¹Deborah. L. Domingue, ¹Faith Vilas, ²Teck Choo, ³Karen R. Stockstill-Cahill, ⁴Joshua T.S. Cahill, ³Amanda R. Hendrix
Icarus (in Press) Link to Article [doi:10.1016/j.icarus.2016.07.011]
¹Planetary Science Institute
²Johns Hopkins University Applied Physics Laboratory
³Planetary Science Institute, 1700 E. Fort Lowell, Suite 106, Tucson, AZ 85719-2395, USA
⁴The Johns Hopkins University Applied Physics Laboratory, 11100 Johns Hopkins Road, Laurel, MD 20723, USA
Copyright Elsevier

Spectrophotometric examination of the Galileo Solid State Imager (SSI) observations from the Galileo spacecraft reveal surface compositional heterogeneities in mineral compositions not related to geologic unit. These include variations in olivine and orthopyroxene content of on the order of 15% and 25%, respectively. Opaque mineral phases across the inter-ridge regions vary in quantity, but consistently modeled better with ilmenite. The macroscale fraction of metallic iron varies subtly (0-10%) in quantity and in grain size (60 – 100 μm). Color properties also vary across the inter-ridge regions, indicating variations in regolith maturity. Comparisons of near-infrared ratio-reflectance suggest changes in regolith maturity that are different from those seen on the lunar surface and asteroid 433 Eros, commensurate with Gaspra’s higher olivine content. Visible to near-infrared slopes compared to near-ultraviolet to visible slopes are indicative of a nanophase iron content of 0.01% – 0.1%. Spectral mixing modeling studies of the SSI color spectra show results consistent with the presence of both microphase (> 50 nm) and nanophase (< 50nm) size iron particulates. While the quantity of microphase and nanophase iron appears to be constant within the sample areas studied, the grain size of the microphase component varies. Agglutinates are present in some areas of the inter-ridge regions, but at low abundances (∼5%).

¹K. Righter,²M. A. Cosca,²L. E. Morgan
Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.12692]
¹Mailcode XI2, NASA Johnson Space Center, Houston, Texas, USA
¹U.S. Geological Survey, Denver Federal Center, Denver, Colorado, USA
Published by arrangement with John Wiley & Sons

The hornblende- and biotite-bearing R chondrite LAP 04840 is a rare kind of meteorite possibly containing outer solar system water stored during metamorphism or postshock annealing deep within an asteroid. Because little is known regarding its age and origin, we determined 40Ar/39Ar ages on hornblende-rich separates of the meteorite, and obtained plateau ages of 4340(±40) to 4380(±30) Ma. These well-defined plateau ages, coupled with evidence for postshock annealing, indicate this meteorite records an ancient shock event and subsequent annealing. The age of 4340–4380 Ma (or 4.34–4.38 Ga) for this and other previously dated R chondrites is much older than most impact events recorded by ordinary chondrites and points to an ancient event or events that predated the late heavy bombardment that is recorded in so many meteorites and lunar samples.