Solar system

Abstract:

The ExoMars Trace Gas Orbiter (TGO) has characterised trace gases in the Martian atmosphere over several Mars years, improving the accuracy of species concentration measurements and observing temporal, vertical and spatial variations. Hydrogen chloride—detected for the first time with TGO [1,2]—has been investigated recently using the mid-infrared channel on the Atmospheric Chemistry Suite (ACS MIR). HCl observations show a strong seasonal variation, with almost all of the detections occurring during the latter half of the year (solar longitudes 180-360°) in the dusty season, when water vapour is present in the Martian atmosphere and ozone concentrations are low. Chlorine-bearing species such as HCl are important to understand in Mars’ atmosphere because on Earth they are involved in numerous processes throughout the planetary system, including volcanism, and they play a key role in atmospheric chemistry, e.g., by influencing concentrations of oxidative species such as oxygen (O2) and ozone (O3).Here, we use the Mars Planetary Climate Model—a 3-D global climate model that includes a photochemical network—to explore the atmospheric HCl observations. We build on existing chlorine photochemical networks [3,4] to investigate potential source and sink mechanisms, focusing in particular on heterogeneous chemistry involving ice aerosols, and exploring the possibility of its role in direct release of HCl to the atmosphere. We also explore how chlorine species are affected indirectly by changes in the abundances of oxidative species (e.g., OH and HO2,and by extension, O and O3),driven by heterogeneous chemistry. Understanding the role of oxidative chemistry on HCl and other trace gases is key to achieving a more complete picture of processes occurring in the present-day Mars atmosphere, as well as processes that have shaped its evolution and habitability.[1] Korablev O. I. et al. (2021). Sci. Adv., 7, eabe4386. [2] Olsen K. S. et al. (2021). Astron. Astrophys., 647, A161. [3] Rajendran, K. et al. (2025). JGR: Planets 130(3), p.e2024JE008537. [4] Streeter, P. M. et al. (2025). GRL 52(6), p.e2024GL111059.

Abstract:

Hydration on the Moon’s surface is widely detected in orbital datasets (e.g. M3 on Chandrayan-1), yet its abundance and physical form (-OH, H2O, frost, and/or ice) remain poorly constrained. The lunar surface is covered in regolith fines, which impacts local thermophysical conditions, obscures underlying volatiles and modifies detectable hydration bands. Our interpretation of hydration form and abundance on the lunar surface is further limited by existing experimental constraints of water-ice spectral behaviour at the regolith interface (photometric effects) and by the restriction of current orbital datasets to the near-infrared (< ~3 µm O–H stretching mode). We are developing a laboratory approach to quantify how dust layering, regolith maturity, grain size, composition, and ice abundance control the spectral expression of water-ice across the near- and mid-infrared (1.8–20 µm), with emphasis on the ~3 and 6 µm diagnostic regions. This poster presents a preliminary experimental set-up developed ahead of the full operation of a custom-built vacuum chamber, Polar Analogue of Dust Overlying Regolith–Ice (PANDOR-I), intended to simulate airless-body and cryogenic polar conditions. In this initial laboratory set-up, the sample compartment of a Bruker 70V Fourier Transform Infrared (FTIR) spectrometer is isolated using potassium bromide (KBr) windows to enable controlled, low-pressure (~0.2 mbar) reflectance measurements of anhydrous and hydrated analogue configurations to (i) characterise the spectral expression of hydration-related structure in the ~3 and 6 µm regions under regolith simulant fines, and (ii) provide benchmark spectra for direct comparison with a Mie–Hapke forward model (band shape,depth, and mixing trends) prior to cryogenic and airless body simulations with PANDOR-I. This preliminary work will establish an empirical reference for model validation and for designing the subsequent PANDOR-I cryogenic experiments, enabling a more robust interpretation of spectrally mixed hydration signatures in forthcoming lunar datasets.

