Other
Scientific paper
Sep 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995metic..30r.545m&link_type=abstract
Meteoritics, vol. 30, no. 5, page 545-546
Other
Crater Ejecta, Impact, Mechanics, Meteorites, Lunar, Mars, Origins, Spall
Scientific paper
In the past decade it has become clear that meteorites falling onto the Earth's surface do not only originate on asteroids or comets, but have also come from the surface of the Moon, Mars, and potentially other major planets or moons in the solar system. One of the most puzzling aspects of a large-planet origin for some meteorites is the relative lack of shock damage in rocks that must have been ejected at speeds of 2.4 km/sec (moon) to 5.0 km/sec (Mars). Older work equated the ejection velocity to the particle velocity behind a shock wave (or, in more sophisticated analyses, to half the particle velocity because of velocity-doubling at the free surface). The known hugoniot relations for, say, basalt, translate these particle velocities to enormous shock pressures: 44 GPa for lunar ejection and 150 Gpa for Mars, which should have pulverized, melted or even partially vaporized the ejected rocks. Although several of the Martian meteorites show moderate degrees of shock (30-40 GPa), some show no detectable signs of shock compression, nor do the lunar meteorites show much evidence for shock upon ejection. Ten years ago I proposed that this situation could be resolved if the process of spallation is important in impact crater ejection [1]. In this process near-surface rocks are protected from high shock pressures simply by virtue of being near the surface. A free surface is, by definition, a surface of zero pressure, and the encroachment of a shock wave cannot change that fact: The shock pressure may rise rapidly with increasing depth, but a near-surface zone will always be present from which material is ejected at high speeds but with little shock damage. This prediction has now been verified directly by laboratory experiments [2], as well as by the discovery of sub-ballistic ejecta from the Ries crater that is composed of lightly-shocked near-surface rocks that were thrown nearly 200 km from the impact site [3]. It also seems that the secondary craters commonly observed in the vicinity of large fresh impacts on the terrestrial planets and satellites may also have been ejected by the spall process, and are potential sources of information about the size-velocity relation of crater ejecta [4, 5], although naturally they pertain to material ejected at much less than escape velocity. Another process that was suggested some time ago, but which has not received much attention until recently, is the role of the rapidly-expanding impact vapor plume in entraining and accelerating surface rocks or lower-velocity spalls [6]. The widespread occurrence of shocked quartz grains in the Chicxulub ejecta suggests some such process and invites further studies, although vapor plume formation itself requires rather high impact velocities that are not likely to be realized in the asteroid belt, but may be important on the moon or Mars. In summary, the ejection of lightly shocked rock debris from the surface of a planet into interplanetary space no longer seems as difficult as it once did. This process is currently supported by the rather strong triad of (1) observation of meteorites from the moon and Mars, (2) theoretical studies of the spall mechanism and perhaps hints of vapor plume acceleration, and (3) experimental observations of fast, lightly shocked ejecta from rock targets. Future work will hopefully flesh out details of the process and gives us hope that we may someday find meteorites from Venus and perhaps the Earth itself. References: [1] Melosh H. J. (1984) Icarus, 59, 234-260. [2] Gratz A. J. et al. (1993) Nature, 363, 522-524. [3] Hofmann B. and Hofmann F. (1992) Eclogae Geol. Helv., 85, 788-789. [4] Vickery A. M. (1986) Icarus, 67, 224-236. [5] Vickery A. M. (1987) GRL, 14, 726-729. [6] Vickery A. M. (1986) JGR, 91, 14139-14160.
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