Do oblique impacts produce Martian meteorites?

Abstract

Geochronological and geochemical characteristics of several achondritic meteorites match those expected of Martian rocks. Several authors have suggested that these meteorites might have originated on Mars, but no satisfactory explanation has been given of how they may have been ejected from the Martian surface. It is suggested here that the oblique impact of large meteoroids may produce ejecta which is entrained with the ricocheting projectile and accelerated to velocities in excess of Martian escape velocity. This suggestion is based on earlier experimental studies of oblique impacts and on the observation of several large Martian craters with the characteristic ‘butterfly’ ejecta pattern produced by low angle impacts. Several acceleration mechanisms may act on the Martian ejecta. At high impact velocities, the ricocheting projectile should be vaporized and fluid dynamic drag should act on the entrained ejecta. The drag equation can be integrated for an idealized representation of ricochet and explosion of the projectile. It is shown that large ejecta fragments, on the order of 1–10% of the initial projectile radius, could be drag accelerated to velocities in excess of the Martian escape velocity. Fragments greater than or equal to about a meter in size will escape the Martian atmosphere if they are launched at velocities significantly in excess of the Martian escape velocity. Impacts capable of launching sizable ejecta are expected to occur at a maximum rate of about 4×10−8 yr−1 on Mars and at a comparable or lower rate on the moon. The long transit time between Mars and earth would lead to establishment of a steady state population of potential Martian meteorites. The much shorter transit time from the moon to earth would cause any lunar ejecta to arrive in spurts which are short in comparison to their repetition rate. Thus the absence of lunar meteorites from our collection could be a simple observational effect due to the short terrestrial lifetime of meteorites. These considerations are preliminary in the sense that several simplifications and assumptions are made. However, they suggest that a Martian origin of the shergottite meteorites is dynamically possible.