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On Mars: Exploration of the Red Planet. 1958-1978

[319] As Klaus Biemann puzzled over argon in the Martian environment, others on the Viking team were tussling with an equally troublesome issue, radar. As a tool to study planetary surfaces at great distances, radar seemed [320] to have immense potential. A signal of a known strength could be transmitted from one of the large radio astronomy antennas on Earth- Arecibo in Puerto Rico, Goldstone in California, or Haystack in Massachusetts- to the moon or Mars. The returned signal could then be compared with known signal characteristics and a judgement made about soil composition, the dimensions of slopes and rocks, and other characteristics of a specific area. Radar promised to give information on a scale of a few centimeters, where orbital imaging would tell the site certification team only about features that were larger than a football field. Radar thus promised to be a powerful tool for certifying landing sites, except that not everyone believed in its promise, making it a controversial issue. Furthermore, this technique could examine only a restricted range of latitudes on Mars.
While the Viking Project Office had been planning all along to use radar as an aid to landing site certification, Carl Sagan, once again acting as a catalyst, forced the issue early in 1973. 6 On 3 February, Sagan wrote to Jim Martin, and beneath the hyperbole of his prose Martin found some specific steps that could be taken to rectify what Sagan saw "as serious short comings in the landing site selection procedures." What worried him most was the interpretation being placed on some of the radar signals received from Mars. Some of Sagan's colleagues saw visually smooth areas as sand or dune fields, but he hypothesized that the low reflectivity of the radar was not due to the scattering effect of sand grains and surface ripples but to the absorption of the signal by a deep layer of dust. "At a recent landing site working group meeting we were all entertained to see a Viking lander sinking up to its eyebrows....While a similar suggestion that lunar landing spacecraft would sink into surface dust has proved erroneous, it by....

The 305-meter-diameter radar dish antenna at the Arecibo Observatory of the National Astronomy and Ionosphere Center nestles in the Puerto Rico hills at left below. At right below, the 64-meter dish antenna of the Deep Space Network's Goldtsone, California, tracking station faces toward space. Arecibo's reflector surface consists of 38 778 aluminum panels, each about 1 by 2 meters, attached to a network of steel cables. The radar feed mechanism, mounted on a 600-ton triangular platform, is suspended by cables above the dish.


