Can Formulas Derived from Pyroxenes and/or HEDs be Used to Determine the Mineralogies of V‐type Asteroids?

1T. H. Burbine,2P. C. Buchanan,3R. L. Klima,4R. P. Binzel
Journal of Geophysical Research, Planets (in Press) Link to Article [https://doi.org/10.1029/2018JE005561]
1Department of Astronomy, Mount Holyoke College, South Hadley, MA, USA
2Department of Geology, Kilgore College, Kilgore, TX, USA
3Johns Hopkins University Applied Physics Laboratory, Laurel, MD, USA
4Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA, USA
Published by arrangement with John Wiley & Sons

We compare methods for determining the pyroxene mineralogies of V‐type near‐Earth asteroids from their reflectance spectra. We evaluate whether band centers derived from the spectra of synthetic pyroxenes can be used to achieve greater analytical accuracy than is achieved through the use of band centers derived from the spectra of basaltic achondrites (howardites, eucrites, diogenites or HEDs). We conclude that band centers derived from the reflectance spectra of synthetic pyroxenes with known mineralogies do not provide useful diagnostic information to derive equations for determining accurate pyroxene compositions of V‐type asteroids. Band centers from the reflectance spectra of HEDs with known pyroxene mineralogies can be used to derive equations for determining accurate pyroxene compositions of V‐type asteroids. HEDs are physical mixtures of a number of different types of pyroxenes and best simulate the surfaces of V‐type asteroids. Formulas using the Band I center appear best for determining asteroid pyroxene mineralogies for V‐type asteroids due to the current difficulty in doing accurate temperature corrections to the Band II center. Most of thes observed V‐type near‐Earth asteroids have interpreted mineralogies similar to eucrites or howardites. One of the observed near‐Earth asteroids could possibly have a surface mineralogy similar to diogenites.

Image Reconstruction Techniques in Neutron and Gamma‐Ray Spectroscopy: Improving Lunar Prospector Data

1J. T. Wilson,1D. J. Lawrence,1P. N. Peplowski,1J. T. S. Cahill,2V. R. Eke,2R. J. Massey,3L. F. A. Teodoro
Journal of Geophysical Research, Planets (in Press) Link to Article [https://doi.org/10.1029/2018JE005589]
1The Johns Hopkins University Applied Physics Laboratory, Laurel, MD, USA
2Institute for Computational Cosmology, Department of Physics, Durham University, Science Laboratories, Durham, UK
3BAER, Planetary Systems Branch, Space Sciences and Astrobiology Division, MS 245‐3, NASA Ames Research CenterMoffett Field, CA, USA
Published by arrangement with John Wiley & Sons

We present improved resolution maps of the Lunar Prospector Neutron Spectrometer thermal, epithermal and fast neutron data and Gamma‐Ray Spectrometer Th‐line fluxes via global application of pixon image reconstruction techniques. With the use of mock data sets, we show that the pixon image reconstruction method compares favorably with other methods that have been used in planetary neutron and gamma‐ray spectroscopy. The improved thermal neutron maps are able to clearly distinguish variations in composition across the lunar surface, including within the lunar basins of Hertzsprung and Schrödinger. The improvement in resolution reveals a correlation between albedo and thermal neutron flux within the basins. The consequent increase in dynamic range confirms that Hertzsprung basin contains one of the most anorthositic parts of the lunar crust, including nearly pure anorthite over a region tens of km in diameter. At Orientale, the reconstructed epithermal neutron data show broad overlap with cpr but there remains a mismatch between measures of regolith maturity that sample the surface and those that probe the near‐subsurface, which is consistent with a complex layering scenario.

Shergottite Northwest Africa 6963: A Pyroxene‐Cumulate Martian Gabbro

1Justin Filiberto et al. (>10)
Journal of Geophysical Research, Planets (in Press) Link to Article [https://doi.org/10.1029/2018JE005635]
1Southern Illinois University, Department of GeologyCarbondale, IL, USA
2School of Environment, Earth and Ecosystem Sciences, Walton Hall, The Open UniversityMilton Keynes, MK, UK
Published by arrangement with John Wiley & Sons

Northwest Africa (NWA) 6963 was found in Guelmim‐Es‐Semara, Morocco, and based on its bulk chemistry and oxygen isotopes, it was classified as a Martian meteorite. On the basis of a preliminary study of the textures and crystal sizes, it was re‐subclassified as a gabbroic shergottite because of the similarity with terrestrial and lunar gabbros. However, the previous work was not a quantitative investigation of NWA 6963; to supplement the original re‐subclassification and enable full comparison between this and other Martian samples, here we investigate the mineralogy, petrology, geochemistry, quantitative textural analyses, and spectral properties of gabbroic shergottite NWA 6963 to constrain its petrogenesis, including the depth of emplacement (i.e., base of a flow versus crustal intrusion).
NWA 6963 is an enriched shergottite with similar mineralogy to the basaltic shergottites, but importantly, does not contain any fine‐grained mesostasis. Consistent with the mineralogy, the reflectance (VNIR and TIR) spectrum of powdered NWA 6963 is similar to other shergottites because they are all dominated by pyroxene, but its reflectance is distinct in terms of albedo and spectral contrast due to its gabbroic texture. NWA 6963 represents a partial cumulate gabbro that is associated with the basaltic shergottites. Therefore, NWA 6963 could represent a hypabyssal intrusive feeder dike system for the basaltic shergottites that erupted on the surface.

