Jupiter’s Hot Youth May Have Melted Its Icy Moons

As a newborn planet, Jupiter glowed brightly in the sky and outshined today’s sun from the perspective of the gas giant’s largest moons. That early radiance—and upcoming visits by multiple spacecraft—may help to solve a 40-year-old mystery about the makeup of those satellites.

For decades scientists have struggled to understand the strange density differences in Jupiter’s four Galilean moons—which, in order from closest to the planet to farthest from it, are Io, Europa, Ganymede and Callisto. Although these natural satellites should have formed from the same feedstock of material and thus have similar compositions, density measurements suggest that Callisto and Ganymede are far icier than Europa, while Io has no ice at all. New research revealed at a conference last month by Carver Bierson, a planetary scientist at Arizona State University, may shed some light on the subject.

Giant planets form by glomming together and compressing enormous volumes of gas and dust. This process releases oodles of excess energy and gives newborn giants a literal youthful glow that can…