The Missing Psyche Family: Collisionally Eroded or Never Formed?

Abstract Asteroid 16 Psyche, the largest M-type asteroid, is widely considered to be the collisionally exposed core of an ∼500-km diameter differentiated parent body which was similar to asteroid 4 Vesta. However, there is no dynamical family associated with Psyche nor are there spectroscopic data for the existence of the mantle or crustal material from the parent body. The usual explanation for the missing material requires that the Psyche parent body was collisionally disrupted early in solar system history, followed by collisional grinding of the family down to below the current observational limit sizes. We test the exposed core hypothesis for the origin of Psyche using a numerical code that simultaneously calculates both the collisional evolution of the asteroid belt and the model family formed by the breakup of the Psyche parent body (PPB). We find that it would take a projectile about 300–350 km in size to thoroughly disrupt a 500-km asteroid (the estimated size of the PPB), and that the probability of such an event occurring in the first 500 Ma of solar system history is only about 1%. While the Psyche model family is found to have been significantly ground down subsequent to its formation, there should be several tens of survivors from the mantle and crust larger than ∼10 km that should be spectroscopically detectable by current technology. Although only a small fraction of the asteroids larger than 10 km have been discovered and observed spectroscopically to date, none have been identified as potential survivors from the PPB (Burbine et al. 1996, Meteor. Planet. Sci. 31, 607–620). Given the low probability of the disruption of a Vesta-like body and the lack of either dynamical or observational confirmation of a family or material from the parent body, we think it more likely that Psyche has possibly been shattered by impacts but not catastrophically disrupted. In this case, it would be a plausible candidate parent body for the mesosiderites. The exposed-core scenario more probably applies to other, smaller (diameter ≲100 km) M-type asteroids, which could be the parent bodies of the iron meteorites. However, this interpretation raises the interesting problem of why among the larger asteroids only Vesta and the PPB would have been fully differentiated.

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