If we ever do find extraterrestrial life in the solar system, it's probably much more likely to look like a few cells than a walking-and-talking green man. Nonetheless, finding any kind of life beyond Earth would be extraordinary. Here are our best hopes:
1. Enceladus
The sixth-largest moon of Saturn has been called the most promising bet for life thanks to its welcoming temperature and the likely presence of water and simple organic molecules. The surface of the icy moon is thought to be about 99 percent water ice, with a good chance of liquid water beneath. Observations from the Cassini probe's 2005 flyby of Enceladus suggest the presence of carbon, hydrogen, nitrogen and oxygen — organic molecules thought to be necessary to develop life. And the moon seems to have a boiling core of molten rock that could heat the world to the toasty temperatures needed to give rise to life. (Image: Cassini Imaging Team, SSI, JPL, ESA, NASA)
2. Europa
Jupiter's moon Europa also seems a possible stomping ground for E.T. due to its potential water and volcanic activity. Though the surface seems to be frozen, many suspect that buried underneath is an ocean of liquid water. Volcanic activity on the moon could provide life-supporting heat, as well as important chemicals needed by living organisms. Microbial life could potentially survive near hydrothermal vents on Europa, as it does on Earth.(Image: Galileo Project, JPL, NASA; reprocessed by Ted Stryk)
3. Mars
As far as planets go, by far the front-runner for life is our next-door neighbor, Mars. The red planet is the most Earth-like of solar system planets, with a comparatively similar size and temperature range as our own planet. Large bodies of water ice lie on Mars' poles, and there's a reasonable chance of liquid water beneath the surface. The puny atmosphere on the planet is not strong enough to shield the planet against lethal solar radiation, though microbes could potentially exist beneath the surface. Evidence also suggests that Mars may have been even more habitable in the past. Geologic features imply that liquid water once flowed across the surface, and volcanic activity, now dead, once flourished, recycling chemicals and minerals between the surface and the interior. (Image: NASA and the Hubble Heritage Team (STScI/AURA))
4. Titan
Saturn's largest moon looks suspiciously like it might have hosted life, because its thick atmosphere is rich in compounds that often mark the presence of living organisms. For instance, Titan's air is filled with methane, which is usually destroyed by sunlight. On Earth, life constantly replenishes methane, so it might similarly be responsible for the methane on Titan. Titan is rather cold, however, and if liquid water exists, it must be deep beneath the frozen surface. (Image: NASA)
5. Io
Jupiter's moon Io is one of the few solar system moons to support an atmosphere, and it contains complex chemicals promising for life. Volcanism on the moon also makes it warmer than many others — another good sign. Io is still a long shot, though, because its location inside Jupiter's magnetic field means it is constantly being pelted with lethal radiation. Its violent surface also seems inhospitable, with temperatures often too cold to support life, as well as molten hot spots that are equally deadly.(Image: The Galileo Project, JPL, NASA)
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