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The Short Wavelength End of the Space Weathering of Planetary Surfaces

Monday, June 28, 2010
Vilas (MMT Observatory)

Reflectance spectroscopy is a major remote sensing probe of a planetary body's surface mineralogy by Earth-based and spacecraft telescopes. Reflectance spectra of planetary surfaces are generated by the interaction of diffuse sunlight with the top <1 mm of surface material. Despite this small depth, differences in mineralogy of atmosphereless planetary bodies are observed. In the inner Solar System, "space weathering" affects spectra of Solar System bodies by darkening and reddening their surface materials, as well as degrading absorption features, at VNIR wavelengths. At UV/VIS wavelengths, a bluing of the spectral reflectance and, in some cases, a spectral reversal is observed. These effects are well documented for the Moon, where they are apparent in spectra of natural lunar soils, but not seen in spectra of powdered lunar rock samples. The cause of this weathering is likely grain coatings caused by vapor deposition of submicroscopic iron, through solar wind irradiation, micrometeorite or heavy ion bombardment of the bodies's surfaces. Space weathering has been proposed as the source of spectral differences between ordinary chondrite meteorites and their proposed parent bodies, S-class asteroids. The manifestations of space weathering on remotely-sensed data obtained of solid planetary surfaces will affect the analysis of all ground-based and space-based reflectance spectra.

Our previous work has shown that the effect in the UV/blue spectral region for S-type asteroids is consistent with the addition of iron or iron-bearing minerals. Opaque materials (such as elemental iron or ilmenite) are dominated by surface scattering, controlled by Fresnel reflection, and are therefore spectrally flat over a wide range of wavelengths. Thus, compared to mafic silicate minerals (such as pyroxenes and feldspars, or "non-opaques"), opaque, iron-bearing minerals can be relatively bright at FUV-NUV wavelengths. In the 150-450 nm range, iron-bearing minerals also vary from non-opaques in spectral shape, where the non-opaques experience a decrease in brightness as they transition from reflectance dominated by volume scattering to reflectance dominated by surface scattering, and opaques tend to be spectrally flat. Therefore, in the 150-450 nm range, we expect surfaces consisting of iron-bearing opaques to be less spectrally red and potentially brighter than surfaces with lower amounts of iron-bearing minerals. We have observed the short-wavelength bluing in S-type asteroid spectra when the reddening at longer wavelengths is not apparent, and suggest that the UV/blue spectral region is a more sensitive indicator of the onset of space weathering.

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