A Tale Of Twin Moons

Callisto and Ganymede, the two outermost great moons of Jupiter, should be twins. They're the same size, and they formed at the same time, of the same material. They are, however, quite different.

Dr. Amy Barr of the Planetary Science Directorate of the Southwest Research Institute in Boulder, Colorado, thinks she knows why.

Audio: CBC Radio's Quirks & Quarks 1/30/10, Host: Bob McDonald. Illustration by Seraph3D... From the Jovian Fly-By sequence in "Aliens of the Deep," an Imax film directed by James Cameron.