Life in the Solar System

The three most likely candidates for life in the solar system (besides Earth) are the planet Mars, the Jovian moon Europa, and Saturn's moon, Titan.[38][39][40][41][42] This speculation is primarily based on the fact that (in the case of Mars and Europa) the planetary bodies may have liquid water, a molecule essential for life as we know it for its use as a solvent in cells.[43] Water on Mars is found in its polar ice caps, and newly-carved gullies recently observed on Mars suggest that liquid water may exist, at least transiently, on the planet's surface,[44] [45] and possibly in subsurface environments such as hydrothermal springs as well. At the Martian temperatures and pressures, such liquid water is likely to be highly saline.[46] As for Europa, liquid water likely exists beneath the moon's icy outer crust.[47] This water may be warmed to a liquid state by volcanic vents on the ocean floor (an especially intriguing theory considering the various types of extremophiles that live near Earth's volcanic vents), but the primary source of heat is probably tidal heating.[48][49]

Another planetary body that could potentially sustain extraterrestrial life is Saturn's largest moon, Titan.[42] Titan has been described as having conditions similar to those of early Earth; according to bbc.co.uk, "The atmosphere on Titan could be identical to that of the early Earth when life began".[50] On Titan, scientists have discovered the first liquid lakes outside of Earth, but they are made of ethane and methane, not water.[51] Additionally, Saturn's moon Enceladus may have an ocean below its icy surface.[52]