Habitable Worlds


Habitability in the Solar System chart!

A complete summary of the astronomical objects of the solar system in which the past or current existence of any form of life has been considered.  Original September 2022 work by Pablo Carlos Budasi.

Habitability in the Solar System

Text transcript:


Planetary Habitability is the measure of a planet's or a natural satellite's potential to develop and maintain environments hospitable to life. As the existence of life beyond Earth is unknown, planetary habitability is mostly referenced by extrapolation of conditions on Earth and the characteristics of the Sun and Solar System which appear favorable to life's flourishing. Life on other worlds is most likely to include microbes, and any complex living system elsewhere is likely to have arisen from and be founded upon microbial life. Research and theory in this regard is a component of a number of natural silences, such as astronomy, planetary science, and astrobiology. Below is a list of solar system bodies where potential habitability has been considered:

On Mercury, the spacecraft Messenger found evidence of water ice. There may be scientific support, based on studies reported in March 2020, for considering that parts of the planet may have been habitable, and perhaps that life forms, albeit likely primitive microorganisms, may have existed on its surface. 

Venus was considered to be similar to Earth for habitability in the early 20th century. Observations since the beginning of the Space Age revealed that the Venusian surface temperature is around 467 °C (873 °F), making it inhospitable for Earth-like life. Likewise, the atmosphere of Venus is almost completely carbon dioxide, which can be toxic to Earth-like life. 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. Furthermore, Venus Bikely had liquid water on its surface for at least a few million years after its formation. 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 fod there was no absorption line.

Earth is the only place in the Universe known to harbor life. As our planet is close to the inner edge of the habitable zone and lacks a tidal flexing energy source, it only scores 82% in the habitability index. The "faint young Sun paradox" is the fact that the Son has become 30% more luminous since life first evolved on Earth, but the oceans have not boiled. There are two main theories to solve this paradox: the first is that Earth could possess feedback mechanisms that prevent the climate from ever wandering to fatal temperatures. The second is that, out of a large number of planets, perhaps some just make it through by luck. It is possible that a mixture of these two circumstances is playing a role in the fact that the Earth has remained habitable for 4 billion years. Although human activity has reduced habitability in several ecosystems causing the Holocene extinction, it is still far from rendering the planet completely uninhabitable. In 600 million years, the natural increase in Sun's luminosity will bring a higher rate of weathering of silicate minerals and a decrease in the level of CO in the atmosphere, eventually leading to the extinction of plants, which will lead to the extinction of almost all animal life. In about one billion years, the Earth's atmosphere will become a "moist greenhouse" causing the evaporation of the oceans and the end of plate tectonics and the carbon cycle. In 4 billion years, the surface of the Earth will be heated to the point of melting, making conditions uninhabitable for all life forms. 

The Moon could have had a magnetic field, an atmosphere, and liquid water sufficient to sustain life on its surface 3.5 to 4 billion years ago. Warm and pressurized regions in the Moon's interior might still contain liquid water. As of 2022, no native lunar life has been found, including any signs of life in the samples of Moon rocks and soil.

Life on Mars has been long speculated Liquid water is widely thought to have existed on Mars in the past, and now can occasionally be found as low-volume liquid brines in shallow Martian soil. The origin of the potential biosignature of methane observed in the atmosphere of Mars is unexplained, although hypotheses not involving life have been proposed. There is evidence that Mars had a warmer and wetter past. Dried-up riverbeds, polar ice caps, volcanoes, and minerals that form in the presence of water have all been found. Evidence obtained by the Curiosity rover studying Aeolis Palus, Gale Crater in 2013 strongly suggests an ancient freshwater lake that could have been a hospitable environment for microbial life. Furthermore, present conditions on the subsurface of Mars may support life. Current studies on the planet by Curiosity and Perseverance rovers are searching for evidence of ancient life, including a biosphere based on autotrophic, chemotrophic, and/or chemolithoautotrophic microorganisms, as well as ancient water, including fluvio-lacustrine environments (plains related to ancient rivers or lakes) that may have been habitable. The search for evidence of taphonomy (fossils), and organic carbon is now a primary objective in Mars exploration.

Ceres, the only dwarf planet in the asteroid belt, has a thin water vapor atmosphere. The vapor could have been produced by ice volcanoes or by sublimating ice from the surface. The presence of water on Ceres had led to speculation about its current and past habitability. Although the dwarf planet might not have living things today, there could be signs it harbored life in the past.

Jupiter was computed by Carl Sagan and others in the 1960s and 1970s considering conditions for hypothetical microorganisms living in its atmosphere. The intense radiation and other conditions, however, do not appear to permit encapsulation and molecular biochemistry, so life there is thought unlikely. In contrast, some of Jupiter's moons may have habitats capable of sustaining life.

