¹Iris Weber,¹Aleksandra N.Stojic,¹Andreas Morlok,¹Maximilian P.Reitze,^1,5Kathrin Markus,¹ Harald Hiesinger,²Sergey G.Pavlov,³Richard Wirth,³ Anja Schreiber,⁴Martin Sohn,²Heinz-Wilhelm Hübers,⁵Jörn Helbert
Earth and Planetary Science Letters (in Press) Link to Article [https://doi.org/10.1016/j.epsl.2019.115884]
¹Westfälische Wilhelms Universität Münster, Institut für Planetologie, Wilhelm – Klemm Str. 10, 48149 Münster, Germany
²German Aerospace Center (DLR), Institute of Optical Sensor Systems, Rutherfordstr. 2, 12489 Berlin, Germany
³Helmholtz-Zentrum Potsdam, Deutsches Geoforschungszentrum (GFZ), Telegrafenberg, 14473 Potsdam, Germany
⁴Hochschule Emden/Leer, Constantiaplatz 4, 26723 Emden, Germany
⁵German Aerospace Center (DLR), Institute of Planetary Research, Rutherfordstr. 2, 12489 Berlin, Germany
Copyright Elsevier

We studied space-weathering effects caused by micrometeorite bombardment simulated by pulsed intense infrared laser, generating ∼15 mJ per pulse in high vacuum. For our investigation, we selected a natural olivine (San Carlos olivine (Fo₉₁)) and a natural pyroxene (Bamble orthopyroxene (En₈₇)) as important rock forming minerals of the Earth upper mantle as well as key planetary minerals. Irradiated areas of powdered pressed samples were examined by optical reflection spectroscopy in a broad optical and infrared wavelength range (visible-, near-, and mid-infrared) and transmission electron microscopy to identify changes due to micrometeorite impacts. The present study aims to investigate especially the effects of micrometeorite bombardment on reflectance spectra in the mid-IR in preparation for future space missions, as well as for the MERTIS experiment onboard the BepiColombo mission.

For both irradiated samples, we found a reduction in albedo and in the reflectance of characteristic Reststrahlen bands and an increase of the transparency feature. VIS and NIR spectra of both minerals show the typical darkening and reddening as described for other space-weathered samples. TEM studies revealed that space-weathered layers in olivine and pyroxene differ in their respective thickness, ∼450 nm in olivine, 100-250 nm in pyroxene, as well as in developed “nanostratigraphy” of laser-ablated material, like nanophase iron (npFe).

In conclusion, our spectral and structural findings were compared to samples in which space weathering was caused by different processes. A comparison with these data demonstrates that there is no difference in optical reflectance spectroscopy, but a significant difference in the microstructure of minerals due to the weathering source in space, as there are solar wind and solar flares cause other structural and chemical changes as the bombardment with micrometeorites.

¹Jiachao Liu,¹Jie Li
Earth and Planetary Science Letters (in Press) Link to Article [https://doi.org/10.1016/j.epsl.2019.115834]
¹Department of Earth and Environmental Sciences, University of Michigan, Ann Arbor, MI, 48109, USA
Copyright Elsevier

Paleomagnetic records revealed that the early Moon had an Earth-like core dynamo, which was likely driven by thermochemical buoyancy force associated with core solidification. The cause for the cessation of the ancient lunar dynamo at about 3.56 Ga ago remains controversial, partly because the composition and temperature of the Moon are not well constrained and the solidification process of its core remains poorly understood. Here we report experimental data at 5.1 GPa showing that the liquidus temperatures of the Fe–Ni–S system are ∼50–150 K lower than that of the Fe–S system, implying that a Ni-bearing core could remain molten to lower temperatures. Calculating the liquidus temperature gradient using previous data at 3 GPa and the new results at 5.1 GPa, we find that an Fe-S core containing less than ∼4.0 wt.% S would freeze from the center of the Moon. At higher S contents, the core would precipitate solid Fe near the core-mantle boundary. Based on the prevailing lunar core models, a change in core solidification from the bottom-up regime to the top-down regime during the lunar history is possible only if its bulk S content falls between about 2.0 and 4.0 wt.%.