Dusty Rocks in Gale Crater: Assessing Areal Coverage and Separating Dust and Rock Contributions in APXS Analyses

1Mariek E. Schmidt, 2,3Glynis M. Perrett, 1Samantha L. Bray, 1Nicholas J. Bradley, 1,4Rebekka E. Lee, 5Jeff A. Berger, 5John L. Campbell, 6Cathy Ly, 2Steven W. Squyres, 5Dustin Tesselaar
Journal of Geophysical Research, Planets Link to Article [https://doi.org/10.1029/2018JE005553]
1Department of Earth Sciences, Brock University, St. Catharines, Ontario, Canada
2Cornell Center for Astrophysics and Planetary Science, Cornell University, Ithaca, NY, USA
3Wilfrid Laurier University, Department of Physics and Computer Science, Waterloo, Ontario, Canada
4Lafarge Aggregates, Mississauga, Canada
5Department of Physics, University of Guelph, Guelph, Ontario, Canada
6Cornell University, Ithaca, NY, USA
Published by arrangement with John Wiley & Sons

A thin, patchy layer of airfall dust covers rock surfaces examined by the Mars Science Lab (MSL) rover Curiosity and complicates interpretation of textures in Mars Hand Lens Imager (MAHLI) images and compositions determined by Alpha Particle X‐ray Spectrometer (APXS). Using three image processing methods, we estimate dust coverages for MAHLI images of APXS targets to Sol 1512. Dust coverages of ‘as is’ rock targets range from 6 to 77% (±5 to 10% estimated error). Targets brushed by the Dust Removal Tool (DRT) range to lower coverages than ‘as is’ targets, but quality depends on surface type; brushed mudstones have the narrowest range and lowest coverages (11‐25%), while sandstones vary, ranging to higher coverages (12‐58%). Groups of rocks with similar compositions (APXS classes) can have strong correlations between dust coverage and SO3/Cl (up to r=0.985). Dust can also strongly affect the lightest elements measured (Na to Ca). By comparing paired ‘as is’ and DRT analyses, using the determined dust coverages, and finding a best fit dust thickness (generally ~10 μm), we model relative contributions of the dust and bedrock to extrapolate dust‐free compositions for homogeneous APXS classes. The dust is basaltic with high S and Cl. Dust‐free rocks have higher SiO2 and Na2O (up to 6.5 wt% and 0.5 wt% higher, respectively) and lower SO3 and CaO (up to 5.5 wt% and 1.3 wt% lower, respectively) than dusty equivalents. Dust most influences compositions that are very different from average Mars, including the alkali‐rich, MgO‐poor Jake M class.

 

Aqueous Processes from Diverse Hydrous Minerals in the Vicinity of Amazonian‐Aged Lyot Crater

1,2Lu Pan, 1,3Bethany L. Ehlmann
Journal of Geophysical Research, Planets (in Press) Link to Article [https://doi.org/10.1029/2017JE005461]
1Division of Geological and Planetary Sciences, California Institute of TechnologyPasadena, CA, USA
2Laboratoire de Geologie de Lyon, Université Claude Bernard Lyon 1Villeurbanne, France
3Jet Propulsion Laboratory, California Institute of TechnologyPasadena, CA, USA
Published by Arrangement with John Wiley & Sons

Amazonian‐aged Lyot crater is the best‐preserved and deepest peak‐ring impact crater (diameter, D=220km) in the northern lowlands of Mars. Morphological features including scouring channels emanating from its ejecta and small channels within the crater have been examined previously to understand hydrological activity associated with the crater. In this study, we analyze images acquired by the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) on board the Mars Reconnaissance Orbiter (MRO) to investigate the mineralogical record in Lyot and its surroundings, which are presently enriched in ground ice, to understand the associated aqueous processes, their relative timing, and a possible role for ground ice in hydrous mineral formation. We find diverse hydrous minerals, including Fe/Mg phyllosilicates, chlorite, illite/muscovite and prehnite in Lyot crater walls, central peak, and ejecta, as well as in two craters to the west of Lyot. The exposure and distribution of the hydrous minerals suggests they are related to the impact process, either exposed by the excavation of hydrothermally altered rocks or formed through syn‐depositional hydrothermal alteration immediately after impacts. The Lyot impact induced channel formation to the north, but no mineralogical evidence of aqueous alteration associated with the channels is observed. The sinuous channels within Lyot, diverted by bedrock units with hydrous mineral detections, did not cause mineralization but likely represent the last stage of water activity in Lyot crater. The separate episodes of water activity indicate flow of liquid water on Mars’ surface during the Amazonian but limited to no aqueous alteration to generate hydrous minerals.

