Degassing of early-formed planetesimals restricted water delivery to Earth.

The timing of delivery and the types of body that contributed volatiles to the terrestrial planets remain highly debated 1,2 . For example, it is unknown if differentiated bodies, such as that responsible for the Moon-forming giant impact, could have delivered substantial volatiles 3,4 or if smaller, undifferentiated objects were more probable vehicles of water delivery 5-7 . Here we show that the water contents of minerals in achondrite meteorites (mantles or crusts of differentiated planetesimals) from both the inner and outer portions of the early Solar System are ≤2 μg g -1  H 2 O. These are among the lowest values ever reported for extraterrestrial minerals. Our results demonstrate that differentiated planetesimals efficiently degassed before or during melting. This finding implies that substantial amounts of water could only have been delivered to Earth by means of unmelted material.