David J. Lawrence^1,*, Patrick N. Peplowski¹, Thomas H. Prettyman², William C. Feldman², David Bazell¹, David W. Mittlefehldt³, Robert C. Reedy², Naoyuki Yamashita²

¹The Johns Hopkins University Applied Physics Laboratory, Laurel, Maryland, USA
²Planetary Science Institute, Tucson, Arizona, USA
³NASA Johnson Space Center, Houston, Texas, USA

Surface composition information from Vesta is reported using fast neutron data collected by the gamma ray and neutron detector on the Dawn spacecraft. After correcting for variations due to hydrogen, fast neutrons show a compositional dynamic range and spatial variability that is consistent with variations in average atomic mass from howardite, eucrite, and diogenite (HED) meteorites. These data provide additional compositional evidence that Vesta is the parent body to HED meteorites. A subset of fast neutron data having lower statistical precision show spatial variations that are consistent with a 400 ppm variability in hydrogen concentrations across Vesta and supports the idea that Vesta’s hydrogen is due to long-term delivery of carbonaceous chondrite material.

Reference
Lawrence DJ, Peplowski PN, Prettyman TH, Feldman W, Bazell D, Mittlefehldt DW, Reedy RC and Yamashita N (in press) Constraints on Vesta’s elemental composition: Fast neutron measurements by Dawn’s gamma ray and neutron detector. Meteoritics & Planetary Science
[doi:10.1111/maps.12187]
Published by arrangement with John Wiley & Sons

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Joel A. Hurowitz^a and Woodward W. Fischer^b

^aDepartment of Geosciences, Stony Brook University, 100 Nicolls Road, Stony Brook, NY 11794-2100, joel.hurowitz@stonybrook.edu, 631-632-6801
^bDivision of Geological and Planetary Sciences, California Institute of Technology, 1200 East California Boulevard, Pasadena, CA 91125

The nature of ancient hydrological systems on Mars has been the subject of ongoing controversy, driven largely by a disconnect between observational evidence for flowing water on the Martian surface at multiple scales and the incompatibility of such observations with theoretical models that predict a cold early Martian environment in which liquid water is unstable. Here we present geochemical data from the Mars Exploration Rovers to evaluate the hydrological conditions under which weathering rinds, soils, and sedimentary rocks were formed. Our analysis indicates that the chemistry of rinds and soils document a water-limited hydrologic environment where small quantities of S-bearing fluids enter the system, interact with and chemically alter rock and soil, and precipitate secondary mineral phases at the site of alteration with little to no physical separation of primary and secondary mineral phases. In contrast, results show that the sedimentary rocks of the Burns Formation at Meridiani Planum have a chemical composition well-described as a mixture between siliciclastic sediment and sulfate-bearing salts derived from the evaporation of groundwater. We hypothesize that the former may be derived from the recently investigated Shoemaker Formation, a sequence of impact breccias that underlie the Burns Formation. This result has important implications for the style of chemical weathering and hydrology recorded by these sedimentary materials, revealing long-range transport of ions in solution in an open hydrological system that is consistent only with subsurface or overland flow of liquid water.

Reference
Hurowitz JA and Fischer WW (in press) Contrasting styles of water-rock interaction at the Mars Exploration Rover landing sites. Geochimica et Cosmochimica Acta
[doi:10.1016/j.gca.2013.11.021]
Copyright Elsevier

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A.A. Fraeman^a, S.L. Murchie^b, R.E. Arvidson^a, R.N. Clark^c, R.V. Morris^d, A.S. Rivkin^b and F. Vilas^e

^aWashington University in St. Louis, 1 Brookings Dr, Campus Box 1169, St. Louis, MO, 63130, United States
^bThe Johns Hopkins University Applied Physics Laboratory, 11101 Johns Hopkins Road, Laurel, MD, 20723, United States
^cUS Geological Survey, Box25046 Denver Federal Center, Denver, CO 80225, United States
^dARES, NASA Johnson Space Center, 2101 NASA Parkway, Houston, TX 77058, United States
^ePlanetary Science Institute, 1700 East Fort Lowell, Suite 106, Tucson, AZ, 85719, United States

Absorption features on Phobos and Deimos in the visible / near infrared wavelength region (0.4 – 3.9 μm) are mapped using observations from the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM). Fe²⁺ electronic absorptions diagnostic of olivine and pyroxene are not detected. A broad absorption centered around 0.65 μm within the red spectral units of both moons is detected, and this feature is also evident in telescopic, Pathfinder, and Phobos-2 observations of Phobos. A 2.8 μm metal-OH combination absorption on both moons is also detected in the CRISM data, and this absorption is shallower in the Phobos blue unit than in the Phobos red unit and Deimos. The strength, position, and shape of both of the 0.65 μm and 2.8 μm absorptions are similar to features seen on red-sloped, low-albedo primitive asteroids. Two end-member hypotheses are presented to explain the spectral features on Phobos and Deimos. The first invokes the presence of highly desiccated Fe-phyllosilicate minerals indigenous to the bodies, and the second invokes Rayleigh scattering and absorption of small iron particles formed by exogenic space weathering processing, coupled with implantation of H from solar wind. Both end-member hypotheses may play a role, and in-situ exploration will be needed to ultimately determine the underlying causes for the pair of spectral features observed on Phobos and Deimos.

Reference
Fraeman AA, Murchie SL, Arvidson RE, Clark RN, Morris RV, Rivkin AS and Vilas F (in press) Spectral Absorptions on Phobos and Deimos in the Visible/Near Infrared Wavelengths and Their Compositional Constraints. Icarus
[doi:10.1016/j.icarus.2013.11.021]
Copyright Elsevier

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M. Shyam Prasad^1,*, N. G. Rudraswami¹ and Dipak K. Panda²

¹National Institute of Oceanography, CSIR, Panaji, India
²Physical Research Laboratory, Ahmedabad, India

Flux of micrometeorites is estimated by using cosmic spherule counts from a seafloor area of 2.50 m² from the Indian Ocean. The spherules are recovered from sediment samples in close-spaced locations from the Indian Ocean after sieving 293 kg of sediment. The terrestrial age of the spherules has a range of 0–~50,000 years. The spherules have a size range of 57–750 µm (average size 265 ± 92 µm). The diameter of the spherules increases from scoriaceous-barred-cryptocrystalline-glassy types. The time-averaged flux of the spherules is 160 t/yr, a sizeable mass (>60%) resides in the >300 µm fraction; the slope of distribution is similar to that of Deep-Sea Spherules but significantly different from other collections which have lower average diameters. It is observed here, a significant population of cosmic dust resides in the larger sizes which can be recovered by sampling large areas in time and space. The spherule textures are similar to that of unbiased collections from the polar regions, indicating that the textural types of cosmic dust that have been raining on the Earth during the last 50 kyr have been constant regardless of size. Major element chemistry of a majority of the spherules show elemental ratios that are close to a CM or CI chondritic parent body; a single spherule (0.2% of the population) suggests an achondritic parent body. Unbiased collections spanning large areas temporally and spatially enlarge the inventory of the Earth-crossing meteoroid complex and provide valuable inputs for models on cosmic dust accretion.

Reference
Prasad MS Rudraswami NG and Panda DK (in press) Micrometeorite flux on Earth during the last ~50,000 years. Journal of Geophysical Research – Planets
[doi:10.1002/2013JE004460]
Published by arrangement with John Wiley & Sons

Link to Article