Although it is believed that Callisto is not geologically active, the probable existence of an ocean under the Callisto's surface leaves the possibility that it may harbor life. Current understandings of the evolution of Callisto permit a liquid-water layered, or ocean, to exist within it. Callisto is on the list of possible places where life could exist in our solar system beyond Earth due to its subsurface oceans. The icy surface was first suggested in 1998.
More recently, scientists have suggested the ocean may be deeper than was initially thought to be on Callisto and the possibility that the ocean does not exist. The Galileo spacecraft has also revealed that Callisto could harbor subsurface oceans of liquid water located 100 kilometers or more beneath the Callisto's surface.
Like other Galilean moons, Callisto has a plentiful water supply as surface ice (or possibly liquid water beneath the surface). While the discovery of an atmosphere and a possible ocean on Callisto might not actually indicate any extraterrestrial life, those features nonetheless make the moon (which is also, by a considerable margin, the most heavily cratered body in the Solar System) one of the more intriguing Jupiter satellites. In addition to its potential for a subsurface ocean, Callisto is the only Galilean moon that is far enough out from Jupiter's strong magnetosphere not to be subjected to damaging radiation levels.
Callisto is further from Jupiter and is not affected by Jupiter's gravity, which means Callisto does not have nearly as much tidal heating either, which would have contributed to the energy needed to melt the Callisto's surface. This means Callisto also experiences less tidal influences compared to other Jovian moons. Also, it does not suffer from the terrors of the Jovian Main Radiation Belt (which can irradiate and therefore kill a significant fraction of life). Energy is a different story, and only radioactive elements heat a possible subsurface ocean. At the same time, neighboring Europa has also got some tidal energy due to its more excellent proximity to Jupiter. Evidence for such an ocean includes the Jovian magnetic field, which shows no signs of penetrating the Callisto's surface.
Evidence for a subsurface ocean is first shown by regular fluctuations in Callisto's magnetic field when it is orbiting Jupiter. Much like Europa and Ganymede, as well as Saturn's moons Enceladus, Mimas, Dione, and Titan, the potential presence of a subsurface ocean on Callisto has led many scientists to speculate on the possibility of life. Its scientific goals for Callisto include finding layers of an ocean, or water storage, mapping its surface, looking at its atmosphere, and understanding the internal structure of the Jovian moon, Callisto. Other papers focus on aspects like the potential water beneath its surface, refining a count of the craters on its surface, and investigating the atmosphere.
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