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Porosity Evolution in Metallic Asteroids: Implications for the Origin and Thermal History of Asteroid 16 Psyche

Some M‐type asteroids have measured bulk densities much lower than expected based on their metal‐rich surfaces. In particular, the density of the largest M‐type asteroid 16 Psyche would require a bulk porosity of ∼52 vol% if it has a pure iron composition. We determine that a pure iron Psyche must have cooled to and remained below 800 K to maintain sufficient porosity for that porosity to persist until present. Iron bodies smaller than Psyche could preserve long‐lived high porosities (>40%), yet even the smallest M‐type asteroids would require temperatures below 925 K. Our results indicate that all iron asteroids must cool to and remain at low temperatures prior to a porosity‐adding event in order to retain porosity on timescales longer than a few million years. Accordingly, a pure iron Psyche would not have sufficiently cooled to retain high porosities when large porosity adding events are most likely to have occurred in the early solar system. Hence, Psyche is not likely to be a pure iron exposed core. However, this structure may be more viable for M‐type asteroids smaller than Psyche which cool more quickly.