M-type asteroids, of which 16 Psyche is the largest, are generally believed considered to have metal-rich surfaces. The masses, and therefore bulk densities, of these bodies are still largely unconstrained, but a few have inferred densities much lower than that of iron requiring either a very large bulk porosity (>40 vol%) or a substantial lower density component. In particular, Psyche’s density of 4,000±200 kg/m3 is lower than the uncompressed densities of Mercury, Venus, and Earth. This The current Psyche density estimate would requires a pure iron Psyche to have a bulk porosity of ~52 vol%.

M-type asteroids are generally considered to be the parent bodies of iron meteorites. Many of the magmatic iron meteorites show evidence for fractional crystallization indicative of differentiated parent bodies as well as cooling rates that imply a lack of an overlying insulating mantle. Therefore, a commonly invoked formation hypothesis for M-type asteroids is that these bodies are remanent stripped cores of differentiated bodies. These stripped cores could be produced by one or more hit-and-run collisions within the first ~1.5 Myr years of the Solar System after which the newly exposed cores would continue to cool and could be further fractured by later impacts.

By considering the effects of self-gravity and viscous closure of pore space, we are investigating the thermal history and origin scenarios that permit such high porosities to be retained in pure iron bodies. These results will enable us to place strong constraints on the origin and thermal history of M-type asteroids, like Psyche.

Collaborators:  This work is led by Brown DEEPS graduate student Fiona Nichols-Fleming in collaboration with Alexander J. Evans, Brandon. C. Johnson and Michael Sori.