ASTRONOMY Ganymede and Callisto, two of Jupiter’s 63 moons, formed at the same time and are nearly identical in size and composition. Yet Ganymede has two distinct layers: a rock core and an icy surface shaped by tectonic activity. Callisto, meanwhile, is a heterogeneous mixture of the two materials and hasn’t changed for billions of years.

Astronomers have attributed the dichotomy to a difference in temperature on the two moons, but they could never figure out why Ganymede is hotter. In March, scientists at the Southwest Research Institute in Colorado calculated that the energy from ancient lunar impacts could have transferred heat to the moons in different quantities.

Nearly four billion years ago, asteroids and comets bombarded our solar system. Jupiter’s powerful gravitational field pulled in objects zooming by. Because Ganymede is closer to Jupiter than Callisto, it would have experienced twice as many lunar impacts as its twin, the researchers say. The heat from those impacts melted Ganymede’s ice, and rocks suspended in it fell toward the moon’s core, shedding their gravitational energy in the form of heat and melting the ice further. The process continued until all of Ganymede’s rocks surrounded the core and the ice refroze above it. Callisto, however, did not experience enough heat from impacts to complete this process, and was left in its heterogeneous state.