PFS/MEX limb observations of 4.3-µm CO2 non-LTE emission in the atmosphere of Mars

Abstract

We present PFS-MEX limb observations of the CO2 non-local thermodynamic equilibrium (non-LTE) emission at 4.3 µm in the atmosphere of Mars collected in more than six Martian years. With unprecedented spatial and temporal coverage, and relatively high spectral resolution, this unique dataset promises to improve our understanding of the upper atmosphere of Mars. The former allows analyses of the emission as a function of tangent altitude, solar zenith angle, season, latitude, local time, and thermal condition of the atmosphere. The latter allows unambiguous identification of several emission bands of different isotopologues. We selected observations in the altitude range 50–200 km.No emission was detected for altitudes higher than 170 km. The spectral shape of the non-LTE emission changes dramatically with the altitude of the tangent point, reflecting the different contribution of the major and minor CO2 bands and isotopologues to the total emission at different heights. For altitudes higher than 130 km the observed spectrum is dominated by the second hot (SH) bands of the main isotopologue 12C16O2 (also referred to as 626 SH). At lower altitudes, the contribution of the isotopic 13C16O2 second hot bands (636 SH) to the observed spectrum gradually increases, and is maximum around 70–80 km. Similar consideration apply to the fourth hot bands of the 12C16O2 (626 FRH), and particularly those from the (2001x) levels, whose contribution is maximum around 80–90 km. The 626 SH bands can be observed up to an altitude 160–170 km, and their emission is peaked around 120–130 km. The 626 FRH and 636 SH bands are not observed above 130–140 km. Both the first hot (FH) and the fundamental band (FB) of the main isotopologue show a peculiar behavior. Indeed, these emissions can be observed at all altitudes, from 50 km up to 170 km. The intensity of the FH band increases linearly with decreasing height, while the intensity of the FB band is essentially constant at all altitudes, and rapidly decreases above 150 km.For a fixed altitude, the solar zenith angle (SZA) is the main parameter affecting the intensity and the spectral shape of the non-LTE emission. For SZA between 0 and 40° the intensity of the emission does not show significant variations. For SZAs larger than 40° the observed emission decreases rapidly with increasing SZA, following a cosine-like relation. The different illumination also affects the spectral shape of the non-LTE emission spectrum. High incidence angles tend to increase the relative contribution of weaker bands compared to stronger/optically thicker bands. For a fixed SZA, we found variation of the intensity of the emission with local time, in response to variations of the thermal structure of the atmosphere.Latitudinal variation of the intensity of the CO2 non-LTE are also investigated. The maximum intensity is observed around the sub-solar latitudes, where the solar flux is maximum. The intensity of the emission and the altitude at which the maximum emission is observed also changes with the season. The altitude where the maximum intensity of the 626 SH bands is observed decreases from 120–130 km at the perihelion (Ls = 251°), down to ∼85 km at the southern winter solstice (Ls = 90°). This is explained by the variability of the thermal structure (scale heights) of the Martian atmosphere with the season, as a response to the changing solar flux. The altitude of a given pressure level depends on the thermal structure of the atmosphere which, in turn, depends on the season. On the contrary, the pressure level of the peak emission does not depend on the scale heights, as it is mainly controlled by the CO2 column density above the peak.These results, while on one hand confirm and provide more insights and constraints to some aspects of the non-LTE processes on Mars, on the other hand further stimulate and challenge current theoretical models, possibly bringing closer the moment in which the measurements could be inverted to derive important information about the upper mesosphere and lower thermosphere of Mars.