The early impact histories of meteorite parent bodies

Abstract

Abstract-We have developed a statistical framework that uses collisional evolution models, shock physics modeling, and scaling laws to determine the range of plausible collisional histories for individual meteorite parent bodies. It is likely that those parent bodies that were not catastrophically disrupted sustained hundreds of impacts on their surfacescompacting, heating, and mixing the outer layers; it is highly unlikely that many parent bodies escaped without any impacts processing the outer few kilometers. The first 10-20 Myr were the most important time for impacts, both in terms of the number of impacts and the increase of specific internal energy due to impacts. The model has been applied to evaluate the proposed impact histories of several meteorite parent bodies: up to 10 parent bodies that were not disrupted in the first 100 Myr experienced a vaporizing collision of the type necessary to produce the metal inclusions and chondrules on the CB chondrite parent; around 1-5% of bodies that were catastrophically disrupted after 12 Myr sustained impacts at times that match the heating events recorded on the IAB/winonaite parent body; more than 75% of 100 km radius parent bodies, which survived past 100 Myr without being disrupted, sustained an impact that excavates to the depth required for mixing in the outer layers of the H-chondrite parent body; and to protect the magnetic field on the CV chondrite parent body, the crust would have had to have been thick (approximately 20 km) to prevent it being punctured by impacts.

Lobate and flow-like features on asteroid Vesta

“…While these volumes are substantially less than for similar-sized craters on the Moon (estimated as 65-140 km 3 using similar scaling and an impact velocity of 17.5 km/s), these model results are consistent with the estimates of melt volumes from mapping putative melt features around Marcia crater ( $ 0.4-11 km 3 ). Thus, both image analyses and modeling calculations suggest that there should be some relatively small deposits of impact melts associated with some large craters on Vesta, especially those Davison et al (2010Davison et al ( , 2013 for impacts between dunite projectiles and a dunite surface (both initially at T o ¼300 K) with porosities of 0% and 20%. Melt volume (scaled to projectile volume) as a function of impact velocity is plotted for two different porosities, along with a best-fit line for dunite from the simulations of Wünnemann et al (2008) for melt number U 2 /E m greater than $ 30.…”

Efficient early global relaxation of asteroid Vesta

“…Because the Melosh (1977) model applies only to figures with small deviation from equilibrium, the comparisons with this model are based on a Vesta with much smaller degree of disequilibrium (f − f eq =0.005) than shapes used in this study (Section 3.1). The early Vestan crust was likely heavily fractured due to impactor bombardment (Davison et al, 2013), and fracture healing processes that operate on the Earth and the Moon likely did not occur as extensively on Vesta due to the asteroid's low interior pressures and absence of igneous activity after the first several 10s of My after Vesta's formation Yamaguchi et al, 1996). We therefore assume that the Vestan lithosphere is strengthless under extension (i.e., cohesionless).…”

“…The hydrostatic flattening factor f eq is a function of internal mass distibution and the rotation rate. The rotation rate of early Vesta may have changed several times due to giant impacts with similar magnitudes as the Rheasilvia event (Davison et al, 2013). The true rotation rate for very early Vesta is therefore uncertain.…”

“…Such large impacts may have recreated significant non-hydrostatic topography during a brief early window; there is a 12-31% probability that Vesta was struck by a Rheasilvia basin-forming impactor sized object during the first 100 My (Davison et al, 2013). Because the cooling of the Vestan interior occurred gradually (see Section 2.2), Vesta would have been able to return to a closely hydrostatic figure if a large impact occurred sufficiently early.…”

The Collisional Evolution of Undifferentiated Asteroids and the Formation of Chondritic Meteoroids

“…The collision-induced compaction of the bodies of the asteroid belt must be considered when comparing the properties of asteroids with those of meteorites. Davison et al (2013) showed that most of the mass of the asteroid belt disappeared after only 100 Myrs and that the size distribution of the present asteroid belt has not significantly changed from that time and can therefore be considered constant for most of the lifetime of the Solar System. Taking the size distribution of the present asteroid belt (De Elia & Brunini 2007), one can show that for instance an asteroid of 100 km in radius has been bombarded more than 10 14 times by fragments with radii between 0.1 m and 22 km during the last 4.5 Gyrs, with the smaller impactors being much more numerous than the large ones.…”