Jupiter's moon Europa has been the subject of speculation about the existence of life due to the strong possibility of a liquid water ocean beneath its ice surface. Hydrothermal vents on the bottom of the ocean, if they exist, may warm the water and could be capable of supplying nutrients and energy to microorganisms. It is also possible that Europa could support aerobic macrofauna using oxygen created by cosmic rays impacting its surface ice. The case for life on Europe was greatly enhanced in 2011 when it was discovered that vast lakes exist within Europa's thick, icy shell. Scientists found that ice shelves surrounding the lakes appear to be collapsing into them, thereby providing a mechanism through which life-forming chemicals created in sunlit areas on Europa's surface could be transferred to its interior. In 2013, NASA reported the detection of "clay-like minerals" (specifically, phyllosilicates), often associated with organic materials, on the icy crust of Europe. The presence of the minerals may have been the result of a collision with an asteroid or comet, according to scientists. The Europa Clipper, which would assess the habitability of Europa, is planned for launch in 2024. Europa's subsurface ocean is considered the best target for the discovery of extraterrestrial life.

Jupiter's Ganymede is thought to have a magnetic field, with ice and subterranean oceans stacked up in several layers, including salty water as a second layer on top of its rocky iron core. In March 2015, scientists reported that measurements of how the aurorae moved confirmed that Ganymede has a subsurface ocean. The evidence suggests that this ocean might be the largest in the entire Solar System. There is some speculation on the potential habitability of Ganymede.

Due to lo's proximity to Jupiter, it is subject to intense tidal heating which makes it the most volcanically active object in the Solar System. The outgassing from erupting umbrella-shaped plumes generates a trace atmosphere. Hydrogen sulfide has been proposed as a hypothetical solvent for life and is quite plentiful on lo, and may be in liquid form a short distance below the surface.

Callisto, the other Galilean moon, is thought to have o salty-water subsurface ocean, heated by tidal forces. Halophiles may be able to thrive in this type of ocean. As with Europa and Ganymede, the idea has been raised that habitable conditions and even extraterrestrial microbial life may currently exist in Callisto's ocean. However, the environmental conditions necessary for life appear to be less favorable on Callisto than on Europa, the main reasons being the lack of contact with rocky material and the lower boat flux from the interior of Callisto.

Like Jupiter, Saturn is not likely to host life. The planet is comprised almost entirely of hydrogen and helium, with only trace amounts of water ice in its lower cloud deck. Temperatures at the top of the clouds can dip down to -150 C. Temperatures do get warmer as you descend into Saturn's atmosphere, but the pressures increase top. When temperatures are warm enough to have liquid water, the pressure of the atmosphere is the same as several kilometers beneath the ocean on Earth. Saturn moons Enceladus, Titan, and Dione have been speculated to have habitats supportive of life. Hydrocarbons have been detected across the surface of Saturn's moon Hyperion.

Enceladus 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. It has been suggested that Enceladus is the most appropriate body as a starting point for the dissemination of life in the Solar System.

Saturn's Titan has an atmosphere that is considered similar to that of the early Earth, although somewhat thicker. The surface is characterized by hydrocarbon lakes, cryovolcanos, and methane rain and snow. Like Earth, Titan is shielded from the solar wind by a magnetosphere, in this case, generated by its parent planet for most of its orbit. The shield's influence on Titan's atmosphere is considered sufficient to facilitate the creation of complex organic molecules. The remote possibility of an exotic methane-based biochemistry life has been hypothesized.

Saturn's Dione has been suggested to have an internal water ocean under 100 kilometers of crust. Based on data gathered and simulations, the tens of kilometers deep ocean would likely be in contact with the moon's rocky core, making it suitable for microbial life in the aspect of available nutrients. This ocean has probably existed for the moon's entire history, meaning there has potentially been plenty of time for life to take root and evolve beneath Dione's battered, icy shell.

Triton's high orbital inclination with respect to Neptune's equator drives significant tidal heating, which suggests a layer of liquid water or a subsurface ocean.

Charon moon of Pluto has a possible internal ocean of water and ammonia, based on cryovolcanic activity.

Models of heat retention and heating via radioactive decay in smaller icy Salar System bodies suggest that Rhea, Titania, Oberon, Triton, Pluto, Eris, Sedna, and Orcus may have oceans underneath solid icy crusts approximately 100 km thick. Of particular interest in these cases is the fact that the models indicate that liquid layers are in direct contact with the rocky cores, which allows efficient mixing of minerals and salts into the water. This configuration contrasts with the oceans that may be inside larger icy satellites like Ganymede, Callisto, or Titan, where layers of high-pressure phases of ice are thought to underlie the liquid water layer.

Exoplanets' and exomoons' surface planetary habitability is thought to require orbiting at the right distance from the host star for liquid surface water to be present, in addition to various geophysical and geodynamical aspects, atmospheric density, radiation type and intensity, and the best star's plasma environment. As of September 2022, 5,084 exoplanets have been confirmed, of which about 70 have a potentially habitable profile in terms of being in their circumstellar habitable zone and having a suitable mass and radius. JWST or a future space telescope could pick up a strong indication of possible life if it finds signs of an atmosphere like our own (oxygen, carbon dioxide, methane). Future telescopes might even pick up signs of photosynthesis or gases/molecules suggesting the presence of animal life. Intelligent, technological life might create atmospheric pollution, as it does on our planet, also detectable from afar.

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