When the Earth Had Two Moons: Cannibal Planets, Icy Giants, Dirty Comets, Dreadful Orbits, and the Origins of the Night Sky

Asphaug, a University of Arizona professor of planetary science, takes readers on an initially promising, ultimately somewhat disappointing tour of the solar system. Exploring a variety of planetary forms—including comets, asteroids, moons, and the sun—he depicts the system in its entirety as a dynamic, complex, and deeply mysterious place. For instance, there’s Iapetus, one of Saturn’s eight moons, dubbed a “walnut moon” because of a 20-kilometer-high ridge that circles its equator, about which Asphaug concludes, “Ideas for how it formed are rather crazy, but one of them is true, or none of them are crazy enough.” Asphaug integrates his own research with that of others, yielding a firsthand account of cutting-edge science, including his titular hypothesis that early in Earth’s history it had two moons. He posits that the smaller satellite crashed into the moon, leading to the stark differences in composition between its two sides. He also hypothesizes about the origin of life, discussing how microscopic organisms might well have migrated among celestial objects. Asphaug makes for an enthusiastic and erudite guide to these and other subjects, but his overly technical language will likely frustrate general readers attracted by his compelling material. (Oct.)