Astronomy and Astrophysics – Astronomy
Scientific paper
Sep 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008dps....40.5201b&link_type=abstract
American Astronomical Society, DPS meeting #40, #52.01; Bulletin of the American Astronomical Society, Vol. 40, p.492
Astronomy and Astrophysics
Astronomy
Scientific paper
Estimates of the amount of material needed to produce planetesimals/embryos in the main belt region suggest the asteroid belt has lost more than 99.9% of its primordial mass. This loss likely occurred in two phases: the ejection of 99% of the main belt's mass over its first 100 My via gravitational interactions between planetary embryos and planets, and the ejection of 90% of the remaining mass 500 My later via sweeping resonances within the context of the Nice model. Still, this scenario leaves ample time for the survivors to undergo collisional evolution, particularly because the main belt was > 10 times more massive during its first 600 My than it was over the last 3.9 Gy. Surprisingly, though, the main belt shows few signs that it was ever affected by an early bombardment. Consider the following. (i) Vesta, a D = 530 km diameter differentiated asteroid, has one 460 km diameter impact basin. If Vesta had been hit by additional large impactors, it should show more basins. (ii) Only 2% of D > 140 km asteroids have sizable "SMAshed Target Satellites" (SMATS). This fraction is consistent with a paucity of impacts in the early main belt. (iii) Meteorite shock degassing ages recorded using the 39Ar-40Ar system show few signs of heating (impact) events between 4.1-4.4 Gy ago, though earlier and later heating events are clearly recorded. We suggest the solution to this mystery may come from new planetesimal formation models that indicate asteroids were born big (i.e., jumping from sub-meter sizes to 100-1000 km objects over short time scales). Using collisional evolution simulations, we will show that top-heavy size-frequency distributions take hundreds of My to build up sufficient numbers of midsize fragments to initiate substantial grinding. This delay is long enough to explain many curious main belt constraints.
Bottke William
Levison Hal
Morbidelli Alessandro
Nesvorný David
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