Asteroids fail to retain cometary impact signatures

TL;DR

Despite evidence of numerous cometary impacts on Vesta from dynamical models, no cometary xenon signatures are found in HED meteorites, suggesting impacts did not deposit detectable cometary material due to high impact speeds and weak gravity.

Contribution

This study combines simulations and measurements to show that cometary impacts on Vesta did not leave detectable xenon signatures in meteorites, challenging previous assumptions.

Findings

Vesta likely experienced many cometary impacts according to simulations.

No xenon cometary signature found in HED meteorites.

High impact speeds and weak gravity prevented retention of cometary material.

Abstract

A bombardment of comets is thought to have occurred in the inner solar system as a result of a dynamical instability among the giant planets after gas disk dispersal. Vesta, the second largest asteroid in the main asteroid belt, likely differentiated before gas disk dispersal, implying its crust witnessed the cometary bombardment. The composition of HED meteorites, which represent fragments of Vesta's crust, could therefore have been altered by cometary impacts. Here we combine noble gas mass spectrometry measurements, N-body simulations, collision rate calculations, and impact simulations to estimate the cometary contribution to Vesta. While our dynamical simulations indicate that Vesta likely underwent a significant number of collisions with large comets, we find no xenon cometary signature in HED meteorites. This apparent contradiction arises due to the fact that cometary impacts were at high speeds and Vesta's weak gravitational attraction made it incapable of retaining cometary material. Smaller asteroids are even less likely to retain such material. Therefore, if a cometary xenon signature is ever detected in an asteroid belt object, it must have been acquired during formation, within the same source region as comet 67P/Churyumov-Gerasimenko, and have been implanted later into the asteroid belt.