Distinguishing between Shock-darkening and Space-weathering Trends in Ordinary Chondrite Reflectance Spectra

Abstract

Space-weathering as well as shock effects can darken meteorite and asteroid reflectance spectra. We present a detailed comparative study on shock-darkening and space-weathering using different lithologies of the Chelyabinsk LL5 chondrite. Compared to space-weathering, the shock processes do not cause significant spectral slope changes and are more efficient in attenuating the orthopyroxene 2 μm absorption band. This results in a distinct shock vector in the reflectance spectra principal component analysis, moving the shocked silicate-rich Chelyabinsk spectra from the S-complex space into the C/X complex. In contrast to this, the space-weathering vector stays within the S complex, moving from Q type to S type. Moreover, the 2 μm to 1 μm band depth ratio (BDR) as well as the 2 μm to 1 μm band area ratio (BAR) are not appreciably affected by shock-darkening or shock melting. Space-weathering, however, causes significant shifts in both BDR and BAR toward higher values. Application of the BDR method to the three distinct areas on the asteroid Itokawa reveals that Itokawa is rather uniformly space-weathered and not influenced by regolith roughness or relative albedo changes.

Investigating S-type asteroid surfaces through reflectance spectra of ordinary chondrites

“…The shock pressures estimated to be needed for this process corresponds to shock stages between S5 and S6 (Kohout et al, 2020). Some small melt pockets and shock veins were visible in the thin sections of samples BTN 00302 and LAR 06279.…”

Physical Characterization of Near-Earth Asteroid (52768) 1998 OR2: Evidence of Shock Darkening/Impact Melt

“…Shock-darkening also reduces the albedo and suppresses mineralogical band features but typically does not redden the spectrum (Kohout et al 2014). Shock-darkening also changes physical aspects of meteorites and asteroid regolith, such as porosity and magnetic susceptibility, which cannot be detected through current remote sensing techniques (Kohout et al 2014(Kohout et al , 2020a(Kohout et al , 2020bReddy et al 2014).…”

“…The second stage occurs at pressures between 90 and 150 GPa, where bulk melting takes place and troilite and metal are finely dispersed within the silicate material. The latter is sometimes referred to as impact melt to differentiate the two phases of shockdarkening (Kohout et al 2020a(Kohout et al , 2020b. The two phases cannot effectively be distinguished with remote sensing, and we primarily use the term "shock-darkening" to refer to both types of impact heating-related darkening of an asteroid surface.…”

Connecting Asteroids and Meteorites with visible and near-infrared spectroscopy

“…These were measurements of slabs and many were shocked, labeled as dark or black ordinary chondrites. While these spectral matches are interesting, they likely only represent a minority of C-or X-complex asteroids (Britt and Pieters 1991;Reddy et al 2014;Kohout et al 2014Kohout et al , 2020.…”

“…3.6). Shock darkening is a process known to decrease the strength of silicate absorption bands and lower the albedo of the material (Britt and Pieters 1991;Reddy et al 2014;Kohout et al 2014Kohout et al , 2020, causing the spectrum to look more C-type-like than S-typelike. Black (shock darkened) chondrites account for nearly 14 percent of ordinary chondrite falls (Britt and Pieters 1991).…”