Weird Moons of the Solar System
Moons in our Solar System get little respect. The latest discoveries from planets like Mars and Saturn grab headlines. New comets invade the public consciousness. Meteors dazzle skygazers, and little Pluto attracts a fan club.
Moons, in comparison, seem boring. The sheer number of these natural satellites in our Solar System—currently 173 circling planets and hundreds more orbiting smaller bodies—makes them overwhelming to study. From afar, the moons look like cratered, gray rocks frozen in time and space. As astronomers look closer with more sophisticated telescopes and unmanned spacecraft, however, they are beginning to discover these objects’ unique and colorful identities.
Jupiter (67 moons) has a moon Io, home to hundreds of active volcanoes that reshape its surface. When the New Horizons spacecraft flew past the satellite in 2007, it captured the plume of the Tvasghtar volcano rising 200 miles above Io’s north pole. The crust of Jupiter’s moon, Europa, is made up of ice blocks that scientists think have broken apart; these features are one piece of geologic evidence to suggest that this satellite likely has a subsurface ocean.
Several small moons orbit within or near the rings of Saturn (62 moons in all). The gravity of tiny Daphnis is great enough to perturb the particles in Saturn’s rings and create wavelike patterns. Saturn’s moon Enceladus has weird “tiger stripes,” 80-mile long fissures coated with fresh ice, arcing across the satellite’s south pole. When scientists mapped daytime temperature data for Saturn’s moon Mimas, they discovered an unexpected visitor: Pac-Man. Instead of the expected smoothly varying temperatures, one side of Mimas is divided into a warm part (on the left) and a cold part (on the right) with a sharp, V-shaped boundary between them. The cold side includes the giant Herschel Crater, which could be responsible in some way for the larger region of cold temperatures that surrounds it.
Iapetus, Saturn’s two-faced moon, resembles a walnut on one side thanks to dark ejecta coating its surface and a ridge that runs along its equator. Spongy Hyperion, maybe the weirdest-shaped moon in the Solar System, has faced a violent past. Scientists think that the saturnine satellite’s bizarre appearance is due to its unusually low density, which helps preserve the original shapers of Hyperion’s craters by limiting the amount of impact ejecta coating the moon’s surface.
Moons don’t just orbit planets. More than 150 known asteroids also retain satellites. Astronomers discovered the first asteroidal moon in 1993. The Galileo spacecraft, on its way to Jupiter, flew near the potato-shaped asteroid Ida and spied a tiny spherical rock circling 55 miles from Ida’s surface. Astronomers named this new moon Dactyl.
When Voyager 2 passed Uranus’ moon Miranda in 1986 (Uranus has 27 known moons) it saw a world of diverse terrain, from subdued hills to linear valleys and ridges to partly curved troughs.
Neptune (14 known moons) has a “cantaloupe moon” Triton, such an oddball that scientists now believe it originates from the more distant Kuiper Belt and was alter captured by the eighth planet.
Forget the planets. For sheer strangeness, there is nothing like the many moons in our Solar System.
 See Dean Regas, “Weird Moons of the Solar System,” Astronomy (42, 6, June 2014, pp. 22-27)