Planetary satellites come in two flavors. The regular satellites generally have small inclinations and eccentricities and orbit within a few 10s of planetary radii of their planets. These properties are consistent with formation in circum-planetary disks and, to some extent, each satellite system (of the major planets) presents a miniature model of the circum-solar disk from which the planets condensed. The irregular satellites, by contrast, commonly possess large inclinations and eccentricities and have orbits sometimes measured in hundreds of planetary radii. These properties, particularly the fact that many irregular satellites orbit in a sense opposite to that of planetary rotation, suggest an origin by capture.
The principal questions posed by the regular satellites concern the mode of their formation. Standard accretion models, when applied to the compact, dense circumplanetary disks, predict implausibly short formation times. UCLA scientists work to understand accretion physics and the incorporation of volatiles into these bodies, including the trapping of ice near the poles of Earth’s Moon. UCLA scientists have discovered compelling evidence for liquid water oceans on several satellites of Jupiter, using measurements of the disturbance of Jupiter’s magnetic field caused by water’s conductivity.
The principal questions posed by the irregulars are 1) how were they captured and 2) from where and 3) when? The standard model (frictional capture through gas drag) has problems and attention has recently turned to the possibility of 3-body (or N-body) capture. UCLA astronomers address these questions both through a program of satellite discovery (that has yielded more than half of all known planetary moons) and through determination of the physical properties.