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© NASA/JPLComet strikes may have warmed Ganymede enough for its ice and rock to fully separate
Heavy pummelling by icy comets could explain why Jupiter's two biggest moons - apparently close kin - look so different inside.

At first glance, Ganymede and Callisto are virtually twins. The colossal moons are similar in size and mass, and are a roughly 50:50 mixture of ice and rock.

However, visits by the Galileo spacecraft beginning in 1996 tell a different story. Ganymede's interior boasts a solid rock core surrounded by a thick layer of ice, while ice and rock are still mingled in parts of Callisto. That suggests Callisto was never warm enough for its ice to melt and allow all of its rock to fall to the centre and form a core.

Tides on moon

There is no shortage of explanations to account for this difference in heat. The debris that formed Ganymede may have collided at higher energies than the stuff that became Callisto, causing Ganymede to be molten from the start. Or perhaps Ganymede once followed a different orbit around Jupiter that raised tides on the moon, causing friction that heated up the interior.

But the conditions in these scenarios must be just right to reproduce the moons' interiors. "They're very restrictive stories, because they're basically fine-tuning the models to get Ganymede to go one way and Callisto the other," says Amy Barr of the Southwest Research Institute in Boulder, Colorado.

Now Barr and Robin Canup, also of the Southwest Research Institute, have come up with an alternative: the moons are different because they were pummelled to differing degrees by wayward comets.

Comets would have bombarded the moons during what's known as the "late heavy bombardment", a cataclysmic period between 3.9 and 3.8 billion years ago.

Shock wave

Barr and Canup modelled the effect of comet impacts and found that the pressure of a shock wave can create a bowl-shaped region of liquid water beneath the surface. Rock fragments in this water will sink to the bottom of the liquid, and form a "blob" of rock that is denser than the ice-rock mixture beneath it. Over thousands of years, these rock blobs would slowly sink to the core of a moon.

Since Ganymede orbits Jupiter at almost half the distance of Callisto, it would have experienced more collisions. That's because Jupiter's gravity would have pulled in large quantities of passing debris. As a result, the pair suggest, the less-battered Callisto was not heated as much and the separation of ice and rock would have been incomplete.

"The most attractive feature of their story is that it [creates an effect] that really is a lot different," says David Stevenson of Caltech. Extending the model to look at the interiors of moons like Saturn's moon Titan, which has an interior structure between that of Ganymede and Callisto, may be an interesting test, he adds.

The picture should become clear after 2026, when a pair of orbiters planned by NASA and the European Space Agency, are due to arrive at Jupiter to study the planet and its moons.

Journal reference: Nature Geoscience, DOI:10.1038/ngeo746