Jupiter's moon Europa may habour vast reservoirs of liquid water within a few kilometres of its surface according to a paper published this week in Nature. The idea that there might be oceans of liquid water beneath Europa's surface has long fascinated astronomers: its surface is covered by a layer of water ice thought to extend to a depth of 100 km. And even though Europa might be expected to be a cold place – it lies five times further away from the Sun than than the Earth – tidal forces from Jupiter's strong gravitational field are continually warping it, generating friction that constantly warms Europa's interior.

A liquid ocean beneath Europa's surface might even be able to support life, but there has been doubt about how deep these oceans might be. Most people have thought they would have to be tens of kilometres beneath the surface. Writing in Nature, Britney Schmidt and her colleagues at the University of Texas tackle this question by looking at a pair of near-circular features spotted by the Galileo spacecraft twenty years ago. One is a dome of ice, elevated 200 metres above the surrounding terrain; the other is a depression, sunken 800 metres below it. Until now, no satisfactory model has explained their formation.

Inspired by melt patterns around Iceland's sub-glacial volcanoes, Schmidt proposes a model in which warm convection currents in the ice beneath Europa's surface carry enough heat to melt the relatively salt-rich ice a few kilometres beneath the surface. As that ice melts, it contracts, producing a huge amount of suction on the ice above, which eventually generates a depression on the surface. The ice also cracks under the force, allowing an inflow of new material. In the second phase of the model, when the ice cools again and the liquid water refreezes, the downward pull on the surface is reversed. This would restore the ice to its original height, except that the inflow of new material means that there is now an excess of ice, which is pushed up into a dome. To generate the features seen by the Galileo probe, a volume of water similar to the Great Lakes in the US, melting at a depth of 3 kilometres, would be needed.

This model has some interesting consequences: firstly that there is liquid water much closer to the surface than anyone had previously thought, though it is probably not home to life given its transient existance. More crucially, we seem to be seeing one of these features in the process of forming, still in its depressional phase, and that suggests that Europa remains geologically active. If there are ongoing processes which could carry the chemical building blocks needed for life from the surface to the oceans below, that's another box ticked in the conditions needed for life. But, given how far it might be beneath the surface, any space missions to look for it will remain in the realm of science fiction for many decades to come.

The sky on 28 Apr 2024

Source

Schmidt, B.E., et al., Nature (2011)

Transcript of a news story presented on Naked Astronomy, 25th November 2011.

Image credit

© NASA/Voyager 1

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