Transient HCl in the atmosphere of Mars

Abstract

A major quest in Mars’ exploration has been the hunt for atmospheric gases, potentially unveiling ongoing activity of geophysical or biological origin. Here, we report the first detection of a halogen gas, HCl, which could, in theory, originate from contemporary volcanic degassing or chlorine released from gas-solid reactions. Our detections made at ~3.2 to 3.8 μm with the Atmospheric Chemistry Suite and confirmed with Nadir and Occultation for Mars Discovery instruments onboard the ExoMars Trace Gas Orbiter, reveal widely distributed HCl in the 1- to 4-ppbv range, 20 times greater than previously reported upper limits. HCl increased during the 2018 global dust storm and declined soon after its end, pointing to the exchange between the dust and the atmosphere. Understanding the origin and variability of HCl shall constitute a major advance in our appraisal of martian geo- and photochemistry.

Annual Appearance of Hydrogen Chloride on Mars and a Striking Similarity With the Water Vapor Vertical Distribution Observed by TGO/NOMAD

“…Dust optical maps from Montabone et al (2015Montabone et al ( , 2020 show a ratio of dust optical depths between perihelion and aphelion of ∼5 (even in a low dust year), which could be sufficient to either preclude the formation of HCl in northern summer, or to keep it below detectability. However, detection limits reported in Korablev et al (2021) (0.1-0.2 ppbv) and here (0.3 ppbv) are much lower than simply scaling the observed HCl abundances by a factor of 5. It is possible that a certain threshold of dust abundance is required for the formation of HCl.…”

A Review of Sample Analysis at Mars-Evolved Gas Analysis Laboratory Analog Work Supporting the Presence of Perchlorates and Chlorates in Gale Crater, Mars

“…Although O 2 is a radiolysis product of surface perchlorates salts [79], the O 2 production rate from perchlorate radiolysis is too slow to account for the measured seasonal O 2 variation in Gale crater [80]. Additionally, atmospheric HCl was detected by spectrometers on board the Trace Gas Orbiter (TGO) and was present at higher concentrations globally after dust storms, particularly the strong dust storm during Mars Year 34 [81]. HCl detections by TGO have been attributed to chlorine-bearing species (i.e., chlorides, oxychlorines) in atmospheric dust reacting with water [81].…”

“…Additionally, atmospheric HCl was detected by spectrometers on board the Trace Gas Orbiter (TGO) and was present at higher concentrations globally after dust storms, particularly the strong dust storm during Mars Year 34 [81]. HCl detections by TGO have been attributed to chlorine-bearing species (i.e., chlorides, oxychlorines) in atmospheric dust reacting with water [81].…”

The Preliminary Study of Dust Devil Tracks in Southern Utopia Planitia, Landing Area of Tianwen-1 Mission

“…In addition to the significance of the cleaning event, knowing the meteorological characteristics of dust devils is conducive to understanding the climate of Mars. One source of HCl is aerosol chemistry occurring with the dust particles lofted into the atmosphere; therefore, transient HCI in the Martian atmosphere may be associated with a fugitive dust event [14,15]. The dust content in the atmosphere is an important parameter in an atmospheric circulation model.…”