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Production of Cosmic Dust by Hydrous and Anhydrous Asteroids: Implications for the Production of Interplanetary Dust Particles and Micrometeorites

Introduction: In our Solar System the asteroid belt is compositionally zoned, with the asteroids similar in reflection spectrum to the carbonaceopus meteorites dominating the outer-half of the main belt. In the outer-half of the main belt about one-half of the carbonaceous asteroids are hydrous, based on their infrared spectra [1]. The asteroids in the inner-half of the main belt are predominantly anhydrous. Thus, about onequarter of the main belt asteroids are hydrous. Because dust particles from the main-belt asteroids generally encounter the Earth with lower relative velocities than dust from comets [2], asteroidal dust experiences much less severe atmospheric entry heating and much greater gravitational focusing. Both factors contribute to a significant overabundance of asteroidal over cometary dust in Earth collections [2]. Thus, much of the cosmic dust collected at Earth is expected to be from asteroids rather than comets. With about one-forth of the main belt asteroids being hydrous, we would expect about one-forth of the cosmic dust from the asteroids to be hydrous if the production mechanism were unbiased. There has been a long-standing puzzle in understanding the relationship between the interplanetary dust particles (IDPs), typically 5 μm to 50 μm cosmic dust particles collected from the Earth’s stratosphere, and the micrometeorites, typically 50 μm to millimetersize cosmic dust collected from the polar ices. A majority of the IDPs are anhydrous, consistent with the composition of the main belt. However, most of the polar micrometeorites are reported to be hydrous [3]. The dominant mechanism for the production of cosmic dust by asteroids is believed to be hypervelocity impact, either by cratering or catastrophic disruption. Tomeoka et al. [4] compared results from shockrecovery experiments on the hydrous CM meteorite Murchison and the anhydrous CV meteorite Allende. They determined that the onset of fracturing occurred at a significantly lower shock pressure in Murchison than in Allende, and suggested hydrous asteroids might contribute far more dust to the interplanetary medium then would be produced by anhydrous asteroids impacted under the same conditions. We have followed up on this work by disrupting samples of anhydrous [5] and hydrous meteorites using the NASA Ames Vertical Gun Range (AVGR). Measurements: We have performed disruption experiments on four hydrous meteorite targets, Murchison and three CM2 meteorites recovered from the Antarctic. The projectiles in these experiments were 1/8 th