¹Andreas Morlok,²Alexander Sehlke,¹Aleksandra Stojic,³Alan Whittington,¹Iris Weber,¹Maximilian P. Reitze,¹Harald Hiesinger,⁴Joern Helbert
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2024.116078]
¹Institut für Planetologie, Wilhelmstr.10, 48149 Münster, Germany
²NASA Ames Research Center, Moffett Field, CA 94035, USA
³Department of Earth and Planetary Sciences, The University of Texas at San Antonio, USA
⁴Institute for Planetary Research, DLR, Rutherfordstrasse 2, 12489 Berlin, Germany
Copyright Elsevier

We studied a series of hermean lava analogs in the mid-infrared (2.5 μm–18 μm) to provide characteristic spectra for enstatite basalt, the Northern Volcanic Plains and Na-rich Northern Volcanic Plains. Our aim is to provide spectra for the interpretation of the data expected from Mercury from the MERTIS (MErcury Radiometer and Thermal Infrared Spectrometer) instrument on the ESA/JAXA BepiColombo mission.

Bulk powder spectra show bands of glass with a dominating broad Si-O-Si stretching feature around 10 μm. Crystalline components are mainly enstatite and forsterite with Reststrahlen Bands (RBs) around 9.3 μm, 9.6–9.9 μm, 10.0 μm, and 10.3–10.7 μm. Increasing intensity of crystalline features in the spectra reflect the increase in the crystallites in glass with decreasing temperature of equilibration and quenching. Micro-FTIR data allowed to extract spectral of individual components and glass. The position of the Christiansen Feature (CF) has only a weak correlation with the degree of crystallinity.

Correlations are observed between the Christiansen Feature (CF) and the bulk SiO₂ content of the materials, as does the correlation of this feature with the compositional index SCFM = SiO₂/(SiO₂ + CaO + FeO + MgO) on an atomic basis. This study also confirms the correlation line of glass-rich, irradiated Mercury analogs in these systems (Weber et al.,2023), indicating a similar spectral response of the glass rich materials expected for the surface of Mercury. The position of the strongest silicate main band (MB) compared to the SiO₂ content, confirms a trend for samples formed in experiments simulating high velocity impacts fall on a different trend line than analog samples formed in magmatic processes.

A comparison of the results to an Earth-based hermean surface spectrum showed similarities to spectra obtained for NVP samples.

¹A.R. Poppe,²I. Garrick-Bethell,³S. Fatemi,⁴C. Grava
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2024.116079]
¹Space Sciences Laboratory, University of California at Berkeley, Berkeley, CA 94720, USA
²Department of Earth and Planetary Sciences, Univ. of California Santa Cruz, Santa Cruz, CA, USA
³Department of Physics, Umeå University, Umeå, Sweden
⁴Southwest Research Institute, San Antonio, TX, USA
Copyright Elsevier

The ratio of 40Ar/36Ar trapped within lunar grains, commonly known as the lunar antiquity indicator, is an important semi-empirical method for dating the time at which lunar samples were exposed to the solar wind. The behavior of the antiquity indicator is governed by the relative implantation fluxes of solar wind-derived 36Ar ions and indigenously sourced lunar exospheric 40Ar ions. Previous explanations for the behavior of the antiquity indicator have assumed constancy in both the solar wind ion precipitation and exospheric ion recycling fluxes; however, the presence of a lunar paleomagnetosphere likely invalidates these assumptions. Furthermore, most astrophysical models of stellar evolution suggest that the solar wind flux should have been significantly higher in the past, which would also affect the behavior of the antiquity indicator. Here, we use numerical simulations to explore the behavior of solar wind 36Ar ions and lunar exospheric 40Ar ions in the presence of lunar paleomagnetic fields of varying strengths. We find that paleomagnetic fields suppress the solar wind 36Ar flux by up to an order-of-magnitude while slightly enhancing the recycling flux of lunar exospheric 40Ar ions. We also find that at an epoch of
2 Gya, the suppression of solar wind 36Ar access to the lunar surface by a lunar paleomagnetosphere is
somewhat fortuitously
nearly equally balanced by the expected increase in the upstream solar wind flux. These counterbalancing effects suggest that the lunar paleomagnetosphere played a critical role in preserving the correlation between the antiquity indicator and the radioactive decay profile of indigenous lunar 40K. Thus, a key implication of these findings is that the accuracy of the 40Ar/36Ar indicator for any lunar sample may be strongly influenced by the poorly constrained history of the lunar magnetic field.