Evidence of a significant rotational non-LTE effect in the CO<sub>2</sub> 4.3 µm PFS-MEX limb spectra

Abstract

Abstract. Since January 2004, the planetary Fourier spectrometer (PFS) on board the Mars Express satellite has been recording near-infrared limb spectra of high quality up to the tangent altitudes ≈ 150 km, with potential information on density and thermal structure of the upper Martian atmosphere. We present first results of our modeling of the PFS short wavelength channel (SWC) daytime limb spectra for the altitude region above 90 km. We applied a ro-vibrational non-LTE model based on the stellar astrophysics technique of accelerated lambda iteration (ALI) to solve the multispecies and multi-level CO 2 problem in the Martian atmosphere. We show that the long-standing discrepancy between observed and calculated spectra in the cores and wings of 4.3 µm region is explained by the non-thermal rotational distribution of molecules in the upper vibrational states 10011 and 10012 of the CO 2 main isotope second hot (SH) bands above 90 km altitude. The redistribution of SH band intensities from band branch cores into their wings is caused (a) by intensive production of the CO 2 molecules in rotational states with j > 30 due to the absorption of solar radiation in optically thin wings of 2.7 µm bands and (b) by a short radiative lifetime of excited molecules, which is insufficient at altitudes above 90 km for collisions to maintain rotation of excited molecules thermalized. Implications for developing operational algorithms for massive processing of PFS and other instrument limb observations are discussed.

RADIS: A nonequilibrium line-by-line radiative code for CO2 and HITRAN-like database species

“…Finally, many nonequilibrium, state-specific LBL CO 2 radiation codes can be found in the atmospheric sciences community. For instance, groups working on the simulation of Mars atmosphere radiation (where CO 2 is excited by the solar flux, as measured by space experiments such as MarsExpress), developed nonequilibrium LBL codes for CO 2 [22,23]. These codes usually have fully coupled collisional-radiative (CR) models, i.e.…”

The Problem of CO2 Reabsorption in Emission Spectra

“…The experimental spectrum in Figure 5 can be matched to the hot ν 2 + ν 3 -ν 2 (01111-01101) and 2ν 3 -ν 3 (00021-00011) bands of CO 2 , as well as to the 2ν 2 + ν 3 -2ν 2 (10012-02201) (lower display, plotted in green, blue and pink) spectrum. In the non-LTE model [13] , the vibrational temperature of the hot bands 2ν 3 -ν 3 and 2ν 2 + ν 3 -2ν 2 was set to 700 K, while other bands were modelled using T = 555 K. The 2ν 3 -ν 3 band is an especially important feature of non-LTE CO 2 within the Martian atmosphere [13] where the overpopulation of the upper state is caused by solar radiation pumping at 2.7 μm. The fundamental band of CO 2 around 2300 cm -1 was also detected in non-LTE emission via fluorescence of solar radiation in Venus's atmosphere by VIRTIS spectrometer [14] .…”