|
Microcraters in aluminum foils exposed by
Stardust
Hörz F., Borg J., Bradley J. P., Bridges J.,
Brownlee D. E., Burchell M. J., Cole M. J., Dai Z. R.,
Djouadi Z., Floss C., Franchi I. A., Graham G. A., Green S.
F., Heck P., Hoppe P., Kearsley A. T., Leitner J., Leroux
H., Teslich N., Marhas K. K., Schwandt C. S., See T. H.,
Stadermann F. J., Stephan T., Troadec D., Tsou P., and
Zolensky M. E. (2006)
Lunar Planet. Sci. XXXVII, Abstract #1148.
ABSTRACT
The Stardust Mission exposed some 1039 cm2 of
aerogel and some 153 cm2 of aluminum-foil to the
particle flux of comet Wild 2. The expected population of
microcraters on these foils represents a substantial science
opportunity, complementing the analysis of individual
particles trapped in aerogel. Owing to the much higher shock
stresses upon encounter of the dense aluminum foils, the
projectile remnants residing in the bottom, walls, and rim
area of these foil craters will be more severely altered,
most likely molten and possibly vaporized, however, compared
to those recovered from aerogel. Nevertheless, beginning
with the investigation of multi-layer thermal blankets from
the Solar Maximum Mission and extending into the Long
Duration Exposure Facility, and more recent opportunities on
space exposed surfaces, it was demonstrated that
compositionally diverse particle types can be distinguished
readily on aluminum (and other) substrates with modern
analytical instruments.
In addition, the size frequency distribution and total
flux of particles in the comets coma - using passive
collector instruments that are returned to Earth - is much
better determined with non-porous targets than with highly
porous materials. Experimental evidence and empirical
analyses of spaceexposed aerogels revealed that it is
difficult, if not impractical, to extract the initial
particle size or mass from either the detailed morphologic
characteristics of a penetration track, or from the size of
the trapped residue in aerogel. The physical cohesion of a
prospective impactor (e.g., a single micro-crack)
substantially controls the penetration outcome in aerogel,
because projectiles of otherwise identical bulk-properties
can fragment or completely disaggregate, producing
dramatically different track morphologies that range from
classical carrotshaped tracks to cylindrical cavities and
bulbous pits. In contrast, the cratering flow fields and
thus the morphology of small microcraters in non-porous
targets are relatively invariant. As a consequence, the
crater populations on the Stardust aluminum foils will
become the primary features, vastly superior to penetration
tracks, to deduce initial size or mass distributions and
associated fluxes of the dust-particles encountered during
Stardusts flyby of comet Wild 2. These determinations
constitute the primary objective of the Stardust Cratering
Team, and they will greatly complement the flux measurements
of the active experiments on board the space craft.
|