Upper limits of HO2 in the atmosphere of Mars from the ExoMars Trace Gas Orbiter

Monthly Notices of the Royal Astronomical Society Oxford University Press 532:4 (2024) 4429-4435

Authors:

Juan Alday, Alexander Trokhimovskiy, Denis A Belyaev, Anna A Fedorova, James A Holmes, Manish R Patel, Jonathon P Mason, Franck Lefèvre, Kevin S Olsen, Franck Montmessin, Oleg Korablev, Lucio Baggio, Andrey Patrakeev

Abstract:

Odd-hydrogen (HOx⁠) species have a crucial role in regulating the chemistry of the atmosphere of Mars and are important to understand some of the most fundamental aspects regarding its atmospheric composition such as the long-term stability of CO2⁠⁠. Despite the key role of these species for our understanding of the Martian photochemistry, there is little observational evidence constraining their abundances. In this study, we use infrared solar occultation observations from the Atmospheric Chemistry Suite aboard the ExoMars Trace Gas Orbiter to search for spectral signatures of HO2⁠ in the atmosphere of Mars. In our analysis of the data, we retrieve vertical profiles of pressure, temperature, and water vapour mixing ratio, but are unable to confidently detect the presence of HO2⁠ features in the spectra. We report upper limits of 15 ppbv (5σ⁠), which represents an order of magnitude improvement with respect to previous investigations. Comparing the derived upper limits with the expectations from 3-dimensional Global Climate Models, we find that approximately an order of magnitude improvement in the instrument sensitivity would be required to detect this molecule and/or constrain the models.

Carbon Cycle Instability for High-CO 2 Exoplanets: Implications for Habitability

The Astrophysical Journal American Astronomical Society 970:1 (2024) 32

Authors:

RJ Graham, RT Pierrehumbert

Abstract:

Implicit in the definition of the classical circumstellar habitable zone (HZ) is the hypothesis that the carbonate-silicate cycle can maintain clement climates on exoplanets with land and surface water across a range of instellations by adjusting atmospheric CO2 partial pressure (pCO2). This hypothesis is made by analogy to the Earth system, but it is an open question whether silicate weathering can stabilize climate on planets in the outer reaches of the HZ, where instellations are lower than those received by even the Archean Earth and CO2 is thought likely to dominate atmospheres. Since weathering products are carried from land to ocean by the action of water, silicate weathering is intimately coupled to the hydrologic cycle, which intensifies with hotter temperatures under Earth-like conditions. Here, we use global climate model simulations to demonstrate that the hydrologic cycle responds counterintuitively to changes in climate on planets with CO2-H2O atmospheres at low instellations and high pCO2, with global evaporation and precipitation decreasing as pCO2 and temperatures increase at a given instellation. Within the Maher & Chamberlain (or MAC) weathering formulation, weathering then decreases with increasing pCO2 for a range of instellations and pCO2 typical of the outer reaches of the HZ, resulting in an unstable carbon cycle that may lead to either runaway CO2 accumulation or depletion of CO2 to colder (possibly snowball) conditions. While the behavior of the system has not been completely mapped out, the results suggest that silicate weathering could fail to maintain habitable conditions in the outer reaches of the nominal HZ.

Observed seasonal changes in Martian hydrogen chloride explained by heterogeneous chemistry

Astronomy and Astrophysics EDP Sciences 687 (2024) A191

Authors:

Benjamin M Taysum, Paul I Palmer, Kevin Olsen, Mikhail Luginin, Nikolay Ignatiev, Alexander Trokhimovskiy, Alexey Shakun, Alexey V Grigoriev, Franck Montmessin, Oleg Korablev

Abstract:

Aims. The aim of this work is to show that the seasonal changes and vertical distribution profiles of hydrogen chloride (HCl) on Mars, as observed by the ExoMars Trace Gas Orbiter, are consistent with the production of gas-phase chlorine atoms from airborne dust and a subsequent rapid uptake of HCl onto water ice particles.

Methods. A 1D photochemistry model was equipped with a chlorine reaction network and driven by dust, water ice, and water vapour profiles measured by the ExoMars Trace Gas Orbiter instrumentation in Mars year 34. The release of Cl and O atoms from airborne dust via the hydration and photolysis of perchlorate within dust grains was parameterised using prior laboratory studies, and the heterogeneous uptake of chlorine species onto dust and water ice was included for processes known to occur in Eartha's atmosphere.

Results. Observed seasonal variations in Martian HCl are reproduced by the model, which yielded low HCl abundances (<1 ppbv) prior to the dust season that rise to 26 ppbv in southern latitudes during the dust season. Structured atmospheric layers that coincide with locations where water ice is absent are also produced. As a consequence of the Cl atoms released via our proposed mechanism, the atmospheric lifetime of methane is shortened by two orders of magnitude. This suggests that the production of Cl induced by the breakdown of hydrated perchlorate via UV radiation (or another electromagnetic radiation) in airborne Martian dust, consistent with observed profiles of HCl, could help reconcile reported variations in methane with photochemical models.

Characteristics and Changes in Ammonia Abundance Features in Jupiter’s Upper Troposphere 2022-2023

Copernicus Publications (2024)

Authors:

Steven Hill, Patrick Irwin, Charlotte Alexander, John Rogers

Constraining Enceladus' Endogenic Emission

Copernicus Publications (2024)

Authors:

Carly Howett, Francis Nimmo, John Spencer, Georgina Miles