Why does life thrive on Earth? It鈥檚 a simple formula: take one rocky world, add water, then set it down in a sweet spot in the sun鈥檚 Goldilocks zone, where it鈥檚 neither too hot nor too cold, but just right.
These temperate conditions keep water on our planet liquid 鈥� an assumed precondition for the emergence of life, and a major reason why we have never found it elsewhere in our solar system. But what if this Goldilocks story is just a fairy tale?
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Start with Mars. It seems very likely that liquid water once flowed here, and possibly still does. NASA鈥檚 Curiosity rover has spotted what seem to be salty water channels and grooves on the planet鈥檚 rocky surface. Perhaps we shouldn鈥檛 be too surprised 鈥� conventional calculations put Mars within the outer edge of our solar system鈥檚 Goldilocks zone. Whether any of this means the Red Planet ever played host to life remains moot.
But lately we have found evidence of liquid water sloshing around in places where it has no business. We have long suspected that there are subsurface oceans on Jupiter鈥檚 moon Europa. Then, last year, the Hubble Space Telescope confirmed that a salty liquid water ocean lurks beneath the icy crust of another of its moons, Ganymede. And the space probe, which has been orbiting Saturn for the past decade, found hints of subsurface oceans on several of Saturn鈥檚 frozen moons, including Mimas and Enceladus. It even flew through a plume of ice particles ejected from Enceladus.
What it found in the water spells trouble for the Goldilocks tale of life: it contains silica. 鈥淭he only way you can get that is if you have liquid water in contact with rock at very high temperatures 鈥� you need almost the boiling point of water,鈥� says , a scientist working on Cassini at NASA鈥檚 Jet Propulsion Laboratory in California.
This is further proof that Enceladus is geothermally active, and probably has geothermal vents on its ocean floor 鈥� a tantalising idea as many believe life began in a similar environment on Earth. It might mean life can exist in parts of the solar system where we didn鈥檛 think it was possible. We will find out more when results are published from a fly-by in October last year, when Cassini zipped through a jet of water just 50 kilometres from Enceladus鈥檚 southern pole , a key signature of hydrothermal vents on Earth.
What powers the inner warmth of these ice balls? In Enceladus鈥檚 case, it orbits Saturn exactly twice in the time another moon, Dione, circles once. The moons consistently approach each other at the same point in their orbits, increasing the gravitational squeeze on Enceladus鈥檚 core and so heating it up.
This kind of 鈥渢idal heating鈥� is probably common in our solar system and beyond. The implications are tremendous, says Spilker, not least in our search for life in other solar systems, where water has been detected in the atmospheres of several planets.
It鈥檚 entirely possible that the Goldilocks formula for habitability no longer holds water. Instead, perhaps we should be thinking of habitable worlds as more like raisins in a fruit cake 鈥� they can crop up randomly, almost anywhere.
Read more: The 6 greatest mysteries of the solar system
(Image: NASA/JPL-Caltech)
This article appeared in print under the headline 鈥淲here can life exist?鈥�
