Nitrogen cycle on Mars: upper limits for NH3 and HCN as derived by NOMAD on ExoMars/TGO

The Nadir and Occultation for MArs Discovery (NOMAD, [1]) spectrometer has been collecting Mars observations since 2018, providing a massive amount of information regarding its atmospheric composition, its vertical structure and bridging the gap between the previous knowledge of the lower atmosphere and the data from other missions (e.g., MAVEN) regarding atmospheric escape. The capability of the Solar Occultation (SO) channel to map the vertical structure of the atmosphere with very high (>1000) signal to noise ratio, very high spectral resolution (>17000) and high vertical sampling (0.5 to 2 km) is valuable in many contexts and has already allowed major new discoveries in the atmosphere of Mars. Among those, particularly significant ones include the contribution to the first detection of HCl in the atmosphere [2] and the characterization of its seasonal cycle and correlation with water vapor [3]. In addition, NOMAD data has been used to put stringent constraints on the upper limits for the long-searched CH₄ and other hydrocarbons [4].

Continuing the exploration of trace species is of fundamental importance because it enables to gain new insights into unknown aspects of how Martian atmospheric chemistry works, by revealing active cycles and exchanges between atmosphere and surface. In this work, we present results related to the quantification of stringent upper limits for two nitrogen species of interest, NH₃ and HCN. Even though a nitrogen cycle on Mars is not expected, we aim at providing a quantification of upper limits for those species in different seasons and on a global scale, with the possibility to provide information to drive future observations and atmospheric modeling. Quantification of upper limits for those species was recently provided by ACS on board TGO [5], and by earlier ground-based studies (e.g. [6]), yet in this work we greatly expand the number of observations to full Martian Years and on a global scale, with the aim of exploring a wider base of data.

Mapping of NH₃ and HCN upper limits will be performed by using diffraction order 148 in NOMAD data. This order covers the spectral interval 3326-3353 cm^-1 and contains strong spectral signatures of both gases. We analyze a wide dataset comprising more than 300,000 spectra taken at all altitudes between surface and 70 km, and at all latitudes, longitudes and seasons, between April 2018 and February 2024. Once CO₂ and H₂O abundances and rotational temperatures are fitted, the residual spectra are used to derive upper limits for NH₃ and HCN, with the methods described in [4]. In this work, we will present the derived upper limits and draw some conclusions about their variability and implications for atmospheric modeling and future observation planning.

References
[1]             A. C. Vandaele et al., Space Science Reviews, vol. 214, no. 5, Aug. 2018, doi: 10.1007/s11214-018-0517-2.
[2]             O. Korablev et al., Science Advances, vol. 7, no. 7, p. eabe4386, Feb. 2021, doi: 10.1126/sciadv.abe4386.
[3]             S. Aoki et al., Geophys. Res. Letters, vol. 48, no. 11, p. e2021GL092506, 2021, doi: 10.1029/2021GL092506.
[4]             E. W. Knutsen et al., Icarus, vol. 357, p. 114266, Mar. 2021, doi: 10.1016/j.icarus.2020.114266.
[5]             A. Trokhimovskiy et al., Icarus, vol. 407, p. 115789, Jan. 2024, doi: 10.1016/j.icarus.2023.115789.
[6]             G. L. Villanueva et al., Icarus, vol. 223, no. 1, pp. 11–27, Mar. 2013, doi: 10.1016/j.icarus.2012.11.013.