Could life exist on one of Jupiter’s moons?

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Subscribe to enjoy similar stories. Besides Earth itself, Mars is the most-studied planet in the solar system. One reason for the abundance of probes and landers, of course, is that Mars is relatively close.

Another is that Mars appears to have once had plenty of liquid water on its surface. And where there is water, astrobiologists whisper about the possibility of life. But Mars and Earth are not the only places in the solar system that either have, or have had, water.

On October 14th a NASA probe called Europa Clipper blasted off from Florida. As its name suggests, the mission’s target is Europa, one of the biggest of Jupiter’s 95 known moons. Europa is a snowball slightly smaller than Earth’s moon.

It has an atmosphere that is thin to the point of non-existence, a crust of water ice and a surface temperature of around -180°C. But scientists think a vast ocean exists beneath the ice, kept liquid by friction produced as Europa is kneaded by Jupiter’s powerful gravity. Over the past few decades scientists have become steadily more excited about the life-bearing potential of such “icy moons".

Besides Europa, these include Ganymede and Callisto, two other Jovian moons; Enceladus, which orbits Saturn; and Triton, the biggest satellite of Neptune. Europa’s icy crust is thought to be tens of kilometres thick. Europa Clipper will, therefore, not be able to tell whether there actually are any aliens swimming around in the depths.

Instead, its job is to assess whether the moon is the sort of place where life might plausibly arise. One of the probe’s tasks will be to characterise the size and saltiness of the ocean. NASA’s present best guess is that it varies from 60km to 150km deep.