Space weathering of silicates simulated by successive laser irradiation: In situ reflectance measurements of Fo₉₀, Fo₉₉₊, and SiO₂

Abstract

Pulsed-laser irradiation causes the visible-near-infrared spectral slope of olivine (Fo 90 and Fo 99+ ) and SiO 2 to increase (redden), while the olivine samples darken and the SiO 2 samples brighten slightly. XPS analysis shows that irradiation of Fo 90 produces metallic Fe. Analytical SEM and TEM measurements confirm that reddening in the Fo 90 olivine samples correlates with the production of "nanophase" metallic Fe (npFe 0 ) grains, 20-50 nm in size. The reddening observed in the SiO 2 sample is consistent with the formation of SiO or other SiO x species that absorb in the visible. The weak spectral brightening induced by laser irradiation of SiO 2 is consistent with a change in surface topography of the sample. The darkening observed in the olivine samples is likely caused by the formation of larger npFe 0 particles, such as the 100-400 nm diameter npFe 0 identified during our TEM analysis of Fo 90 samples. The Fo 90 reflectance spectra are qualitatively similar to those in previous experiments suggesting that in all cases formation of npFe 0 is causing the spectral alteration. Finally, we find that the accumulation of successive laser pulses cause continued sample darkening in the Vis-NIR, which suggests that repeated surface impacts are an efficient way to darken airless body surfaces.

Impact Characteristics of Different Rocks in a Pulsed Laser Irradiation Experiment: Simulation of Micrometeorite Bombardment on the Moon

“…On the contrary, the nanophase metal particles are distributed nonuniformly on the surface of the olivine. They are concentrated at the edges of ripple‐shaped ridges (Figures e and f) which is consistent with Loeffler et al () studies. For the Gebel Kamil iron meteorite, the characteristics of melt and vapor deposits are similar to those of kamacite in Huaxi ordinary chondrite (Figures g and h).…”

“…Those of silicate minerals which are dozens of microns away from kamacite usually have a four‐layer structure (Figures b and c), in which many npFe, identified by the interplanar spacing, are observed in the outer three layers (Figures d and e). However, silicate minerals near kamacite show a two‐layer structure in which a vapor‐deposited layer covers the substrate (Figure f) which is consistent with Loeffler et al () studies. In this layer, npFe of several nanometers to hundreds of nanometers are also identified by their interplanar spacing (Figures g and h).…”

“…In our work, the obvious difference with previous studies such as Christoffersen et al (2016) is that the amorphous layer is divided into partially amorphous layer and completely amorphous layer which is similar to Loeffler et al (2016) studies. And the vapor-deposited layer only can be easily observed in some places, such as the olivine and pyroxene near kamacite, and this layer, when present, is enriched in npFe or Fe-Ni (Figure 8a), which is similar to Noble et al (2011Noble et al ( , 2016) studies (Figure 8b).…”

Coordinated Nanoscale Compositional and Oxidation State Measurements of Lunar Space‐Weathered Material

“…Thus, while experiments provide data on individual aspects of space weathering, they cannot replicate the complexity of natural materials and offer only qualitative points of comparison. Distinct sublayers were formed on an olivine sample altered in a 99‐pulse experiment (Loeffler et al, ) that resemble the layers in sample 71501‐ol in having both nominally npFe 0 ‐free layers somewhat depleted in Fe and other sublayers rich in 5–20 nm npFe 0 . Depletion of Fe is consistent with vapor phase fractionation of olivine (Fazio et al, ).…”

Astrochemistry in the Age of Broadband Radio Astronomy

“…In fact, multiple laboratories have recently or will very soon become operational in order to test ice surface chemistry under varying radiative conditions (Ioppolo et al 2013;Allodi et al 2013;Paardekooper et al 2014), chemistry on bare carbonaceous or silicate surfaces (e.g. Loeffler et al 2016, and references therein), and radical-driven gas phase kinetics (C. Endres 2015, private communication). As the measurements from these experiments become available, they can be included in models.…”