INSIGHTS INTO REGOLITH DYNAMICS FROM THE
IRRADIATION RECORD PRESERVED IN HAYABUSASAMPLES. L. P. Keller1 and E. L. Berger2,1. 1Robert M. Walker
Laboratory for Space Science, Code KR, ARES, NASA/JSC,Houston, TX 77058. E-mail: Lindsay.P.Keller@nasa.gov.
2GeoControl Systems, Inc./JETS, Houston, TX 77058.
Introduction: The rates of space weathering processes arepoorly constrained for asteroid surfaces, with recent estimatesranging over 5 orders of magnitude [e.g. 1,2]. The return of thefirst surface samples from a space-weathered asteroid by theHayabusa mission and their laboratory analysis provides “groundtruth” to anchor the timescales for space weathering. We determine the rates of space weathering on Itokawa by measuring solar flare track densities and the widths of solar wind damaged
rims on grains. These measurements are made possible throughnovel focused ion beam (FIB) sample preparation methods [3].
Methods: Particles RA-QD02-0192 and RA-QD02-0211
were embedded in low viscosity epoxy and thin sections wereprepared using ultramicrotomy. Imaging, diffraction, and compositional data were obtained using a JEOL 2500SE 200 kV
field-emission scanning-transmission electron microscope. We
determine accurate solar flare track densities and rim widths fromthe FIB sections of the particles.
Results: Both particles are olivine-rich (Fo70), contain solar
flare tracks, and are surrounded by space weathered rims [4]. The
rims are continuous, show varying widths, and are nanocrystalline, not amorphous [4]. Interestingly, RA-QD02-0211 exhibits asolar flare particle track gradient across the particle, ranging from
3.4x109 cm-2 down to 9.2x108 cm-2. The solar wind damaged rim
is widest (80 nm) on the side of the grain with the highest track
density and is thinner (40 nm) on the opposite side of the grain.
The track density corresponds to a surface exposure of 105 years
based on the track production rate we determined for olivine at 1
AU (3.5 x104 cm-2) [5]. Particle RA-QD02-0192 also shows asolar flare track density gradient 2.9- to 1.1 x109 cm-2 and similar
rim widths as in -0211. The track density in -0192 corresponds toan exposure of ~8x104 years.
Discussion and Conclusions: The solar flare track density
gives a direct measure of the exposure time that the grain resided
within a few mm of the parent body surface, whereas the solar
wind effects are limited to the outer ~100 nm of grains that had adirect exposure to the solar wind. The heterogeneous distributionof the space weathering effects on -0192 and -0211 is consistent
with both particles maintaining a relatively fixed orientation in
the Itokawa regolith while they were irradiated by incoming solar
wind and solar flare ions. Provided these results are confirmed bymeasurements on many more grains, they imply that the regolith
at the Muses-C region on Itokawa was stable to mm-depths forthe last ~105 years. We are modeling the direct space exposure
time for such grains based on the width of their solar wind damaged rims [6].
References: [1] Willman, M. et al. 2010. Icarus 208:758772. [2] Vernazza, P. et al. 2009. Nature 458:993-995.
[3] Berger, E. L. & Keller L. P. 2014. LPS XLV, #1485. [4] Keller, L. P. & Berger, E. L. 2014. EPS 66:71. [5] Berger, E. L. &Keller L. P. 2014. Goldschmidt mtg. [6] Christoffersen R. andKeller L. P. 2014. this volume.