Presentation Abstract

Title The Irregular Satellites: The Most Collisionally Evolved Populations in the Solar System
Author Block William Bottke1, D. Nesvorny1, D. Vokrouhlicky2, A. Morbidelli3
1Southwest Research Inst., 2Institute of Astronomy, Charles University, Czech Republic, 3Observatoire de la Cote d'Azur, France.
Abstract The irregular satellites are dormant comet-like objects that reside on prograde and retrograde orbits around the giant planets. Their debiased size distributions (SFDs) are surprisingly similar to one another, with the observed populations at Jupiter, Saturn, and Uranus having remarkably shallow power-law slopes for diameter D > 8-10 km. Recent modeling work indicates they were dynamically captured during the Nice model, a violent reshuffling event of the giant planets ~3.9 billion years ago that led to the clearing of a ~35 Earth mass disk of comet-like objects. Close encounters between the giant planets allowed some comets scattered out of the disk to be captured via three-body reactions. This implies the irregular satellites should be closely related to other dormant comet-like populations that were produced at the same time from the same disk (e.g. Trojans asteroids). A critical problem with this idea, however, is that the Trojan SFD does not look like that of the irregular satellites.
Here we use numerical codes to investigate whether collisional evolution among the irregular satellites is sufficient to explain this difference. Starting with Trojan asteroid-like SFDs and testing a range of physical properties, we found that our model populations literally self-destruct over hundreds of My and lose 99% of their starting mass. The survivors evolve to low-mass SFDs similar to those observed, where they stay in steady state for Gy. This explains why the irregular satellites of the giant planet have similarly-shaped SFDs. Our work also indicates that collisions produce ~0.001 lunar masses of dark dust at each giant planet, and that non-gravitational forces should drive most of it onto the outermost regular satellites. We argue this scenario most easily explains the ubiquitous veneer of dark non-icy carbonaceous chondrite-like material seen on many prominent outer planet satellites (e.g., Callisto, Iapetus, Titan).

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41st DPS Program published in BAAS volume 41 #3, 2009.