While Jupiter’s two largest moons, Ganymede and Callisto, are nearly the same size, they’re far from identical twins. Now, in a Nature Geoscience study, Amy Barr and her team might have figured out this tale of two similar moons with very different histories.

Voyager and Galileo mission images showed Ganymede, seen here on the right, to be a geologically active place, with a surface that scientists think changes through tectonic processes like those that we have here on the Earth. Callisto, seen on the left, looks totally different: Its rock and ice have not mixed in the same way, and it doesn’t seem to have such active geology, despite being approximately the same size as Ganymede. For 30 years, researchers have wondered what process could have got enough heat into Ganymede to drive its geological evolution without setting off Callisto as well [ScienceNOW Daily News].

Trying to crack the puzzle, Barr turned to comets. Just less than 4 billion years ago, astronomers hypothesize, a wave of comets careened across the solar system during a phase called the late heavy bombardment. Barr’s team of scientists modeled how that comet storm would have affected Jupiter’s satellites. Ganymede, located about 500,000 miles closer to giant Jupiter than Callisto, bore the brunt of its parent’s heavy hands. Jupiter’s extreme gravity (a 150-pound person would weigh 355 pounds on Jupiter) tugged more comets toward Ganymede and caused them to crash at higher speeds than it did for Callisto [Discovery News].

That was the turning point, Barr says. If all those cosmic snowballs pounded Ganymede with enough energy, the formation of its core and other geological processes could have become self-sustaining. Says Barr: “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” [SPACE.com].

Castillo would not have been so lucky. Shortchanged on impact energy, Callisto would not have melted enough to achieve “runaway” heating during separation, leaving it cold and without a core [ScienceNOW Daily News]. Thus, Barr’s team says, the moon has been dead, geologically speaking, ever since.

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Image: NASA