O. Ruesch¹ et al. (>10)*
*Find the extensive, full author and affiliation list on the publishers website.

¹1Institut für Planetologie, Westfälische Wilhelms-Universität, Münster, Germany

The magmatism characterizing the early history of the asteroid Vesta has long been investigated with the mafic and ultramafic meteorites howardite, eucrite, and diogenite (HED). The lack of geologic context for the meteorites, however, has limited its understanding. Here we use the visible to near-IR (VIR) orbital observations of Vesta’s surface to detect relative enrichments in olivine and to study the associated geologic features. Because the near-IR signature of olivine on Vesta’s surface is subtle relative to the widespread pyroxene absorption bands, a method was developed to distinguish olivine enrichments from admixture of pyroxenes with high Fe²⁺/M1, dark material, and potential Fe-bearing glass. Relative enrichment of olivine (~<50–60 vol %) is found in 2–5 km wide, morphologically fresh areas. Our global survey reveals a dozen of these areas clustering in the eastern hemisphere of Vesta. The hemispherical coincidence with a widespread, low enrichment in diogenite-like pyroxene suggests the presence of a distinct compositional terrain. On the central mound of the Rheasilvia impact basin, no olivine enrichment was found, suggesting the absence of an olivine-dominated mantle above the basin’s excavation depth or, alternatively, a low amount of olivine homogeneously mixed with diogenite-like pyroxenes. Rare olivine-enriched areas in close proximity to diogenite-like pyroxene are found as part of material ejected by the Rheasilvia impact. Such cooccurrence is reminiscent of local, ultramafic lithologies within the crust. The possible formation of such lithologies on Vesta is supported by some HED meteorites dominated by olivine and orthopyroxene.

Reference
Ruesch O et al. (2014) Detections and geologic context of local enrichments in olivine on Vesta with VIR/Dawn data. Journal of Geophysical Research: Planets

Link to Article [10.1002/2014JE004625]

D. L. Blaney¹ et al. (>10)*
*Find the extensive, full author and affiliation list on the publishers website.

¹Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California, USA

A suite of eight rocks analyzed by the Curiosity Rover while it was stopped at the Rocknest sand ripple shows the greatest chemical divergence of any potentially sedimentary rocks analyzed in the early part of the mission. Relative to average Martian soil and to the stratigraphically lower units encountered as part of the Yellowknife Bay formation, these rocks are significantly depleted in MgO, with a mean of 1.3 wt %, and high in Fe, averaging over 20 wt % FeO_T, with values between 15 and 26 wt % FeO_T. The variable iron and low magnesium and rock texture make it unlikely that these are igneous rocks. Rock surface textures range from rough to smooth, can be pitted or grooved, and show various degrees of wind erosion. Some rocks display poorly defined layering while others seem to show possible fractures. Narrow vertical voids are present in Rocknest 3, one of the rocks showing the strongest layering. Rocks in the vicinity of Rocknest may have undergone some diagenesis similar to other rocks in the Yellowknife Bay Formation as indicated by the presence of soluble calcium phases. The most reasonable scenario is that fine-grained sediments, potentially a mixture of feldspar-rich rocks from Bradbury Rise and normal Martian soil, were lithified together by an iron-rich cement.

Reference
Blaney DL et al. (2014) Chemistry and texture of the rocks at Rocknest, Gale Crater: Evidence for sedimentary origin and diagenetic alteration. Journal of Geophysical Research: Planets

Link to Article [10.1002/2014JE004650]

Micah J. Schaible andRaúl A. Baragiola

Laboratory of Atomic and Surface Physics, University of Virginia, Charlottesville, Virginia, USA

Hydroxyl on the lunar surface revealed by remote measurements has been thought to originate from solar wind hydrogen implantation in the regolith. The hypothesis is tested here through experimental studies of the rate and mechanisms of OH bond formation due to H⁺implantation of amorphous SiO₂ and olivine in ultrahigh vacuum. The samples were implanted with 2–10 keV H⁺, in the range of solar wind energies, and the OH absorption band at ~2.8 µm measured by transmission Fourier transform infrared spectroscopy. For 2 keV protons in SiO₂, the OH band depth saturated at fluences F ~5 × 10¹⁶ H⁺/cm² to a maximum 0.0032 absorption band depth, corresponding to a column density η_s = 1.1 × 10¹⁶ OH/cm². The corresponding values for 5 keV protons in olivine are >2 × 10¹⁷/cm², 0.0067, and 4.0 × 10¹⁶ OH/cm². The initial conversion rate of implanted H⁺ into hydroxyl species was found to be ~90% and decreased exponentially with fluence. There was no evidence for molecular water formation due to proton irradiation. Translating the laboratory measurements in thin plate samples to the granular lunar regolith, it is estimated that the measurements can account for a maximum of 17% relative OH absorption in reflectance spectroscopy of mature soils, consistent with spacecraft observations in the infrared of the Moon.

Reference
Schaible MJ and Baragiola RA (2014) Hydrogen implantation in silicates: The role of solar wind in SiOH bond formation on the surfaces of airless bodies in space. Journal of Geophysical Research: Planets

Link to Article [10.1002/2014JE004650]

P. Senthil Kumar¹, K. J. Prasanna Lakshmi¹, N. Krishna¹, R. Menon¹, U. Sruthi¹, V. Keerthi², A. Senthil Kumar², D. Mysaiah¹, T. Seshunarayana¹ and M. K. Sen¹

¹National Geophysical Research Institute, Council of Scientific and Industrial Research, Hyderabad, India
²National Remote Sensing Centre, Indian Space Research Organisation, Hyderabad, India

Impact fragmentation is an energetic process that has affected all planetary bodies. To understand its effects in basalt, we studied Lonar Crater, which is a rare terrestrial simple impact crater in basalt and analogues to kilometer-scale simple craters on Mars. The Lonar ejecta consists of basaltic fragments with sizes ranging from silt to boulder. The cumulative size and mass frequency distributions of these fragments show variation of power index for different size ranges, suggesting simple and complex fragmentation. The general shape of the fragments is compact, platy, bladed, and elongated with an average edge angle of 100°. The size distribution of cobble- to boulder-sized fragments is similar to the fracture spacing distribution in the upper crater wall, indicating the provenance of those large fragments. Its consistency with a theoretical spallation model suggests that the large fragments were ejected from near surface of the target, whereas the small fragments from deeper level. The petrophysical properties of the ejecta fragments reflect the geophysical heterogeneity in the target basalt that significantly reduced the original size of spall fragments. The presence of Fe/Mg phyllosilicates (smectites) both in the ejecta and wall indicates the role of impact in excavating the phyllosilicates from the interior of basaltic target affected by aqueous alteration. The seismic images reveal a thickness variation in the ejecta blanket, segregation of boulders, fractures, and faults in the bedrock beneath the crater rim. The fracturing, fragmentation, and fluvial degradation of Lonar Crater have important implications for Mars.

Reference
Kumar SP, Prasanna Lakshmi KJ, Krishna N, Menon R, Sruthi U, Keerthi V, Kumar AS, Mysaiah D, Seshunarayana T and Sen MK (2014) Impact fragmentation of Lonar Crater, India: Implications for impact cratering processes in basalt. Journal of Geophysical Research: Planets
Link to Article [10.1002/2013JE004543]