Space weathering on Eros: Constraints from albedo and spectral measurements of Psyche crater

Abstract

Abstract-We present combined multi-spectral imager (MSI) (0.95 pm) and near-infrared spectrometer (NIS) (0.8-2.4pm) observations of Psyche crater on S-type asteroid 433 Eros obtained by the NearEarth Asteroid Rendezvous (NEAR)-Shoemaker spacecraft. At 5.3 km in diameter, Psyche is one of the largest craters on Eros which exhibit distinctive brightness patterns consistent with downslope motion of dark regolith material overlying a substrate of brighter material. At spatial scales of620 m/ spectrum, Psyche crater wall materials exhibit albedo contrasts of 3 2 4 0 % at 0.946 pm. Associated spectral variations occur at a much lower level of 4-8% (+2-4%). We report results of scattering model and lunar analogy investigations into several possible causes for these albedo and spectral trends: grain size differences, olivine, pyroxene, and troilite variations, and optical surface maturation. We find that the albedo contrasts in Psyche crater are not consistent with a cause due solely to variations in grain size, olivine, pyroxene or lunar-like optical maturation. A grain size change sufficient to explain the observed albedo contrasts would result in strong color variations that are not observed. Olivine and pyroxene variations would produce strong band-correlated variations that are not observed. A simple lunar-like optical maturation effect would produce strong reddening that is not observed. The contrasts and associated spectral variation trends are most consistent with a combination of enhanced troilite (a dark spectrally neutral component simulating optical effects of shock) and lunar-like optical maturation. These results suggest that space weathering processes may affect the spectral properties of Eros materials, causing surface exposures to differ optically from subsurface bedrock. However, there are significant spectral differences between Eros' proposed analog meteorites (ordinary chondrites and/or primitive achondrites), and Eros' freshest exposures of subsurface bright materials. After accounting for all differences in the measurement units of our reflectance comparisons, we have found that the bright materials on Eros have reflectance values at 0.946pm consistent with meteorites, but spectral continua that are much redder than meteorites from 1.5 to 2.4 pm. Most importantly, we calculate that average Eros surface materials are 3040% darker than meteorites.

Spectral variability on primitive asteroids of the Themis and Beagle families: Space weathering effects or parent body heterogeneity?

“…The so-called ordinary chondrite paradox, that is lack of asteroids similar to the ordinary chondrites, which represent 80% of meteorite falls, has been solved. These meteorites are now clearly related to S-type asteroids, as proved by direct measurements of the NEAR and HAYABUSA missions on the near Earth asteroids Eros and Itokawa (Clark et al 2001;Noguchi et al 2011) and laboratory experiments on irradiation of silicates/ordinary chondrites (Moroz et al 1996;Sasaki et al 2002;Brunetto & Strazzulla 2005;Noble et al 2007). Spectral differences between S-type asteroids and ordinary chondrites are caused by space weathering effects, which produce a darkening in the albedo, a reddening of the spectra, and diminish the silicates absorption bands on the asteroids surfaces, exposed to cosmic radiation and solar wind.…”

Asteroid (21) Lutetia: Disk-resolved photometric analysis of Baetica region

“…Sarnus has a more intriguing behavior, since it shares characteristics of Gallicum and Danuvius Labes, it is spectrally closer to the latter, but it has similar normal albedo to Gallicum Labes. If compared to Psyche Crater in (433) Eros (Clark et al, 2001), the correlation is opposite, the variegation is larger (32-40%), but spectrally, the dark material at the bottom of the crater and the bright material on the walls, show no more than 4-8% of difference. Thus, we have confirmed that the variegation and spectral differences between crater walls are significant, as hinted by the small phase angle images of (21) Lutetia.…”

Evidence for the effects of space weathering spectral signatures on low albedo asteroids

“…In dashed black lines are the +σ and -σ from the mean. We chose this type of ratio plot to be consistent with other studies showing the spectral effects of surface alteration on higher albedo asteroids (Clark et al 2001). Asteroid data were smoothed before ratioing to avoid high frequency variations and focus only on low frequency behavior.…”

“…The effects of space weathering on S-type asteroids have been widely analyzed (Moroz et al 1996;Clark et al 2001;Brunetto et al 2005;Ishiguro et al 2007). Because S-types have high albedos and strong absorption band features, space weathering effects are easier to detect.…”