DN VALUE (INFRARED)
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scattering, then the shadow would be well defined with sharp "corners" in the profile at the onset of eclipse and at the point when all of Phobos was in front of the Sun as seen from the Martian surface. The actual shadow profile is smoothed and rounded and spread out compared to the non-scattering ideal (as seen in Figure 6.3). By using a numerical scattering model of the martian atmosphere (e.g., Yung, 1976) in a computer model that also takes into account the geometry involved, model shadow profiles can be produced.
The zero phase angle observations simplify the geometry somewhat
By comparing these model profiles to the actual profiles, the dust loading of the
atmosphere as described by its optical depth can in principal be determined. Parameters such as particle size, distribution with height, and single scattering albedo may have to be assumed based upon existing estimated values for martian atmospheric dust. It is possible that some of these parameters can be constrained based upon the shadow shape as well.
Comparing results for the different shadow occurrences will give a check of the internal consistency of the method. Doing the same analysis using Viking visible images will give a check of the general validity of the method. The morning and evening limb profiles also contain information about the atmosphere that can be independently compared with the shadow results.
7 .2.3 Aeolian Studies
Aeolian processes are currently active on Mars and have been proposed as the cause of many of the thermal inertia variations on Mars [e.g., Christensen and Moore, 1992]. Thus, although I have identified two morphologic features (ejecta blankets and channels) whose primary thermal nature is likely not of aeolian origins, most of the thermal variations on Mars are still thought to have aeolian causes. The first part of this study would be to analyze and characterize aeolian features using the Termoskan data set alone. Types of aeolian features observed within the data include: relatively high inertia, presumably aeolian sand intracrater deposits, the largest of which were studied with IRTM
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data [e.g., Christensen, 1983]; wind streaks, which were studied at length in the visible [e.g., Veverka et al., 1981] and the largest of which were studied using IRTM data;
miscellaneous aeolian appearing splotches; and what appear to be regional aeolian deposits and erosional surfaces.
The second part of the study would involve comparison of the Termoskan data with Viking visible images and Viking IRTM data to look for temporal variations that have occurred since Viking. Temporal variations, particularly those involving temporary blanketing, were observed during the time period of Viking observations. Termoskan gives the opportunity to look for changes that occur on the scale of years (between the late 70's and 1989). The analysis could start by looking at Viking data from the same season (very early northern spring) in order to try to eliminate seasonal variations. Then, data can be compared from all seasons to try to distinguish changes that occur on a seasonal scale from those that are really the work of about ten years of aeolian modification. Eventually, the study of these features can be expanded to future missions' data in order to get an even larger spread in time, but with Termoskan serving as a
"middle" point
7.2.4 Radar Stealth Region
Muhleman et al. [1991] found a large (tens to hundreds of km) region west of Pavonis Mons that showed no detectable polarized or depolarized radar return using the highest resolution ground based radar study of Mars to date. They named the region, Stealth. To explain the observations, they proposed a several meter thick ash tuff surface layer devoid of volume scatterers. Termoskan obtained extremely high resolution (0.3 km per pixel) thermal and visible data for part of this region. Termoskan provides one potential test of the ash tuff hypothesis. One would expect an ash tuff that gives virtually no radar return to have very homogeneous thermal properties as well as low thermal
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inertia. To assess the thermal inertia homogeneity, the visible channel will need to be carefully anchored to Viking albedo observations to insure accuracy.
7 .2.5 Comparison of Termoskan Data with Phobos '88 ISM Data
ISM (Infrared Spectrometer for Mars) was a French near-infrared (VIMS-like) scanning spectrometer flown aboard the Soviet Phobos '88 mission [Bibring et al., 1989 and 1991]. Like Termoskan, ISM obtained a limited number of swaths in the equatorial region before the failure of the spacecraft. The overlap of the Termoskan and ISM coverages is shown in Figure 7 .1. The Termoskan data can be compared with the ISM data in order to study possible correlations between composition (ISM) and surface thermal properties (Termoskan). Comparison of these two data sets is particularly powerful because they were obtained at the same exact season and closely in time of day.
For example one can look for correlations between low inertia features and strong hydration features that may be indicative of clay rich dust deposits. Similarly, hydration features are of particular interest in looking at the thermally distinct channels seen by Termoskan. The ISM data available in the literature (e.g., Erard et al., 1991) will make for an interesting first comparison with Termoskan. Then, comparison with the digital data can be done.