Earth is teeming with life in the least likely places – in rocks kilometres below the ground, in acid or radioactive ponds, in volcanic springs under the sea, and in lakes beneath the Antarctic. The tenacity of life in such ‘unearthly’ environments has led scientists to believe that maybe we’ll also find life elsewhere in the solar system, such as on Saturn's moon, Enceladus, above. Mars is generally considered the most likely candidate. Here are some other possibilities.



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From a distance, Saturn’s moon Enceladus looks to be frozen solid. But NASA has detected a liquid water sea beneath its hard, icy exterior.
If life exists in this sea, space probes might be able to sample it without landing. Geysers on Enceladus shoot plumes of frozen water – and perhaps lifeforms – high into space.

Image Courtesy of NASA/JPL/SSI

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With a surface temperature of 480°C, Venus is an unlikely place to look for life. But the planet once had an ocean, which was evaporated by a runaway greenhouse effect. The planet’s atmosphere is to blame. It is 96.5% carbon dioxide, trapping heat inside a thick CO2 blanket. But that very atmosphere may also harbour life.

Image Courtesy of NASA/JPL

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Although the surface of Venus is now scorched and dry, 50 kilometres above the ground the planet has a thick cloud layer composed of sulfuric acid droplets. The cloud is so dense it reflects most of the sunlight – but that makes the cloud temperatures pleasantly Earth-like.
If life ever emerged in a Venusian sea, might it now inhabit acid droplets in the clouds? On Earth, microbes have been found living happily in acidic puddles and they also inhabit our highest clouds. Perhaps they’ve found the best of both worlds on Venus?

Image Courtesy of NASA/JPL

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If life requires liquid water then Europa would be the perfect place to look. The biggest sea in the solar system might lie beneath the scarred surface of Jupiter’s large frozen moon. It may carry more than twice the volume of liquid water found on Earth.

Image courtesy of NASA/JPL-Caltech/SETI Institute

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Beneath the frozen surface, scientists suspect the sea floor is pierced by hydrothermal vents – volcanic hot springs.
On Earth the energy and minerals gushing from similar springs support ocean ecosystems teeming with life. Researchers suspect they were the cradles of life on Earth. Could the same be true of Europa?

Image courtesy of NASA/JPL

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Jupiter’s Ganymede is a monster of a moon, the largest in our solar system. It’s bigger than Mercury and Pluto, and three quarters the size of Mars.

Image Courtesy of NASA/JPL

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Ganymede, like Europa, probably has subsurface water beneath its solid ice skin. Until recently scientists believed this water was sandwiched between two layers of ice, isolating it from nutrients and the energy needed to power life. But in May 2014 NASA scientists re-examined Ganymede’s structure and decided the moon is more like a club sandwich, with multiple layers of water and ice. Crucially, the lowest layer, lying next to the moon’s rocky core, is water. If an odd hydrothermal vent is found there perhaps Ganymede, too, might turn out to harbour life.

Image courtesy of NASA/JPL/Brown University