10.14 - The Origin of the Natural Satellites

Abstract

This chapter examines the state-of-the-art in the understanding of the origin of all the natural satellites in the solar system. Each satellite system is unique, so different processes must be invoked in the formation of each systems several of which have been proposed for the origin of the Moon. The proposals have varying degrees of uncertainties, where we are reasonably confident that giant impacts of the primary led to the formation of the Moon and Pluto's system of three satellites, but the process or processes leading to the distributions of size, density and composition of Sation's satellites remain a mystery. Another secure conclusion is that the regular satellites (those in circular, equatorial orbits) were accreted in a dissipative disk of gas, and solids, although many details remain uncertain (e.g., for the Saturnian system). Observed properties of the Jovian satellites are consistent with an extended accretion period near the end of the accretion of the planet itself where the disk was “starved” in the sense of having considuably less mass than that required to make the satellites while they were being constructed from a waving influx of mass. Formation in a dissipative disk also applies to Mars' satellites in spite of compositions that are inferred to be markedly different from that of Mars. That Neptune's irregular satellite Triton was captured most probably in its retrograde orbit by a gentle tidal disruption of a passing binary pair is also a relatively secure conclusion. Perturbations of the pre-existing satellite system around Neptune by the massive Triten as its high initial eccentricity was damped by tidal friction caused their destruction. The small satellites inside Triten's orbit are thus later generations reaccreted after being shattered, same repeatedly, both by collisions among themselves and by cometary impacts. The Uranus system is a special case because of the very high planetary obliquity. No adiabatic process has been proposed that could tilt Uranus slowly enough to pressure the small orbital inclinations of its satellites relative to the planetary equator, and a proposal of a giant collision leading to the large obliquity, possibly destroying an existing satellite system for reaccretion in a resulting equatorial disk, has not been extensively investigated. The distribution of orbital semimajor axes, eccentricities and inclinations of the irregular satellites are consistent with their having been captured from the heliocentric swarm by any of several energy dissipating processes occurring within the Hill Sphere. Close satellites of asteroids or Kuiper belt objects were most likely formed in collisions, whereas the distant satellites were probably captured by three body interactions within the local Hill sphere. Many problems remain for future investigation as we continue the attempts to place more constraints on the theories of origin of the natural satellites.