High-Dispersion Spectroscopic Obervations of Mars. I. The CO_{2} Content and Surface Pressure

From high-dispersion infrared McDonald and Lick Observatories' spectrograms of Mars we have determined a CO2 abundance of 90 + 27 (estimated error) m-atm (STP) at 200 K, chosen as a representative lower-atmosphere temperature. We used the weak 5P3 CO2 lines near X 8700 A. An abundance of 90 m-atm of CO2 gives a surface partial pressure of 6.6 mb. The observation and reductions are discussed. By comparing our weak-line CO2 abundance with Sin ton's and Kuiper's measures of the saturated 1.6- and 2.0- bands we find a total surface pressure of P, = 8 1 4 (estimated error) mb from the 1.6- band and P, =1618mb from the 2.0- band. The technique of analysis follows Kaplan, Mllnch, and Spinrad and Owen and Kuiper. The lower pressure (1.6- data) is to be preferred. A compromise value is P, = 10__+10 mb. The result is compared to the Mariner IV occultation data which yield P, = S or 6 mb if the temperature of the lower Martian atmosphere is 180 K. The difference in surface pressure is not substantial; the occultation results coupled with our CO2 abundance suggest an almost pure CO2 atmosphere for Mars.