Catastrophic Disruption of Meteorites: Implications for Asteroids and Interplanetary Dust G. J. Flynn and T. VanVeghten SUNY - Plattsburgh D. D. Durda Southwest Research Institute S. Hart and E. Asphaug University of California -- Santa Cruz

Stone Meteorites and Their Parent Bodies Meteorites are generally accepted to be samples of asteroids, thus the physical properties of meteorites are likely to be indicative of those of asteroids. Most stone meteorites (the ordinary and carbonaceous chondrites) are dominated by distinct subunits – matrix, chondrules and inclusions. The chondrules and inclusions are large (~mm size) and strong, while the matrix is usually fine- grained and weaker. Allende Section Scale: Cube is 1 cm on edge. Red Arrows indicate chondrules Green Arrows indicate inclusions

Impact Disruption of Analog Materials A detailed understanding of the response of these porous, inhomogeneous meteorites to hypervelocity impact is required to understand asteroid fragmentation. Several years ago, we performed a series of impact disruption experiments at the AVGR, using as targets samples of a Hawaiian vesicular basalt with olivine phenocrysts. This material is at least morphologically similar to the chondritic meteorites. Hawaiian Olivine Basalt Translucent green millimeter –size olivine in a porous matrix.

Collection of Fragmentation Debris For each shot we deployed four “passive detectors” around the target to collect primary debris from the disruption. Each passive detector contained Al foils of three thicknesses (7, 13 and 51  m) to monitor the size-frequency distribution, and two aerogel blocks to collect the small (<125  m) fragments. In addition, all the material was collected from the chamber after each shot, and the mass- frequency distribution of the fragments larger than 0.02 grams was measured. Passive Detector

Results from Analog Materials We found an under-abundance of olivine chondrule material in the <25  m size fragments captured in the aerogel passive detectors (Durda & Flynn, Icarus, 1998). We found a change in the slope of the mass-frequency distribution of the fragmentation debris at roughly the mass of the chondrules (Durda & Flynn, Icarus, 1998). Both results are consistent with the target exhibiting preferential failure at the chondrule-matrix boundary, with many strong chondrules being ejected intact or in large pieces, while matrix contributed mostly to the small debris. Computed microtomography on stone meteorites frequently shows a low-density region around the chondrules suggesting they may exhibit similar failure along the chondrule-matrix boundary (Flynn et al., LPSC XXXI, 2001). To test this, we have now disrupted seven stone meteorites.

Meteorite Targets Three moderately weathered ordinary chondrite (OC) finds from North Africa were disrupted in November 2001: * a gram sample of the L6 OC NWA791, * a 248 gram sample of the unclassified OC NWA620, and, * a gram sample of the unclassified OC MOR001. Four unweathered falls were disrupted in May 2002: * a 105 gm sample of the highly-friable L4 OC Saratov, * two samples, one gm and the second gm, of the L5/6 OC Mbale, and, * a 70.9 gm sample of the CV3 carbonaceous chondrite meteorite Allende. Each target was struck by an ~5 km/sec Al projectile fired from the NASA Ames Vertical Gun – a speed comparable to the mean collision velocity in the main belt.

Chemical Analysis of Particles in Aerogel The individual particles collected in aerogel were analyzed in-situ using the X-Ray Microprobe at beamline X26A of the National Synchrotron Light Source at Brookhaven National Laboratory, using techniques we developed to analyze particles collected in aerogel on the MIR space station. Because the distribution of debris from each shot is not isotropic, only a few aerogel cells contain enough fragments (~50 fragments) >10  m in size in order to determine chemical composition trends. We use a microscope to measure two dimensions  size = (x+y)/2 Particles in Aerogel

Distinguishing Matrix From Chondrules We distinguish olivine chondrules from matrix by the significantly higher Ni content of the matrix, which is well-documented in meteorite literature. A single in-situ analysis takes only a few minutes with synchrotron-based x-ray fluorescence because of the high Fe and Ni contents of both materials.

Chemical Analysis of OC Fragments Twelve of the 19 fragments 0.05 (high-Ni), while only 4 of the 29 fragments >25  m had Ni/Fe count rate ratios >0.05. Eight of 18 fragments 0.05, while only 1 of the 15 fragments >25  m had Ni/Fe count rates >0.05. Eleven of 30 fragments 0.05, while only 2 of the 12 fragments >25  m had Ni/Fe count rates >0.05.

