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Jupiter (right) and the Galilean satellites (right to left) Io, Europa, Ganymede, and Callisto. Cutaways show the interior states of Ganymede and Callisto after many impacts by icy planetesimals during the late heavy bombardment.
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updated 1/25/2010 1:55:36 PM ET 2010-01-25T18:55:36

Each of Jupiter's more than 60 moons has its own unique character, but scientists have often wondered at the striking differences between the surfaces and interiors of two of the gas giant's largest moons, Ganymede and Callisto.

A new study, detailed in the Jan. 24 issue of the journal Nature Geoscience, might have found an explanation for the disparate features of these Galilean moons: Ganymede was pummeled by more and faster comets impacts than its sister moon billions of years ago.

While Ganymede and Callisto are similar in size and both made up of a mixture of ice and rock, data from both the Galileo and Voyager missions show that they sport different looks, both on the inside and outside.

But just why the two moons looked so different is a problem that planetary scientists have been grappling with for 30 years. The solution to the problem could shed light on how our solar system, and planets in general, evolved.

"Similar to Earth and Venus, Ganymede and Callisto are twins, and understanding how they were born the same and grew up to be so different is of tremendous interest to planetary scientists," said Amy Barr of the Southwest Research Institute Planetary Science Directorate.

Ganymede has a surface that shows evidence of resurfacing by tectonic processes — the same forces that continually reshape the surface of the Earth. The moon also has a large rock/metal core, showing that its constituent materials separated out over time, with the heavier stuff settling to the interior of the planet (just as the iron present in Earth settled to the core, while the lighter rocky materials floated to the surface).

The surface of Callisto, on the other hand, shows no signs of resurfacing, and the separation of rock and ice within it seems to be incomplete.

Barr and her colleague Robin Canup created a model that looked at the possible role of comet impacts in the evolution of these two moons. The model simulated the impacts and rocky core formation and found that Ganymede and Callisto's evolutionary paths diverged around 3.8 billion years ago, during a period in the solar system's life called the Late Heavy Bombardment. (The pockmarked surface of Earth's moon shows that this period was dominated by large impacts).

In the model, Jupiter's strong gravity focuses comets that swing into the neighborhood into the paths of Ganymede and Callisto.

When a comet impacted either moon, the mixed ice and rock that made up the surface would have created a pool of liquid water, allowing rock in the melt pool to sink to the moon's center.

Because Ganymede is closer to Jupiter, it was hit by twice the number of impactors as Callisto was. The proximity to Jupiter also meant that the comets colliding with Ganymede were going faster than those that hit Callisto.

The model shows that if the impacts to Ganymede released enough energy, the process of rock sinking and core formation could have become self-sustaining.

"Impacts during this period melted Ganymede so thoroughly and deeply that the heat could not be quickly removed. All of Ganymede's rock sank to its center the same way that all the chocolate chips sink to the bottom of a melted carton of ice cream," Barr said. "Callisto received fewer impacts at lower velocities and avoided complete melting."

These model findings help link the evolution of Jupiter's moons to the overall evolution of the solar system and the history of bombardment of Earth's own moon.

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