Constraining the thermal history of asteroids Ryugu and Bennu in near-Earth orbit through dehydration of hydrous magnesium phosphate (MgHPO₄·3H₂O)

Abstract

Ryugu and Bennu samples contain hydrous Mg phosphates, which likely formed during aqueous alteration and were subsequently dehydrated likely due to surface heating on the asteroids. To understand dehydration kinetics of hydrous Mg phosphates, we conducted dehydration experiments on MgHPO₄·3H₂O at low pressures. We found that dehydration occurs upon heating, and the reaction stalls at specific degrees of dehydration depending on the temperature, indicating that the reaction rate significantly decreases as dehydration progresses. This behavior is attributed to an increase in activation energy for dehydration, likely caused by a decrease in the coordination number of Mg with the release of H₂O molecules coordinated to Mg. Our dehydration model based on the first-order reaction well reproduces the experimental results and suggests that MgHPO₄·3H₂O would not be completely dehydrated under the current daytime surface temperatures of Ryugu and Bennu at their perihelia, whereas it would be fully dehydrated at 190 degree C within 10⁵ years. The presence of hydrous Mg phosphates in both asteroid samples therefore implies that they have not been heated above 190 degree C for 10⁵ years.