In the 11 years since the Galileo spacecraft transmitted its last pictures of Jupiter and its satellites, those who study the icy Jovian moon Europa have been left with a puzzle. Pieces of the Europan crust appear to have cracked and drifted apart over time, with new regions of pristine water-ice crust forming in the gaps. Those so-called dilational bands make up tens of percent of the total surface area. And the paucity of craters on Europa’s nearly unblemished face suggests that the whole surface has been recycled over the past 40 million to 90 million years, just 1–2% of the moon’s age.

And yet there was no clear sign of any process capable of consuming surface area at the same rate as it’s produced. Europa as a whole is not getting any bigger. So where is the extra surface going?

Simon Kattenhorn (formerly of the University of Idaho) and...

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