Planetary Climate Dynamics

Abstract:

Recent studies of the climate and weather on Mars have benefitted greatly from the development of publicly available “reanalysis” datasets. These are quantitative reconstructions of the three-dimensional, multivariate, time-varying state of the Martian atmosphere, obtained by combining observations of atmospheric variables, usually from remote sensing platforms in orbit around the planet, with a numerical model simulation of the entire atmospheric circulation and surface, using a statistical-dynamical algorithm known as an “assimilation”. The result represents an estimate of the evolving state of the entire global weather and climate that makes optimal use of both direct observations and physical knowledge of the physics and chemistry of the atmosphere and surface (as contained in the design of the numerical model), taking into account statistical uncertainties in both measurements and model simulations. This approach thereby takes account of measurement uncertainty, the incomplete coverage of observations in both space and time and enables the estimation of variables that cannot be measured directly through the internal dynamical consistency of the numerical model. At least four such datasets have been produced during the past decade or so [1-6], based on recent spacecraft observations from the Thermal Emission Spectrometer (TES) on NASA’s Mars Global Surveyor (MGS), the Mars Climate Sounder (MCS) instrument on NASA’s Mars Reconnaissance Orbiter (MRO), the Atmospheric Chemistry Suite thermal infrared channel (ACS-TIRVIM) on ESA's Trace Gas Orbiter and the Emirates Mars InfraRed Spectrometer (EMIRS) on the Emirates Mars Mission (EMM), using different assimilation algorithms. In most cases so far, the data assimilated has been limited to retrieved surface temperature and atmospheric temperature profiles, together with column integrated dust opacities (CIDO) and certain other trace gases. Although this leads to reasonable agreement between different assimilations and with independent, out of sample measurements, the neglect of measurements of the vertical structure of atmospheric dust loading leads to significant errors in modelled radiative heating and cooling rates that temperature assimilation is then required to correct. Such a correction is highly undesirable, quite apart from misrepresenting the structure and transport of dust aerosol by the circulation, and increases the likelihood of systematic errors in the reconstructed circulation. In recent work [4], the Analysis Correction system used for the Mars Analysis Correction Assimilation (MACDA) reanalysis [1] has been extended to enable the assimilation of both CIDO and profiles of dust opacity obtained from limb-sounding instruments such as MCS. The results have enabled phenomena such as the climatological elevated dust layers during northern hemisphere summer to be captured in a reanalysis (see Fig. 1), which has been elusive in previous work. In new work presented here, this approach has now been applied to more than 12 Mars years of observations, from early MY24 to the beginning of MY36 (to date), based mainly on retrievals of temperature and dust opacity from the MGS/TES and MRO/MCS instruments that have been assimilated into the UK version of the LMD Mars Planetary Climate Model. This dataset will shortly be available publicly via the UK Centre for Environmental Data Analysis (https://www.ceda.ac.uk/). Here we present an overview of the new reanalysis and illustrate some of its results with the aim of alerting researchers to this new resource for future studies. PLR and AV acknowledge support from the UK Space Agency. The authors are grateful to Armin Kleinböhl and the MCS science team for advice and early access to retrieved data.Figure 1: Snapshots of the zonally averaged structure of the Martian atmosphere during northern late summer in Mars Year 28, showing the zonal and vertical wind, temperature and dust distribution, showing the presence of a persistent elevated dust layer from the MACDA2 reanalysis.[1] Montabone, L., Marsh, K., Lewis, S. R., Read, P. L., Smith, M. D., Holmes, J., et al. (2014). The Mars Analysis Correction Data Assimilation (MACDA) dataset V1.0. Geoscience Data Journal, 1(2), 129–139. https://doi.org/10.1002/gdj3.13[2] Greybush, S. J., Kalnay, E., Wilson, R. J., Hoffman, R. N., Nehrkorn, T., Leidner, M., et al. (2019). The Ensemble Mars Atmosphere Reanalysis System (EMARS) version 1.0. Geoscience Data Journal, 6(2), 137–150. https://doi.org/10.1002/gdj3.77[3] Holmes, J. A., Lewis, S. R., & Patel, M. R. (2020). OpenMARS: A global record of Martian weather from 1999 to 2015. Planetary and Space Science, 188, 104962. https://doi.org/10.1016/j.pss.2020.104962[4] Ruan, T., Young, R. M. B., Lewis, S. R., Montabone, L., Valeanu, A., & Read, P. L. (2021). Assimilation of both column- and layer-integrated dust opacity observations in the Martian atmosphere. Earth and Space Science, 8(12), e2021EA001869. https://doi.org/10.1029/2021EA001869[5] Young, R. M. B., Millour, E., Guerlet, S., Forget, F., Ignatiev, N., Grigoriev, A. V., et al. (2022). Assimilation of temperatures and column dust opacities measured by ExoMars TGO-ACS-TIRVIM during the MY34 Global Dust Storm. Journal of Geophysical Research: Planets, 127, e2022JE007312. https://doi.org/10.1029/2022JE007312[6] Young, R. M. B., Millour, E., Forget, F., Smith, M. D., Aljaberi, M., Edwards, C. S., et al. (2022). First assimilation of atmospheric temperatures from the Emirates Mars InfraRed Spectrometer. Geophysical Research Letters, 49, e2022GL099656.

