Impact jetting of water ice, with application to the accretion of icy planetesimals and Pluto

Abstract

Jetting can occur during oblique impacts of water ice bodies at relative velocities as low as ∼500 m s−1, because of the low Hugoniot elastic limit and high compressibility of ice compared to rock. In jetted ice, incipient melting, complete melting, and incipient vaporization occur, upon release to low pressure, at impact velocities of 1.3, 2.0, and 2.7 km s−1, respectively, much less than the 3.4, 4.4, and 5.3 km s−1, required in head‐on collisions. Uncertainties in the shock equation‐of‐state may allow complete melting during jetting at relative velocities as low as 1.2 km −1. Because jet speeds exceed impact speeds, often by a factor of several, during the accretion of icy bodies greater than a few 100 km in radius there may be a significant loss of icy material. This is more true if the accreting body is large enough to differentiate so that its surface layers are closer to pure ice in composition, and especially true if bodies of comparable size are involved, which emphasizes the obliqueness of the collision. I suggest that it is jetting during a Charon‐forming collision (and not vaporization) that may account for Pluto‐Charon's relatively large rock/ice ratio, should the C/O ratio of the solar nebula turn out to be too low to sufficiently raise the rock/ice ratio of outer solar nebula condensates by formation of non‐condensable CO.