IMPACT DELIVERY OF WATER DURING IMPACTS BY HYDRATED ASTEROIDS: INSIGHTS FROM HYPERVELOCITY IMPACT EXPERIMENTS

Multiple lines of evidence indicate that impacts by hydrated asteroids and comets ferry water throughout the solar system. This evidence includes spacecraft observations (e.g., LCROSS, Schultz et al., 2010); hydrated clasts in some meteorites (e.g., Zolensky et al., 1996), shock physics codes (e.g., Ong et al., 2010), and experiments (e.g., Harris and Schultz, 2011).

New experiments performed at the NASA Ames Vertical Gun Range measured how efficiently impacts deliver water. In addition, these experiments directly reveal how impact-delivered water is hosted within impact products, something that shock physics codes currently cannot resolve. In these experiments, serpentine projectiles were fired into dehydroxlated powdered pumice (a regolith analog) at a variety of impact angles and at speeds comparable to those in main asteroid belt. Hence, impact speed need not to be scaled when extrapolating these experiments, although size-dependent factors (e.g., strain rate) may play an additional role.

These new experiments indicate that up to 60% of the hydroxyl carried by serpentine projectiles can be trapped on the target. This water is contained in two reservoirs: lightly-shocked projectiles relics and impact glasses, as revealed by a combination of thermogravimetry, geochemical mixing models, x-ray diffraction, and near-infrared spectroscopy. These analyses also elucidate the relative contribution of these two reservoirs to the total amount of water delivered during impacts at typical main belt collision speeds.

Based on these experiments, both impact melt and projectile relics host water derived from hydrated impactors and serve as reservoirs for impactor-derived water on asteroids such as Vesta. Other larger asteroids also should be laden with significant meteoritic debris as a natural consequence of their impact histories and dynamical conditions in the main asteroid belt (Daly and Schultz, 2015, 2016). Hence, many asteroids may be plastered with hydroxyl and molecular water hosted not only in partly devolatilized projectile relics but also in impact melts produced by the impacts of volatile-rich asteroids and meteoroids.