Isotopes of chlorine from HCl in the Martian atmosphere

¹ Space Research Institute (IKI) RAS Moscow, Russia
e-mail: trokh@iki.rssi.ru
² Department of Physics, University of Oxford, Oxford, UK
³ Laboratoire Atmosphères, Milieux, Observations Spatiales (LATMOS/CNRS), Paris, France

Received: 29 March 2021
Accepted: 30 April 2021

Abstract

Hydrogen chloride gas was recently discovered in the atmosphere of Mars during southern summer seasons. Its connection with potential chlorine reservoirs and the related atmospheric chemistry is now of particular interest and actively studied. Measurements by the Atmospheric Chemistry Suite mid-infrared channel (ACS MIR) on the ExoMars Trace Gas Orbiter allow us to measure the ratio of hydrogen chloride two stable isotopologues, H³⁵Cl and H³⁷Cl. This work describes the observation, processing technique, and derived values for the chloride isotope ratio. Unlike other volatiles in the Martian atmosphere, because it is enriched with heavier isotopes, the δ³⁷Cl is measured to be − 7 ± 20°, which is almost indistinguishable from the terrestrial ratio for chlorine. This value agrees with available measurements of the surface materials on Mars. We conclude that chlorine in observed HCl likely originates from dust and is not involved in any long-term, surface-atmosphere cycle.