Dehydration of MgHPO4·3H2O (newberyite) under low pressure conditions and its implications for the surface thermal history of Ruygu and Bennu

Spacecraft-returned samples from the C-type asteroid (162173) Ryugu and the B-type asteroid (101955) Bennu record various processes from presolar chemistry and early-stage aqueous alteration processes on their parent planetesimals to ongoing geological processes on their surfaces. Hydrous magnesium phosphate is a common phosphate in Bennu, whereas it is present but rare in Ryugu. Because phosphates in both asteroids formed during aqueous alteration on their parent planetesimals, the difference in the abundance of hydrous magnesium phosphate indicates that the two asteroids experienced different aqueous chemistries. Hydrous magnesium phosphate grains in both asteroid samples show varying degrees of dehydration, which may have been resulted from heating at later evolutionary stages, such as solar heating in their Earth-crossing orbits. This study aims to elucidate the dehydration behavior of hydrous magnesium phosphate on near-Earth asteroids, we conducted kinetic dehydration experiments on MgHPO4·3H2O (newberyite), a likely precursor mineral phase before dehydration, under low-pressure conditions (~200 and ~10–4 Pa). Dehydration of MgHPO4·3H2O occurs efficiently at lower temperatures under pressures lower than at 1 atm, likely due to the more effective escape of H2O molecules from the sample. The dehydration reaction stalled at different extents of dehydration depending on temperature, suggesting that the reaction rate decreases significantly as dehydration progresses. We demonstrate that this reaction stall in the experiments reflects an increase in the activation energy for dehydration caused by a decrease in the coordination number of Mg atoms as H2O molecules detach. The kinetic dehydration model developed in this study successfully reproduces the temporal evolution of the degree of dehydration at each temperature ranging from 50 to 300°C. The dehydration model suggests that hydrous magnesium phosphates on the surfaces of Ryugu and Bennu would experience only partial dehydration under their current orbits over their dynamical lifetimes as near-Earth asteroids. The model also suggests that the samples from Ryugu and Bennu have never been heated above 190°C – the radiation equilibrium temperature in a Venus-crossing orbit – which could place a constraint on their orbital evolution after migration from the main asteroid belt.