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

We present combined multi-spectral imager(MSI) (0.95 mum) and near-infrared spectrometer (NIS) (0.8-2.4 mum) observations of Psyche crater on S-type asteroid 433 Eros obtained by the Near-Earth 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 of 620 m/spectrum, Psyche crater wall materials exhibit albedo contrasts of 32-40% at 0.946 mum. 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 0.946 mum consistent with meteorites, but spectral continua that are much redder than meteorites from 1.5 to 2.4 mum. Most importantly, we calculate that average Eros surface materials are 30-40% darker than meteorites.