SP-425 The Martian Landscape


Desert Drifts


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Figure 29

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Figure 30


[38] It is entertaining to compare these three views (figs. 29, 31, and 32) of the same region. The first was taken near noon, the second in early morning, and the third in late afternoon. Because of changes in lighting, each picture reveals different features within the scene.

Figure 30 should be compared with figure 29. The two were taken some nine months apart. The lighting is approximately similar. However, the details are not the same. Refer to the large boulder, nicknamed "Big Joe." Just below it, at the right, there is a small region of slumped soil visible in figure 30 but not in figure 29. For a geologist, trained to study the changing shape of planetary surfaces, this event is unusually exciting. It represents the first evidence of contemporary geomorphological change that we have witnessed on any planet other than [39] Earth, or on the Moon. As insignificant as this one small slump is, if the event were repeated every few years, or even every few hundred years, the cumulative effect over geological periods, measured in millions of years, would be pervasive.

Figure 31 was the first of the three panoramas to be obtained. The rising Sun backlights the entire scene, sharply delineating drifts of sediment and shadowing a prominent boulder about 2 m across and 9 m from the spacecraft. This is probably the most publicized picture taken during the entire Viking mission. Within a day after it was released it appeared on the front page of virtually every major paper in the United States, and many other papers around the world. One editor, determined to reproduce the long panorama at optimum size, rotated the picture 90° and printed it sideways. The ultimate compliment came from a friend of mine who could look back on a distinguished career as a photographer for Life during the heyday of that magazine. After the picture was first described at a special news conference, he came up and remarked, "That's a good picture." "Of course," I responded, thinking primarily of its technical qualities. "No," he returned. "You don't understand. It's really a good picture."

The origin of Big Joe has not been established. The rock is coarsely granular and is banded in one, possibly two, directions. It may be a breccia fragment, thrown out by a nearby meteoroid impact. Supporting this interpretation is the fact that Big Joe actually is two closely matched boulders side by side. The cleft between the boulders is visible as a notch in the shadow (fig. 32). The two-boulder occurrence appears more consistent with splitting on impact than with erosion in place. Also, some analysts have remarked on the apparent ring-like arrangement of small blocks to the right of Big Joe. However, detailed mapping of the blocks fails to document any circular geometry.

Note that Big Joe is capped with fine-grained material. This may be a remnant of a much thicker cover of dust that formerly covered the entire region to a depth of several meters.


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Figure 31

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Figure 32