Physics
Scientific paper
Dec 2002
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2002agufm.p61a0339n&link_type=abstract
American Geophysical Union, Fall Meeting 2002, abstract #P61A-0339
Physics
6061 Remote Sensing
Scientific paper
The current paradigm for the optical alteration of soils due to space weathering processes involves the deposition of surface correlated nanophase iron (npFe0), deposited both as a vapor from micrometeorite impacts and via solar wind sputtering. These alterations due to npFe0 will effect all surfaces that contain iron-bearing minerals and are exposed to the harsh environment of space, i.e. the Moon, Mercury, asteroids. Space weathered materials containing npFe0 have distinct optical properties. In addition to reducing the strength of spectral bands, space weathering adds a "characteristic continuum" to spectra that is generally darker and redder than the original material. The exact shape of this continuum, however, is dependent on both the amount and the size of the npFe0 present. Miniscule amounts of npFe0 (< 0.1wt%) cause the spectra to become sharply curved in the visible, while leaving the near-IR relatively unaffected. As npFe0 accumulates (0.2-0.4 wt%), the continuum becomes less curved and significantly redder through the entire Vis-NIR range, eventually becoming nearly linear. These effects have been observed with natural lunar soils (Noble et al, MAPS 2001). With additional npFe0 the spectra darken further, progressively losing redness until finally, with >1.0 wt% npFe0, the spectrum becomes dark and featureless. The size of the npFe0 is also important to the spectral properties. Small npFe0 grains cause reddening, while larger grains only result in darkening. To better constrain the size where this reddening to darkening transition occurs, we have created synthetic analogs using silica gel impregnated with various amounts of npFe0. Because these gels contain defined pore sizes, we can somewhat control the size of npFe0 created, allowing comparison of samples with different npFe0 sizes. Initial results suggest that the transition from reddening to darkening occurs between 15 and 25 nm.
Noble Sarah K.
Pieters Carlé M.
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