CO2 and temperature retrievals in the Mars atmosphere from solar occultation by NOMAD-SO and ACS-MIR: performance and cross validation.

Abstract

We present simultaneous retrievals of vertical profiles of CO2 and temperature obtained from a small sample of solar occultation scans by the NOMAD-SO [1] and ACS-MIR [2] instruments on board the ExoMars Trace Gas Orbiter (TGO). The orbits or scans selected from each instrument's observations are sufficiently close in time and space so that the atmospheric variability plays a minor role and a meaningful comparison of the performance of both instrument channels is possible. This is an on-going work devoted to a proper validation of both instruments. We will present a small selection of retrievals than can be used for a critical analysis of the results obtained and as a first step into the mutual validation of NOMAD-SO and ACS-MIR.

1. Instruments and observations

Our work is focussed in the NOMAD-SO and the ACS-MIR channels, those specifically devoted to operational observations in solar occultation. Several groups in the NOMAD and ACS teams are performing retrievals of species abundances from these signals and some are reported in this conference [3,4]. We focus on the retrieval of CO2 and temperature in the 2.7 µm range, the strongest IR bands by this gas in the spectra region coverd by SO and ACS [5]. The spectra correspond to calibrated transmittances derived from SO difraction orders 164 and 165 and from MIR position 4 spectra. About a dozen strong CO2 lines are observed in these spectra up to very high tangent altitudes, around 170-180 km. The present study targets at both solar occultation signals, NOMAD-SO and ACS-MIR, but unfortunately, due to some unexpected miss-alignments among the different channels in each instrument and between NOMAD and ACS, an entirely simultaneous observation of the solar disc as it emerges or hides behind the atmosphere is not possible with both SO and MIR channels. Instead this work study the inversions from the two channels in nearly coincident solar occultations, with small changes in latitude, longitude, solar longitude and time.

2. Retrieval Scheme

We have performed the present retrievals using an inversion code previously used in the Earth upper atmosphere for routine operations of the MIPAS instrument on board the Envisat mission [6]. The inversion scheme, which has been recently adapted to Mars for limb sounding in emission from the OMEGA spectrometer on board the Mars Express spacecraft [7], is adapted here for occultation observations. At the core of the inversion scheme is the forward model KOPRA (Karlsruhe Optimized and Precise Radiative transfer Algorithm [8] ) and the inversion processor (RCP), jointly developed by the Institute for Meteorology and Climate Research (IMK) and the IAA [9].

As mentioned above we used here CO2 calibrated transmittances obtained from routine processing by the PI teams. But before injection into the inversion algorithm, a pre-processing of every orbit or scan is performed in order to correct every spectrum of spurious effects. These are essentially two: residual spectral shifts and artificial bending in single spectra. The magnitude of these two effects changes from order to order and between SO and ACS. In spite of these differences, the pre-processing is conceptually similar to both signals, and is actually performed with the same Python code. Further, the ILS associated to each of the signals is best reproduced by a double gaussian, whose detailed description (parameters describing their FWHM and peak ratios, for example) requires also a precise determination by the user, as apparently it slightly varies with time and observing conditions.

It is specially useful to obtain both density and temperature information from a single scan, and this is the purpose of the present study. The simultaneous retrieval from a given scan actually derives a CO2 density profile, and after an iterative process, a temperature profile assuming hydrostatic equilibrium. An adjustment for the pointing is also intruduced, and the aerosol extinction and possible interference from species like H2O are also considered during the inversion to obtain the best fits.

3. Results

We will discuss the fist comparisons of CO2 and temperatures from NOMAD-SO and ACS-MIR, using a common pre-processing and inversion code. The retrievals perform in a similar manner, as in both cases the number of lines, the ro-vibrational bands and the spectral region are similar. Small differences in noise and spectral resolution have some impact but overall both channels permit sounding the Mars atmosphere up to the upper thermosphere with just one single scan. Averaging kernels indicate effective vertical resolutions around 4-6 km in the middle atmosphere (50-100 km), both in CO2 and temperature. The inversion is affected by heavy dust loading and in general degrades below about 30 km and above 150 km due to saturation and noise, respectively.

Acknowledgements

The IAA/CSIC team acknowledges financial support from the State Agency for Research of the Spanish MCIU through the ‘Center of Excellence Severo Ochoa’ award for the Instituto de Astrofísica de Andalucía (SEV-2017-0709). MALV was funded by grant PGC2018-101836-B-100 (MCIU/AEI/FEDER, EU). ExoMars is a space mission of the European Space Agency (ESA) and Roscosmos. The NOMAD experiment is led by the Royal Belgian Institute for Space Aeronomy (IASB-BIRA), assisted by Co-PI teams from Spain (IAA-CSIC), Italy (INAF-IAPS), and the United Kingdom (Open University). US investigators were supported by the National Aeronautics and Space Administration.

 

References

[1] Vandaele et al., Space Science Reviews 214 (5), 2018 [2] Korablev et al., Space. Sci. Rev. 214, 7 (2018). [3] Loic et al., Update on CO₂ and temperature profiles retrievals from NOMAD-SO on board ExoMars TGO, contribution to this conference, 2020. [4] Shohei et al., Water vapor vertical profiles on Mars: Results from the first full Mars Year of TGO/NOMAD science operations, contribution to this conference, 2020. [5] Lopez-Valverde et al., Space Sci Rev (2018) 214:29 [6] Funke, B., et al. , Atmos. Chem. Phys., 9(7), 2387–2411, 2009.Funke et al., ACP, 9, 2387–2411, 2009. [7] Jimenez-Monferrer, Icarus in press, 2020 [8] Stiller et al., JQSRT, 72, 249–280, 2002 [9] von Clarmann et al., J. Geophys. Res. 108, 4746, 2003