[Previous] | [Session 5] | [Next]

[5.07] Spectroscopic Mapping of Mars CO Mixing Ratio: Detection of Seasonal Asymmetry

V. A. Krasnopolsky (Catholic University of America)

The CO mixing ratio on Mars was mapped in northern summer (L_S = 112^o) using the IRTF/CSHELL long-slit spectrograph. The spatial resolution of 500 km is intermediate between those for the CO observations from the Phobos orbiter and the previous ground-based observations. The spectral resolving power of 4x10⁴ is high enough to measure equivalent widths of individual lines. The R8-R13 lines of the CO (3-0) band at 1.57 \mum and some CO₂ lines of comparable strength were observed. This made it possible to determine the CO mixing ratios and cancell out the effects of airmass 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. 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, elevational, and latitudinal variabilities 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. This depletion is much smaller than that of a factor of 5-8 from 0 to 20 km observed from the Phobos orbiter. 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 8.3x10^-4 northward of the subsolar latitude at 23N to the south from this latitude where CO reaches a value of 12.5x10^-4 at 50S. This asymmetry is explained by the intense condensation of CO₂ on the south polar cap at L_S =112^o. The amount of CO₂ ice accumulated on the south polar cap is equal to ~ 1 mbar of global-mean CO₂ gas according to the MOLA observations and 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. Further study of seasonal variations of this asymmetry should be made and provide a crucial test for the general circulation models.

This work is supported by NASA Planetary Astronomy and Mars Data Analysis Programs.

If the author provided an email address or URL for general inquiries, it is as follows:

Bulletin of the American Astronomical Society, 34, #3< br> © 2002. The American Astronomical Soceity.