Miniaturized x-ray spectrometers (two isotope sources, four proportional counter detectors) will be soft-landed on Mars in the 1975 Viking program for a “first look” at the planetary surface. In tests of analytical performance on unknown samples provided by the U.S. Geological Survey, 11 elements (Mg, Al, Si, K, Ca, Ti, Fe, Rb, Sr, Zr, and “O” [the sum of weight percents, elements of atomic number 1 through 11]) were detected and measured with accuracies which compare favorably with those obtained by wet-chemical methods. Upper concentrational limits were established for several other elements (including P, S, and Cl); limits also were established for CO2. Geochemical and petrologic interpretations derived from the x-ray results are in accord with similar interpretations based upon wet-chemical analyses of the unknown samples in three important respects: (1) determination of elemental differentiation trends, (2) determination of normative mineralogy and rock type, and (3) determination of critical parameters needed to resolve closely related geologic materials.

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