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

Analytical scanning electron microscopy of solar cells returned from the Hubble Space Telescope (HST) at the end of HST Service Missions SM-1 (1993) and SM-3B (2002) has revealed abundant remains of micrometeoroids. We have documented the most common residue compositions, and in this paper we suggest how they relate to mineral phases, and show how it is possible to estimate the proportion of the original micrometeoroid preserved. From a total of 273 impacts examined and analysed, we found 61/162 impacts on solar cells from SM-1 were produced by micrometeoroids, as were 45/111 from SM-3B. In each survey approximately 25% of damage features could not be assigned to a particular origin (micrometeoroid or space debris). A cumulative micrometeoroid flux curve for randomly selected cells shows impact features ranging from 3 to nearly 3800 pm in size. To assist interpretation of space exposed surfaces, impact residues from known meteoritic and terrestrial analogue mineral phases were produced by light gas gun assisted acceleration of buckshot projectiles into solar cell targets at 5.5-6.3 km s(-1).

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 alummous 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 pm, although the mass flux is concentrated in grains of more than 50 pm 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 mu m. 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 Cl group. The relative contribution of cometary as opposed to asteroidal particle sources cannot yet be assessed from this data set.