SOLAR ION PROCESSING OF ITOKAWA GRAINS:RECONCILING MODEL PREDICTIONS WITH SAMPLEOBSERVATIONS.
R. Christoffersen1 and L. P. Keller2. 1Jacobs, NASA Johnson
Space Center, Mail Code XA, Houston, TX 77058, USA. E-mail:roy.christoffersen-1@nasa.gov. 2NASA Johnson Space Center,Mail Code XA, Houston, TX 77058.
Introduction: Analytical TEM observations of Itokawa
grains reported to date show complex solar wind ion processing
effects in the outer 30-100 nm of pyroxene and olivine grains[1,2,3]. The effects include loss of long-range structural order,formation of isolated interval cavities or “bubbles”, and othernanoscale compositional/microstructural variations [1,2,3]. Noneof the effects so far described have, however, included complete
ion-induced amorphization. To link the array of observed relationships to grain surface exposure times, we have adapted ourprevious numerical model for progressive solar ion processing
effects in lunar regolith grains [4] to the Itokawa samples. The
model uses SRIM [5] ion collision damage and implantation calculations within a framework of a constant-deposited-energy
model for amorphization [6]. Inputs include experimentallymeasured amorphization fluences [7], a 2π steradian variable ionincidence geometry required for a rotating asteroid, and a numerical flux-versus-velocity solar wind spectrum based on [8].
Results: The model depth versus ion damage relations exhibit a cross-over point where He+ largely replaces H+ in producing
damage effects extending below 25 nm. For the typical ionprocessed rim widths of 50-70 nm in Itokawa pyroxenes and olivines, the last stages of rim widening are therefore controlled by
the lower He+ fluxes in the solar wind [8]. Incorporating the experimental critical amorphization fluence for solar wind energy
He+ in olivine [7], the model predicts complete amorphization toa depth of 60-70 nm in Itokawa olivines in as little as ~2000
years of direct solar wind exposure. This is in line with limits forthe amount of implanted He+ that SRIM predicts the olivine
structure could reasonably hold before extensive He+ implantation cavities or “bubbles” should form. (Only limited such cavities/bubbles are observed in the Itokawa olivines [1,2,3].) Ion
amorphization data for pyroxene comparable to [7] are lacking,but a large difference in exposure time relative to olivine is notexpected based on behavior in higher-energy ion irradiations [9].
Discussion: Our model predicts that olivine grains exposedto the solar wind on the surface of Itokawa should acquire fully
amorphous rims, 60-70 nm wide, on a timescale of a few 103
years, yet the olivine rims observed to date are nanocrystalline,
not amorphous [1,2,3]. Provided the model results are correct,
they suggest that the nanocrystalline rims on Itokawa olivine
grains developed very rapidly (<2000 years). The fact that thisvalue is orders of magnitude shorter than solar flare track exposure ages [3,10] is not fully understood, but may be because solar
flare tracks can continue to accumulate under burial to mm
depths, whereas solar wind ion damage cannot.
References: [1] Noguchi T. et al. 2014. MAPS, 49, 188. [2]Thompson et al. 2014. EPS 66, 89. [3] Keller, L. P. and Berger E.
L. 2014. EPS, 66, 71. [4] Chamberlin, S. et al. 2008. Abstract #2302, 36th LPSC. [5] Ziegler J.F et al. 2008. SRIM, Lulu Press.
[6] Wang L. M and Ewing R. C. 1992. MRS Bulletin, 17, 38. [7]Carrez P. et al. 2002, MAPS, 37, 1599. [8] Reisenfeld D. B. et al.
2013. Space Sci. Rev. 175, 25. [9] Christoffersen, R. and Keller
L.P. 2011, MAPS, 46, 950. [10] Keller, L. P. and Berger E. L.
2014. This volume.