The chemical composition of micrometeoroids impacting upon the solar arrays of the Hubble Space Telescope

Mg- and Fe-rich residues were found in 30/61 impacts from SM-1 and 26/45 from SM-3B, with variable Mg:Fe ratio, usually lacking Ca, and likely to be from olivine or low-Ca pyroxene. Only in a few examples is it possible to determine the divalent cation to silicon ratio, and thereby positively identify olivine or pyroxene. Vesicular Fe-, Mg-, Ni- and S-rich residues, found in eight impacts from SM-1 and 5 from SM-3B, closely resemble residue from light gas gun shots of phyllosilicate-rich meteorite grains, and may be from a layered silicate such as serpentine or smectite interlayered with tochilinite. Fe- and S-rich immiscible melt droplets, low in nickel, are probably of troilite origin. Fe-, Ni- and P-rich residue is almost certainly from the phosphide schreibersite, and iron-nickel metal residues show an elemental ratio characteristic of kamacite. One Mg-, Cr-, Fe- and O-rich residue suggests a spinel precursor. Ca-rich particles found within the spall zone of several craters closely resemble residue from calcium carbonate. Mg sulfates are also present. Very little aluminous silicate residue was found (one residue from each survey). One extraordinarily well-preserved assemblage contains residues from five mineral components and may represent impact by a chondrule fragment. Derivation of incident particle sizes from impact feature dimensions, by use of calibrated damage equations, reveals that the majority of impacting micrometeoroids had diameters of less than 10 μm, although the mass flux is concentrated in grains of more than 50 μm diameter. In one well-preserved crater, the mass of residue was calculated to be 60 ng, approximately 25% of the particle mass as suggested by experimental crater size calibration. The smallest impacts were produced by grains of between 600 nm and 1.3 μm. The most common residue assemblages suggest that the majority of micron to millimetre scale micrometeoroids have an origin from chondritic material, similar to interplanetary dust particles, micrometeorites, and possibly the hydrous carbonaceous chondrites of the CM, CR or CI group. The relative contribution of cometary as opposed to asteroidal particle sources cannot yet be assessed from this data set.