, reducing carbon sequestration. With most carbon dioxide staying in the atmosphere for good, the greenhouse effect worsened even more.

Nevertheless, between the altitudes of 50 and 65 kilometers, the pressure and temperature are Earth-like, and it may accommodate thermoacidophilic extremophile microorganisms in the acidic upper layers of the Venusian atmosphere. According to this theory, life would have started in Venusian oceans when the planet was cooler, adapt to other environments as it did on Earth, and remain at the last habitable zone of the planet. The putative detection of an absorption line of phosphine in Venus's atmosphere, with no known pathway for abiotic production, led to speculation in September 2020 that there could be extant life currently present in the atmosphere. Later research attributed the spectroscopic signal that was interpreted as phosphine to sulfur dioxide, or found that in fact there was no absorption line.

Earth
Earth is the only celestial body known for sure to have generated living beings, and thus the only current example of a habitable planet. At a distance of 1 AU from the Sun, it is within the circumstellar habitable zone of the Solar system, which means it can have oceans of water in a liquid state. There is also a great amount of elements required by lifeforms, like carbon, oxygen, nitrogen, hydrogen, and phosphorus. The Sun provides energy for most ecosystems on Earth, processed by vegetals with photosynthesis, but there are also ecosystems in the deep areas of the oceans that never receive sunlight and thrive on geothermal heat instead.

The atmosphere of Earth also plays an important role. The ozone layer protects the planet from the harmful radiations from the Sun, and free oxygen is abundant enough for the breathing needs of terrestrial life. Earth's magnetosphere, generated by its active core, is also important for the long-term habitability of Earth, as it prevents the solar winds from stripping the atmosphere out of the planet. The atmosphere is thick enough to generate atmospheric pressure at sea level that keeps water in a liquid state, but it is not strong enough to be harmful either.

There are further elements that benefited the presence of life, but it is not completely clear if life could have thrived or not without them. The planet is not tidally locked and the atmosphere allows the distribution of heat, so temperatures are largely uniform and without great swift changes. The bodies of water cover most of the world but still leave large landmasses and interact with rocks at the bottom. A nearby celestial body, the Moon, subjects the Earth to substantial but not catastrophic tidal forces.

Following a suggestion of Carl Sagan, the Galileo probe studied Earth from the distance, to study it in a way similar to the one we use to study other planets. The presence of life on Earth could be confirmed by the levels of oxygen and methane in the atmosphere, and the red edge was evidence of plants. It even detected a technosignature, strong radio waves that could not be caused by natural reasons.

The Moon
Despite its proximity to Earth, the Moon is mostly inhospitable to life. No native lunar life has been found, including any signs of life in the samples of Moon rocks and soil. In 2019, Israeli craft Beresheet carrying tardigrades crash landed on the Moon. While their "chances of survival" were "extremely high", it was the force of the crash –and not the Moon's environment –that likely killed them.

The atmosphere of the Moon is almost non-existent, there is no liquid water (although there is solid ice at some permanently shadowed craters), and no protection from the radiation of the Sun.

However, circumstances could have been different in the past. There are two possible time periods of habitability: right after its origin, and during a period of high volcanic activity. In the first case, it is debated how many volatiles would survive in the debris disk, but it is thought that some water could have been retained thanks to its difficulty to diffuse in a silicate-dominated vapor. In the second case, thanks to extreme outgassing from lunar magma the Moon could have an atmosphere of 10 millibars. Although that's just 1% of the atmosphere of Earth, it is higher than on Mars and may be enough to allow liquid surface water, such as in the theorized Lunar magma ocean. This theory is supported by studies of Lunar rocks and soil, which were more hydrated than expected. Studies of Lunar vulcanism also reveal water within the Moon, and that the Lunar mantle would have a composition of water similar to Earth's upper mantle.

This may be confirmed by studies on the crust of the Moon that would suggest an old exposition to magma water. The early Moon may have also had its own magnetic field, deflecting solar winds. Life on the Moon may have been the result of a local process of abiogenesis, but also from panspermia from Earth.

Dirk Schulze-Makuch, professor of planetary science and astrobiology at the University of London considers that those theories may be properly tested if a future expedition to the Moon seeks markers of life on lunar samples from the age of volcanic activity, and by testing the survival of microorganisms at simulated lunar environment that try to imitate that specific Lunar age.

