DN VALUE (INFRARED)
5.3 Fine Thermal Structure: Arsia Mons
The flanks of Arsia Mons show much fine thermal structure as seen Figure 5.8 and as roughly mapped from the Termoskan data by Crumpler eta/. [1990]. The far reaching finger-like thermal/visible structures extending radially from the caldera appear warmer
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Figure 5.8: Termoskan visible (top) and thermal (bottom) images showing Arsia Mons and its flanks. Note the fine thennal structure emanating from the caldera that corresponds to flow units
[Crumpler eta/., 1990]. The thermal inertia difference may be primary or secondary.
113 53 Fine Thermal Structure: Arsia Mons
and brighter than the surrounding terrain. These finger-like features may correspond to the ends of a flow unit, as mentioned by Crumpler et al. [1990]. The brighter, warmer fingers that appear to have emanated either in or near the caldera have lower inertia than the region that they have flowed into.
It is surprising to find fine thermal structure corresponding to flows in this region of low inertias [Palluconi and Kieffer, 1981], low rock abundances [Christensen, 1986a], and presumed current dust deposition [Kieffer et al., 1977; Zimbelman and Kieffer, 1979;
Palluconi and Kieffer, 1981; Christensen, 1986b], although much of the region is radar bright [Muhleman et al., 1991]. There are several possible explanations: 1 - physical differences in the flows themselves cause the difference as suggested by Crumpler et al.
[1990]; 2 - the surface rock abundances on the flows, possibly due to the flows themselves, differ on the lower and higher inertia units; 3 - different properties of the flow units cause a preferential trapping of aeolian material on one unit as opposed to the other;
4 - the greater age of the higher inertia unit has allowed more time for the bonding of fines; or 5 - the lower topography of the higher inertia unit has induced greater bonding of fines due to greater availability of water from subsurface or other sources. Although it is difficult to separate these processes with available data, the low inertias and low rock abundances in the region lead me to favor explanations that involve secondary, presumably aeolian, material, possibly covering a radar bright substrate, i.e., explanations 3, 4, or 5.
This region is a good example of two significant general features within the Termoskan data. First, fine thermal structure is seen at the limit of resolution of the data.
This is true even of panorama 1 with its 300 rn/pixel resolution. This argues against any type of general aeolian blanketing in the regions observed. Before Termoskan, there were some questions about whether we would see significant thermal structure at these scales.
The Termoskan results bode well not only for Termoskan analyses, but also for Mars Observer TES and Mars '94 Termoskan 2. In some cases fine thermal structure
may
be unrelated to local geology. However, in cases such as these apparent flow features, it isMiscellaneous Topical Studies 114
related to the geology. Fine thermal structure does not appear everywhere in the data.
Most of the ancient cratered highlands that were observed in the fourth observing session appear remarkably bland in the thermal.
Second, as can be seen in Figure 5.8, the individual low inertia fingers that probe the higher inertia material are part of a much longer thermal inertia boundary that extends for hundreds of km encircling southern Arsia Mons. Several large scale, relatively sharp thermal inertia boundaries are seen in the data. These include the highland-lowland boundary discussed in the last section and a boundary cutting across Syria Planwn.
Although these boundaries may have had different origins, Termoskan has provided new clues about them by identifying their sharpness at or near the Termoskan resolution.
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Chapter 6
Dancing with my shadow, and letting my shadow lead.-Warrant
THE SHADOW OF PHOBOS ON MARS
This chapter describes analyses of Termoskan observations of the shadow of Phobos on Mars. Most of the shadow observations discussion (Section 6.2) appeared in Murray et al. [1991]. Here, I present three models of the shadow induced cooling of the Martian surface. Model 1 and its results were presented originally in abstracts Betts et al.
[1990a; 1990b] and then formally published with several other preliminary Termoskan analyses in Murray et al. [1991]. That model and its results are presented in much greater detail here. Models 2 and 3, their descriptions, and their results have not yet been published, but I intend to submit them for publication in the near future.
Section 6.1 introduces the shadow observations and analyses. It also gives some historical background. Section 6.2 describes the shadow observations and the nature of the shadow on the surface. This is followed by a section describing in detail the three thermal models, including the inputs that were used from the Termoskan data. Section 6.4 compares the Termoskan thermal data with the model results to derive thermal inertias.
Section 6.5 discusses the results, and the last section of this chapter discusses the three other shadow occurrences and potential for future Phobos shadow research. Readers who are not interested in the details of the models may wish to read at least the Introduction (Section 6.1) before skipping to the results (6.4) section. Obviously, reading the observations section (6.2) and the model description section (6.3) will enhance overall understanding.