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.
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.
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.
[4] The spacecraft MESSENGER found evidence of water ice on Mercury, within permanently shadowed craters not reached by sunlight.
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.
[10] 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.
[12] 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.
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.
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.
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 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.
[30] 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.
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.
[31] 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.
[36] The origin of the potential biosignature of methane observed in the atmosphere of Mars is unexplained, although hypotheses not involving life have been proposed.
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.
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.
[45][46][47] It is even conjectured that Ceres could be the source of life on Earth by panspermia, as its small size would allow fragments of it to escape its gravity more easily.
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.
[56] 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.
[59] 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.
[58] 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.
[58] 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.
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.
[64] Enceladus, the sixth-largest 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.
[65][66] 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.
In 2024, based on orbital data from the Cassini–Huygens mission, Mimas was calculated to contain a large tidally heated subsurface ocean starting ~20–30 km below the heavily cratered but old and well-preserved surface, hinting at the potential for life.
[72] 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.
[80] The moon Triton, however, was thoroughly shown to have cryovolcanism on its surface, as well as deposits of water ice and relatively young and smooth geology for its age, raising the possibility of a subsurface ocean.
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.
