The composition of the Martian atmosphere: Minor constituents

The Mariner 6 and 7 infrared spectrometers provided data which, in principle, determine upper limits on the possible atmospheric abundance of every gaseous substance that was undetected but which has recognized absorptions in the accessible spectral region, 1.9 to 14.4 μ. Through supporting laboratory determinations of curves of growth under pressure broadening conditions appropriate to Mars, upper limits can be specified (expressed first in cm-atm, STP, for a vertical column and then in parts per million) for the following gases: NO₂ (<0.0016, 0.23); NH₃ (<0.0031, 0.44); C₃O₂ (<0.0032, 0.45); SO₂ (?0.0037, 0.52); OCS (<0.0040, 0.56); NO (<0.0050, 0.70); O₃ (<0.0061, 0.86); CH₄ (<0.026, 3.7); N₂O (<0.13, 18); HCl (<3.7, 520); HBr (?8.2, 1150); H₂S (<27, 3800). In addition, considerations of band contours, moments of inertia, and experimental absorption coefficients permit us to place useful upper limits on twenty-seven additional substances that were not detected and for which curves of growth have not been measured: less than 1.0ppm (or, <0.0071cm-atm); SF₆, NF₃, Cl₂CO, CF₄, CHF₃, Br₂CO, SiF₄; less than 6.0ppm (or, <0.035 cm-atm); C₂F₆, CO₃, F₂CO, C₂H₄, BF₃, CS₂, CHCl₃, C₂H₆, H₂CO, CH₃F, C₆H₆, CH₂I₂; less than 50ppm (or, <0.35 cm-atm): C₂H₂, HCN, CH₂Br₂, CH₂Cl₂, CH₃Br, PH₃, CH₃I, and CH₃Cl. These upper limits are compared to earlier limits based upon terrestrial measurements. The present limits placed upon the possible atmospheric content of the molecules C₃O₂ and CO₃ are of particular interest because of their relevance to the photochemistry of the Martian atmosphere.