[321]....no means follows that quicksand is not a hazard for Mars. "He reminded the project manager that the Soviets had suggested that quicksand might have been the cause for Mars 3 's failure.
As a consequence, Sagan made some "explicit recommendations." First, he believed more serious theoretical work was needed to understand better the meaning of returned radar data. Second, Earth-looking radar on satellites and aircraft could bounce signals off terrain thought to be analogous to that on Mars, and as a data base was established scientists could compare radar returns from unknown Martian surface areas with known Earth terrains. Third, Sagan thought that major support should be given to Arecibo, Haystack, and Goldstone Observatories so they could examine Mars in detail during the 1973 and 1975-1976 oppositions. He noted that the Arecibo staff was resurfacing its 300-meter radar dish and would be installing a new transmitter. Once these renovations were completed, the observatory would "have a very impressive Mars mapping capability, which should be exploited to the fullest."
Turning to visual imaging, Sagan repeated his concern that smooth surfaces at the 100-meter scale might be rough at 10 centimeters. Had lunar surface data been analyzed to determine if there was any relationship between roughness at the two scales? Hal Masursky's people might look into this matter, and similar correlation of Earth photos should also be studied. He seriously doubted that one could make judgments about the nature of the surface or the scale of the lander from any photographs the orbiter was likely to produce. Sagan believed that radar, properly understood and interpreted, was likely to be more useful in site certification than all the photographs that would be taken. 7
Sagan's concerns were important ones. Jim Martin and Tom Young considered his recommendations, and on 23 March 1973 Martin wrote to Edgar M. Cortright, director of the Langley Research Center. Martin planned to take three actions as a consequence of Sagan's letter. Arecibo, Goldstone, and Haystack radar facilities would make nearly simultaneous observations of the same areas on Mars during 1973. Since the latitude base that could be studied was limited to 10° to 20° south, none of the candidate sites could be examined, but the information would be valuable because it would contribute to the specialists' understanding of radar's potential in such investigations. The Arecibo team also agreed to make studies in the 1975-1976 period and prepare a quick analysis of its data in the weeks before the scheduled landings.
The second action taken by the Viking Project Office was to set up a radar study team, which would undertake to eliminate some of the ambiguity in interpreting radar data. On l March, Tom Young and Jerry Soffen met with Von R. Eshleman and G. Leonard Tyler of Stanford University's Center for Radar Astronomy, where they had been engaged in an active program of analyzing and interpreting lunar radar studies. Tyler agreed to lead the team that would work toward improving interpretation of Mars radar information. Martin told Cortright, "As you are aware, some of the [322] areas with low radar reflectivity are candidate landing sites. We must better understand the meaning of the low radar reflectivity to assure that the current sites are acceptable or guide the selection of proper alternatives." Tyler had his work cut out for him, and Martin arranged for a retreat at which a small group could consider thoroughly the implications of radar studies for Viking. 8
Tyler presented the results of his study to the landing site working group meetings at Langley on 4 November 1974. Basing his conclusions on data obtained from all three radar facilities, Tyler noted that correlation between radar features and Project Mariner imagery was poor. His study on the large scale; Mars tended to have greater variation in surface reflectivity than Earth or the moon; Mars appeared smoother than the moon to the radar; the 100-meter resolution of the orbiter camera system seemed likely to give appropriate information for extrapolating down to the scale of the lander; and data for the 15° to 20° south band of the planet could not be applied to latitudes in the north without variation. Jim Porter, keeping minutes for this meeting, reported that both Tyler and his colleague Gordon Pettengill "laced their presentations strongly with tutorial material which greatly enhanced the ability of the group to understand and correctly interpret their findings."
After listening to Tyler, the landing site working group was unanimous in the opinion that the A and B sites were still the best targets. Although the four targets A-1, A-2, B-l, and B-2 were still believed to be in the correct order of precedence (the Chryse site, A-1, receiving a strong vote of confidence), the team became less enthusiastic in its endorsement of the B sites. They also raised some questions about the C sites that had been located recently at 9° south. The need for new sites had been raised in early 1974 when some of the working group members began to get nervous about what the orbiter's cameras might find. Should the prime and backup sites prove unsatisfactory or if operational difficulties should develop with the spacecraft that would require the selection of some other safe landing spot, they wanted a pair of "super safe" sites where radar, photographic, and topographic information indicated that the spacecraft would have the best chance of landing undamaged. A special subcommittee* had been established to look into possible C sites and make recommendations as early as possible. 9
The work of the C site subcommittee took longer than the working group anticipated. After meeting in December 1974, the group met again on 6 February 1975 at the Jet Propulsion Laboratory to recommend the study of three latitude bands (8.5°S, 4°S, and 4°-6°N) that would be visible to either the Goldstone or Arecibo radars during August to November 1975. The radar specialists would observe each of these regions as it became [323] accessible and recommended sites based on combined radar and visual criteria to the Landing Site Staff, the new name of the certification team, in September 1975. They would repeat the process in November after the 4° north coverage. From these observations, the Landing Site Staff would develop a final recommendation in April for Tom Young, who had become mission director. A detailed alternate mission design (for the C sites) would be developed between December 1975 and May 1976 by Viking flight team members at JPL.
A general feeling among the subcommittee members was that the second mission should be targeted for one of the C sites, since the available radar data indicated that some regions on Mars were very unsafe for landers. The B sites were so far north that radar coverage would never be possible. Norm Crabill wrote in the minutes of the 6 February 1975 meeting that apparently radar data could be used to reject sites, but it was doubtful that it was sufficient to confirm a site. On the other hand, Sagan and some of his colleagues did not want to rely on photos alone. Despite all their earlier work, the landing site specialists were still nervous about their efforts to find suitable landing points for Viking. 10 Putting aside nagging uneasiness, the Science Steering Group and the Landing Site Staff met in a joint session at Langley to consider the recommended process for selecting the C sites. After more discussion of radar as a tool, further explanations of this complex business by Len Tyler, and additional considerations of the argon problem, the joint group approved the proposed plan for C site selection. 11

* Subcomittee members included Chairman H.Masursky, N. L. Crabill, J. D. Porter, L. Kingsland, G. L. Tyler, T. Owen, H. Moore, G. A. Soffen, and G. A. Briggs.