The Origin of the Natural Satellites

Abstract

This critical review of the current thoughts on the origin of the natural satellites in the solar system includes discussions of the virtues and caveats of numerous theories and points out problems in need of further research. The widely accepted origin of the Moon as resulting from a giant impact of a Mars-sized body on the primitive Earth has been complicated by the need to accommodate several examples of elements with identical isotope ratios in the Earth and Moon. Problems in the dynamic evolution of the Earth–Moon system remain to be resolved in the two modified impact schemes suggested to allow identical compositions of the Moon and Earth's mantle. Mars' satellites formed from a disk of debris, the possible different compositions of the satellites from Mars notwithstanding. The dynamic evolution of the satellites to the present configuration after formation is shown to be consistent with such an origin, although the origin of the disk remains uncertain. A model of Jupiter's satellite formation in a starved accretion disk in the last stages of the formation of Jupiter, where several generations of satellites form that are lost by migration into Jupiter, produces a system within the constraints of observation. But problems remain in the delivery of solids to the disk, justifying the values of the turbulent viscosity parameter α assumed and the treatment of a time-varying opacity during the accretion process. The Saturn system of satellites is the least understood of all the systems. Nearly all the mass of the satellites is within the single large satellite Titan, and the small, generally ice-rich satellites interior to Titan show a nonmonotonic ice to rock ratio as a function of orbital radius. Tethys must be nearly pure ice. A scheme for getting icy satellites close to Saturn where one would have expected rocky satellites involves spawning the satellites from a nearly pure ice particle ring, where the latter would result from the tidal stripping of the outer ice layers from a large, differentiated satellite as the latter spirals toward its destruction in Saturn. But a viable evolution of satellites so created coupled with other satellites still migrating toward Saturn into the current configuration and the deposition of rock into some of these satellites in an uneven distribution leaving a pure ice Tethys has yet to be demonstrated. The origin of the satellites of Uranus is complicated by the apparent necessity of a giant impact to account for Uranus' large obliquity. An existing satellite system would be destroyed during the impact process, but the equatorial disk so formed by the debris can rotate in the same retrograde direction as Uranus' spin only if Uranus already had a significant obliquity before the final impact. In this latter case, a satellite system like that observed could be reassembled. The understanding of the Neptune system configuration is more secure than that for any other satellite system. The capture of the large retrograde satellite Triton by the disruption of a binary asteroid as it passes close to Neptune is dynamically sound and probable. The small prograde satellites interior to Triton's orbit are reaccreted objects after the destruction of the original system during the tidal decay of Triton's initially very eccentric orbit. Like the Moon, Charon, the large satellite of Pluto, was almost certainly the result of a giant impact to account for the large system-specific angular momentum. However, the origin of the four small satellites exterior to Charon's orbit cannot be the result of transport of collisional debris to their current positions locked in mean motion resonances with Charon as the latter's orbit tidally expands to the current state of dual synchronous rotation. The origin of the small Pluto satellites is still a mystery. The distribution of irregular satellites with distant orbits around the four major planets is consistent with their being captured objects from the planetesimal population in the solar nebula, where capture is achieved through several effective mechanisms.