# Revised upper limits for abundances of NH3, HCN and HC3N in the Martian atmosphere

A. Trokhimovskiy, A.A. Fedorova, Franck Lefèvre, O. Korablev, Kevin S. Olsen, J. Alday, D. Belyaev, Franck Montmessin, A. Patrakeev, N. Kokonkov

► To cite this version:

A. Trokhimovskiy, A.A. Fedorova, Franck Lefèvre, O. Korablev, Kevin S. Olsen, et al.. Revised upper limits for abundances of NH3, HCN and HC3N in the Martian atmosphere. Icarus, 2024, 407 (January), pp.115789. 10.1016/j.icarus.2023.115789. insu-04211677

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# Revised upper limits for abundances of NH₃, HCN and HC₃N in the Martian atmosphere

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A. Trokhimovskiy¹, A. A. Fedorova¹, F. Lefèvre², O. Korablev¹, K.S. Olsen³, J. Alday⁴,

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D. Belyaev¹, F. Montmessin², A. Patrakeev¹, N. Kokonkov¹

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¹ Space Research Institute (IKI) RAS, Moscow, Russia

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e-mail: trokh@cosmos.ru

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² LATMOS/CNRS, Guyancourt, France

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³ Department of Physics, University of Oxford, Oxford, UK

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⁴ The Open University, Milton Keynes, UK

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## Abstract

The Atmospheric Chemistry Suite (ACS) onboard the ExoMars Trace Gas Orbiter (TGO) 15spacecraft has been studying Mars' atmosphere since 2018. The sensitivity of the middle 16infrared channel (MIR) allows it to address many ardent topics and it is capable of improving 17and establishing upper limits for many trace species. In this work we present analysis of 18transmittance spectra in the 3332.5-3338.6 cm⁻¹ range with 30,000 resolution (λ/Δλ), 19covering absorptions lines of three nitrogen-bearing species: ammonia (NH₃), hydrogen cyanide 20(HCN) and cyanoacetylene (HC₃N). According to existing models, all of those are not expected 21to be present in a CO₂-rich Martian atmosphere, but outgassing or unknown chemistry sources 22cannot be discounted. The upper limits of 14, 1.5 and 11 ppbv are obtained for NH₃, HCN and 23HC₃N from individual occultation measurements during the warm and dusty perihelion season 24of martian year 36. For the ammonia and hydrogen cyanide the upper limits are improved 25compared to previously published results. A search for cyanoacetylene on Mars is reported for 26the first time.

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Keywords: IR spectroscopy; Mars, atmosphere; Atmospheres, composition

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## Main

Molecular nitrogen is the second most abundant species (2.8%) after CO₂ in the Martian 32atmosphere (Franz et al., 2017). Soil contains oxidised nitrogen-bearing compounds (Stern et 33al., 2015; Sutter et al., 2017). Also the chemically-derivatized molecules ammonia (NH₃) and 34hydrogen cyanide (HCN) were detected from sand, among others, with the wet chemistry 35experiments of the Sample Analysis at Mars (SAM) instrument on NASA's Curiosity rover 36(Millan et al., 2022). The combination of an elevated abundance of derivatized ammonia and 37several N-bearing organic species detected in the follow-up analyses suggested that 38indigenous N-bearing molecules are present in the soil. At the same time, expected 39atmospheric processes involving nitrogen are limited (Lefèvre and Krasnopolsky, 2017), 40because the strong N–N bond makes N₂ a relatively inert species. Odd-nitrogen species, N and 41NO, can be produced as a result of N₂ dissociation at high altitudes only. The nitrogen 42chemistry in the lower atmosphere thus greatly depends on the downward fluxes. As calculated 43by the general circulation model of Moudden and McConnell (2007), main nitrogen species 44below 40 kilometres are NO and NO₂, varying in tiny abundances from 0.1 ppbv to 0.25 ppbv. 45The current knowledge for NO₂ is <10 ppbv (Maguire, 1977) and less than 1.7 ppb for NO 46(Krasnopolsky, 2006) The sensitivity of the mid-infrared channel of the Atmospheric Chemistry 47Suite (ACS MIR) approaches the expected NO₂ 0.2 ppbv absorption features in the 2915 cm⁻¹
48band, which will be a subject of a separate study. Not considering any release processes, other
49nitrogen components are modelled to be orders of magnitude less abundant, like the pptv level
50concentrations of HNO₄ near 20 km in altitude.

