Space Weathering of FeS Induced via Pulsed Laser Irradiation

Abstract

Here we present results from pulsed laser irradiation of troilite samples in an effort to simulate space weathering on airless bodies via micrometeorite impacts. We find that the spectral trends observed in directly irradiated samples and samples with a vapor-deposited coating are different than those found in silicate minerals previously studied. For instance, direct laser irradiation causes our troilite samples to initially brighten, but continued irradiation causes darkening and a decrease in spectral slope. In contrast, our samples with a vapor-deposited coating show a continuous increase in spectral slope and overall albedo as the deposit thickness increases. Observation using both digital imaging and electron microscopy of our directly irradiated samples leads us to conclude that topography effects likely become important after a relatively high number of laser pulses in our directly irradiated samples, causing the apparent darkening and decrease in spectral slope. Thus, we conclude that the spectral changes observed relevant to space weathering via micrometeorite impacts are an increase in spectral slope and an increase in the albedo of troilite. Future studies will investigate whether these trends are generally representative of other sulfide-bearing minerals and of weathering trends in other components found in the asteroid regolith.Plain Language Summary Carbonaceous asteroids are the most common asteroid found in our solar system. While it is generally accepted that the surfaces of these objects are altered over time by impacts via energetic particles, it is not currently understood how this alteration will change the spectral properties of the surface. In this study, we investigated how the spectral reflectance of troilite (FeS), one component that will likely be found on the surface of these types of asteroids, was altered by pulsed laser irradiation, which is used to mimic micrometeorite impacts that are regularly occurring on asteroid surfaces. The main finding in our study that is different from what has been found in previous studies with other relevant minerals, such as silicates, is that the overall reflectance spectrum brightens as the sample is altered. These results, combined with previous ones, will help astronomers interpret reflectance spectra taken of carbonaceous asteroids and possibly other extraterrestrial objects containing troilite.

Weak spectral features on (101995) Bennu from the OSIRIS-REx Visible and InfraRed Spectrometer

“…Troilite (FeS) and certain phases of graphitized carbon can show an absorption that is often centered at shorter wavelengths, as seen in panel B of Fig. 3 (Cloutis et al 1994(Cloutis et al , 2011aApplin et al 2018;Prince et al 2020).…”

Space Weathering Effects in Troilite by Simulated Solar‐Wind Hydrogen and Helium Ion Irradiation

“…Changes in surface micro‐roughness and chemistry may both play a role in altering the optical reflectance, slope, and albedo of troilite and other minerals present on the surface of Psyche (Yon & Pieters, 1987 ). The size and relative abundance of troilite grains in the regolith as well as gardening rates and other concurrent sulfur‐depleting processes (e.g., Prince et al., 2020 ), will influence the detectability of weathering‐induced sulfur depletion by both instruments, while not being directly measured.…”

“…Other minerals have received less attention. Only a small number of studies have investigated space‐weathering effects on iron meteorites or their major mineral components (Bezaeva et al., 2015 ; Harries & Langenhorst, 2014 ; Loeffler et al., 2008 ; Matsumoto et al., 2020 , 2021 ; Prince et al., 2020 ; Wu et al., 2017 ). While there have been extensive engineering studies on the effect of the space environment on structural metals for spaceflight applications, these synthetic materials are poor analogs for iron meteorites due to their orders‐of‐magnitude smaller crystal sizes and differing microstructures, both resulting from the inability to synthetically crystallize materials over geologic timescales (Petrovic, 2011 ).…”

Bulk synthesis of stoichiometric/meteoritic troilite (FeS) by high‐temperature pyrite decomposition and pyrrhotite melting

“…4f). As for most recent studies, scientists either used troilite out from iron meteorites (Dibb et al., 2021) to have the most spectrally genuine elements or they purchased synthesized FeS powder (Prince et al., 2020). In the latter case, the FeS powder used in the study does not show the typical brownish‐gold appearance that is usually observed in genuine troilites (see this work Figs.…”