Nonthermal hydrogen loss at Mars: Contributions of photochemical mechanisms to escape and identification of key processes

Hydrogen loss to space is a key control on the evolution of the Martian atmosphere and the desiccation of the red planet. Thermal escape is thought to be the dominant loss process, but both forward modeling studies and remote sensing observations have indicated the presence of a second, higher-temperature "nonthermal" or "hot" hydrogen component, some fraction of which also escapes. Exothermic reactions and charge/momentum exchange processes produce hydrogen atoms with energy above the escape energy, but H loss via many of these mechanisms has never been studied, and the relative importance of thermal and nonthermal escape at Mars remains uncertain. Here we estimate hydrogen escape fluxes via 47 mechanisms, using newly-developed escape probability profiles. We find that HCO$^+$ dissociative recombination is the most important of the mechanisms, accounting for 30-50% of the nonthermal escape. The reaction CO$_2^+$ + H$_2$ is also important, producing roughly as much escaping H as momentum exchange between hot O and H. Total nonthermal escape from the mechanisms considered amounts to 39% (27%) of thermal escape, for low (high) solar activity. Our escape probability profiles are applicable to any thermospheric hot H production mechanism and can be used to explore seasonal and longer-term variations, allowing for a deeper understanding of desiccation drivers over various timescales. We highlight the most important mechanisms and suggest that some may be important at Venus, where nonthermal escape dominates and much of the literature centers on charge exchange reactions, which do not result in significant escape in this study.Comment: 47 pages, 4 figures, 3 tables. Accepted manuscript. An edited version of this paper was published by AG

HCO+ Dissociative Recombination: A Significant Driver of Nonthermal Hydrogen Loss at Mars

Hydrogen escape to space has shaped Mars' atmospheric evolution, driving significant water loss. An unknown fraction of atmospheric H lost acquires its escape energy from photochemical processes, with multiple observational studies suggesting much higher densities of such &ldquo;hot&rdquo; H than models predict. Here, we show that a previously unconsidered mechanism, HCO + dissociative recombination, produces more escaping hot H than any previously studied process, potentially accounting for more than 50% of escape during solar minimum aphelion conditions and &sim;5% of the expected long-term average loss. This hot H is predicted to impact observed brightness profiles negligibly, posing a significant challenge to the interpretation of spacecraft remote sensing observations. This mechanism's efficiency is largely due to the high (63%&ndash;83%) albedo ofthe planet to H at 1&ndash;10 eV energies, indicating the likely importance of dozens of similar photochemical mechanisms for the desiccation of Mars, Venus and planets throughout the universe.</p

Geophysical and atmospheric evolution of habitable planets

The evolution of Earth-like habitable planets is a complex process that depends on the geodynamical and geophysical environments. In particular, it is necessary that plate tectonics remain active over billions of years. These geophysically active environments are strongly coupled to a planet's host star parameters, such as mass, luminosity and activity, orbit location of the habitable zone, and the planet's initial water inventory. Depending on the host star's radiation and particle flux evolution, the composition in the thermosphere, and the availability of an active magnetic dynamo, the atmospheres of Earth-like planets within their habitable zones are differently affected due to thermal and nonthermal escape processes. For some planets, strong atmospheric escape could even effect the stability of the atmosphere

Photometry of the Didymos System across the DART Impact Apparition

On 2022 September 26, the Double Asteroid Redirection Test (DART) spacecraft impacted Dimorphos, the satellite of binary near-Earth asteroid (65803) Didymos. This demonstrated the efficacy of a kinetic impactor for planetary defense by changing the orbital period of Dimorphos by 33 minutes. Measuring the period change relied heavily on a coordinated campaign of lightcurve photometry designed to detect mutual events (occultations and eclipses) as a direct probe of the satellite’s orbital period. A total of 28 telescopes contributed 224 individual lightcurves during the impact apparition from 2022 July to 2023 February. We focus here on decomposable lightcurves, i.e., those from which mutual events could be extracted. We describe our process of lightcurve decomposition and use that to release the full data set for future analysis. We leverage these data to place constraints on the postimpact evolution of ejecta. The measured depths of mutual events relative to models showed that the ejecta became optically thin within the first ∼1 day after impact and then faded with a decay time of about 25 days. The bulk magnitude of the system showed that ejecta no longer contributed measurable brightness enhancement after about 20 days postimpact. This bulk photometric behavior was not well represented by an HG photometric model. An HG 1 G 2 model did fit the data well across a wide range of phase angles. Lastly, we note the presence of an ejecta tail through at least 2023 March. Its persistence implied ongoing escape of ejecta from the system many months after DART impact

The Event Tunnel: Interactive Visualization of Complex Event Streams for Business Process Pattern Analysis Abstract

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Altitude profiles of O₂ on Mars from SPICAM stellar occultations

International audienceWe determine the first altitude profiles of O2 in the important photochemical region below 120 km in the atmosphere of Mars by analyzing Mars Express/SPICAM ultraviolet observations of six occultations of stars by the atmosphere. Over the range of 90–130 km the altitude-averaged mixing ratio of O2 relative to the major constituent CO2 varies in space and time in the range of 3.1×10-3 to 5.8×10-3, with a mean value of 4.0×10-3. This mean value exceeds by a factor of 3–4 those reported earlier for the lower atmosphere. However, some of the O2 abundance and mixing ratio profiles determined here are similar to those measured by Viking in 1976 in the upper atmosphere

Altitude profiles of O₂ on Mars from SPICAM stellar occultations

International audienceWe determine the first altitude profiles of O2 in the important photochemical region below 120 km in the atmosphere of Mars by analyzing Mars Express/SPICAM ultraviolet observations of six occultations of stars by the atmosphere. Over the range of 90–130 km the altitude-averaged mixing ratio of O2 relative to the major constituent CO2 varies in space and time in the range of 3.1×10-3 to 5.8×10-3, with a mean value of 4.0×10-3. This mean value exceeds by a factor of 3–4 those reported earlier for the lower atmosphere. However, some of the O2 abundance and mixing ratio profiles determined here are similar to those measured by Viking in 1976 in the upper atmosphere

Martian Temperature Profiles Measured by MAVEN and MRO from 20 to 160 km

International audienceWe combined the temperature profile retrieved from the Imaging UltraViolet Spectrometer (IUVS) on Mars Atmosphere and Volatile EvolutioN (MAVEN) [1] and from the Mars Climate Sounder (MCS) on Mars Reconnaissance Orbiter (MRO) [2] to obtain a measured temperature profiles spanning the altitude range from the lower (~20 km) up to the upper (~160 km) atmosphere. A temperature profile that covers such a large altitude range will allow us to investigate coupling among atmospheric regions both for static structures controlled by radiative exchange and for propagating structures such as tides and waves. To construct the merged temperature profile we required IUVS and MCS measurements at similar locations (latitude, longitude, and local time). The latitude and local time distribution of the measurements is shown