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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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The New Horizons Long Range Reconnaissance Imager (LORRI) captured these two images of Jupiter's outermost large moon, Callisto, as the spacecraft flew past Jupiter in late February. New Horizons' closest approach distance to Jupiter was 2.3 million kilometres, not far outside Callisto's orbit, which has a radius of 1.9 million kilometres. However, Callisto happened to be on the opposite side of Jupiter during the spacecraft's pass through the Jupiter system, so these images, taken from 4.7 million kilometres and 4.2 million kilometres away, are the closest of Callisto that New Horizons obtained.

CALLISTO
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Credit: NASA

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