Global maps of H2O, HCl and isotopic signatures in the Martian atmosphere: synergies between NASA/IRTF and TGO/ExoMars observations.

The first detection of HCl in the atmosphere of Mars was recently achieved by the IR spectrometers (ACS and NOMAD) onboard the ExoMars Trace Gas Orbiter (TGO), revealing the presence of chlorine species in the Martian atmosphere. The two instruments report HCl abundance ranging from 1 to 4 ppbv, for Ls between 250 and 350 ^[1,2,3,4]. Perchlorate salts ^[5] (ClO₄^-) and chloride-bearing minerals ^[6] are known to be widely present on the surface, so lifted surface materials may be involved in the release of Cl in the atmosphere.
The TGO measurements showed that HCl detections are mainly confined in the specific period around the southern summer season, leading to a possible chemical pathway connected with lifted dust and the release of water vapor from the southern polar cap. Yet sources and sinks of Martian HCl are still unknown.

Orbital and rover measurements do not permit rapid hemispheric mapping, but this is achieved with ground-based observatories in a few hours, permitting accurate measurements of the global distribution of trace gases and fundamental understanding of the distribution of present volatile reservoirs on Mars, and allowing to quantify their evolution and atmospheric loss.
Importantly, with the advent of new near-IR high resolution instrumentation (e.g., Keck/NIRSPEC-II and NASA-IRTF/iSHELL) ground-based observatories are much more sensitive than before, enabling a significant contribution to understand the origin and the variability of organics on Mars. With their much broader spectral grasp, these new high resolution spectrometers also permit simultaneous mapping of trace gases, and isotopes.

In 2021 we started a comprehensive Mars observing campaign with the NASA IRTF/iSHELL spectrometer.
We performed full rapid hemispheric mapping of water, trace gases, HCl and its isotopes, with a highly temporal cadence and extended seasonal coverage from the beginning of the Martian Spring Equinox Ls ~ 356, in early Feb/2021 to about the Summer Solstice Ls ~ 79 at the end of Jul/2021.
This strategic seasonal sampling has the primary objective to properly characterize the Martian atmospheric water cycle, since the polar cap vaporizes between Ls = 0 – 70. Moreover, HCl will be searched close to the Spring Equinox, enabling further investigations of the newly suggested correlation between the release of water and HCl evidenced by TGO measurements ^[1,2,3,4].

By combining the 2D-maps obtained with iSHELL with the vertical profiles measured by TGO, we will obtain a novel 3D-picture of H₂O, HDO HCl and trace gases, and their circulation.

References
[1] Korablev, O., Olsen, K. S., Trokhimovskiy, A., Lefèvre, F., Montmessin, F., Fedorova, A. A., et al. (2021). Transient HCl in the atmosphere of Mars. Science Advances, 7, eabe4386. https://doi.org/10.1126/sciadv.abe4386
[2] Olsen, K. S. et al. "Seasonal reappearance of HCl in the atmosphere of Mars during the Mars year 35 dusty season", Astronomy & Astrophysics, A161, Doi:10.1051/0004-6361/202140329 (2021)
[3] Aoki, S., et al. (co-authors Villanueva, G., Liuzzi, G.) “Annual appearance of hydrogen chloride on Mars and a striking similarity with the water vapor vertical distribution observed by TGO/NOMAD”, Geophys. Res. Letters, 48, doi:10.1029/2021GL092506.
[4] Liuzzi G., Villanueva, G., Faggi, S., Kofman, V. et al. “Probing the Atmospheric Cl Isotopic Ratio on Mars: Implications for Planetary Evolution and Atmospheric Chemistry”, Geophys. Res. Letters, 48, doi:10.1029/2021GL092650.
[5] Hecht et al., 2009, Science, 325, 64–67.
[6] Osterloo et al., 2010, J. Geophys. Res., 115, E10012.