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

Certification Team
[317] In August 1973, Jim Martin selected Hal Masursky to lead the landing site certification team, with Norman L. Crabill from Langley as his deputy. This group, which functioned as an operational organization rather than a planning body, included members from orbiter imaging, infrared thermal mapping, Mars atmospheric-water detection, and mission planning and analysis teams, as well as radio astronomers. 1 Together they designed a strategy for landing site certification, which R. C. Blanchard of the Viking Project Office presented at the February 1974 meeting of the Science Steering Group. Blanchard broke the certification process down into four periods: l. Pre-Mars orbit insertion (MOI) for Viking 1 . 2. Post-MOI and prelanding for Viking 1 . 3. Postlanding for Viking 1 ; pre-MOI for Viking 2 . 4. Post-MOI and prelanding for Viking 2 . Blanchard noted that before the first Viking spacecraft orbited Mars, new sources of data that might possibly affect the landing sites could include Earth-based radar studies of the planet, Soviet missions flown before June 1976, and scientific observations made by Viking as it approached Mars. Analyzing all new information would help them make a "go/no-go" decision concerning the desirability of landing at the prime site latitude and, they hoped, would contribute to ``A-1" site (first choice for first lander) certification.
[318] Viking 1 would make extensive observations of the prime site, with special emphasis on the low-altitude photographs obtained during the close approach (periapsis). In addition, two or three picture pentads (groups of five photos) would be taken on each revolution to permit comparison of images taken at different exposures (due to the elevation angle of the sun). The A-1 site would also be studied by the orbiter water-vapor detector and infrared thermal-mapping instrument to determine if the scientists' preconceived notions about the target were valid. Viking 1 would also observe the second lander's primary target (E- l) from low altitude with two picture swaths and one high-altitude pentad. Should the A-1 site be found acceptable (certified), then the lander would be targeted for that site. If it was not acceptable, then the backup site (A-2) would be examined. Once a landing area was chosen, orbiter trim maneuvers would fix the spacecraft's periapsis near that site.
During the third period, postlanding for Viking1 and preorbit insertion for Viking 2, information sources available to Earth control would include B-1 site data from the first orbiter, entry and landed science data from the first lander, evaluation of the first site certification procedure, and approach observations made by Viking 2 . The team would then make its commitment to the B mission target. Once the second craft was in orbit, the men would confirm a B- l site using additional data from the second orbiter and the further assessment of Viking 1 science results. Blanchard assured the Science Steering Group that the A-1 and B-l targets chosen by the landing site working group definitely would be used, unless compelling arguments materialized to require a change. Further, the scientists were reminded that the certification team would continue to be influenced strongly by considerations of safety during the first landing, but hoped that during the second landing it could look for a more scientifically interesting site even if less safe than the first. 2
The first new data the Viking team received came from the Soviet missions.
Soviet Attempts to Investigate Mars
Much to the dismay of everyone working on Viking, the four flights the Soviets sent to Mars in 1973 raised as many issues as they settled. Mars 4 and 5 were launched on 22 and 25 July, followed by Mars 6 and 7 on 5 and 9 August. Mars 4 came within 2100 kilometers of the Red Planet on 10 February 1974 but failed to go into orbit when the braking engine did not fire. On 12 February, Mars 5 went into orbit. As no effort was made to detach landers, Western observers assumed that these two Soviet craft were designed to operate as orbiting radio links between landers aboard Mars 6 and 7 and tracking stations on Earth. Mars 7 approached its target on 9 March, but the descent module missed the planet by 1300 kilometers when some onboard system malfunctioned. On 12 March, the remaining vehicle separated from its carrier ship, which then went into orbit around the sun. [319] Mars 6 descended directly to the surface and provided telemetry for 120 seconds before it crashed. 3
Soviet scientists reporting on the descent and crash-landing of Mars 6 calculated that it landed at 23°54' south latitude and 19°25' longitude in the region called Mare Erythraeum. The landing site was "situated in the central part of an extensive lowland region," part of the global zone of depression extending for several thousand kilometers north and south of the Martian equator. Most of the landing zone (about 75 percent) was heavily cratered. Part of this terrain analysis was based on Mariner 9 data, but the characteristics of the actual landing zone were determined by the radar-altimeter readings obtained during the parachute descent of the Soviet craft. Additionally, Mars 6 instruments indicated "several times'" more water vapor in the atmosphere than previously estimated, news over which Viking scientists were cautiously optimistic, since it enhanced the possibility of discovering some kind of life forms. Mars 5 photographs provided additional data on the planet's surface features, and while most of the Soviet findings correlated with previous knowledge and predictions there was one major anomaly. 4
One of the experiments carried on the Mars 6 lander was a mass spectrometer designed to determine the gaseous composition of the Red Planet's atmosphere. Although the recorded mass spectrum data were not recovered, engineering data on the operation of the vacuum pump appeared to indicate unexpected quantities of noncondensable gases. Soviet scientists interpreted the data as an indication that the atmosphere might contain as much as 15 to 30 percent argon (contrasting with l percent in Earth's atmosphere). The Americans had been operating on the assumption that the thin Martian atmosphere contained less than 3 percent argon. A concentration approaching 15 to 30 percent would force some rethinking about Mars and about Klaus Biemann's mass spectrometer experiment. It would mean that the Martian atmosphere had been much denser in the past than the specialists had believed. That would have made the existence of liquid water possible, but it posed a question what had happened to those atmospheric gases? That was the puzzler. A great concentration of argon would also require some changes in the use of the gas chromatograph-mass spectrometer, since inert gases like argon tended to impede its operation. Obviously, the Soviet Mars missions had not answered many of the U.S. questions, but they had added another element of excitement to the first Viking landing. Everyone would watch closely the results of the entry science team's experiment to see just how much argon it detected as the A lander made its way to the surface. 5