Tobias G. Meier

Ultra-short period lava worlds offer a unique window into the coupled evolution of planetary interior and atmospheres under extreme irradiation. In this study, we investigate the mantle dynamics, nightside volcanism, and volatile outgassing on lava world K2-141 b (1.54 R_⊕, 5.31 M_⊕) using two-dimensional convection models with tracer-based volatile tracking. Our simulations explore a range of interior configurations, including models with and without plastic yielding, basal versus mixed heating, core cooling, and melt intrusion. In models without plastic yielding (i.e. with a strong lithosphere), we find that mantle upwellings form at the substellar and antistellar points, while downwellings form near the day-night terminators at the boundary between the magma ocean and cold, solid nightside. These downwellings facilitate the recycling of crustal material, representing a form of asymmetric, single-lid tectonics. The resulting magma ocean thickness varies from 200 to 300 km depending on the model parameters, corresponding to about 2-3 % of the planet’s radius. Continuous nightside volcanism produces a basaltic crust and gradually depletes the mantle of volatiles. We find that over a billion years, volcanic eruptions can outgas tens of bars of CO₂ and H₂O. We show that even relatively large volcanic eruptions on the nightside produce thermal emission signals of no more than 1 ppm, remaining below the current detectability threshold in thermal phase curves. However, for most models, outgassing rates are increased near the day-night terminators and future studies should assess whether such localised outgassing could lead to atmospheric signatures in transmission spectroscopy.

The discovery of exoplanets has uncovered a vast spectrum of planetary types, from enormous gas giants to smaller, rocky worlds akin to Earth. Among these, super-Earths are prevalent and are believed to exhibit a range of tectonic regimes. A portion of these have ultra-short periods and orbit their stars in mere hours to days, resulting in synchronous rotation with their host star. This establishes a surface temperature dichotomy like that seen on LHS 3844b, a bare-rock super-Earth with a radius approximately 1.3 times that of Earth, where temperatures reach 1040 K at the point receiving the most intense sunlight on the dayside and drop close to 0 K on the nightside.We use StagYY to model mantle convection on LHS 3844b in a 2D spherical-annulus geometry. Our models incorporate a temperature-dependent yield stress that captures both near-surface and deep lithospheric rheological variations, rather than assuming a fixed effective yield stress as in previous studies. We represent the effects of various temperature-dependent microphysical processes by varying the temperature dependence of the yield stress slope. The yield stress components in our models are systematically varied to examine their impact on tectonic style and mantle dynamics.Parameterisation of the brittle component is based on the proposition that temperature-dependent frictional weakening plays a factor in the tectonic regimes of Earth and Venus. On Earth, where low surface temperatures create a geothermal gradient that keeps much of the crust below 400°C, frictional heating can reduce the friction coefficient at high slip velocities. In contrast, Venus’ elevated surface temperatures maintain a higher friction coefficient, which helps suppress plate tectonics. In deeper lithospheric regions, elevated temperatures favour ductile deformation, which would normally weaken the lithosphere. However, these higher temperatures can also promote grain growth, counteracting dynamic strain localisation and thereby strengthening the rock.We find that hemispheric temperature differences strongly influence lithospheric strength and deformation on LHS 3844b: the colder nightside allows brittle failure to persist over greater depths, whilst the hotter dayside promotes ductile flow at shallower depths due to a much thinner lithosphere. Importantly, we find that an increased temperature dependence of the ductile yield stress amplifies the hemispheric contrast in the planet's tectonic behaviour.

The dawn of high-resolution observations with the James Webb Space Telescope will enable spatially resolved observations of ultrashort-period rocky exoplanets. Some of these planets orbit so closely to their star that they lack an atmosphere [1], which gives direct access to their surfaces and opens a window to infer their geodynamics [2]. The physical parameters of the ultrashort-period sub-Earth GJ 367 b have been observationally constrained to a planetary radius of about 0.72 to 0.75 Earth-radii and a mass between 0.48 and 0.55 Earth-masses, implying a density of 6200 to 8500 kg/m3 [3, 4], which puts this planet in a Mercury-like interior regime with a thin mantle overlying a fractionally large core.The dayside temperature ranges between 1500 to 1800 K, thus suggesting the presence of a permanent magma ocean or dayside magma pond on the surface, induced by stellar irradiation. The large uncertainty on the age of the stellar system, between 30 Myr [4] and about 8 Gyr [3], however, introduce severe uncertainties related to the compositional and thermal evolution of the planetary mantle. In this study we perform global 2D spherical annulus StagYY simulations [5, 6] of solid-state mantle convection and surface melting with the goal to constrain the geometric and compositional properties of the planet. Constraining the spatial dimensions of thermodynamic properties of partially molten, atmosphere-less planets like GJ 367 b offers unique opportunities to constrain the compositional fractionation during magma ocean epochs and provides avenues to constrain the delivery and loss cycle of atmophile elements on strongly irradiated exoplanets.References:[1] L. Kreidberg and 18 co-authors. Absence of a thick atmosphere on the terrestrial exoplanet LHS 3844b. Nature, 573:87–90, 2019.[2] T. G. Meier, D. J. Bower, T. Lichtenberg, P. J. Tackley, and B.-O. Demory. Hemispheric Tectonics on LHS 3844b. Astrophys. J. Lett., 908:L48, 2021.[3] K.W.F. Lam and 78 co-authors. GJ 367 b: A dense, ultrashort-period sub-earth planet transiting a nearby red dwarf star. Science, 374:1271–1275, 2021.[4] W. Brandner, P. Calissendorff, N. Frankel, and F. Cantalloube. High-contrast, high-angular resolution view of the GJ367 exoplanet system. Mon. Notices Royal Astron. Soc., 513:661–669, 2022.[5] J. W. Hernlund and P. J. Tackley. Modeling mantle convection in the spherical annulus. Phys. Earth Planet. Int., 171:48–54, 2008.[6] P. J. Tackley. Modelling compressible mantle convection with large viscosity contrasts in a three-dimensional spherical shell using the yin-yang grid. Phys. Earth Planet. Int., 171:7–18, 2008.

