Spectroscopic mapping of Mars CO mixing ratio: Detection of north‐south asymmetry

Abstract

The CO mixing ratio on Mars was mapped in early northern summer (LS = 112°) using the IRTF/CSHELL long‐slit spectrograph. The spatial resolution of 500 km is intermediate between those from the Phobos orbiter and the previous ground‐based observations. The spectral resolving power of 4 × 104 is high enough to measure equivalent widths of individual lines. The R8‐R13 lines of the CO (3‐0) band at 1.57 μm and some CO2 lines of comparable strength were observed. This made it possible to determine the CO mixing ratios and to cancel out the effects of air mass and dust opacity. Extraction of the CO and CO2 abundances from the observed line equivalent widths was made using the standard curve of growth approach with a special care for the CO2 lines with high lower level energies. Some formulas, which describe the effects of the low‐level energy on weak absorption lines in Mars' reflection spectra, have been derived. The MGS MOLA and TES data on the surface elevation and thermal structure of the atmosphere during the observation were applied to improve the uncertainty of the derived CO mixing ratios, which was equal to 28% for 520 individual points on the disk. Isolines of the CO mixing ratio on Mars' disk and the latitude‐longitude map were used to study the local time, elevation, and latitudinal variations of CO on Mars. The CO mixing ratio is constant within ±7% relative to local time from 700 to 1800. A decrease by a factor of 1.18 in the CO mixing ratio from elevation of −6 km to 3 km was observed. Probably, it is due to a systematic error. The most interesting result is the detection of a seasonal (winter‐summer) asymmetry in the CO mixing ratio. This asymmetry is displayed as a latitudinal increase in the CO mixing ratio from a constant value of 8.3 × 10−4 northward of the subsolar latitude at 23°N to the south from this latitude where CO reaches a value of 12.5 × 10−4 at 50°S. This asymmetry is explained by the intense condensation of CO2 on the south polar cap at LS = 112°. General circulation models predict a weak meridional circulation in southern winter, which cannot prevent the buildup of the CO mixing ratio. The amount of CO2 ice accumulated on the south polar cap is equal to ≈1 mbar of global‐mean CO2 gas according to the general circulation models of the polar processes. If this gas were released and mixed in the southern atmosphere up to the subsolar latitude, then the CO mixing ratio would be equal in both hemispheres. Photochemical accumulation of CO during the southern winter due to freezing out of water vapor does not exceed 4%, that is, an order of magnitude weaker than the observed effect. Further study of seasonal variations of the CO asymmetry should be made by observations and simulated by general circulation models.