Abstract:

Survive or not survive, that is the question of the 500-hour JWST Rocky Worlds DDT Program. Whether a terrestrial planets’ atmosphere can suffer under the intense XUV of its host, or if it completely escapes, these are the questions we explore. Zahnle & Catling (2017) defined the Cosmic Shoreline, but recent observations from JWST reveal airless worlds around M-stars, calling for a refinement of this “receding” shoreline (Pass et al. 2025). M-stars spend a longer time in pre-main sequence, subjecting their orbiting worlds to some higher intensity XUV activity. This complicates our present understanding of this shoreline. Investigating chemical effects of planet-star interactions could be the key to a more complete picture of this shoreline.  We investigate the interplay between photochemistry, mixing, and escape of carbon dioxide atmospheres under intense and mild XUV fluxes as follow on work to both Johnstone et al. (2018) and Nakayama et al. (2022). We expand on this work by adopting thermal structure models from Nakayama et al. (2022) and apply them to identify key chemical pathways for escape. We create a reduced C-O chemical network including neutral and ionic species to identify these pathways. As photochemistry simulations take into account many reactions, these 1D calculations are too computationally expensive to be done in 3D. Although rudimentary at best, the mixing parameter– eddy diffusion term, K_zz, comprises the dynamical element of 1D photochemical simulations. Here, we consider the mixing of photochemical products in competition with escape to explore the chemical pathways of retention and loss. We compare the photochemical model results for active and inactive cases for the Trappist-1 system planets. Then, using the resulting composition-dependent heating and cooling rates for Trappist-1 planets, we assess their propensity for efficient atomic line cooling versus escape. We follow the work of Chatterjee & Pierrehumbert (2024) in this assessment.  Finally, using our pathway analysis, we find an analytical formula for calculating an energy-limited escape boundary for these planets based on composition.  It is important here to note the limitations of 1D work. First, there exists an exchange of rigor between modelling chemistry and dynamics. Insights from this work are ripe for implementation into 3D GCMs, especially in response to incorporating UV-driven processes for thermospheric modelling mentioned in Ding and Wordsworth (2019). Second, interaction with the interior is important in the early phase of planetary formation, i.e., the magma ocean phase. Due to exchange between atmosphere and magma early in the planet’s formation, incorporation with an interior-atmosphere model would better constrain higher pressure chemical abundances. Although this work focuses on the upper atmosphere, extrapolation to the surface environment is a key goal for understanding a planet.  Considering planet-star interaction is imperative for the selection of targets for observation. However, it is also important when considering anomalous detections of atmospheres around planets predicted to not have an atmosphere. This could be a first step in determining an atmosphere as non-primary and/or distinguishing between an airless planet and one with high altitude haze.

Abstract:

