Early accretion of water during planet formation: Insights from impact experiments

The timing of volatile delivery to the inner solar system continues to be debated. Data from angrites [1] and eucrites [2] point to the accretion of water in the inner solar system within the first few million years of solar system history. High temperatures in the inner part of the protoplanetary disk may have precluded the stability of water, thereby requiring that water be delivered from the outer solar system, perhaps during a Grand Tack-like giant planet migration. In such a scenario, bodies from the outer solar system enter the inner solar system at extremely high velocities. These impact velocities may have been high enough to vaporize iron [3] and would have far exceeded the velocities required to devolatilize the carbonaceous chondrite (CC)-like impactors that isotopic evidence [e.g., 2,4] indicates delivered many of the volatiles to the inner solar system. However, the mechanisms that trap impactor-derived water remain poorly constrained. We conducted hypervelocity impact experiments to address two fundamental questions. First, how much of the water carried by CC-like impactors can be trapped in impact products? Second, where does impactor-derived water reside? We focus on oblique impacts (30 and 45°), which create a wide range of P/T conditions in the both the impactor and target [e.g., 5]. Impact experiments reveal that impact melts and breccias capture up to 30% of the water carried by CC-like impactors under impact conditions typical of the main asteroid belt and the early phases of planet formation. This impactor-derived water resides in two distinct reservoirs: quenched impact melts and projectile survivors. Quenched impact melt hosts the bulk of the delivered water, and in these materials molecular water dominates over hydroxyl. Entrapment of water within impact glasses and melt-bearing breccias likely contributed to the early accretion of water during planet formation. As such, water and other volatiles may have been sequestered within growing planets, with significant implications for geodynamics and planetary evolution.

[1] Sarafian et al. 2017. Geochim. Cosmochim. Acta 212, 156-166.

[2] Sarafian et al. 2014. Science 346, 623-626.

[3] Johnson et al. 2016. Science Advances 2, e1601658.

[4] Saal et al. 2013. Science 340, 1317 - 1320.

[5] Schultz and Eberhardy 2015. Icarus 248, 448-462.