Abstract:

Whether a planet's volcanic gas is oxidising or reducing is inherited from redox conditions in the planet's mantle. It is often presumed that reactions between iron species control mantle oxygen fugacity. However, iron alone need not be the sole dictator of how oxidising the interior of a planet is. Carbon is a powerful redox element, with great potential to feed back upon the mantle redox state as it melts. Despite Earth being carbon-poor, it has been proposed that the oxygen fugacity of Earth's upper mantle is in part controlled by carbon (Holloway et al., 1992; Stagno et al., 2013); a slightly-higher volatile endowment could make carbon-powered geochemistry inescapable. Indeed, a number of known rocky exoplanets are predicted to have formed with carbon contents greater than Earth (Bergin et al., 2023). We offer a framework for how carbon is transported from solid planetary interior to atmosphere, tracking redox couplings between carbon and iron. We also incorporate a coupled 1D energy- and mass-balance model to provide first-order predictions of the rate of volcanism. We show that carbon-iron redox coupling would maintain interior oxygen fugacity in a narrow range: more reducing than Earth magma, but not reducing enough to prevent CO2 outgassing entirely.Bergin, E. A., Kempton, E. M.-R., Hirschmann, M., Bastelberger, S. T., Teal, D. J., Blake, G. A., Ciesla, F. J., & Li, J. (2023). Exoplanet Volatile Carbon Content as a Natural Pathway for Haze Formation. The Astrophysical Journal, 949, L17. Holloway, J. R., Pan, V., & Gudmundsson, G. (1992). High-pressure fluid-absent melting experiments in the presence of graphite: Oxygen fugacity, ferric/ferrous ratio and dissolved CO2. European Journal of Mineralogy, 4(1), 105–114. Stagno, V., Ojwang, D. O., McCammon, C. A., & Frost, D. J. (2013). The oxidation state of the mantle and the extraction of carbon from Earth’s interior. Nature, 493(7430).

Abstract:

To extract, or 'retrieve' atmospheric properties from the observed radiance spectra from a planetary atmosphere requires software that can generate the expected radiances from a guessed atmospheric model, compare the radiances with those measured, determine how the model should be updated to reduce any discrepancy between the modelled and observed radiances, and then iterate these steps until these differences are minimised. One such retrieval model is NEMESIS (Nonlinear optimal Estimator for MultivariatE Spectral analySIS), which was initially developed by myself and my colleagues in the 1990s, and which has since been continually updated and enhanced. NEMESIS has now been used in more than 300 papers retrieving atmospheric properties from observed thermal and solar-reflected radiance spectra from all the planetary atmospheres in our solar system and also some beyond. NEMESIS uses the Optimal Estimation framework for atmospheric retrievals and is written in FORTRAN. Recently, more Bayesian frameworks have become computationally possible and favoured, especially for exoplanetary retrievals where prior constraints are almost entirely absent. Hence, NEMESIS has recently been updated to Python (ArchNEMESIS), and combined with PyMultiNest to allow nested sampling retrievals that can better explore the degeneracy between different atmospheric properties. I will review how NEMESIS retrievals have improved our understanding of planetary atmospheres over the last 30 years and how the development of ArchNEMESIS has breathed new life into the NEMESIS/ArchNEMESIS project. 

Abstract:

Plain Language Summary: The High‐resolution Volatiles and Minerals Moon Mapper (HVM3) is one of two science instruments on the Lunar Trailblazer smallsat mission, whose science goal is to understand the distribution, abundance, and form of water on the Moon, as well as the lunar water cycle. HVM3 uses patterns in infrared light reflection and absorption at different wavelengths to detect water and minerals in rocks and soils on the Moon's surface. In July 2025 the Lunar Trailblazer mission end was declared. Here, we detail the formulation and testing of algorithms for making water and mineral maps in preparation for the anticipated HVM3 returned data using existing Moon Mineralogy Mapper (M3) and Deep Impact spacecraft lunar data sets, which are similar types of instruments. We demonstrate that presented spectral parameters can distinguish lunar minerals of interest and therefore, capture lunar mineral diversity well. We also show that a newly developed water spectral parameter can be used as a reliable indication of lunar surface water presence, thereby demonstrating the value of expected HVM3 maps for the broader scientific community as well as planning future exploration of the Moon.