Physics
Scientific paper
Dec 2010
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010agufm.p52b..04l&link_type=abstract
American Geophysical Union, Fall Meeting 2010, abstract #P52B-04
Physics
[5464] Planetary Sciences: Solid Surface Planets / Remote Sensing
Scientific paper
The optical effects of exposure of lunar materials to space are well-characterized (e.g [1]) and similar effects are expected on Mercury. In the lunar case perhaps the most notable effect is strong reddening of lunar materials relative to powdered lunar rocks. There is consensus that this is due to the effect of extremely fine grained (10’s of nm) particles of iron that occur in rims on mineral grains generated during micrometeorite impact and sputtering [2]. Noble et al. 2007 [3] studied silica gel samples infused with fine grained iron and observed lunar-like optical effects. Hapke 2001 [4] used Maxwell-Garnett theory to combine the optical properties of iron metal with the optical properties of the host lunar material to successfully produce the reddening and darkening effects observed. Lucey and Noble 2008 [5] tested the Hapke 2001 formulation against the Noble et al. (2007) data and found good agreement when particles were small with respect to the wavelength. However, particles that were larger than about 50 nm showed sharp deviations from the predictions of the Hapke 2001 treatment. At present there is no theoretical treatment of iron metal inclusions that darken, but do not redden. The Hapke 2001 formulation for modeling the optical effect of iron particles within grains relates the complex index of refraction of the host mineral, iron and particle size of the host to the single scattering albedo to account for absorption due to subwavelength particle inclusions, and adds this to his previously defined absorption coefficient. This absorbing effect, based on equivalent medium Maxwell-Garnett theory, assumes the particles are much smaller than the wavelength, and hence has no particle size dependence. While [5] showed Hapke’s model for space weathering works well for very small particles, the larger particles in the Noble et al. 2007 experiments did not conform well to predictions. To introduce a wavelength dependent term we replace Hapke’s term that captures small absorbing inclusion effects (tg) with absorption computed from Mie Theory. This modification quantitatively predicts the behavior of the Noble et al. 2007 experiments very well. We applied this model to spectra of Mercury obtained on the first two flybys [6]. We find that Mercury has experienced more space weathering than the Moon because submicroscopic iron is substantially more abundant on Mercury. Mercury’s low albedo drives this result; to match both the spectra and albedo submicroscopic iron that is larger than the wavelength is required in much higher abundances than the Moon. Opaque minerals alone cannot explain the low albedo, and in fact opaque minerals are not required for good fits, though albedo variations on Mercury probably demand the presence and variation of opaques. [1] Fischer, E. M., and C. M. Pieters (1994), Icarus, 111, 475- 488. [2] Pieters, C. M., L. A. Taylor, S. K. Noble, L. P. Keller, B. Hapke, R. V. Morris, C. C. Allen, D. S. McKay, and S. Wentworth (2000), Meteorit. Planet. Sci., 35, 1101- 1107. [3] Noble, S.K., C.M. Pieters, and L. P. Keller, Icarus, doi:10.1016/j.icarus.2007.07.021, 2007. [4] Hapke, B. (2001), J. Geophys. Res., 106(E5), 10,039-10,074. [5] Lucey, P. G. and Noble S. K. (2008) Icarus, doi: 10.1016/j.icarus.2008.05.008. [6] W. E. McClintock et al., Science 321, 92 (2008).
Lucey Paul G.
Riner Miriam A.
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