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RECOMMENDATIONS ON POST-VIKING BIOLOGY STRATEGY FOR MARS 1. Viking has neither confirmed nor ruled out current or past Martian life. Organic compounds have not been detected. Although all three biology ex- periments have yielded signals that indicate chemical activity, the interpreta- tion of the signals remains ambiguous or inconclusive. Abiogenic explanations seem likely for at least two of the experiments and are probable for the third. We believe that it is preferable to predicate future strategy on the assumption that the signals are not biological in origin. 2. We recommend that the next phase in the biological exploration of Mars should be to acquire and characterize soil samples from areas likely to contain sediments and from ice-regolith interfaces. Locating these areas and locating sites that are shielded from the powerful atmospheric ultraviolet radiation and the powerful surface oxidants will require subsurface sampling. 3. Subsurface sampling can be achieved by soft landers or by hard landers such as penetrators. The choice between the two modes and the attendant costs depend in part on the sampling depths that have to be attained. The required depths are currently unknown. We urge NASA to conduct intensive studies to estimate these depths from data returned from the current Viking and from observations from Earth. 4. The samples acquired from the subsurface of Mars should be charac- terized with respect to organic compounds, carbon and sulfur isotope ratios, the amount and state of water, the presence of water-soluble electrolytes, and the existence of nonequilibrium gas compositions. 5. Several of the sample characterizations that we consider of paramount biological importance are also considered to be of major geochemical impor- tance by the Space Science Board's Committee on Planetary and Lunar Ex- ploration (COMPLEX). 6. The acquisition and characterization of samples could be carried out by soft landers, by hard landers, or by both. Soft landers can carry the instru- ments necessary to characterize the samples, but there are serious questions as to whether the landers can reach most of the areas on the surface of Mars that are prime candidates for sampling. Hard landers can probably impact on these prime areas, but there are serious questions as to whether they can carry the instruments necessary to characterize the samples adequately. We consider 23

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it urgent that NA SA study this paradox in detail to determine how best to resolve it. Its resolution should precede a decision on the next mission. 7. At least one mission would be required for the first, characterization phase of exploration. The greater the extent of reduced carbon, liquid water, soluble electrolytes, and gas and isotopic disequilibrium the subsurface sam- ples possess, the greater the priority that should be accorded the initiation of a second phase of post-Viking biological exploration of Mars—a detailed search for evidence of present or past life. 8. If it is decided to initiate these detailed biological studies, we recom- mend that they be conducted on samples returned to Earth. We recommend against attempting to perform the detailed studies remotely on the surface of Mars. 9. The current recommendation of the Space Science Board1'p'19 is that "The long-term objectives of exobiology and surface-chemistry investigation are best served by the return of an unsterilized surface sample to earth" and "We, therefore, recommend that Mars surface-sample return (MSSR) be adopted as a long-term goal.. .." Our recommended strategy is consistent with this policy, but it should be noted that our strategy emphasizes subsurface samples and recommends that the priority accorded biological exploration on a sample return mission should depend on the results of the prior characterization mission or missions. In 1974 the Board also recommended as preparation for a sample return mission and as an interim cost-effective program "... a Mariner polar orbiter mission and Pioneer survivable hard lander and probe mission...." Although the proposed interim mission would provide invaluable information on geo- chemical and geophysical characteristics of the Martian surface and interior, either the mission or its scientific payload would have to be supplemented to provide the biological characterizations required prior to decisions on the specifics of the sample return mission. 24