Chemical Analysis of Fragments From Unweathered Meteorites -- Allende The results are even more striking for the Allende and Saratov shots. Twelve of the 15 fragments 0.05, while only 4 of the 24 fragments >35  m had Ni/Fe count rate ratios >0.05.

Chemical Analysis of Fragments From Unweathered Meteorites -- Saratov For the Saratov shot, 6 of the 7 fragments 0.05 (high-Ni), while none of the 15 fragments >10  m in size had a Ni/Fe count rate ratio >0.05. The preliminary Mbale results are very similar to the three weathered OCs we measured earlier – with an ~25  m division between high-Ni and normal fragments.

Implications for the Interplanetary Dust These results indicate that high-Ni (volatile-rich) matrix material is over-represented in the 25  m fraction of these chondrite meteorites. These small particles are rapidly removed from the “debris trail” near the asteroid, spiraling towards the Sun because of Poynting-Robertson drag before most of the larger particles are broken up by collisions. Thus, IDPs <25  m collected at Earth may be biased towards the matrix composition of the asteroid, while larger particles, collected as micrometeorites from the polar ices, may preferentially sample chondrule material. This may explain the chemical and mineralogical differences between IDPs and polar micrometeorites, and why IDPs are more volatile-rich than even CI carbonaceous chondrites.

Size Differences Among Meteorites The difference in the transition size from matrix fragments to olivine fragments between Saratov (where the transition occurs at ~10  m) and Allende (~35  m), with the three weathered OCs and the two Mbale OC samples transitioning at ~25  m, may reflect differences in the micro-structure of these materials. Saratov contains very little matrix occurring in small patches, thus it may be difficult to produce 10 to 25  m fragments of Saratov matrix. Allende has both a higher abundance of matrix [CV3 meteorites like Allende have 35 to 50% matrix] and larger size matrix regions, which may be why it produces many matrix fragments up to ~35  m. Most OC meteorites have matrix contents between the low content of Saratov and the high content of Allende.

Mass-Frequency Distribution of Fragments White sheets were deployed over the walls and floor of the AVGR chamber to facilitate the recovery of all debris. The debris from each shot was recovered from the AVGR chamber. The debris was sieved to sort fragments by size.

Mass-Frequency Distributions In each case the mass- frequency distribution of the fragments was determined by weighing each fragment down to ~0.02 grams. Most of the meteorites show a change in the slope of the mass-frequency distribution at roughly the mass of the chondrules, indicating that the disruption cannot be modeled as a single power-law. The distribution of the Saratov fragments is best fit by a single power-law, while the Allende distribution shows a change of slope at ~0.2 grams.

Implications for Asteroid Size Distribution A long-standing axiom, going back to Dohnanyi (1969), suggests that for a population of mutually colliding particles the details of the fragmentation size distribution are unimportant in affecting the overall size distribution of the entire population, assuming the collision cascade is self- similar, with size-independent impact strengths. However, Durda (1993) and Durda and Dermott (1996, 1997) showed that if the assumption of size-independent impact strengths is relaxed, the evolved size distribution can differ substantially from the canonical Dohanyi power law. Durda (1993) and Campo Bagatin et al., (1994) demonstrated that a sharp cutoff in the numbers of the smallest particles can induce deviations from the conventional power law size- frequency distribution of the large objects. Thus, the under-abundance of small particles that we measured for the impact disruptions of meteorites could have significant effects on the sizes of the largest asteroids in the main-belt.

Conclusions These results indicate that many meteorites suffer preferential failure along the chondrule-matrix boundary, an effect that should be considered in modeling the cratering and disruption of their asteroidal parent bodies. Except for the unusually friable OC Saratov, the mass- frequency distributions of the fragments from these stone meteorite disruptions are best fit by two power-law segments, with a noticeable change in slope at approximately the mean chondrule size. This result may have important implications for the size distribution of the largest asteroids. The results also suggest that fragments <25  m in size from hypervelocity impacts onto chondritic asteroids significantly over-sample the volatile-rich matrix material. Thus the IDPs collected at Earth may not be representative of the bulk composition of their parent asteroids, but may preferentially sample the volatile-rich matrix of the parent.