Ganymede and Callisto are the largest of Jupiter's so-called Galilean satellites, the four moons of the giant planet that were discovered 400 years ago, in January 1610, by Italian astronomer Galileo Galilei. Ganymede, the largest moon in the solar system, even bigger than the planet Mercury, boasts its own magnetic field and bears the marks of past tectonic activity. But Callisto, of roughly equal size and with a similar makeup of rock and ice, has neither a magnetic field nor an apparent history of tectonics—the moons' geologic histories have proceeded very differently.

Ganymede seems more evolved, so to speak—its constituents appear to have differentiated further than those of Callisto. Specifically, most of its rock and metal have migrated to the core, whereas those components are more widely distributed throughout Callisto, which appears to host a smaller core as a result.

The circumstances that could have led Ganymede to differentiation without fully affecting its sibling moon have been debated for years. One suggestion is that Ganymede's orbital history included a phase in which the moon experienced strong gravitational tides that heated the icy body and allowed the rock and metal to coalesce at its center.

In a paper published online Sunday in Nature Geoscience, planetary scientists Amy Barr and Robin Canup of the Southwest Research Institute in Boulder, Colo., propose an alternative scenario: heating by cometary impacts, which should have been plentiful several hundred million years after the moons formed, could have liberated the materials that now constitute Ganymede's core. (Scientific American is part of Nature Publishing Group.) Callisto orbits much farther from Jupiter and so would have endured less bombardment from comets drawn in close to Jupiter by the massive planet's gravitational pull.

Each time a comet strikes an icy satellite, Barr explains, a portion of the moon's surface melts from the heat of the impact; the heavier metallic and rocky constituents mixed in sink to the bottom of the melt pool. With enough impacts providing sufficient melting, the sinking rocks' gravitational potential energy is released as heat, producing more melting, and the separation of rock and ice becomes self-sustaining, a process known as "runaway differentiation."

During the solar system's period of intense impacts about 3.8 billion years ago known as the late heavy bombardment, tremendous amounts of cometary material would have been flying around Jupiter and the outer gas-giant planets. Barr and Canup estimate that Ganymede's proximity to Jupiter, the latter of which acts as something of a gravitational sink, led to Ganymede's experiencing double the impacts of Callisto, and at higher velocities, to boot. "Ganymede gets 3.5 times as much energy in the late heavy bombardment as Callisto," Barr says. That energy differential, Barr and Canup realized, could account for Ganymede's much more complete state of differentiation—the so-called Ganymede–Callisto dichotomy.

By their calculations, a broad range of starting conditions for the source population of comets could produce Ganymede's full differentiation but stop short of runaway differentiation at Callisto. Importantly, the debris disk described by the so-called Nice model, a popular dynamical simulation for the solar system's evolution, would do the job. "There is a huge range of masses of planetesimal disks that lead to the formation of the dichotomy," Barr says, noting that prior hypotheses for the divergent histories of Ganymede and Callisto required fine-tuning of parameters or worked for only a very narrow set of circumstances. "This fits in with what is already known about dynamical sculpting in the outer solar system, and it works for a broad range of parameters," she says.

Planetary scientist William McKinnon of Washington University in Saint Louis notes that work in recent years has complicated a competing explanation for the dichotomy, in which tidal heating during the orbital evolution of the Jovian moons melted Ganymede enough to differentiate it. Some research has in fact shown a strong dynamical preference for Ganymede to have settled quickly into its present orbital resonance with the moons Io and Europa. "And if that's true then there is no later special time for Ganymede to be tidally heated," McKinnon says. The fact that Barr and Canup's model dovetails with a primordial development of the moons' orbits makes it attractive, he adds.

The new hypothesis is "a completely plausible explanation," McKinnon says. "What they've shown is that the effect of a strong late heavy bombardment might be the answer."