¹Andreas Morlok,^2,3Christopher Hamann,⁴Dayl Martin,¹Iris Weber,⁴Katherine H.Joy,¹Harald Hiesinger,⁴Roy Wogelius,¹Aleksandra Stojic,⁵Joern Helbert
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2019.113410]
¹Institut für Planetologie, Wilhelm-Klemm-Str. 10, 48149 Münster, Germany
²Museum für Naturkunde, 10115 Berlin, Germany
³Bundesanstalt für Materialforschung und –prüfung, 12489 Berlin, Germany
⁴School of Earth and Environmental Sciences, University of Manchester, Oxford Road, Manchester M13 9PL, UK
⁵Institute for Planetary Research, DLR, Rutherfordstrasse 2, 12489 Berlin, Germany
Copyright Elsevier

We obtained mid-infrared spectra and major-element analyses of glasses produced in pulsed laser experiments of basalt. Materials from pits excavated in a basalt slab, as well as of a larger, separated melt droplet were studied. The results of this study show that these glasses exhibits spectral features clearly distinguishable from the unprocessed starting material. Spectra and chemistry show changes, which could be the result of not only melting but also vaporization.

Christiansen Features (CF) for the melt glass in the laser-excavated pits are at 8.3–8.5 μm, and a dominating Reststrahlen Band (RB) at 10.1–10.5 μm in wavelength. The spectra of the powdered glass droplet has a CF at 8.8–8.9 μm and a RB at 10.3–10.5 μm. The spectra are clearly different from the spectra of the surrounding starting material, which shows CF between 8.0 and 8.3 μm, and ample RBs between 9.3 μm and 14.7 μm, typical olivine, plagioclase and pyroxene features.

The results reflect the chemical composition, which shows significant losses of volatiles like K2O and Na₂O, as well as of moderate volatiles like FeO, SiO₂, and MgO. Refractories TiO₂, Al₂O₃, and CaO tend to be enriched compared to the bulk starting composition. This indicates loss of material through evaporation.

While the spectra of size fractions of the powdered bulk melt glass droplet follow this trend in general, but, because of contamination by the experimental set-up, CaO was found to be strongly enriched in contrast to the other refractories TiO₂ and Al₂O₃.

At least the composition of the glasses in the laser-excavated pits could serve as an ‘endmember’ for the sequence of glassy materials expected to be produced in high energy impact processes involving a basaltic target.

Correlation of CF with SiO₂ contents and the SCFM (SiO₂/(SiO₂ + CaO + FeO + MgO)) index show similar behaviour of the pit melts like found in earlier studies. However, when the position of the RB in the pit glass is correlated with the SiO₂ content, the result shows a different trend compared with earlier studies. Consequently, the data presented in this study could help distinguishing between surface regions formed by volcanic processes and such modified by high-velocity impacts, where evaporation could play a central role.

This is of high interest for remote sensing studies of Mercury, which, because of its proximity to the Sun, was probably affected by high-velocity impacts to a very high degree.

¹Mikhail Yu.Zolotov
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2019.113404]
¹School of Earth and Space Exploration, Arizona State University, Tempe, AZ 85287-1404, USA
Copyright Icarus

Results of Ceres’ exploration with the Dawn spacecraft are modeled and discussed in terms of rock/organic/elemental composition, density and porosity in the interior, and formation, migration and geological evolution of the body. Carbon-rich surface composition is used to assess phase and elemental composition of the interior. The consistent bulk density and surface composition suggest an abundant organic matter within the body. Ceres is modeled as a chemically uniform mixture of CI-type carbonaceous chondritic rocks and 12–29 vol% of macromolecular organic matter. Water ice, gas hydrates or high porosity (>10%) are not required to explain bulk density. Ceres may not have a partially differentiated interior structure because gravity and shape could be explained by compaction of chemically uniform materials. Gravity data suggest a two-layer structure with an abrupt density change. Gravity may not reflect the current global density distribution in the interior because the implied bulk porosity >9% and grain density > 2380 kg m⁻³ disagree with organic-rich compositions. In contrast, Ceres’ polar flattening indicates mild density gradients that could be explained by two-layer and gradual compaction models. The flattening implies grain density of 2200–2350 kg m⁻³ that is consistent with the organic-rich interior. Viscosity of warmed rock-organic mixtures at depth could account for the observed relaxation of long wavelength topography. The organic-rich composition together with abundant surface carbonates, NH₄-bearing phases suggests Ceres’ formation at larger heliocentric distances and later than CI chondrites. Ceres-forming materials could have been more water-rich than parent bodies of CI chondrites and excessive water could have been lost from the body. A majority of Ceres’ surface compounds could have formed through water-rock-organic reactions in a middle interior followed by collisional stripping of an upper interior.

^1,2,3Meng‐Hua Zhu,²Kai Wünnemann,⁴Ross W.K. Potter,⁵Thorsten Kleine,⁶Alessandro Morbidelli
Journal of Geophysical Research, Planets (in Press) Link to Article [https://doi.org/10.1029/2018JE005826]
¹Space Science Institute, Macau University of Science and Technology, Taipa, Macau
²Museum für Naturkunde, Leibniz Institute for Evolution and Biodiversity Science, Berlin, Germany
³CAS Center for Excellence in Comparative Planetology, China
⁴Department of Earth, Environmental and Planetary Sciences, Brown University, Providence, RI, USA
⁵Institut für Planetologie, University of Münster, Münster, Germany
⁶Département Lagrange, University of Nice–Sophia Antipolis, Nice, France
Published by arrangement with John Wiley & Sons

The Moon exhibits striking geological asymmetries in elevation, crustal thickness, and composition between its nearside and farside. Although several scenarios have been proposed to explain these asymmetries, their origin remains debated. Recent remote sensing observations suggest that (1) the crust on the farside highlands consists of two layers: a primary anorthositic layer with thickness of ~30‐50 km and on top a more mafic‐rich layer ~ 10 km thick; and (2) the nearside exhibits a large area of low‐Ca pyroxene that has been interpreted to have an impact origin. These observations support the idea that the lunar nearside‐farside asymmetries may be the result of a giant impact. Here, using quantitative numerical modeling, we test the hypothesis that a giant impact on the early Moon can explain the striking differences in elevation, crustal thickness, and composition between the nearside and farside of the Moon. We find that a large impactor, impacting the current nearside with a low velocity, can form a mega‐basin and reproduce the characteristics of the crustal asymmetry and structures comparable to those observed on the current Moon, including the nearside lowlands and the farside’s mafic‐rich layer on top of a primordial anorthositic crust. Our model shows that the excavated deep‐seated KREEP (potassium, rare‐earth elements, and phosphorus) material, deposited close to the basin rim, slumps back into the basin and covers the entire basin floor; subsequent large impacts can transport the shallow KREEP material to the surface, resulting in its observed distribution. In addition, our model suggests that prior to the asymmetry‐forming impact, the Moon may have had an 182W anomaly compared to the immediate post‐giant impact Earth’s mantle, as predicted if the Moon was created through a giant collision with the proto‐Earth.