Efficient degassing of early-formed planetesimals: Water delivery to Earth via unmelted material

The timing of delivery and the types of bodies that contributed volatiles to the terrestrial planets remain highly debated. For example, it is unknown if differentiated bodies, such as that responsible for the Moon-forming giant impact, could have delivered substantial volatiles, or if smaller, undifferentiated objects were more likely vehicles of water delivery. Measurements of water contents of nominally anhydrous minerals and melt inclusions in ungrouped achondrite meteorites (mantles/crusts of differentiated planetesimals) from both the inner and outer portions of the early Solar System are extremely low. Furthermore, measurements of water in minerals and quenched melts in ureilites demonstrate efficient degassing of water from the ureilite parent body (UPB), even though the UPB did not have a global magma ocean. Our results demonstrate that partially melted planetesimals efficiently degassed prior to or during melting. This finding implies that water could only have been delivered to Earth via unmelted material.