Turning Meteorites into Rock: Constraints on Asteroid Physical Evolution

Introduction: Chondrites have been compared to terrestrial sandstones, formed by the physical accumulation of unrelated grains followed by a lithification process. Tectonics and the simple weight of overlying layers provide the conditions for lithification of sandstones on Earth. What plays this role on meteorite parent bodies? Present day asteroids cannot compact meteorites by their own self-gravity; lithostatic pressures even in the center of Ceres are far too small. Furthermore, it is likely that most asteroids are loose rubble piles, shattered and re-accreted. A high degree of impacting is evident in the meteorites: most meteorites are breccias, and the most common shock state of meteorites is S3 [1]. What does this tell us about the possible physical histories of the asteroids? Our measurements of the porosities, and model porosities, of ordinary chondrites [2] can be combined with other physical studies of meteorites such as shock state to put numerical limits on possible models for the lithification of the meteorites and their parent bodies. We propose here three suggestions concerning meteorite lithification. It is possible that all or none of the proposed processes may actually have occured in the early solar system. But confirming or eliminating any of these models can put useful constraints on our ideas about the accretion of material in the early solar system. Porosity and Shock: Our previous work examining over 300 ordinary chondrites has shown that most ordinary chondrites have primordial porosities ranging from 5% to 15%, with a few rare cases having porosities up to 30%. We find no correlation between the metamorphic state of meteorites and their porosities. Other workers using optical methods to estimate the shock state of meteorites [1] have noted a lack of correlation between shock state and metamorphic state; thus one might expect, logically, that there should be no correlation between shock state and porosity. In fact, as seen in Figure 1, the average porosity does indeed stay constant at around 10% for all shock levels. But there is a distinct second order effect: as shock state increases, the spread of the observed porosities about this mean decreases. While meteorites of shock state S1 and S2 average at 10% porosity with a spread up to 30%, those at shock state S5 and S6 average 10% porosity with very little spread away from that average. Clearly something subtle is going on here. Porosity and Pressures: The lack of correlation between metamorphic state and porosity is not surprising; no asteroid is large enough for burial depth to change porosity. Experiments [3] on sandstones with original porosities ranging from 15% to 30% show that confining pressures of 0.3 0.4 GPa are required to fill their pore spaces with comminuted rock. By comparison, the central pressure of asteroid 1 Ceres is about 0.2 GPa. Note also that such compressed sandstone is still filled with microcracks and pore space of to up to 10% porosity. Experimentally shocked mineral powders (initial porosity 30%-35%) [4] reduced their porosity to about 10% when shocked to 2.5 GPa. Increasing the shock pressure only slightly decreased porosity from this point. This is consistent with impact processes in the early solar system being a source of compaction for the ordinary chondrites. If so, however, the persistence of some high porosity meteorites would still demand explanation. In both cases, the experimental evidence is interesting but not conclusive. The behavior of sandstones is not consistent from sample to sample, and notoriously difficult to model. And the results of the shock experiments may been influenced by the use of closed containers, which may have overamplified the effect of the shock. Why are there meteorites? Numerous workers have now pointed out that asteroids are likely loose piles of rubble (see references in [2]), and a number of models have shown how repeated impacts can fragment asteroids without totally dispersing them [5]. But all these models assume that one starts with a solid rocky body, impacted by other solid rocky bodies. Where did these original un-rubblized asteroids come from? 0 0.05 0.1 0.15 0.2 0.25 0.3