Planetary remote sensing of regolith surfaces requires use of theoretical models for interpretation

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Radiative Transfer Modeling of Laboratory Thermal
Infrared Emissivity Spectra K. M. Pitman (SSI),
in collaboration with M. J. Wolff (SSI) and G. C.
Clayton (LSU)
Planetary remote sensing of regolith surfaces
requires use of theoretical models for
interpretation of constituent grain physical
properties. In this work, we critically
evaluated and assessed the efficacy of hybrid
computational models (Mie theory numerical
radiative transfer solutions) with comparison to
a trusted set of laboratory quartz emissivity
spectra in the thermal IR (2000-200 cm-1)
wavenumber regime. We established a (previously
undefined) statistical metric to rate successful
model-lab emissivity spectral fits (RMS lt 10 for
micron-sized ?-quartz particles) and directly
compared the performance of two methods for dense
packing of particles diffraction subtraction and
static structure factor corrections. Iteratively
back calculating single grain scattering
properties from lab measured emissivity values,
we showed that neither of the two packing
corrections when applied to single scattering
albedo and asymmetry parameter values generated
via Mie theory resulted in a model-lab fit.
Future work is planned to replace Mie theory, a
widely used but poor approximation to grain
shape, with atmospheric model algorithms that
better approximate grain shapes in planetary
regolith.
The results were presented at AAS-DPS (Nov. 2004)
and published as Application of modern
radiative transfer tools to model laboratory
quartz emissivity (Pitman et al. 2005, J.
Geophys. Res., in press) and will appear as a
chapter in K. Pitmans doctoral dissertation,
Radiative transfer modeling of thermal infrared
emissivity spectra applications to Martian
regolith observations (Louisiana State
University UMI Publications). Laboratory
emissivity spectra (solid line) for small and
large quartz particles as compared to model
emissivity spectra with and without dense packing
corrections. Corrections for packing worsen the
model-lab fit in both cases.
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Directional Emissivity Effects Observed in
MGS-TES EPF Sequences K. M. Pitman (SSI), in
collaboration with M. J. Wolff (SSI) and J. L.
Bandfield (Mars Space Flight Facility, ASU)
High Albedo (Dusty) Surface
The Emission Phase Function (EPF) sequences
collected by the Mars Global Surveyor Thermal
Emission Spectrometer (MGS-TES) constitute a
substantial thermal IR remote sensing radiance
dataset at angles that are observed from top down
(e 0o, where e is the angle of emergence from
the Martian surface) and at e 30o, 50o, and 70o
angles from the vertical. Within moderate to
high albedo (bright) regions (upper panel), the
EPF sequences clearly display a dependence on e
similar trends are also observed in a smaller
number of EPF sequences acquired over low albedo
Mars regions (lower panel). These observations
suggest that as the angle of observation
increases, the emissivity values decrease. As
of July 2005, we have identified approximately
400 MGS-TES EPF spectra which show subtle
directional emissivity effects. This sample set
is biased toward high albedo surfaces because low
albedo surfaces are nonuniform. Results on the
MGS-TES EPF spectra, tied to field and laboratory
emissivity measurements of Martian terrestrial
analog materials as reported in K. Pitmans
dissertation, are planned for a 2006 Geophysical
Research Letter.
Low Albedo Surface