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On Mars: Exploration of the Red Planet. 1958-1978
 
 
SAFE HAVENS: SELECTING LANDING SITES FOR VIKING
 
 
 
[277] Since the basic goal of Viking was to conduct scientific experiments on the surface of Mars, the selection of landing sites was recognized early as a topic of major importance. Once the decision was made in November- December 1968 to make a soft landing from Mars orbit, the project engineers and scientists began a long colloquy in which they weighed the demands for lander safety (a crashed lander equaled no science) against the desire to land at locations most attractive scientifically. At first, the discussions were necessarily general in tone; the scientific knowledge (in terms of both physical data and visual images) was still very limited. Mariner 4 , flying by the planet in July 1965, had yielded new information and the first extraterrestrial images, revealing a heavily cratered, moonlike surface. From Mariner 4 's perspective, the planet appeared to have eroded very little. Some scientists concluded that this meant there had not been much wind or water activity on the surface. Other scientists pointed out that Mariner 4 had sampled only l percent of the Martian surface; they wanted to see the other 99 percent, and they wanted to see it more closely.
 
The Viking Project Science Steering Group began to consider the interplay between landing sites and Viking lander science during its first meeting in February 1969. Mariner 4 had raised as many questions as it had answered, and data from Mariner 69 (Mariner 6 and 7 ), soon to be launched, would not be available until next year. Donald G. Rea, deputy director of planetary programs in the Office of Space Science at NASA Headquarters, during this first Science Steering Group meeting raised the landing site question when he asked for thoughts on how best to use the orbiter in support of the landed science program. Thomas Mutch, a geologist, began the discussion. The lander imaging team he headed had not considered landing site selection, since members thought orbital images were of little value in the site selection process. They assumed that orbital photographs would not be able to pick up geological features smaller than a football stadium (i.e., resolutions in the 100- to 1000-meter range). Ground-based scientists could not possibly see the lander or smaller scale hazards that could affect its safety, and Mutch's team did not believe that orbital pictures would help them pick either a good science site or a good landing spot.1
 
[278] Wolf Vishniac of the biology team disagreed. Orbiter imaging could provide a valuable means for differentiating between places of low and high biological potential. He also believed that a possible strategy for selecting a landing site might be to set one craft down in a dark area and the other in a light area. The difference between Mutch's evaluation and that expressed by Vishniac was in itself illuminating. Mutch was thinking in, terms of the small-scale features (measured in centimeters) that the lander would be able to see. Vishniac was basing his comments on the large-scale light and dark features observed through Earth-based telescopes. Between these two scales lay an unknown range of Martian topographical features that would mean the difference between a safe landing and a crash.
 
The second steering group meeting, at Stanford University a month later, heard additional possibilities for using orbiter imaging in selecting a landing zone. On the large scale, Seymour Hess, the Viking meteorologist, expressed the hope that the orbiter could find a large, flat area on which the lander could he placed, so his weather station would function more effectively. He preferred a place with no surface "relief for 10's to 100's of kilometers." Surely the orbiter images could spot such a tableland. But Klaus Biemann, a chemist from MIT, noted that in the search for life forms, as well as in the molecular analysis his team would make, it was preferable that the first lander sit down in a warm, wet, low site. His ideal site demanded the fewest degrees below freezing, the highest traces of water in whatever form it might be found, and the highest atmospheric pressure (i.e., the lowest elevation) possible; life would most likely survive under those conditions.
 
In addition to the imaging system, the Water-vapor mapping and thermal mapping experiments being planned would give the Viking team clues to the best sites while the lander was still attached to the orbiter, but the exact role of the orbiter would become clear only with time. Defining the mission occupied the Science Steering Group for the remainder of 1969 and most of 1970. 2 By August 1970, Jim Martin believed "that the definition of Viking landing site characteristics, the definition of data and data analysis needed to support the selection of sites, and the integration of engineering....capabilities and constraints" should be more coordinated. 3 A. Thomas Young, Viking Program Office science integration manager, led a landing site working group,* which met for the first time as a body at MIT on 2 September 1970. Martin opened the proceedings, indicating that "the actual Viking landing sites would be selected through this group."
 
C. Howard Robins, Jr., deputy mission analysis and design manager, reminded the group that the Viking system requirements were not being developed for a single ideal mission. Instead, his teams were planning for a [279] broad spectrum of missions based on the desire to set the lander down anywhere in the latitude band 30° south. The hypothetical landing sites being used to develop the "preliminary reference mission" had not been selected for their scientific merit. They had been chosen simply to give the analysis and design specialists something to work with in creating spacecraft design requirements. Finally, he reported that his office would develop the "operational mission design," which would guide the conduct of the real missions, by working hand in hand with the landing site working group.
 
The working group members began to discuss the desirable features and characteristics of Viking landing sites with Tom Voting suggesting that initially they ignore any potential system or mission constrains. Carl Sagan led off the brainstorming session considering the problem in terms of three primary areas of investigation-biology, geology, and meteorology. Comments on biology centered on the availability of water, atmospheric and surface temperatures, and ultraviolet radiation. Each of these three variables could affect the possibility of finding life forms.
 
The meteorologists wished to observe four related phenomena over a period of time-seasonal darkening, the daily night-day cycle, long-term meteorological variations, and the annual polar-cap regression process. They also hoped the lander could be in a position to observe dust devils, ground fog, and ice clouds. William Baum of the Lowell Observatory's Planetary Research Center presented a status report on Earth-based motion studies of clouds on Mars. Cloud patterns were being mapped under the International Planetary Patrol Programs hourly each day, and recent daily photographs had shown significant changes, but he could not say how these alterations might be correlated with seasonal or other patterns.
 
The first working group meeting closed with a discussion of the relationship between the Mariner 71 mission (Mariner 9, launched 8 May 1971) and Viking. Dan Schneiderman, Mariner 71 project manager. hoped Viking personnel members would participate in that mission as observers during the first 100 days and thereafter as users of the orbital cameras to look for potential Viking landing sites. Martin assured the working group members that they would have an opportunity in October to discuss topics of common interest between Mariner 71 and Viking. 4
 
* Other members of the working group were C. H. Robins and G. A. Soffen, Langley Research Center; W. A. Baum, Lowell Observatory; A. Binder, Science Applications Institute; G. A. Briggs and C. B. Farmer, JPL; H. Kieffer, University of California at Los Angeles; J. Lederberg, Stanford University; H. Masursky and H. J. Moore, U. S. Geological Survey; and C. Sagan, Cornell University.