51 In this work we present upper limits of somewhat unexpected molecules for Mars: 52ammonia, hydrogen cyanide and cyanoacetylene derived from ACS MIR data. The first two 53have been already searched for on Mars, the third has not yet been considered.

54 As in the case of methane (CH₄), the ammonia could be considered as a biomarker. But 55in contrast to methane, whose lifetime is considered to be on the order of a hundred years 56(Lefèvre and Krasnopolsky, 2017), ammonia is not a stable molecule in the atmosphere of Mars 57and would be quickly photolyzed within hours. Hydrogen cyanide is also not expected to be 58produced in the Martian atmosphere (Mancinelli and Banin, 2003): its permanent presence 59would require a continuous source, like active volcanoes or biota, both actively sought, but 60never detected on Mars.

61 The first NH₃ 3 ? upper limit of 8 ppbv was set by the Mariner 9 Infrared Interferometer 62Spectrometer (IRIS; Maguire, 1977). The results were obtained from an average of spectra 63from all over the planet, covering a full Martian year (MY), starting from Ls = 293° in MY 9. In 642004, the detection of ammonia in the atmosphere of Mars by the Planetary Fourier 65Spectrometer (PFS) was announced; the report was not followed by a publication. The NH₃ 66was then sought using NIRSPEC at Keck-2 observatory (Villanueva et al., 2013). The 67observations at Ls = 352° in MY 27 provided a 57 ppbv 3 ? upper limit, and at Ls = 83° in MY 6830, a 45 ppbv upper limit. In parallel, they have established HCN upper limits of 4.5 and 2.1 69ppbv, respectively.

70 HC₃N was not considered as a goal for Martian atmospheric surveys, while it has a 71notable astrochemical importance (e.g. Thelen et al., 2019; Jiang et al., 2017). The most recent 72version of the spectroscopy database HITRAN2020 (Gordon et al., 2022) includes, for the first 73time, ro-vibrational transitions relative to all seven vibrational modes of HC₃N up to 3400 cm⁻¹, 74and rotational data in the ground state and many vibrational states of all normal modes. In this 75work, we used the recently documented band around 3327 cm⁻¹ to set the first upper limit on 76HC₃N in Mars atmosphere.

77 The ACS package for the Trace Gas Orbiter (TGO) is a part of the ExoMars mission 78(Korablev et al., 2018; Vago et al., 2015). ACS includes the mid-infrared channel (MIR), a 79versatile instrument for the spectral range 2.3-4.2 µm. It is a cross-dispersion echelle 80instrument dedicated to solar occultation measurements. The resolving power of 30,000 and a 81signal-to-noise ratio (SNR) of 5,000 per pixel per 2 s acquisition make it perfect for detecting 82weak absorptions (Trokhimovskiy et al., 2020; Olsen et al., 2020). We use the ACS MIR 83measurements obtained using secondary grating position 13, which includes 27 diffraction 84orders (190-216) encompassing a spectral range of 3178-3643 cm⁻¹. Order 199 covers a range 853330.5-3355.1 cm⁻¹, where all three gases, NH₃, HCN, and HC₃N, have absorption lines. This 86range also contains CO₂ and H₂O lines, which are used for additional pixel-to-wavenumber 87calibration. To retrieve upper limits in this work, we used a narrower 3332.5-3338.6 cm⁻¹ fitting 88window (see Figure 1 Panel A for model spectra of sought gases and data example).

