¹U. Raut,¹P. L. Karnes,^1,2K. D. Retherford,¹M. W. Davis,³Y. Liu,^1,2G. R. Gladstone,¹E. L. Patrick,¹Thomas K. Greathouse,⁴A. R. Hendrix,¹P. Mokashi
Journal of Geophysical Research, Planets (In Press) Link to Article [https://doi.org/10.1029/2018JE005567]
¹Space Science and Engineering Division, Southwest Research Institute, San Antonio, TX, USA
²Department of Physics and Astronomy, The University of Texas at San Antonio, San Antonio, TX, USA
³Lunar and Planetary Institute, Houston, TX, USA
⁴Planetary Science Institute, Tucson, AZ, USA
Published by arrangement with John Wiley & Sons

We report new measurements of the far‐ultraviolet (FUV) bidirectional reflectance of Apollo soil 10084 from the Southwest Research Institute ultraviolet reflectance chamber. The bidirectional reflectance distribution function of this mare soil, enriched in Ti and Fe content, is rather featureless in the FUV wavelength region of 115–180 nm, except for a small blue slope, which is attributed to the effects of space weathering. This soil preferentially backscatters FUV photons as indicated by the angular distribution of the bidirectional reflectance. The phase curves are fitted with a simplified Hapke photometric model to derive the average volume single scattering albedo and scattering phase function of the mare lunar grains. The albedo values and the backscattering nature reported here are consistent with Lunar Reconnaissance Orbiter’s Lyman‐Alpha Mapping Project ultraviolet imaging spectrograph observations, despite expected morphological differences.

¹Jason C. Cook et al. (>10)
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2018.05.024]
¹Pinhead Institute, Telluride, CO, USA
Copyright Elsevier

On July 14, 2015, NASA’s New Horizons spacecraft encountered the Pluto-system. Using the near-infrared spectral imager, New Horizons obtained the first spectra of Nix, Hydra, and Kerberos and detected the 1.5 and 2.0 μm bands of H2O-ice on all three satellites. On Nix and Hydra, New Horizons also detected bands at 1.65 and 2.21 μm that indicate crystalline H2O-ice and an ammoniated species, respectively. A similar band linked to NH3-hydrate has been detected on Charon previously. However, we do not detect the 1.99 μm band of NH3-hydrate. We consider NH4Cl (ammonium chloride), NH4NO3 (ammonium nitrate) and (NH4)2CO3 (ammonium carbonate) as potential candidates, but lack sufficient laboratory measurements of these and other ammoniated species to make a definitive conclusion. We use the observations of Nix and Hydra to estimate the surface temperature and crystalline H2O-ice fraction. We find surface temperatures  < 20 K ( < 70 K with 1-σ error) and 23 K ( < 150 K with 1-σ error) for Nix and Hydra, respectively. We find crystalline H2O-ice fractions of 78−22+12% and  > 30% for Nix an Hydra, respectively. New Horizons observed Nix and Hydra twice, about 2-3 hours apart, or 5 and 25% of their respective rotation periods. We find no evidence for rotational differences in the disk-averaged spectra between the two observations of Nix or Hydra. We perform a pixel-by-pixel analysis of Nix’s disk-resolved spectra and find that the surface is consistent with a uniform crystalline H2O-ice fraction, and a  ∼ 50% variation in the normalized band area of the 2.21 μm band with a minimum associated with the red blotch seen in color images of Nix. Finally, we find evidence for bands on Nix and Hydra at 2.42 and possibly 2.45 μm, which we cannot identify, and, if real, do not appear to be associated with the ammoniated species. We do not detect other ices, such as CO2, CH3OH and HCN.

¹G.L.Christenson et al. (>10)
Earth and Planetary Science Letters 495, 1-11 Link to Article [https://doi.org/10.1016/j.epsl.2018.05.013]
¹University of Texas Institute for Geophysics, Jackson School of Geosciences, Austin, USA
Copyright Elsevier

Joint International Ocean Discovery Program and International Continental Scientific Drilling Program Expedition 364 drilled into the peak ring of the Chicxulub impact crater. We present P-wave velocity, density, and porosity measurements from Hole M0077A that reveal unusual physical properties of the peak-ring rocks. Across the boundary between post-impact sedimentary rock and suevite (impact melt-bearing breccia) we measure a sharp decrease in velocity and density, and an increase in porosity. Velocity, density, and porosity values for the suevite are 2900–3700 m/s, 2.06–2.37 g/cm3, and 20–35%, respectively. The thin (25 m) impact melt rock unit below the suevite has velocity measurements of 3650–4350 m/s, density measurements of 2.26–2.37 g/cm3, and porosity measurements of 19–22%. We associate the low velocity, low density, and high porosity of suevite and impact melt rock with rapid emplacement, hydrothermal alteration products, and observations of pore space, vugs, and vesicles. The uplifted granitic peak ring materials have values of 4000–4200 m/s, 2.39–2.44 g/cm3, and 8–13% for velocity, density, and porosity, respectively; these values differ significantly from typical unaltered granite which has higher velocity and density, and lower porosity. The majority of Hole M0077A peak-ring velocity, density, and porosity measurements indicate considerable rock damage, and are consistent with numerical model predictions for peak-ring formation where the lithologies present within the peak ring represent some of the most shocked and damaged rocks in an impact basin. We integrate our results with previous seismic datasets to map the suevite near the borehole. We map suevite below the Paleogene sedimentary rock in the annular trough, on the peak ring, and in the central basin, implying that, post impact, suevite covered the entire floor of the impact basin. Suevite thickness is 100–165 m on the top of the peak ring but 200 m in the central basin, suggesting that suevite flowed downslope from the collapsing central uplift during and after peak-ring formation, accumulating preferentially within the central basin.

¹Anne Pommier
Earth and Planetary Science Letters 496, 37-46. Link to Article [https://doi.org/10.1016/j.epsl.2018.05.032]
¹UC San Diego, Scripps Institution of Oceanography, Institute of Geophysics and Planetary Physics, La Jolla, CA, USA
Copyright Elsevier

Electrical experiments were performed on core analogues in the Fe–S system and on FeSi2 up to 8 GPa and 1850 °C in the multi-anvil apparatus. Electrical resistivity was measured using the four-electrode method. For all samples, resistivity increases with increasing temperature. The higher the S content, the higher the resistivity and the resistivity increase upon melting. At 4.5 GPa, liquid FeS is up to >10 times more resistive than Fe-5 wt.% S and twice more resistive than FeSi2, suggesting a stronger influence of S than Si on liquid resistivity. Electrical results are used to develop crystallization-resistivity paths considering both equilibrium and fractional crystallization in the Fe–S system. At 4.5 GPa, equilibrium crystallization, as expected locally in thin snow zones during top-down core crystallization, presents electrical resistivity variations from about 300 to 190 microhm-cm for a core analogue made of Fe-5 wt.%S, depending on temperature. Fractional crystallization, which is relevant to core-scale cooling, leads to more important electrical resistivity variations, depending on S distribution across the core, temperature, and pressure. Estimates of the lower bound of thermal resistivity are calculated using the Wiedemann–Franz law. Comparison with previous works indicates that the thermal conductivity of a metallic core in small terrestrial bodies is more sensitive to the abundance of alloying agents than that of the Earth’s core. Application to Ganymede using core adiabat estimates from previous studies suggests important thermal resistivity variations with depth during cooling, with a lower bound value at the top of the core that can be as low as 3 W/m K. It is speculated that the generation and sustainability of a magnetic field in small terrestrial bodies might be favored in light element-depleted cores.