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National Geographic : 1973 Feb
Contents
changing patterns of light and dark are the result of dust shifted by the winds.* And today most scientists would agree. Mariner 9, which has been able to look at selected areas repeatedly for nearly a year, seems to prove that windblown dust is the answer to the mystery. It could be that the varying shades result from two different types of minerals. Another explanation is that finely divided dust offers more surface area than large par ticles or rock, scattering more light and thus appearing brighter in color. Consequently, when dust is sifted down from a major storm, it brightens the areas where it lands. But later, when local gusts scour the fine dust away, un derlying areas may appear dark again. Or per haps varying wind speeds determine whether small bright particles or heavy dark particles will be moved in a given area. Thus, in either case, seasonal wind variations might account for seasonal changes in brightness. Mariner's pictures pinpoint many irregular dark splotches, as well as long roughly paral lel streaks, both dark and light, streaming away from craters, ridges, or hills. Careful examination shows that areas covered with these splotches and streaks coincide with some of the classical markings of Mars. If the riddle of the large-scale variable markings may be solved, another has taken its place. What causes the localized streaks? At first glance it would seem that the bright *Dr. Sagan wrote of "Mars: A New World to Explore," in the December 1967 NATIONAL GEOGRAPHIC. streaks represent light-colored dust streaming out of craters. And, in other areas, the dark crater tails might be interpreted as wind "shadows," where crater rims have inter rupted the passage of bright material blow ing over the area (page 253). But how does one account for both dark and light streaks appearing at the same time? Could both dark and light materials blow out of craters in the same area? Still more puz zling: How do dark and light materials re main separate over the eons? Clearly, there is no lack of Martian mysteries to occupy us. W WATER NO LONGER courses through the arroyo, but, from the bluff above, its former passage is clear enough. Slumping walls have not con cealed the terraces left by successive floods that cut ever deeper into the banks. In the bottom, old-time channels weave their sinu ous ways, sometimes blending, sometimes crisscrossingto leave a braided pattern. At intervals along the bluff, tributariesen ter the main stream. From those tributaries multitudes of smaller branches reach far across the arid plain, as though in search of water long since disappeared. Probably the biggest surprise from Mariner 9 is the appearance in some of the pictures of meandering "riverbeds." With their branch ing tributaries, they resemble dry water courses of earth (pages 260-61). "There is simply no way to account for 249
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