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Abstract:

Mars Express (MEX) is one of the most productive planetary missions of the European Space Agency (ESA). This low cost (∼150 M€) mission has been instrumental in shaping the planetary community in Europe and has contributed to paving the way for many subsequent ESA endeavours. During more than two decades, Mars Express has collected a wealth of data in all disciplines of Martian science. This paper concludes the Topical Collection “Mars Express: Pioneering Two Decades of European Science and Exploration of Mars” prepared under the auspices of the International Space Science Institute. It briefly describes various aspects of the mission (leaving details to dedicated articles), summarizes the major science achievements, discusses the lessons learned from 20 years of Mars Express operations, and bridges with future Mars science and exploration.

Abstract:

Plain Language Summary: Hydrogen chloride is a gas emitted by volcanoes on Earth. It has been hunted on Mars as a sign of recent volcanic activity, and was found with the ExoMars Trace Gas Orbiter (TGO), whose main objective is to find rare gases in the Martian atmosphere that tell us about biological or geological activity there. This commentary examines the recent results presented by Faggi et al. (2025), https://doi.org/10.1029/2025je009105 on a campaign to measure HCl in the Martian atmosphere from the Earth. From a telescope on Earth, the measurements cover the whole surface of Mars revealing how HCl is distributed and how that changes over a year. Here, we discuss the context of these results and their implications for chlorine deposits seen on the surface.

Abstract:

Some oxidized compounds in Martian soils may form through heterogeneous electrochemistry (HEC) stimulated by electrostatic discharge (ESD) during dust storms and dust devils. To test this hypothesis, we conducted medium-strength ESD experiments in a Mars simulation chamber and analyzed the Cl, O, and C isotopic compositions of the resulting chloride, (per)chlorate, and carbonate products. These ESD products exhibit substantial mass-dependent depletions in heavy isotopes: ε 37Cl from -11.3 ‰ to +2.0 ‰, ε 18O from -34.5 ‰ to -12.9 ‰, and ε 13C around -11.4 ‰. These results, when compared with isotopic measurements from recent Mars missions (ESA’s ExoMars Trace Gas Orbiter and the Sample Analysis at Mars (SAM) instrument package aboard NASA’s Curiosity rover) and Martian meteorites, indicate that HEC induced by Martian dust activities can account for a substantial portion of the (per)chlorates and carbonates identified at the surface of Mars.

Abstract:

Atmospheric convection behaves differently in hydrogen-rich atmospheres compared to higher mean molecular weight atmospheres due to compositional gradients of tracers. Previous 1D studies predict that when a condensable tracer exceeds a critical mixing ratio in H2-rich atmospheres, convection is inhibited, leading to the formation of radiative layers where the temperature decreases faster with height than in convective profiles. We use 3D convection-resolving simulations to test whether convection is inhibited in H2-rich atmospheres when the tracer mixing ratio exceeds the critical threshold, while including processes neglected in 1D, e.g., turbulent mixing and evaporation. We run two sets of simulations. First, we perform simulations initialized on saturated isothermal states and find that compositional gradients can destabilize isothermal atmospheres. Second, we perform simulations initialized on adiabatic profiles, which show distinct, stable inhibition layers form when the condensable tracer exceeds the critical threshold. Within the inhibition layer, only a small amount of energy is carried by latent heat flux, and turbulent mixing transports a small amount of tracer upward, but both are generally too weak to sustain substantial tracer or heat transport. The thermal profile gradually relaxes to a steep radiative state, but radiative relaxation timescales are long. Our results suggest stable layers driven by condensation-induced convective inhibition form in H2-rich atmospheres, including those of sub-Neptune exoplanets.

Abstract:

Astrobotic's Peregrine Mission-1 spacecraft experienced a propulsion system anomaly that prevented the lander from reaching the Moon. During the mission, several instruments operated successfully in cis-lunar space. Among them, the Peregrine Ion Trap Mass Spectrometer (PITMS) measured both the presence of outgassing water and nitrogen oxides traceable to the MON-25 oxidizer. We performed Direct Simulation Monte Carlo (DSMC) studies of the oxidizer leak on Peregrine to characterize the gas diffusion from the leak to the instrument, mediated by inter-species collisions and gas–surface interaction. We conclude that the latter process was prevalent and that diffusion paths through Peregrine are necessary to explain the PITMS detections. Our DSMC study and estimation of Peregrine's outgassing rate suggest that, at the early stage of the mission, the spacecraft released water at a rate comparable to the Space Shuttle and at a much larger rate than typical spacecraft during science operations. This provides useful information for planning future operations of science instruments on commercial missions.