Chlorine species play a role in the atmospheric chemistry of Earth and Venus. On Earth, hydrogen chloride in the lower atmosphere comes mainly from sea salt aerosols. It can react, e.g., with ozone and hydroxyl radicals. On Venus, HCl also is a dominant chlorine species, and its photolysis produces reactive chlorine species that take part in atmospheric chemistry. In contrast to Earth and Venus, HCl had not been observed in the atmosphere of Mars so far.
Oleg Korablev, Space Research Institute (IKI), Moscow, Russia, and colleagues have detected HCl in the atmosphere of Mars for the first time. The team used data from the ExoMars Trace Gas Orbiter (TGO), a joint project of the European Space Agency (ESA) and the Russian agency Roscosmos. TGO’s primary mission is to measure trace atmospheric species. It carries two spectrometers, the Atmospheric Chemistry Suite (ACS, infrared spectrometer) and the Nadir and Occultation for Mars Discovery suite (NOMAD, infrared and ultraviolet spectrometer).
The spectra recorded by the TGO show absorption features that can be attributed to HCl, with a wide distribution and peak HCl concentrations of 1–4 ppbv (parts per billion volume). The team found that HCl concentrations increased during a planet-wide dust storm on Mars in 2018 and declined soon after its end. This indicates an exchange between the dust and the atmosphere. Continued observation and new atmospheric simulations could lead to a better understanding of this surface-atmosphere interaction and its effect on the planet’s chlorine cycle.
- Transient HCl in the atmosphere of Mars,
Oleg Korablev, Kevin S. Olsen, Alexander Trokhimovskiy, Franck Lefèvre, Franck Montmessin, Anna A. Fedorova, Michael J. Toplis, Juan Alday, Denis A. Belyaev, Andrey Patrakeev, Nikolay I. Ignatiev, Alexey V. Shakun, Alexey V. Grigoriev, Lucio Baggio, Irbah Abdenour, Gaetan Lacombe, Yury S. Ivanov, Shohei Aoki, Ian R. Thomas, Frank Daerden, Bojan Ristic, Justin T. Erwin, Manish Patel, Giancarlo Bellucci, Jose-Juan Lopez-Moreno, Ann C. Vandaele,
Sci. Adv. 2021, 7, eabe4386.