Volatile-rich Asteroids in the Inner Solar System

Bennu (101195), target of the Origins, Spectral Interpretation, Resource Identification, Security, Regolith Explorer
(OSIRIS-REx) mission, is a type-B asteroid with abundant spectral evidence for hydrated silicates, low thermal
inertia “boulders” and frequent bursts of particle emission. We suggest that Bennu’s parent body formed in the
outer solar system before it was perturbed into the asteroid belt and then evolved into a near-Earth object. We show
that this is consistent with models of planetesimal evolution. Bennu has many characteristics expected for a
dormant comet, and could potentially contain a minimum of 1 wt.% adsorbed water if its gross mineralogy is
consistent with the Orgueil meteorite. This is in addition to any ice or water contained within the hydrous minerals
themselves. Based on this hypothesis, we predict the properties of the samples that will be returned to Earth by the
OSIRIS-REx mission, including abundant phyllosilicate minerals, amorphous hydrated silicates and low-density,
very high-porosity aggregates, already observed by the OSIRIS-REx instrument suite. We predict enrichments in
D/H and 15N/14N, high C/Mg ratios (>∼7 wt%), and a greater range of organic compositions than found in
meteorites, including an organic component poor in aromatics, and a more labile organic fraction. Ammonium
salts, ranging from NH4Cl to ammonium–organic acid salts could carry much of the nitrogen in comets, yet only
NH4CH3COO and heavier salts are sufficiently stable to be found in these returned samples. Water adsorbed onto
highly porous grains should also be detected provided that the sample return capsule remains below 300 K during
Earth return, and this water should be isotopically heavy.