Effect of an impact‐generated gas cloud on the acceleration of solid ejecta

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A large amount of petrologic, geochemical, isotopic, and rare gas evidence strongly indicates that the shergottite, nakhlite, and Chassigny (SNC) meteorites originated on a parent body large enough to have sustained recent geological activity, probably Mars. The most straightforward interpretation of the cosmic ray exposure evidence requires that the protoshergottites were at least 5 and, more likely, 15 m in diameter; this large size combined with the 5 km/s escape velocity for Mars presents a formidable dynamic problem. This study investigates the possibility that the entrainment of large blocks of solid ejecta within the rapidly expanding vapor cloud produced by an impact provides sufficient additional acceleration to overcome this problem. A large number of impact parameters, such as impactor and target material, impact velocity, times of ejection, and crater size, were varied to determine their relative effects. Ejection time was found to be the single most important variable: Rocks entrained at very early times are invariably crushed by high dynamic pressures, and rocks ejected at late times, after the gas density has become very low, are little affected by the gas. Only rocks ejected within a fairly narrow window of intermediate times have their velocities significantly changed. Rocks initially ejected at very high velocities, such as those predicted by the Melosh spallation model, are strongly decelerated by the gas. Rocks injected into the gas stream with velocities much different from the average gas velocity at times early enough that the gas density is still significant are often subjected to dynamic pressures great enough to crush them. Thus the net effect of a large, rapidly expanding, impact‐generated gas cloud on solid ejecta is to homogenize the velocity distribution and to reduce the average size. For 30‐km diameter craters the only rocks with final velocities greater than 5 km/s were those whose initial velocities were greater than or approximately equal to 5 km/s. Since the average final velocity increases with crater size, it may be necessary to invoke a crater much larger than 30 km to explain the Martian origin of SNC meteorites, if the proto‐SNCs had diameters of greater than 1 m. On the other hand, if the proto‐SNCs were only a few tens of centimeters or less, gas acceleration of ejecta from a 30‐km‐diameter crater is consistent with the Martian origin of these unusual meteorites.