A Search for O₂ on Mars and Venus: a Possible Detection of Oxygen in the Atmosphere of Mars

We have obtained Doppler-shifted spectra of Mars and Venus in the middle of the oxygen A band. There are two weak absorption features in the Martian spectrum which may be due to the O₂ molecule. If the identification is correct, the abundance of O₂ on Mars is 20 cm atm; otherwise, this value is an upper limit. This amount of oxygen is most probably mixed throughout the entire atmosphere, not con- centrated in a high-altitude photochemical layer. The equilibrium of 03 in the Martian atmosphere is discussed in an appendix, and it is shown that the presence of 20 cm atm of O₂ is not discordant with the observed lack of strong absorption in the spectrum near 2550 A. Similar observations of Venus show no indication of 0₂ and lead to an upper limit of 4 X 1O~ for the O₂/CO₂ mixing ratio. We have not been able to reconcile this result with the findings of the Venera-4 atmospheric probe. The Venus spectrum also contains four weak absorption features, three of which are probably of solar origin; the other may originate in the planet's atmosphere. Recent measurements of the strength of the A band are discussed in Appendix A.