DN VALUE (INFRARED)
6.1 Introduction
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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.
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equinoxes, Phobos casts a completely penumbral shadow on the surface of Mars' equatorial regions during portions of each orbit A passing of the shadow would be viewed by an observer on the surface as a partial eclipse lasting roughly 20 seconds.
Termoskan obtained the first ever thermal images of Phobos' shadow on the surface of Mars. Simultaneous visible images were also obtained.
Termoskan observed the shadow of Phobos on the surface of Mars during two of its four panoramas. There are four distinct shadow occurrences. Presented here is my analysis of the best observed and least complicated occurrence (shown in Figure 6.1).
For this shadow occurrence, I have combined the observed decrease in visible illumination of the surface with the observed decrease in brightness temperature to calculate thermal inertias of the uppermost tenths of a millimeter of the Martian surface. I use three progressively more complicated thermal models. My preferred model (#3) combined with the data implies values of thermal inertia (roughly .9 to 1.4) that are significantly lower than those originally derived from Viking IRTM measurements (2 to 3.5) [Palluconi and Kieffer, 1981]; however, they are similar to those implied for IRTM data by Haberle and Jakosky's [1991] thermal model that includes increased atmospheric re-radiation. Note that the IRTM derived inertias are diurnally derived and are sensitive to centimeter depths.
The shadow derived inertias sample the upper tenths of a millimeter of the surface. Thus, if layering exists at all, it is not very significant
These Phobos shadow studies have an interesting historical background. Tomas Svitek and Bruce Murray at Caltech suggested attempted observations of the shadow to the Soviets long before the Phobos '88 spacecraft reached Mars. They noted that the orbit of the spacecraft was highly advantageous, being equatorial and very nearly that of the moon Phobos. The Soviets indicated that the observations were too complicated and that perhaps they would be considered after the primary mission was completed. However, the spacecraft failed before the primary mission was over. Nonetheless, essentially by chance, Termoskan obtained observations of the shadow as part of routine observations.
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Figure 6.1: Phobos shadow images. Tennoskan visible (top) and thennal (bottom) images showing the analyzed Phobos shadow occurrence on the flanks of Arsia Mons. Note that the shadow is observed first (further West) in the visible, then later (to the East) in the thermal. This is due to the delay in cooling after the onset of the shadow.
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The similarity of the spacecraft to Phobos' orbit combined with Termoskan's anti- solar orientation (zero solar phase angle) conspired to put Termoskan's instantaneous field of view near the location of the shadow as it traveled across Mars' surface. However, because the spacecraft and moon were not actually in the same place, this orientation alone would have missed observing the shadow. "Fortunately," the spacecraft, and hence Termoskan, rocked slightly back and forth. Thus, Termoskan's instantaneous field of view rocked into and out of observing the shadow. This fortuitous combination of factors has allowed a unique analysis of the cooling from the shadow. This analysis gives never before available insight into the nature of the upper millimeter of Mars' surface in selected locations.
Tomas Svitek, working with Bruce Murray, did initial modelling and analysis of the shadow. I then independently produced a thermal model that reproduced Svitek's preliminary results. This model evolved just slightly into the form that is described as Model 1 here.
Since the time that the initial results were published in Murray et al. [1991], I have created two more detailed and realistic models. Here I present those models and their results for the first time. Model 2, my non-isothermal model, does not assume that the pre-eclipse temperatures are constant with depth as Model 1 did. Haberle and Jakosky [1991] compared theoretical considerations to Betts et al., [1990a]. They concluded that atmospheric effects are less important for eclipse derived thermal inertias than for diurnally derived thermal inertias. To test atmospheric effects, I created Model 3 by adding a downward atmospheric flux term to Model 2.