for

[Figure: Figure 10: Results of the psNMF algorithm for the order 136 for NS = 5. The source 1 is identified to the level background (continuum misestimation), the sources 2, 3, 4 and 5 are identified to H2O, either directly either from the adjacent orders. No source seems to be related to CH4.]

[Figure: Figure 10: Results of the psNMF algorithm for the order 136 for NS = 5. The source 1 is identified to the level background (continuum misestimation), the sources 2, 3, 4 and 5 are identified to H2O, either directly either from the adjacent orders. No source seems to be related to CH4.]

We illustrate the approach for typical atmospheric spectroscopy. We first put forward a synthetic test, based on simple linear mixing to give a toy example and to identify the best promising algorithm. The psNMF clearly outperformed MU and BPSS2. Then we proposed a simulation, based on realistic radiative transfer and instrumental effects, applied on NOMAD-SO spectra. The detection limits goes below 500 ppt in favorable conditions, with reduced H2O and low noise level. The same range of detection limits is reach with usual approach of model fitting at a much higher computation cost and analysis effort. Given the simplicity of use, this tool may be relevant to handle large and complex datasets at first glance. As a perspective, analysis of residuals after the non-linear retrieval of the data may lower the detection limits. One can then test if the residuals are simply Gaussian noise, or if they may contain interesting features. Interestingly, a molecular specie not well mixed in the atmosphere can be most easily detected with our approach. The last section presented the results of the application on real NOMAD-SO data, using orders 119, 134 and 136, selected as they are representative of the baseline strategy of measurements in NOMAD, allowing characterization of H2O and potential detection of CH4. The outcome is that no CH4 has been identified, but H2O and CO2 are detected. Interestingly a new set of spectral lines has been discovered in the NOMAD data. These lines has been first detected in the ACS instrument data and attributed to CO2 magnetic dipole transition (Trokhimovskiy et al., 2020). We thus confirm their presence with our current analysis. One way to go back to the data is to pick the real data with the highest source contribution ˙A. Our quicklook analysis is thus only a starting point of a more complete scientific analysis. This second step will require much more prior information (chemical compounds, fundamental spectroscopic constants, radiative transfer model, ...). Future work should apply the proposed approach to other datasets, such as other NOMAD-SO orders, or other spectroscopic datasets (including hyperspectral images) from laboratory measurements, ground based telescopes or space-born spectrometers. The approach is generic enough to treat datasets that can be at first order approximated to a linear mixture.
Acknowledgements We acknowledge support from the “Institut National des Sciences de l’Univers” (INSU), the "Centre National de la Recherche Scientifique" (CNRS) and "Centre National d’Etudes Spatiales" (CNES) through the "Programme National de Planétologie" and the ExoMars TGO programs. The NOMAD experiment is led by the Royal Belgian Institute for Space Aeronomy (BIRA-IASB), assisted by Co-PI teams from Spain (IAA-CSIC), Italy (INAF-IAPS), and the United Kingdom (Open University). This project acknowledges funding by the Belgian Science Policy Office (BELSPO), with the financial and contractual coordination by the ESA Prodex Office (PEA 4000103401, 4000121493), by Spanish Ministry of Science and Innovation (MCIU) and by European funds under 21 grants PGC2018-101836-B-I00 and ESP2017-87143-R (MINECO/FEDER), as well as by UK Space Agency through grants ST/R005761/1, ST/P001262/1, ST/R001405/1 and ST/R001405/1 and Italian Space Agency through grant 2018-2-HH.0.

This work was supported by the Belgian Fonds de la Recherche Scientifique - FNRS under grant number 30442502 (ET-HOME). 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). US investigators were supported by the National Aeronautics and Space Administration. Canadian investigators were supported by the Canadian Space Agency.