News of the WeekPlanetary Science

Did a Battering Rain of Comets Bring Ganymede to Geologic Life?

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Science  29 Jan 2010:
Vol. 327, Issue 5965, pp. 515
DOI: 10.1126/science.327.5965.515-a

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Scientists have come up with a promising explanation of a planetary odd couple: Jupiter's major moons Ganymede and Callisto. The two moons have similar sizes and similar compositions but have followed vastly different paths of development. Once formed, Ganymede separated into layers of ice, rock, and molten metal, and then something reshaped parts of its surface. Meanwhile, Callisto remains the bland, newborn ball of mixed ice and rock it started out as. A study published this week in Nature Geoscience offers a possible explanation for why Ganymede evolved so much more than Callisto did: Ganymede suffered a far worse beating 3.9 billion years ago when a rain of comets and asteroids battered much of the solar system.