¹Hu, J., ¹Sharp, T. G
¹School of Earth and Space Exploration, Arizona State University, Tempe, Arizona, USA

Northwest Africa 757 is unique in the LL chondrite group because of its abundant shock-induced melt and high-pressure minerals. Olivine fragments entrained in the melt transform partially and completely into ringwoodite. Plagioclase and Ca-phosphate transform to maskelynite, lingunite, and tuite. Two distinct shock-melt crystallization assemblages were studied by FIB-TEM analysis. The first melt assemblage, which includes majoritic garnet, ringwoodite plus magnetite-magnesiowüstite, crystallized at pressures of 20–25 GPa. The other melt assemblage, which consists of clinopyroxene and wadsleyite, solidified at ~15 GPa, suggesting a second veining event under lower pressure conditions. These shock features are similar to those in S6 L chondrites and indicate that NWA 757 experienced an intense impact event, comparable to the impact event that disrupted the L chondrite parent body at 470 Ma.

Reference
Hu J, Sharp TG (2016) High-pressure phases in shock-induced melt of the unique highly shocked LL6 chondrite Northwest Africa 757.
Meteoritics & Planetary Science (in Press)
Link to Article [DOI: 10.1111/maps.12672]
Published by arrangement with John Wiley & Sons

¹Rebecca J. Thomas, ^1,2Brian M. Hynek, ³David A. Rothery, ⁴Susan J. Conway
¹Laboratory for Atmospheric and Space Physics, University of Colorado, 3665 Discovery Drive, Boulder, CO 80303, USA
²Department of Geological Sciences, University of Colorado, 399 UCB, Boulder, CO 80309, USA
³Department of Physical Sciences, The Open University, Walton Hall, Milton Keynes, MK7 6AA, UK
⁴Laboratoire de Planétologie et Géodynamique – UMR CNRS 6112, 2 rue de la Houssinière – BP 92208, 44322 Nantes Cedex 3, France

Unusually low reflectance material, within which depressions known as hollows appear to be actively forming by sublimation, is a major component of Mercury’s surface geology. The observation that this material is exhumed from depth by large impacts has the intriguing implication that the planet’s lower crust or upper mantle contains a significant volatile–rich, low–reflectance layer, the composition of which will be key for developing our understanding of Mercury’s geochemical evolution and bulk composition. Hollows provide a means by which the composition of both the volatile and non–volatile components of the low–reflectance material (LRM) can be constrained, as they result from the loss of the volatile component, and any remaining lag can be expected to be formed of the non–volatile component. However, previous work has approached this by investigating the spectral character of hollows as a whole, including that of bright deposits surrounding the hollows, a unit of uncertain character. Here we use high–resolution multispectral images, obtained as the MESSENGER spacecraft approached Mercury at lower altitudes in the latter part of its mission, to investigate reflectance spectra of inactive hollow floors where sublimation appears to have ceased, and compare this to those of the bright surrounding products and the parent material. This analysis reveals that the final lag after hollow–formation has a flatter spectral slope than that of any other unit on the planet and reflectance approaching that of more space–weathered parent material. This indicates firstly that the volatile material lost has a steeper spectral slope and higher reflectance than the parent material, consistent with (Ca,Mg) sulfides, and secondly, that the low–reflectance component of LRM is non–volatile and may be graphite.

Reference
Thomas RJ, Hynek BM, Rothery DA, Conway SJ (2016) Mercury’s Low-Reflectance Material: Constraints from Hollows. Icarus (in Press)
Link to Article [doi:10.1016/j.icarus.2016.05.036]
Copyright Elsevier

¹Daniel M. Applin, ^1,2,3Matthew R.M. Izawa,¹Edward A. Cloutis
¹Hyperspectral Optical Sensing for Extraterrestrial Reconnaissance Laboratory, Dept. Geography, University of Winnipeg, 515 Portage Avenue, Winnipeg, Manitoba, Canada R3B 2E9
²Dept. Earth Sciences, Brock University, 1812 Sir Isaac Brock Way, St. Catharines, Ontario, Canada L2S 3A1
³Planetary Science Institute, 1700 East Fort Lowell, Suite 106, Tucson, AZ 85719-2395

The diversity of oxalate formation mechanisms suggests that significant concentrations of oxalic acid and oxalate minerals could be widely distributed in the solar system. We have carried out a systematic study of the reflectance spectra of oxalate minerals and oxalic acid, covering the 0.2-16 µm wavelength region.. Our analyses show that oxalates exhibit unique spectral features that enable discrimination between oxalate phases and from other commonly occurring compounds, including carbonates, in all regions of the spectrum except for the visible. Using these spectral data, we consider the possible contribution of oxalate minerals to previously observed reflectance spectra of many objects throughout the solar system, including satellites, comets, and asteroids. We find that polycarboxylic acid dimers and their salts may explain the reflectance spectra of many carbonaceous asteroids in the 3 µm spectral region.. We suggest surface concentration of these compounds may be a type of space weathering from the photochemical and oxidative decomposition of the organic polymer found in carbonaceous chondrites. The stability and ubiquity of these minerals on Earth, in extraterrestrial materials, and in association with biological processes make them useful for many applications in Earth and planetary sciences.

Reference
Applin DM, Izawa MRM, Cloutis EA (2016) Reflectance spectroscopy of oxalate minerals and relevance to solar system carbon inventories. Icarus (in Press)
Link to Article [doi:10.1016/j.icarus.2016.05.005]
Copyright Elsevier