Update on the Small Craters Origin of the Martian Meteorites

The small craters model for launch of the Martian meteorites was presented in 1999 [1-3]. The model was based upon hydrodynamic computer code impact simulations showing that the launch efficiency of Martian material should be age-dependent. It was easiest to launch material from intact (young) material, harder from material covered by a thin regolith, and truly difficult from terrain covered by a deep regolith. The minimum required crater diameter for the above was 3 km, 7 km, and 20 km respectively. Even though little of Mars is covered by Shergottite-aged (S) terrain, the smaller size limit for meteorite-launching events means there are actually more candidate source craters on this terrain formed in the last 10 Ma than on the heavlily cratered southern highlands. This bias was sufficient to explain the origin of the 13 Martian meteorites known at the time as fragments from 6 or 7 impact events. The samples in hand were assigned to impact events as follows: 1) EETA79001 launched 0.8Ma, 2) DaG 476 launched 1.3 Ma, 3) Shergotty, Zagami, and QUE94201 all launched 2.1 Ma, 4) ALHA77005, Y793605, and LEW88516 all launched 3.9 Ma, 5) Nakhla, Lafayette, and Governador Valadares all launched 11 Ma, 5a) Chassigny launched 11.6 Ma, possibly with the Nakhlites, and 6) ALH84001 launched 14.4 Ma. Because the model explained both the relative and absolute abundances of the samples in hand, there followed a number of predictions. 1) The new Martian samples should continue to over-represent the youngest terranes. 2) Additional samples from ancient terrain, if any, should be source crater-paired with ALHA84001, rather than representing new impacts. 3) New samples of Nakhlite-age should represent source craters in roughly the same proportion as did the first three Nakhlites and Chassigny in 1999. 4) New meteorites from young terrain should represent additional impacts with much older CRE ages than those already observed. 5) Martian meteorites with two-stage CRE histories should continue to be unknown. Since 1999, 13 additional Martian meteorites have been identified, 10 S and 3 N. None appear to be ancient as is ALHA84001. Thus the first two predictions still stand. CRE data are currently available only for 4 new S and 1 new N. Of those, the Nakhlite NWA 817 has a CRE age consistent with launch in the same event as N/Lafayette/GV [4]. Therefore at most the new samples represent 2 new impact events for 1.3 Ga material. Of the new S, it appears that the SaU stones with a CRE age of about 1.5 Ma can be source-crater paired with the DaG stones[5]. NWA 480, with a CRE age of 2.4 Ma may represent a new event, or be paired with S/Zagami/QUE[4] as can Los Angeles[6]. Dhofar 019 has a CRE age of about 20 Ma and is assigned to its own impact [7]. Hence the new S samples represent at least 1 though no more than 6 new impact events. Hence, the number of source craters for S continues to outpace that of N and one of those has the oldest known CRE age for any Martian meteorite. Thus predictions 3 and 4 still hold. Lastly, none of the new samples are reported to have a two-stage CRE history, matching the last prediction. This last prediction is basic to the small craters model and is exceedingly difficult to explain in a model requiring in-space breakup events. To summarize, the number of known Martian meteorites has doubled since the small craters model was proposed. Geochemical analyses are available for a portion of these, the data from which are entirely consistent with the small craters model. 1. Head and Melosh, (1999) LPSC 30. 2. Head, J.N. (1999) PhD Dissertation, U. Arizona. 3. Head et al., submitted. 4. Marty et al. (2001) MAPS 36 A122-123. 5. Pasch et al. (2000) MAPS 35 A124. 6. Nishiizumi (2000) MAPS 35 A120. 7. Shukolyukov et al. (2000) MAPS 35 A147