A satellite-asteroid mystery and a possible early flux of scattered C-class asteroids

Abstract

Spectrophotometric measurements of the subset of satellites thought to be of capture origin, orbiting Saturn, Jupiter, and Mars, indicate that they typically (all?) have neutral spectra and low albedo, indicating spectral class C. This finding is puzzling in light of evidence that class-C objects are native only to the outer half of the asteroid belt. How did C-like objects approach, and get captured by, planets? Probably Jupiter resonances scattered a high flux of C-type objects out of the belt and throughout much of the primordial solar system. Such scattering could occur only at the close of planet accretion when extended atmospheres could affect capture. The largest flux of scattered objects came from resonances in the most heavily populated regions of the belt near 2.8 AU: in particular, the 7:3 and 5:2 resonance regions primarily populated by C's. Hence, most captured satellites are C's, not D's, S's, etc. This scenario supports and expands upon the capture scenario involving extended protoatmospheres, proposed by J. B. Pollack, J. Burns, and M. Tauper (1979, Icarus 37, 587–611) and D. Hunten (1979, Icarus 37,113–123).