Following seminal studies such as Muñoz’s 2007 work on HD 209458b, which simulated heavy element escape beyond the Roche lobe, one-dimensional hydrocodes have flourished, routinely solving the Euler equations to model transonic outflows across an increasingly diverse population of exoplanets. However, the modelling frontier of escape is often shaped by the hand-off from continuum to rarefied flow (Kn ≳ 0.1) and non-equilibrium processes. Molecular-kinetic techniques, long the workhorse of Solar-System aeronomy, naturally bridge this gap, providing a self-consistent description of collisional, transitional and free-molecular regimes in a single framework. Here we make the case for a concerted push toward large-scale molecular-kinetic simulations of exoplanet outflows, highlighting two end-member scenarios along the escape spectrum where forthcoming observations may allow the theory to be tested and refined.Cosmic Shoreline. Characterising the transition from Jeans (particle-by-particle) escape to subsonic and ultimately transonic bulk outflow remains an open problem in escape theory. The onset of rapid escape (~1 bar Myr⁻¹) as ionising irradiation increases is a key parameter defining the phase boundary between airless and airy rocky worlds—the “Cosmic Shoreline” (Zahnle & Catling 2017; Ji et al. 2025). Johnson et al. (2013) combined an analytic treatment with Direct Simulation Monte Carlo (DSMC; Bird 1994) to derive a critical heating rate for triggering transonic flow, working with the ansatz that the scaling of this transition extends smoothly from Pluto- to Earth-sized bodies. We will present new DSMC simulations that probe this transition for high-molecular-weight atmospheres on Earth-mass and super-Earth planets, refining the dynamics of rapid escape across this regime.Helium triplet and fractionation. Fractionation may help explain some of the non-detections of the neutral-helium triplet (1083 nm) in giant-planet outflows (Schulik & Owen 2024). Multi-fluid hydrodynamics simulations have found that the neutral helium can actually be accelerated by gravity to accrete out of the flow at a downward velocity of ~1 km s⁻¹ (Xing et al. 2023; Schulik & Owen 2024). We note that the ratio of the slip velocity to the thermal speed of the outflow scales with the Knudsen number for collisionality, ΔU/ Vth~ KnHe . Thus, we will discuss how a significant slip velocity may require Kn ≳ 0.1, a regime in which the fractionation process may be better described with molecular-kinetics, possibly with implications for predictions of the transit depth of the helium triplet.Moreover, the Direct Simulation Monte Carlo (DSMC) method offers some desirable properties over hydrocodes: it scales naturally to fully three-dimensional geometries, albeit at significant computational cost, and naturally treats non-equilibrium phenomena such as photoelectron heating and excited-state populations.

Abstract:

Larger-than-Earth exoplanets are sculpted by strong stellar irradiation, but it is unknown whence they originate. Two propositions are that they formed with rocky interiors and hydrogen-rich envelopes (‘gas-dwarfs’), or with bulk compositions rich in water-ices (‘water-worlds’) . Multiple observations of super-Earth L 98-59 d have revealed its low bulk-density, consistent with substantial volatile content alongside a rocky/metallic interior, and recent JWST spectroscopy evidences a high mean molecular weight atmosphere. Its density and composition make it a waymarker for disentangling the processes which separate super-Earths and sub-Neptunes across geological timescales. We simulate the possible pathways for L 98-59 d from birth up to the present day using a comprehensive evolutionary modelling framework. Emerging from our calculations is a novel self-limiting mechanism between radiative cooling, tidal heating, and mantle rheology, which we term the 'radiation-tide-rheology feedback'. Coupled numerical modelling yields self-limiting tidal heating estimates that are up to two orders of magnitude lower than previous calculations, and yet are still large enough to enable the extension of primordial magma oceans to Gyr timescales. Our analysis indicates that the planet formed with a large amount (>1.8 mass%) of sulfur and hydrogen, and a chemically-reducing mantle; inconsistent with both the canonical gas-dwarf and water-world scenarios. A thick atmosphere and tidal heating sustain a permanent deep magma ocean, allowing the dissolution and retention of volatiles within its mantle. Transmission features can be explained by in-situ photochemical production of SO2 in a high-molecular weight H2-H2S background. These results subvert the emerging gas-dwarf vs. water-world dichotomy of small planet categorisation, inviting a more nuanced classification framework. We show that interactions between planetary interiors and atmospheres shape their observable characteristics over billions of years.

Abstract:

The “cosmic shoreline”, a semi-empirical relation that separates airless worlds from worlds with atmospheres as proposed by Zahnle & Catling (2017), is now guiding large-scale JWST surveys aimed at detecting rocky exoplanet atmospheres. We expand upon this framework by revisiting the shorelines using existing hydrodynamic escape models applied to Earth-like, Venus-like, and steam atmospheres for rocky exoplanets, and we estimate energy-limited escape rates for CH4 atmospheres. We determine the critical instellation required for atmospheric retention by calculating time-integrated atmospheric mass loss. Our analysis introduces a new metric for target selection in the Rocky Worlds DDT and refines expectations for rocky planet atmosphere searches in Cycle 4. Exploring initial volatile inventory ranging from 0.01% to 1% of planetary mass, we find that its variation prevents the definition of a unique clear-cut shoreline, though non-linear escape physics can reduce this sensitivity to initial conditions. Additionally, uncertain distributions of high-energy stellar evolution and planet age further blur the critical instellations for atmospheric retention, yielding broad shorelines. Hydrodynamic escape models find atmospheric retention is markedly more favorable for higher-mass planets orbiting higher-mass stars, with carbon-rich atmospheres remaining plausible for 55 Cancri e despite its extreme instellation. Dedicated modelling efforts are needed to better constrain the escape dynamics of secondary atmospheres, such as the role of atomic line cooling, especially for Earth-sized planets. Finally, we illustrate how density measurements can be used to statistically test the existence of the cosmic shorelines, emphasizing the need for more precise mass and radius measurements.