Chryse Site Looks Rough

Chryse Site Looks Rough

By William K. Hartmann

Instead of the featureless, smooth, dusty plain that had been hoped for, the Viking orbiting pictures of Chryse revealed myriad different geologic structures. Although large impact craters were lacking in the area, clusters of small, 100-yards pits were common in some localities. These were apparently formed by ejects blasted out of some large craters. Other high rimmed craters and conical hills may have been volcanic in origin. Large streamlined hills, 20 to 30 miles long, were probably formed by water flow or etched by windblown dust: they showed cliffs indicating erosion. Nearby, some regions showed incredibly intricate complexes of gullies about 100 yards across: landing in such an area might be fatal to Viking. Another landform, possibly related, was a light-floored complex of shallow channels several miles wide and bounded by bluffs, suggesting dangerous slopes. Floors of such channels might be smoother, perhaps covered by light dust.

One problem is site selection was that the Viking lander could be targeted with 99 percent certainty only in an elliptical area about 31 miles by 75 miles. It is difficult to find such a large area in Chryse totally devoid of ominous topography.

Chryse Planitia. Photo: NASA

From June 22 through 26, photos were received showing still more details in the Chryse site. It became clear that many kinds of geologic processing had occurred in the area since the channels deposited their sediments. Geologists could recognize some stratigraphic units of known source, such as crater ejecta, and other units of unknown origin. At a meeting of June 25, all available information was considered by mission planners, who faced the decision of whether to target for the July 4 landing in Chryse. The consensus was that no one could predict whether 20 degrees slops and 10 inch boulders might exist in the region, since no one was sure of the geologic processes that had formed meter scale relief in the region.

The decision was announced on June 27 to delay the landing. The purpose was to allow more time to study the geology of other possible sites, such as the backup site at Tritonis Lacus, in a relatively “featureless” area east of Syrtis Major. Would small scale, secondary craters, gullies and bluffs dominate the small scale geography in all parts of Mars? Or can some smooth area perhaps formed by deposition of sediments, be found? Late June and early July will be spent trying to answer these questions.

(Astronomy, August 1976, vol. 4, #8).