Comet Storm Split Destiny Of Jupiter's Twin Moons

Comet strikes may have warmed Ganymede enough for its
ice and rock to fully separate (Image: NASA/JPL)

From New Scientist:

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.

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