Moe Matsuoka^a, Tomoki Nakamura^a, Yuki Kimura^b, Takahiro Hiroi^c, Ryosuke Nakamura^d, Satoshi Okumura^a, Sho Sasaki^e
aDivision of Earth and Planetary Materials Science, Graduate School of Science, Tohoku University, Aoba, Sendai, Miyagi 980-8578, Japan
^bInstitute of Low Temperature Science, Hokkaido University, Sapporo, Hokkaido 060-0819, Japan
^cDepartment of Geological Sciences, Brown University, Providence, RI 02912, USA
^dNational Institute of Advanced Industrial Science and Technology, 1-1-1 Umezono, Tsukuba, Ibaraki 305-8568, Japan
^eDepartment of Earth and Space Science, Graduate School of Science, Osaka University, Toyonaka, Osaka 560-0043, Japan

We performed pulse-laser irradiation experiments of a primitive meteorite to simulate space weathering by micrometeorite bombardments on C-type asteroids. Pellets of powdered Murchison CM2 chondrite were set in vacuum and exposed to pulse laser with a diameter of 0.5 mm and delivered energies of 5, 10 and 15 mJ. We measured reflectance spectra of unirradiated and irradiated surfaces of the pellets. During analysis the pellet was heated to approximately 100°C and purged in N2 gas in order to reduce absorption of ambient water. The spectra become darker and bluer with increasing laser energies. Their UV reflectance increases and 0.7- and 3-μm band depths decrease from 0 to 15 mJ. The spectral bluing observed in our experiments reproduces the bluing occurred during space weathering of C-type asteroids. High-resolution observation by a transmission electron microscope showed that the laser heating causes preferential melting and evaporation in FeS-rich fine-grained portions, which results in dispersion and deposition of numerous FeS-rich amorphous silicate particles 20-1000 nm in size on the surface of the pellet. In addition, at the laser-irradiated but unmelted areas, heat-induced amorphization and decomposition of serpentine occur. These mineralogical changes make the reflectance spectra of the Murchison CM chondrite darker and bluer.

Reference
Matsuok M, Tomoki Nakamura T, Kimura Y, Hiroi T, Nakamura R, Okumura S, Sho Sasaki S (2015) Pulse-laser irradiation experiments of Murchison CM2 chondrite for reproducing space weathering on C-type asteroids. Icarus (in Press)
Link to Article [doi:10.1016/j.icarus.2015.02.029]

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Kelsey D. Hargrove^a, Josh Emery^b, Humberto Campins^a, Michael S.P. Kelley^c
a Physics Department, University of Central Florida, Orlando, FL 32816
^b Earth and Planetary Science Dept. and Planetary Geosciences Institute, University of Tennessee, Knoxville, TN 37996
^c Department of Astronomy, University of Maryland, College Park, MD 20742-2421

Many members of the Themis family show evidence of hydration in the form of oxidized iron in phyllosilicates (Florczak et al. 1999), and OH-bearing minerals (Takir and Emery 2012). The largest member, (24) Themis, has H2O ice covering its surface (Campins et al., 2010 and Rivkin and Emery, 2010). We have investigated the second largest Themis-family asteroid, (90) Antiope, which Castillo-Rogez and Schmidt (2010) predict to have a composition that includes water ice and organics. We obtained 2-4-μm spectroscopy of (90) Antiope in 2006 and 2008, and we find an absorption in the 3-μm region clearly present in our 2008 spectrum and likely in our 2006 spectrum. Both spectra have rounded, bowl-shaped absorptions consistent with those due to water ice as in the spectrum of (24) Themis, but do not uniquely identify water ice. We also present and compare Spitzer 8-12-μm mid-infrared spectra of (24) Themis and (90) Antiope. We find that (90) Antiope is lacking a “fairy castle” dusty surface, which is in contrast to (24) Themis, other Themis family members (Licandro et al. 2012), and Jupiter Trojans (e.g. Emery et al. 2006). We conclude that the surface structure of (90) Antiope is most similar to Cybele asteroid (121) Hermione (Hargrove et al. 2012).

Reference
Hargrove KD, Emery J, Michael HC, Kelley SP (2015) Asteroid (90) Antiope: Another Icy Member of the Themis Family? Icarus (in Press)
Link to Article [doi:10.1016/j.gca.2015.03.007]

Copyright Elsevier