The chemical composition of micrometeoroids impacting upon the solar arrays of the Hubble Space Telescope
Introduction
It is possible to obtain samples of extraterrestrial fine particles (cosmic dust) by a number of different methods, each of which imposes different limitations on interpretation of the composition and provenance of the collected particles. Due to the high relative velocity of most particles entering Earth’s atmosphere, many undergo thermal and oxidative alteration and may be selectively melted (Alexander and Love, 2001), contaminated, or simply difficult to distinguish from abundant terrestrial dust. Nevertheless, pristine interplanetary dust particles (IDP) have been successfully captured by high altitude aircraft, and have yielded excellent structural and compositional information (Brownlee, 1985, Rietmeijer, 1998, Bradley, 2004). However, the limited numbers of particles captured during such collections do not easily permit measurement of the flux of particle masses and sizes. Micrometeorite collections made from bulk polar ice samples (Taylor et al., 1998) sample short time intervals (years) or integrated temporal flux for a relatively long time interval (centuries), but may themselves show compositional bias due to weathering. Ice collections also require very large sample sizes to reach the area–volume product of deep sea sediment samples, necessary for recognition of statistically significant numbers of the larger (>100 μm) particles, and sufficient for reliable measurement of extraterrestrial mass flux (Peucker-Ehrenbrink and Ravizza, 2000). Are there other collection methods that may yield substantial numbers of particles for examination, from known time intervals and without confusion with terrestrial contamination?
Two different approaches may be tried for deliberate collection of micrometeoroid particles in space: onboard analysis by sophisticated spectrometers (e.g. the Gorid detector, Svedhem et al., 1999); or use of dedicated capture media on spacecraft (e.g. low density silica aerogel, Hörz et al., 2000) followed by return to Earth for analysis. Returned samples can be subjected to an ever-increasing range of sophisticated laboratory instrumentation (Zolensky et al., 2000). However, in all these cases the area and time available for particle capture may be limited (an area-time product of a few m2a at most), the total number of particles collected is likely to be small, and sampling of larger particles limited.
Opportunistic examination of impact residues on non-dedicated surfaces has also proven valuable (Graham et al., 2000, Graham et al., 2001b, Graham et al., 2002). In this present paper we show that hypervelocity impact residues on solar cells from the Hubble Space Telescope (HST) can reveal much about the size, composition, and even the origin, of micrometeoroids.
We have sought to answer the following questions: What are the most common micrometeoroid residue compositions and how do they relate to mineral phases that may have dominated the micrometeoroid particles? How big were the particles? What proportion of an original micrometeoroid particle has survived to be preserved within a crater? What are the origins of the micrometeoroids?
Section snippets
Samples, experimental and analytical parameters
The solar arrays removed from the HST during Service Mission 1 (SM-1) in 1993, and SM-3B in 2002, provided large numbers of particle impacts for analysis. In total, over 60 m2 of array surface was returned by shuttle orbiters, the earlier SM-1 (single) array having been exposed to space in low Earth orbit (LEO) at an altitude of about 615 km for 3.6 years, and the two arrays from SM-3B at a similar altitude for 8.2 years. The survey methodology, areas sampled, and the results from impact feature
Results and interpretation
In both post flight surveys, we were able to determine the type of impactor responsible for approximately 75% of the impact features examined. Distinction of the particle origins as debris or from micrometeoroids was explained in Kearsley et al., 2005. The smaller craters, less than 50 μm Dco were mainly of space debris origin (37 craters formed by micrometeoroids: 87 by space debris, mainly solid rocket motor combustion products). In contrast, most craters of greater than 50 μm diameter
Interpretation of particle origins
Several aspects of the residue preservation need to be taken into account before the apparent compositional assemblages can be compared to known extraterrestrial samples that have not undergone hypervelocity impact. Impact of a micrometeoroid upon a solar cell causes violent, albeit very short-lived (nanosecond to microsecond) processing of the projectile, with rapid quenching of a mixture of melted/condensed cover glass and encapsulation of fine droplets derived from the particle. Despite such
Conclusions and future work
LEO impact craters on solar cells yield abundant residues, from which micrometeoroid mineral precursors can be interpreted. As well as making measurement of micrometeoroid flux, we are now able to ascribe compositional information to impacting particles across a size range from micrometre to hundreds of micrometres diameter. A substantial proportion of the micrometeoroid may be preserved, especially in oblique craters. It is not yet possible to determine the precise source of the abundant mafic
Acknowledgement
Study of SM-1 was funded by ESA Contract No. 13308/98/NL/MV, and study of SM-3B craters by UniSpaceKent ESA Contract No. 16283/02/GD/ESTEC. Giles Graham acknowledges PPARC Case studentship and post-doctoral funding, and his recent work was under the auspices of the U.S. Department of Energy, National Nuclear Security Administration by the University of California, Lawrence Livermore National Laboratory under Contract No. W-7405-Eng-48. We thank Dave Wallis and Mike Cole for performing Light Gas
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