Asteroids fail to retain cometary impact signatures

The inner solar system experienced a period of intense cometary bombardment following a dynamical instability among the giant planets, which occurred after the dispersal of the gas disk. Vesta, the second-largest asteroid in the main asteroid belt, provides a unique opportunity to study this period, as it is believed to have fully differentiated before gas disk dispersal. This differentiation implies that Vesta's crust, which is represented today by HED meteorites, could have recorded evidence of cometary impacts.

To investigate the extent of cometary contributions to Vesta’s crust, we adopted an interdisciplinary approach combining several methodologies including noble gas mass spectrometry measurements, N-body simulations, collision rate calculations, and impact simulations.

Our results show that Vesta likely experienced numerous impacts with large comets. Despite this, we find no xenon cometary signature in HED meteorites. This apparent contradiction can be explained by the fact that cometary impacts were at high speeds and Vesta’s weak gravitational attraction made it incapable of retaining cometary material.  Consequently, smaller asteroids, with even weaker gravity, are even less likely to retain material from cometary collisions. Thus, the detection of cometary xenon in samples returned from an asteroid by a space mission would serve as a smoking gun, pointing to co-formation in a shared source region with comets, and a later implantation into the asteroid belt.