Formation of evolved rocks at Gale crater by crystal fractionation and implications for Mars crustal composition

1Arya Udry,2Esteban Gazel,3Harry Y. McSween Jr
Journal of Geophyisical Research, Planets (in Press) Link to Article [https://doi.org/10.1029/2018JE005602]
1Department of Geoscience, University of Nevada, Las Vegas
2Department of Earth and Atmospheric Sciences, Cornell University
3Department of Earth and Planetary Sciences, University of Tennessee
Published by Arrangement with John Wiley & Sons

The recent discovery of some ancient evolved rocks in Gale crater by the Curiosity rover has prompted the hypothesis that continental crust formed in early martian history. Here we present petrological modeling that attempts to explain this lithological diversity by magma fractionation. Using the thermodynamical software MELTS, we model fractional crystallization of different martian starting compositions that might generate felsic igneous compositions like those analyzed at Gale crater using different variables, such as pressure, oxygen fugacities, and water content. We show that similar chemical and mineralogical compositions observed in Gale crater felsic rocks can readily be obtained through different degrees of fractional crystallization of basaltic compositions measured on the martian surface. The results suggest that Gale crater rocks may not represent true primary liquids as they possibly accumulated and/or fractionated feldspar. In terms of major element compositions and mineralogy, we found that the Gale crater felsic compositions are more similar to fractionated magmas produced in Earth’s intraplate volcanoes than to terrestrial felsic continental crust as represented by tonalite‐trondhjemite‐granodiorite (TTG) suites. We conclude that the felsic rocks in Gale crater do not represent continental crust, as it is defined on Earth.

Spectral analysis of the Cerean geological unit crater central peak material as an indicator of subsurface mineral composition

1,2A.Galiano et al. (>10)
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2018.05.020]
1IAPS-INAF, Via del Fosso del Cavaliere, 100, 00133 Rome, Italy
2 Università degli Studi di Roma Tor Vergata, Via della Ricerca Scientifica, 1, 00133 Rome, Italy
Copyright Elsevier

The dwarf planet Ceres is a heavily cratered rocky body, and complex craters with a central peak are widely observed on its surface. These types of craters form when a large body impacts the surface, generating extreme temperatures and pressures. During the impact event a large volume of rock is raised from the subsurface and a central uplift is formed. The material composing the central uplift is called crater central peak material (ccp) and the spectral analysis of such geologic areas can provide information about the composition of Ceres’ subsurface. Reflectance spectra of 32 ccps, acquired by the VIR spectrometer on board the NASA/Dawn spacecraft, were analysed and shows absorption bands located at about 2.7, 3.1, 3.4 and 4.0 µm which are also common on the Cerean surface. These absorptions are related, respectively, to Mg-phyllosilicates, NH4-phyllosilicates and Mg/Ca-carbonates.
The spectral parameters considered in this work are: spectral slopes estimated between 1.2 µm and 1.9 µm, band depths at 2.7-, 3.1-, 3.4- and 4.0-µm, and band centers near 4.0-µm. The ccps spectral parameters were analysed in conjunction with other Cerean parameters, such as the estimated depth of excavation of the material composing the central peak, in order to search for correlations and information about Ceres’ subsurface.
Central peak material located polewards show stronger 2.7- and 3.1-µm band depths with respect to those at the equatorial region, suggesting that subsurface deposits closer to poles are probably richer in Mg- and NH4-phyllosilicates. The 3.4-µm spectral feature is also deeper in ccps located at poleward latitudes, similar to the phyllosilicates. Conversely, the 4.0-µm band does not show this trend with latitude.
An increase in both 3.1- and 3.4-µm band depths with the estimated depth of excavation indicates that the spectral feature at 3.4-µm is the result of different contributions from carbonates and NH4-phyllosilicates, as expected. However, depending on their relative influence, the shape of the 3.4-µm spectral feature can vary.
Phyllosilicates and carbonates are the resulting products of aqueous alteration of chondritic material and, given the increasing abundance of such minerals (in particular ammoniated phyllosilicates) with depth of excavation, it is likely that our investigation involved subsurface layers nearby the boundary between the volatile-rich crust and the silicate-rich mantle.
Na-carbonate is found in the crater central peak material of Ernutet, Haulani and Ikapati, characterized by an estimated depth of excavation of about 6-9 km, where deposits of sodium carbonates could be locally present.