Mars has been home to extraterrestrial life in our imaginations since the earliest days of science fiction. Percival Lowell believed that the canali described by Schiaparelli were canals built by an advanced Martian civilization. H.G. Wells wrote of a Martian invasion of Victorian England.
Although the Viking landers showed us an apparently lifeless Martian surface, it is possible that Mars once supported life, and may yet do so. After all, it possesses a (thin) atmosphere, essential nutrients, and fairly abundant solar energy. There is evidence that water ice is abundant, and that liquid water once flowed across the surface. Might there be Martian microorganisms hiding beneath the surface, protected from the harsh chemicals and UV radiation?
After Mars, Saturn’s moon Titan may be one of the most popular places in the solar system for speculation about alien life over the past several decades. Observed by Pioneer 11 and Voyager 1 only as a hazy world surrounded by an opaque atmosphere, the Cassini probe and Huygens lander provided much more information about Titan.
Though too cold for liquid water, Titan’s surface is covered by lakes of hydrocarbons. Its atmosphere is mostly nitrogen, along with methane and other hydrocarbons. This has led astrobiologists to speculate about life forms not based on water, but instead using methane or ethane as a solvent.
This moon of Jupiter possesses a subsurface ocean, kept liquid by the heat from tidal forces exerted by Jupiter. This same tidal flexing could create undersea hydrothermal vents, and there is evidence to suggest that the ocean is in contact with a rocky surface. The presence of a liquid ocean, coupled with an energy source in the form of undersea vents, and a rocky undersea surface as a source of minerals, suggests that Europa is a place that could support life.
Proposals have been made for exploration of the Europan ocean, but the high-radiation environment around Jupiter, difficulties involved in boring down to the ocean, and concerns about contaminating the moon with Earth life have prevented such a mission so far.
Yes, the poor non-planet at the edge of our solar system could potentially support life.
Pluto possesses a very thin atmosphere of nitrogen, methane, and carbon monoxide, and though it was once thought that the atmosphere would freeze when Pluto was farthest from the sun, scientists now believe that it remains gaseous throughout Pluto’s orbit.
Additionally, the New Horizons probe revealed the existence of tholins, a type of organic compound, on the surface of Pluto. If the extreme conditions of the surface make it uninhabitable, life might still exist deep beneath the surface, in a possible sub-surface ocean.
It should not be surprising, then, that science fiction writers have from time to time picked Pluto as a potentially life-supporting world.
- Venus – Once imagined as a lush jungle world, we know now that our sister planet suffers from a stifling atmosphere with clouds of sulfuric acid and surface temperatures that can melt lead. Still, extremophiles do exist on Earth, and some speculate that life might survive high in the atmosphere of Venus.
- Ganymede – Another of Jupiter’s moons, Ganymede is suspected to harbor a subsurface ocean with more water than all of Earth’s oceans combined. Might something be swimming beneath the surface of this largest Galilean satellite?
- Jupiter – Hard as it might be to imagine life on a planet with no solid surface, Carl Sagan speculated that balloon-like organisms might float on the Jovian breezes, consuming microorganisms likewise swept around by the gale-force winds. Concept art was even created for his legendary Cosmos series.
- Earth. Ok, this one is a cheat. Not only does the third rock from the Sun support life, we’re also fairly certain that there’s intelligent life to be found here. Somewhere. If you’re searching for life in the solar system, Earth is your safest bet.
En-what? Enceladus, one of the larger moons of Saturn, is yet another world possessing a subsurface ocean. Observations by the Cassini spacecraft suggest a geologically active world, with geysers containing hydrocarbon compounds and indicating the presence of a high-pH ocean.
Though highly alkaline and buried deep within the moon, scientists speculate that this ocean may provide the energy sources and support the complex chemical reactions needed for life to form. Life on Enceladus may be powered by chemosynthesis, with hydrothermal vents being a source of chemical energy for the Enceladan microbes.
Incidentally, if you are ever uncertain about pronunciation, Space.com has a handy guide for pronouncing the names of all the planets and moons in our solar system. Yes, even the eighth planet…