Mars


Mars is the celestial body in the solar system with the most similarities to Earth. A Mars sol lasts almost the same as an Earth day, and its axial tilt gives it similar seasons. There is water on Mars, most of it frozen at the Martian polar ice caps, and some of it underground. However, there are many obstacles to its habitability. The surface temperature averages about -60 degrees Celsius (-80 degrees Fahrenheit). There are no permanent bodies of liquid water on the surface. The atmosphere is thin, and more than 96% of toxic carbon dioxide. Its atmospheric pressure is below 1% than that of Earth. Combined with its lack of a magnetosphere, Mars is open to harmful radiation from the Sun. Although no astronauts have set foot on Mars, the planet has been studied in great detail by rovers. So far, no native lifeforms have been found. The origin of the potential biosignature of methane observed in the atmosphere of Mars is unexplained, although hypotheses not involving life have been proposed.

It is thought, however, that those conditions may have been different in the past. Mars could have had bodies of water, a thicker atmosphere and a working magnetosphere, and may have been habitable then. The rover Opportunity first discovered evidences of such a wet past, but later studies found that the territories studied by the rover were in contact with sulfuric acid, not water. The Gale crater, on the other hand, has clay minerals that could have only been formed in water with a neutral PH. For this reason, NASA selected it for the landing of the Curiosity rover.

The crater Jezero is suspected of being the location of an ancient lake. For this reason NASA sent the Perseverance rover to investigate. Although no actual life has been found, the rocks may still contain fossil traces of ancient life, if the lake had any. It is also suggested that microscopic life may have escaped the worsening conditions of the surface by moving underground. An experiment simulated those conditions to check the reactions of lichen and found that it survived by finding refuge in rock cracks and soil gaps.

Although many geological studies suggest that Mars was habitable in the past, that does not necessarily mean that it was inhabited. Finding fossils of microscopic life of such distant times is an incredibly difficult task, even for Earth's earliest known life forms. Such fossils require a material capable to preserve cellular structures and survive degradational rock-forming and environmental processes. The knowledge of taphonomy for those cases is limited to the sparse fossils found so far, and are based on Earth's environment, which greatly differs from the Martian one.

Ceres
Ceres, the only dwarf planet in the asteroid belt, has a thin water-vapor atmosphere. The vapor is likely the result of impacts of meteorites containing ice, but there is hardly an atmosphere besides said vapor. Nevertheless, the presence of water had led to speculation that life may be possible there. It is even conjectured that Ceres could be source of life on Earth by panspermia, as its small size would allow fragments of it to escape its gravity more easily. Although the dwarf planet might not have living things today, there could be signs it harbored life in the past.

The water in Ceres, however, is not liquid water on the surface. It comes frozen in meteorites and sublimates to vapor. The dwarf planet is out of the habitable zone, is too small to have sustained tectonic activity, and does not orbit a tidally disruptive body like the moons of the gas giants. However, studies by the Dawn space probe confirmed that Ceres has liquid salt-enriched water underground.

Jupiter
Carl Sagan and others in the 1960s and 1970s computed conditions for hypothetical microorganisms living in the atmosphere of Jupiter. The intense radiation and other conditions, however, do not appear to permit encapsulation and molecular biochemistry, so life there is thought unlikely. In addition, as a gas giant Jupiter has no surface, so any potential microorganisms would have to be airborne. Although there are some layers of the atmosphere that may be habitable, Jovian climate is in constant turbulence and those microorganisms would eventually be sucked into the deeper parts of Jupiter. In those areas atmospheric pressure is 1,000 times that of Earth, and temperatures can reach 10,000 degrees. However, it was discovered that the Great Red Spot contains water clouds. Astrophysicist Máté Ádámkovics said that "where there’s the potential for liquid water, the possibility of life cannot be completely ruled out. So, though it appears very unlikely, life on Jupiter is not beyond the range of our imaginations".

Callisto
Callisto has a thin atmosphere and a subsurface ocean, and may be a candidate for hosting life. It is more distant to the planet than other moons, so the tidal forces are weaker, but also it receives less harmful radiation.

Europa


Europa may have a liquid ocean beneath its icy surface, which may be a habitable environment. This potential ocean was first noticed by the two Voyager spacecraft, and later backed by telescope studies from Earth. Current estimations consider that this ocean may contain twice the amount of water of all Earth's oceans combined, despite Europa's smaller size. The ice crust would be between 15 and 25 miles thick and may represent an obstacle to study this ocean, though it may be probed via possible eruption columns that reach outer space.

Life would need liquid water, a number of chemical elements, and a source of energy. Although Europa may have the first two elements, it is not confirmed if it has the three of them. A potential source of energy would be a hydrothermal vent, which has not been detected yet. Solar light is not considered a viable energy source, as it is too weak in the Jupiter system and would also have to cross the thick ice surface. Other proposed energy sources, although still speculative, are the Magnetosphere of Jupiter and kinetic energy.

