Observations of the CO dayglow at 4.7 μm on Mars: Variations of temperature and CO mixing ratio at 50 km

The CO (2-1) and (1-0) dayglow at 4.7 μm was observed on Mars at the peak of northern summer (LS = 110°) using the CSHELL spectrograph at NASA IRTF. There are six (2-1) and two (1-0) emission lines in the observed spectra. They are contaminated by the solar CO lines and some martian and telluric lines. Fitting by synthetic spectra results in intensities of the dayglow lines and reflectivities of Mars at 4.7 μm. Mean reflectivity at 109°W from 50°S to 50°N is 0.15, similar to that observed by Mariner 6 and 7 in four regions on Mars. The CO (1-0) dayglow is excited by absorption of sunlight at 4.7 μm; the emission is optically thick with a non-LTE line distribution and peaks near 87 km. The (1-0) line intensities are converted to the (1-0) band intensity using the line distribution from Billebaud et al. (1991). Mean intensity of the CO (1-0) dayglow is 1.7 MR with a weak limb darkening to 1.3 MR. This dayglow is poorly accessible for diagnostics of the martian atmosphere. The CO (2-1) dayglow is excited by absorption of the sunlight by the CO (2-0) band at 2.35 μm with minor contributions from photolysis of CO2 and the CO (3-0) band at 1.58 μm. The dayglow is quenched by CO2 and peaks at 50 km. Intensities of the observed six (2-1) lines result in rotational temperatures that should be equal to ambient temperatures at 50 km. These temperatures are retrieved from 50°S to 90°N and vary in the range of 140–170 K with a mean value of 153 K. The observed intensities of the CO (2-1) dayglow are corrected for airmass and the surface reflection and give vertical intensities that are equal to 2.1 MR at 20°N to 50°N decreasing to 1.5 MR at 90°N and 1 MR at 45°S. The dayglow intensities depend on CO mixing ratio at 50 km and solar zenith angle. Retrieved CO mixing ratios at 50 km gradually increase from 1100 ppm at 40°S to 1600 ppm at 70°N. This behavior is very different from that observed in the lowest scale height at the same season with increase to southern polar regions because of condensation of CO2 near the south pole (Krasnopolsky, V.A. [2003]. Icarus 165, 315–325). The difference reflects complicated dynamic processes in the atmosphere. This is the first observation of CO in the middle atmosphere of Mars, and the observed behavior of CO should be further studied in both observation and theory. The CO (2-1) dayglow is a tool for remote sensing of temperature and CO at 50 km on Mars using ground-based and spacecraft instruments. The observed CO and temperatures may be used to test photochemical GCMs for Mars.