Vertical Structure and Variability of HCl in the Martian Atmosphere Observed by TGO-NOMAD.

Hydrogen chloride (HCl) is one of the most recent trace species discovered in the Martian atmosphere, renewing interest in the study of its climatology. Since its unambiguous detection by the Trace Gas Orbiter (TGO) in 2021 [1,2], several studies have focused on characterizing the spatial distribution and variability of this trace species and on understanding the processes controlling its presence in the atmosphere. Observations from the two TGO instruments capable of detecting HCl in solar occultation geometry, NOMAD and ACS, have consistently reported detections mainly during the second half of the Martian year, corresponding to the perihelion season and southern summer. These enhanced abundances are followed by a rapid decline toward the end of the year. During aphelion and northern summer, however, HCl detections from orbit remain rare and close to instrumental detection limits. This pattern has been observed repeatedly across multiple Martian years, with no noticeable interannual variability.

Several hypotheses have been proposed to explain the sources and sinks of HCl, its strong seasonal variability, and its preferential occurrence during perihelion. One possible source mechanism involves interactions between the surface and the atmosphere through the lifting of chlorine-bearing dust and subsequent reactions releasing chlorine species into the atmosphere. On the other hand, heterogeneous uptake by water ice particles has been proposed as a plausible sink for HCl [3, 4]. However, systematic observations of temporally and spatially collocated HCl and aerosol (dust and water ice) profiles are needed in order to provide further constraints on these hypotheses.

In this work, we present a climatology of HCl derived from four Martian years of NOMAD Solar Occultation observations. This instrument is an excellent asset for the detection and systematic monitoring of trace species in the Martian atmosphere. It provides high spectral resolution (λ/∆λ ∼ 17000) and high sensitivity thanks to the excellent signal-to-noise ratio achieved in the solar occultation geometry [5]. We investigated the seasonal and latitudinal variability of HCl vertical profiles throughout 5245 solar occultations, of which 291 profiles showed reliable HCl detections. We focused the analysis on the vertical structure and local time dependence of the retrieved profiles during the perihelion season, in addition to the study of correlations between HCl and other atmospheric species such as water vapor and aerosols. 

Our analysis confirms that HCl is primarily observed during southern summer, in agreement with previous studies analyzing orbital [1,2,6] and ground-based observations [7], while no robust detections are identified during the aphelion at tangent altitudes with optimum transmittance levels. Our results show that HCl is generally found at altitudes where water vapor abundances are enhanced, and both species exhibit a strong positive correlation, in agreement with previous studies [8]. We also observe a remarkably similar vertical structure of HCl at the morning and evening terminators. Finally, using collocated aerosol observations, we compare HCl distributions with dust and water ice abundances, finding a positive correlation with dust abundance but no clear correlation with water ice abundance.

References:

[1] Korablev, O., Olsen, K. S., Trokhimovskiy, A., Lef`evre, F., Montmessin, F., Fedorova, A. A., . . . others (2021). Transient HCl in the atmosphere of Mars. Science Advances, 7 (7),eabe4386

[2] Aoki, S., Daerden, F., Viscardy, S., Thomas, I. R., Erwin, J. T., Robert, S., . . . others (2021). Annual appearance of hydrogen chloride on Mars and a striking similarity with the water vapor vertical distribution observed by TGO/NOMAD. Geophysical Research Letters, 48 (11), e2021GL092506

[3] Taysum, B. M., Palmer, P. I., Olsen, K., Luginin, M., Ignatiev, N., Trokhimovskiy, A., . . . Korablev, O. (2024). Observed seasonal changes in Martian hydrogen chloride explained by heterogeneous chemistry. Astronomy & Astrophysics, 687 , A191.

[4] Luginin, M., Trokhimovskiy, A., Taysum, B., Fedorova, A. A., Korablev, O., Olsen, K. S.,. . . Lefevre, F. (2024). Evidence of rapid hydrogen chloride uptake on water ice in the atmosphere of Mars. Icarus, 411 , 115960.

[5] Vandaele, A. C., Lopez-Moreno, J.-J., Patel, M. R., Bellucci, G., Daerden, F., Ristic, B., . . . others (2018). NOMAD, an integrated suite of three spectrometers for the ExoMars trace gas mission: Technical description, science objectives and expected performance. Space Science Reviews, 214 (5), 1–47.

[6] Olsen, K. S., Fedorova, A. A., Kass, D. M., Kleinb¨ohl, A., Trokhimovskiy, A., Korablev, O. I., . . . others (2024). Relationships between HCl, H2O, aerosols, and temperature in the Martian atmosphere: 1. Climatological outlook. Journal of Geophysical Research: Planets, 129 (8), e2024JE008350

[7] Faggi, S., Aoki, S., Liuzzi, G., Villanueva, G., Sagawa, H., Mumma, M. J., . . . Viscardy, S. (2025). Following the HCl cycle over three Martian seasons in Mars year 36. Journal of Geophysical Research: Planets, 130 (11), e2025JE009105.

[8] Olsen, K., Fedorova, A., Kass, D., Kleinb¨ohl, A., Trokhimovskiy, A., Korablev, O., . . . others (2024). Relationships between HCl, H2O, aerosols, and temperature in the Martian atmosphere: 2. Quantitative correlations. Journal of Geophysical Research: Planets, 129 (8), e2024JE008351.