Probing the Atmospheric Cl Isotopic Ratio on Mars: Implications for Planetary Evolution and Atmospheric Chemistry

Following the recent detection of HCl in the atmosphere of Mars by ExoMars/Trace Gas Orbiter, we present here the first measurement of the Cl-37/Cl-35 isotopic ratio in the Martian atmosphere using a set of Nadir Occultation for MArs Discovery (NOMAD) observations. We determine an isotopic anomaly of -6 78 compared to Earth standard, consistent with the -51--1 measured on Mars' surface by Curiosity. The measured isotopic ratio is also consistent with surface measurements, and suggests that Cl reservoirs may have undergone limited processing since formation in the Solar Nebula. The examination of possible sources and sinks of HCl shows only limited pathways to short-term efficient Cl fractionation and many plausible reservoirs of light Cl. Plain Language Summary Recently, HCl has been observed in the Martian atmosphere for the first time by the instruments onboard the European Space Agency (ESA) ExoMars Trace Gas Orbiter. This discovery implies that there is an active chemical and physical cycle of chlorine in the Martian atmosphere. The Cl in HCl is usually present with its two stable main isotopes, the light Cl-35 and heavy Cl-37, their ratio in chlorine-bearing compounds is variable across the Solar System, and can be revealing of the chemistry at play, surface-atmosphere interactions and the degree of evolution of chlorine reservoirs. In this work, we measure the Cl-37 to Cl-35 ratio for the first time in the Martian atmosphere. In average, we find that the measured atmospheric value is slightly lighter than the standard value at Earth (although with large uncertainty), and consistent with that measured at Mars' surface. This indicates that Cl does not actively escape from the Martian atmosphere, and that the Cl chemistry at Mars has little efficiency in changing the isotopic signature of Cl, if at all. The light Cl isotopic composition of surface and atmosphere may indicate that Cl reservoirs on Mars are less processed than other bodies in the Solar System. Key Points Cl isotopic ratio in atmospheric HCl on Mars is measured with Nadir Occultation for MArs Discovery (NOMAD), finding a slight depletion of Cl-37 compared to Earth standard The atmospheric Cl isotopic ratio is compatible with surface values measured by Mars Science Laboratory (MSL), not showing any fractionation beyond uncertainties The majority of possible HCl depletion processes at Mars yield residual HCl with lighter Cl than its source

Automated summary

Following the recent detection of HCl in the atmosphere of Mars by ExoMars/Trace Gas Orbiter, this study presents the first measurement of the Cl-37/Cl-35 isotopic ratio in the Martian atmosphere, showing a slight depletion of Cl-37 compared to the Earth standard. The isotopic ratio measured is consistent with surface values, indicating limited processing of Cl reservoirs on Mars since formation. This suggests that the Cl chemistry in the Martian atmosphere has little efficiency in changing the isotopic signature of Cl.