Ganymede Is Jupiter's Melted, Battered Child

Scientists have figured out a potential reason for the difference between Jupiter's two moons: One of them has been abused so badly that its metaphorical chocolate chips have sunk to the bottom of its ice cream carton. Or something.

For decades, scientists have wondered why Ganymede and Callisto look so different, despite their similar composition and size. But new research carried out at the Southwest Research Institute suggests that the answer has to do with what happened to both moons over 3 billion years ago. According to Dr. Amy Barr, who ran the study with Dr. Robin Canup, Ganymede's closer proximity to Jupiter meant that it saw more action during the Late Heavy Bombardment:

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. Callisto received fewer impacts at lower velocities and avoided complete melting.

Barr and Canup have even created a movie to explain the difference. Barr is suitably excited about the discovery:

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... 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.

One of Jupiter's Moons is Melted! [Space Fellowship]