Composition and Structure of the Martian Atmosphere: Preliminary Results from Viking 1

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Abstract
Results from the aeroshell-mounted neutral mass spectrometer on Viking I indicate that the upper atmosphere of Mars is composed mainly of CO 2 with trace quantities of N 2 , Ar, O, O 2 , and CO. The mixing ratios by volume relative to CO 2 for N 2 , Ar, and O 2 are about 0.06, 0.015, and 0.003, respectively, at an altitude near 135 kilometers. Molecular oxygen (O 2 + ) is a major component of the ionosphere according to results from the retarding potential analyzer. The atmosphere between 140 and 200 kilometers has an average temperature of about 180° ± 20°K. Atmospheric pressure at the landing site for Viking 1 was 7.3 millibars at an air temperature of 241°K. The descent data are consistent with the view that CO 2 should be the major constituent of the lower martian atmosphere.