04.08

Fine particle spectra of palagonite and smectite in the mid-infrared

C. D. Cooper, J. F. Mustard (Brown University)

The composition of the silicate component of the dust on Mars is controversial. Analog materials from iron-exchanged clays to poorly crystalline or amorphous palagonite have been proposed. However a range of crystallinity in Martian material may be expected, where crystallinity is diagnostic of the environment of formation. Spectral analog studies with clays and palagonites have had success in matching ferric absorptions and continuum properties of observed Mars spectra, but have used coarser particle sizes than the 1 expected for the dust on Mars and fail to match all spectral features. We have been investigating the effects of using much finer particles sizes on the spectral properties of clays and palagonites.

Previous results have shown that the diagnostic 2.2 metal-OH band in both montmorillonite and palagonite decreases significantly as particle size approaches that expected for Mars. We are now investigating the effects of particle size in four size separates between 0 and 25 on the mid-infrared reflectance properties of these materials. Both palagonite and smectite exhibit sharp decreases in reflectance and changes in spectral contrast characteristic of Mie scattering in the finest particle sizes measured. The palagonite shows absorptions between 5 and 10 that are similar to those in the smectite, but much broader and weaker. Thus the palagonite shares crystallographic properties with smectite but is apparently less crystalline. The relative strength of these absorptions should be resolvable in TES spectra of Mars, and therefore estimates of the crystallinity of the silicate component of the Martian dust may be possible. These estimates can provide insight into the environmental conditions that existed during the formation of Martian surface material.