Reduction of Spectral Contrast Between Laboratory and Remote Spectral Observations

Details Reduction of Spectral Contrast Between Laboratory and Remote Spectral Observations Reduction of Spectral Contrast Between Laboratory and Remote Spectral Observations

Dec 2002

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2002agufm.p72a0476m&link_type=abstract

American Geophysical Union, Fall Meeting 2002, abstract #P72A-0476

Mathematics

Logic

5400 Planetology: Solid Surface Planets, 5410 Composition, 5464 Remote Sensing, 5470 Surface Materials And Properties

Scientific paper

A significant challenge in "Closing the Loop" is the apparent reduction in spectral contrast observed in remotely acquired data compared to laboratory data. Laboratory measurements of many geologic materials (minerals, rocks, soils) show well defined absorption or emission features linked to the fundamental chemistry and structure of the constituents, forming the basis of remote material identification. High spectral resolution observations of natural surfaces exhibit diagnostic absorption features permitting material identification. However, the strength of the absorption features is attenuated relative to laboratory measurements. While this does not affect the identification of materials which have well defined, strong absorptions, it can affect identification of materials that exhibit weak absorptions. Furthermore, models or analytical approaches to quantify the amount of material present will under estimate the true amounts due to this reduction in spectral contrast. A central question is what are the causes of the reduction in spectral contrast and can they be accommodated? Here I assess three possible causes: particle size, texture, and unaccounted for components. Many laboratory measurements are of well characterized materials of relatively large particle size. Natural surfaces contain a wide range of particle sizes. Theory predicts and many studies have shown that contrast is reduced for small particle sizes. Also, where a range of particle sizes exists, the small particle sizes influence the measured signal greater than their mass fraction in the system. Texture (solid vs. particulate, rough vs smooth) is well known to affect spectral contrast. This is particularly important for thermal emission measurements where cavities behave as small blackbodies, reducing spectral contrast. Smooth surfaces for vis-near infrared wavelengths have much reduced contrast while contrast in increased at thermal emission wavelengths. Unaccounted for constituents probably play the largest role in reducing spectral contrast. Natural surface contain a number of weathering products which are poorly known, and equally poorly crystalline. The lunar example provides guidance for this where the lunar soils show weakened contrast relative to rocks from which they were derived. Detailed detective work revealed that this was due to agglutinates, free iron metal, and very fine grained shocked plagioclase. An assessment of the relative importance of these spectral reduction processes will be presented and possible effects on the goal of "Closing the Loop" will be discussed.

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