Unlike the oceans of Earth, the oceans of Europa would be under a permanent thick ice layer, which may make water aeration difficult. Richard Greenberg of the University of Arizona considers that the ice layer would not be a homogeneous block, but the ice would be rather in a cycle renewing itself at the top and burying the surface ice deeper, which would eventually drop the surface ice into the lower side in contact with the ocean. This process would allow some air from the surface to eventually reach the ocean below. Greenberg considers that the first surface oxygen to reach the oceans would have done so after a couple of billion years, allowing life to emerge and develop defenses against oxidation. He also considers that, once the process started, the amount of oxygen would even allow the development of multicellular beings, and perhaps even sustain a population comparable to all the fishes of Earth.

On 11 December 2013, NASA reported the detection of "clay-like minerals" (specifically, phyllosilicates), often associated with organic materials, on the icy crust of Europa. The presence of the minerals may have been the result of a collision with an asteroid or comet, according to the scientists. The Europa Clipper, which would assess the habitability of Europa, is planned for launch in 2024 and reach the moon in 2030. Europa's subsurface ocean is considered the best target for the discovery of life.

Ganymede
Ganymede, the largest moon in the Solar system, is the only one that has a magnetic field of its own. The surface seems similar to Mercury and the Moon, and is likely as hostile to life as them. It is suspected that it has an ocean below the surface, and that primitive life may be possible there. This suspicion is caused because of the unusually high level of water vapor in the thin atmosphere of Ganymede. The moon likely has several layers of ice and liquid water, and finally a liquid layer in contact with the mantle. The core, the likely cause of Ganymede's magnetic field, would have a temperature near 1600 K. This particular environment is suspected to be likely to be habitable.

Io
Of all the Galilean moons, Io is the closest to the planet. It is the moon with the highest volcanic activity in the Solar System, as a result of the tidal forces from the planet and its oval orbit around it. Even so, the surface is still cold: -143 Cº. The atmosphere is 200 times lighter than Earth's atmosphere, the proximity of Jupiter gives a lot of radiation, and it is completely devoid of water. However, it may have had water in the past, and perhaps lifeforms underground.

Saturn
Similarly to Jupiter, Saturn is not likely to host life. It is a gas giant and the temperatures, pressures, and materials found in it are too dangerous for life. The planet is hydrogen and helium for the most part, with trace amounts of ice water. Temperatures near the surface are near -150 C. The planet gets warmer on the inside, but in the depth where water may be liquid the atmospheric pressure is too high.

Enceladus
Enceladus, a moon of Saturn, has some of the conditions for life, including geothermal activity and water vapor, as well as possible under-ice oceans heated by tidal effects. The Cassini–Huygens probe detected carbon, hydrogen, nitrogen and oxygen—all key elements for supporting life—during its 2005 flyby through one of Enceladus's geysers spewing ice and gas. The temperature and density of the plumes indicate a warmer, watery source beneath the surface. Of the bodies on which life is possible, living organisms could most easily enter the other bodies of the Solar System from Enceladus.

Titan
Titan, the largest moon of Saturn, is the only known moon in the Solar System with a significant atmosphere. Data from the Cassini–Huygens mission refuted the hypothesis of a global hydrocarbon ocean, but later demonstrated the existence of liquid hydrocarbon lakes in the polar regions—the first stable bodies of surface liquid discovered outside Earth. Analysis of data from the mission has uncovered aspects of atmospheric chemistry near the surface that are consistent with—but do not prove—the hypothesis that organisms there, if present, could be consuming hydrogen, acetylene and ethane, and producing methane. NASA's Dragonfly mission is slated to land on Titan in the mid-2030s with a VTOL-capable rotorcraft with a launch date set for 2027.

Uranus
The planet Uranus, an ice giant, is unlikely to be habitable. The local temperatures and pressures may be too extreme, and the materials too volatile.

Neptune
The planet Neptune, another ice giant, is also unlikely to be habitable. The local temperatures and pressures may be too extreme, and the materials too volatile.

Pluto
The dwarf planet Pluto is too cold to sustain life on the surface. It has an average of -232 °C, and surface water only exists in a rocky state. The interior of Pluto may be warmer and perhaps contain a subsurface ocean. Also, the possibility of geothermal activity comes into play. That combined with the fact that Pluto has an eccentric orbit, making it sometimes closer to the sun, means that there is a slight chance that the dwarf planet could contain life.

Kuiper belt
The dwarf planet Makemake is not habitable, due to its extremely low temperatures. The same thing goes for Haumea and Eris.