89 Regular and optimised position 13 measurements have started mid MY 36. The dataset 90analysed includes 135 ACS MIR occultations performed in MY 36 during southern summer at 91the perihelion season (Ls = 212°-360°; TGO orbits 19,656-22,705). The observation period is 92characterised by greater dust load, higher temperatures, elevation of water vapour up to 93120 km (Belyaev et al., 2021; Fedorova et al., 2023). Recently discovered by ACS, hydrogen 94chloride is being observed exceptionally in the perihelion season (Korablev et al., 2021; Olsen 95et al., 2021), emphasising more active atmospheric chemistry. The observed latitudes range
96from -77°S to 90°N, with the majority of occultations occuring at high latitudes (Korablev et al., 1972-2018).

98 The retrieval is performed by the iterative Levenberg-Marquardt iterative algorithm with 99Tikhonov regularisation developed to analyse data from SPICAM IR spectrometer on Mars 100Express (Fedorova et al., 2018) and TGO's ACS near-infrared channel (NIR) data (Fedorova et al., 2011, 2020) and adapted for ACS MIR data (Trokhimovskiy et al., 2021; Fedorova et al., 2022). 102The CO₂ density and temperature profiles were not retrieved, but taken from ACS NIR 103simultaneous measurements (Korablev et al., 2018; Fedorova et al., 2023). The free 104parameters are aerosol extinction and volume mixing ratios of gases. In the absence of 105detection, the uncertainty on the retrieved quantities is given by the covariance matrix of the 106solution, which defines the upper limit. The retrieved values are generally within a 1? away 107from zero; we present the three-sigma upper limits as a confident non-detection case. The 108vertical dependence of the upper limits is typical for occultation experiments and for Mars 109reaches the best values at altitudes of about 10-25 km. At higher altitudes, the line-of-sight 110captures only a miserable number of searched molecules, at lower altitudes the signal is 111diminished by aerosol load. On Figure 1 Panel B we present a collection of three-sigma upper 112limits for NH₃ from different orbits. The vertical pattern for HCN and HC₃N is similar.

113 The best upper limits are achieved at Ls = 233°, latitude 64°N and altitude 10 km, and 114accordingly for NH₃, HCN and HC₃N are 14, 1.5 and 11 ppbv. Tens of observations provide 115upper limits below 33, 3.5 and 30 ppbv at high latitudes in both hemispheres during the whole 116season observed. The upper limit for HC₃N is not restrictive since HCN is typically the most 117abundant nitrile. At lower latitudes, with more aerosols and less occultations available, the best 118upper limits by ACS MIR for NH₃, HCN and HC₃N were 114, 12 and 95 ppbv. These numbers 119were achieved at Ls = 287°, latitude -12°S and altitude 39 km.

120 The upper limits for ammonia and hydrogen cyanide are several times below those 121obtained by Villanueva et al. (2013) for the end of MY 27. Moreover, the ACS data cover the 122entire perihelion season and different latitudes. The upper limits for cyanoacetylene in the 123Martian atmosphere are obtained for the first time. Tracing these species, unexpected on Mars, 124with stringent upper limits is important to monitor potential outgassing events and constrain 125chemical models. As the ExoMars mission continues, ACS will extend the trace species search 126further.

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130 Figure 1. Panel A. Example of ACS MIR data from order 199 and the best fit at tangent 131altitude 18 km, orbit 21352 N1, Ls = 299°, latitude -55°S. The model spectra for NH3, HCN and 132HC3N are shown shifted and with abundances of 50, 8 and 20 ppbv, respectively. The
133noticeable lines fitted in the data are due to CO₂ and are located at 3333.1, 3336.2 and 3337.8 cm⁻¹, the H₂O line is at 3334.6 cm⁻¹ and a doublet around 3336.8 cm⁻¹. Panel B. The vertical 134dependency of the three-sigma upper limits for ammonia from the ACS MIR solar occultation 135observations. The vertical pattern for HCN and HC₃N is similar. Different colours stand for 136distinct occultations.

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