Jupiter's two largest moons look different because Ganymede was pelted with so many comets that it melted while Callisto was spared this bombardment, astronomers say.
Ganymede and Callisto are similar in size — both about the same radius as the planet Mercury — and are composed of a similar mix of rock and ice.
The Voyager spacecraft showed Ganymede to be composed of two different types of terrain, one dark and covered with impact craters and one lighter with grooves and ridges.
Callisto has a far more consistent, pock-marked appearance all over its surface. Scientists have been searching for an explanation for these differences since the Voyager fly-bys 30 years ago.
As well, in the 1990s the Galileo probe found that Ganymede has a large, iron-rich core, which is probably why it's the only moon in the solar system to have its own magnetic field. Callisto is thought to have a small silicate core.
Amy Barr and Robin Canup, researchers at the Southwest Research Institute in San Antonio, used a computer model to show that the two moons diverged in their evolution about 3.8 billion years ago.
They devised a model for melting on the moons' surface by comet impacts during the late heavy bombardment, a time in the solar system's history when a large number of the impact craters seen on the planets and moon, including the Earth's moon, were formed.
"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 centre the same way that all the chocolate chips sink to the bottom of a melted carton of ice cream," said Barr, in a statement.
"Callisto received fewer impacts at lower velocities and avoided complete melting," she said.
Ganymede is closer to Jupiter, so the planet's gravity pulled twice as many comets into Ganymede than into Callisto, and at a higher speed.
Barr and Canup's model showed that on Ganymede, the formation of a core became self-sustaining because of the heavy bombardment, but it didn't on Callisto.
Barr said their work on modelling Jupiter's largest moons could be applied to planets and other moons.
"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," she said.
"Our study shows that Ganymede and Callisto record the fingerprints of the early evolution of the solar system, which is very exciting and not at all expected," said Barr.