¹C. B. Kiddell, ¹E. A. Cloutis, ¹B. R. Dagdick, ¹J. M. Stromberg, ¹D. M. Applin, ¹J. P. Mann
Journal of Geophysical Research, Planets (in Press) Link to Article [https://doi.org/10.1029/2018JE005600]
¹Dept. of Geography, University of Winnipeg, Winnipeg, Manitoba, Canada
Published by arrangement with John Wiley & Sons

Carbonaceous chondrite meteorites (CCs) are among the most primitive materials in the solar system and provide important insights into solar system history and evolution. A number of planetary spacecraft missions will visit asteroids that are thought to compositionally resemble these meteorites. To better assist sample acquisition in terms of how the physical properties of CCs affect their reflectance spectra, we investigated the spectral reflectance properties of solid and powdered CCs, and powder coatings on slabs of a number of CCs, including CB, CH, CK, CM, CO, CR, and CV classes. We found that decreasing grain size leads to increasing reflectance across the ~500‐2500 nm range and steeper spectral slope, regardless of CC type. Powdered CC reflectance spectra are brighter beyond ~500 nm and redder than bare roughened slabs. For powders sprinkled on slabs, as the powder coating gets thicker, spectral slopes get redder.

Optically thick fine‐grained powders are brighter beyond ~500 nm and are as red or redder, than slabs covered with airfall dust (for dust thicknesses up to a few hundred microns). Diagnostic absorption features of CC minerals, particularly those in the 1000 nm region attributable to Fe‐bearing silicates, are ubiquitous regardless of physical properties. Reflectance spectra of terrestrially weathered (i.e., “rusty”) CCs are strongly modified below ~700 nm and in the 900 and 1900 nm regions by these Fe oxyhydroxides. Their effects can be mitigated through chemical treatment, but this may also affect pre‐terrestrial ferric iron‐bearing phases. Some spectral characteristics, such as hydrous and anhydrous silicate absorption bands in CC spectra, are present regardless of physical properties (fine‐grained dust, powders, slabs, dust on slabs). Other spectral characteristics (such as albedo and spectral slope) vary as a function of physical properties, indicates that reflectance spectroscopy could be used to ascribe spectral variations across an asteroid’s surface to either physical or compositional causes. This information can, in turn, be used to inform site selection for asteroid sample return missions, where both composition and physical properties are important drivers. When searching for fine‐grained areas on an asteroid to sample, the best indication would be the brightest and reddest‐sloped spectra.

¹Gene Schmidt, ²Frank Fueten, ³Robert Stesky, ⁴Jessica Flahaut, ⁵Ernst Hauber
Journal of Geophysical Research, Planets (in Press) Link to Article [https://doi.org/10.1029/2018JE005658]
¹IRSPS, Universita “G.D’Annunzio”, Pescara, Italy
²Department of Earth Sciences, Brock University, St. Catharines, Ontario, Canada
³Pangaea Scientific, Brockville, Ontario, Canada
⁴CNRS (institute)/CRPG Nancy (department), France
⁵Institute of Planetary Research, German Aerospace Center (DLR), Berlin, Germany
Published by arrangement with John Wiley & Sons

Hebes Chasma is an 8 km deep, 126 by 314 km, isolated basin that is partially filled with massive deposits of water‐altered strata called interior layered deposits (ILD). By analyzing the ILD’s structure, stratigraphy and mineralogy, a depositional history of Hebes Chasma is interpreted. Three distinct ILD units were found and are informally referred to as the Lower, Upper and Late ILD. These units are distinguished by their layer thicknesses, layer attitudes, mineralogies and erosional landforms. The Lower and Upper ILDs comprise the chasma’s 7.5 km tall, 120 by 43 km, central mound and the Late ILD is located in the valley between the central mound and the chasma’s northern wall. A horizontal unconformity separates the Lower and Upper ILDs and layer attitudes revealed large‐scale shallow folding within the Lower ILD. All ILDs are characterized by both monohydrated and polyhydrated sulfates (MHS and PHS) signatures. Erosional landforms such as hummocks, polygons, and debris flows suggest past glacial activity within the chasma. A scenario involving several ash fall events during various stages of chasma formation is proposed as the dominant setting throughout Hebes’ geologic history.

¹C.Xiang, ¹A.Carballido, ²R.D.Hanna, ¹L.S.Matthews, ¹T.W.Hyde
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2018.10.014]
¹Center for Astrophysics, Space Physics, and Engineering Research, Baylor University, Waco, TX 76798-7316, USA
²Jackson School of Geosciences, University of Texas, Austin, TX 78712, USA
Copyright Elsevier

In order to characterize the early growth of fine-grained dust rims (FGRs) that commonly surround chondrules in carbonaceous chondrites, we perform numerical simulations of dust accretion onto chondrule surfaces. We employ a Monte Carlo algorithm to simulate the collision of dust monomers having radii between 0.5 and 10 μm with chondrules whose radii are between 500 and 1000 μm, in 100-μm increments. The collisions are driven by Brownian motion and solar nebula turbulence. After each collision, the colliding particles either stick at the point of contact, roll or bounce. We limit accretion of dust monomers (and in some cases, dust aggregates) to a small patch of the chondrule surface, for computational expediency. We model the morphology of the dust rim and the trajectory of the dust particle, which are not considered in most of the previous works. Radial profiles of FGR porosity show that rims formed in weak turbulence are more porous (with a porosity of 60-74%) than rims formed in stronger turbulence (with a porosity of 52-60%). The lower end of each range corresponds to large chondrules and the upper end to small chondrules, meaning that the chondrule size also has an impact on FGR porosity. Consistent with laboratory observations of CM chondrites, the thickness of FGRs obtained in the simulations depends linearly on chondrule radius. The collection of single monomers leads to the increase of grain size from the inner to the outer layers of the dust rim. The porosity of FGRs formed by dust aggregates is  ∼ 20% greater on average than that of FGRs formed by single monomers. In general, the relatively high porosities that we obtain are consistent with those calculated by previous authors from numerical simulations, as well as with initial FGR porosities inferred from laboratory measurements of rimmed chondrule samples and rimmed chondrule analogs.