The results, set for publication in Thursday's issue of the journal Nature, grew out of an attempt to explain two different areas within Europa's "chaos terrain" – a landscape of jumbled surface features visible in images from the Voyager and Galileo spacecraft.
The two areas that have puzzled scientists cover significant large patches of the moon's surface and host icebergs with ground-up ices filling the channels in between – much as jumbled ice chunks fill gaps between icebergs as they calve from ice shelves in polar regions on Earth.
One of the two areas appeared as a depression in the surface. The other appeared as a dome.
"Why are these two features so similar but so different?" says Britney Schmidt, a planetary scientist at the University of Texas at Austin who led the research team.
In trying to solve the puzzle, the team was struck by parallels on Earth. Similar features are apparent in ice that forms above under-ice volcanoes, as well as at ice shelves where warm sea water is thought to be thinning the ice from underneath.
The team suggests that plumes of warmer water, heated by Europa's core, migrate up through the thick icy crust until they reach a "sweet spot" where a combination of pressure, temperature, and the composition of the ices allows a subsurface lake to form. Since liquid water takes up less volume than water ice, the icy lid on top of the lake begins to crack and collapse in a kind of frigid sink hole. The surface ice breaks into chunks in the process. As it does, some of it overturns, mixing the potential nutrients into the water below.
That explains the depressions. But what about the domes?
Over time, the lake refreezes and expands. This pushes remaining large ice blocks upward. The brine surrounding the blocks also refreezes, expanding as it does, adding to the dome.
One of the features, Thera Macula, is a depression, suggesting that a Great Lakes-scale body of water some two miles below currently is sculpting the surface, the team says. A dome feature, known as Conamara Chaos, represents the end of another lake's thaw-freeze process.