The prevalence of atmospheres on rocky planets is one of the major questions in exoplanet astronomy, but there are currently no published unambiguous detections of atmospheres on any rocky exoplanets. The MIRI instrument on JWST can measure thermal emission from tidally locked rocky exoplanets orbiting small, cool stars. This emission is a function of their surface and atmospheric properties, potentially allowing detections of atmospheres. One way to find atmospheres is to search for lower dayside emission than would be expected for a blackbody planet. Another technique is to measure phase curves of thermal emission to search for nightside emission due to atmospheric heat redistribution. Here, we compare strategies for detecting atmospheres on rocky exoplanets. We simulate secondary eclipse and phase curve observations in the MIRI F1500W and F1280W filters for a range of surfaces (providing our open-access albedo data) and atmospheres on 30 exoplanets selected for their F1500W signal-to-noise ratio. We show that secondary eclipse observations are more degenerate between surfaces and atmospheres than suggested in previous work, and that thick atmospheres can support emission consistent with a blackbody planet in these filters. These results make it difficult to unambiguously detect or rule out atmospheres using their photometric dayside emission alone. We suggest that an F1500W phase curve could instead be observed for a similar sample of planets. While phase curves are time-consuming and their instrumental systematics can be challenging, we suggest that they allow the only unambiguous detections of atmospheres by nightside thermal emission.

Many super‐Earths are on very short orbits around their host star and, therefore, more likely to be tidally locked. Because this locking can lead to a strong contrast between the dayside and nightside surface temperatures, these super‐Earths could exhibit mantle convection patterns and tectonics that could differ significantly from those observed in the present‐day solar system. The presence of an atmosphere, however, would allow transport of heat from the dayside toward the nightside and thereby reduce the surface temperature contrast between the two hemispheres. On rocky planets, atmospheric and geodynamic regimes are closely linked, which directly connects the question of atmospheric thickness to the potential interior dynamics of the planet. Here, we study the interior dynamics of super‐Earth GJ 486b ( , , K), which is one of the most suitable M‐dwarf super‐Earth candidates for retaining an atmosphere produced by degassing from the mantle and magma ocean. We investigate how the geodynamic regime of GJ 486b is influenced by different surface temperature contrasts by varying possible atmospheric circulation regimes. We also investigate how the strength of the lithosphere affects the convection pattern. We find that hemispheric tectonics, the surface expression of degree‐1 convection with downwellings forming on one hemisphere and upwelling material rising on the opposite hemisphere, is a consequence of the strong lithosphere rather than surface temperature contrast. Anchored hemispheric tectonics, where downwellings und upwellings have a preferred (day/night) hemisphere, is favored for strong temperature contrasts between the dayside and nightside and higher surface temperatures.

The vigour and style of mantle convection in tidally-locked super-Earths may be substantially different from Earth's regime where the surface temperature is spatially uniform and sufficiently cold to drive downwellings into the mantle.The thermal phase curve for super-Earth LHS 3844b suggests a solid surface and lack of a substantial atmosphere. The dayside temperature is around 1040 K and the nightside temperature is around 0 K, which is unlike any temperature contrast observed at present day for planets in the Solar System. On the other hand, the thermal phase curve of super-Earth 55 Cnc e suggests much hotter temperatures with a nightside temperature around 1380 K and a substellar point temperature around 2700 K. The substellar point is also substantially shifted eastwards, which requires efficient energy circulation in the atmosphere.Here, we use constraints from thermal phase curve observations to model the interior mantle flow. To constrain the surface temperature of 55 Cnc e, we use the results from general circulation models varying the atmospheric composition and optical depth.Depending on how strong the surface temperature contrast is, this can lead to hemispheric tectonic regimes. Such a regime could influence a planet's atmosphere through interior-exterior coupling mechanisms (e.g. volcanic outgassing). We run geodynamic simulations of the interior mantle flow using the mantle convection code StagYY. The models are fully compressible with an Arrhenius-type viscosity law where the mantle is modelled with an upper mantle, a perovskite-layer and a post-perovskite layer. The lithospheric strength is modelled through a plastic yielding criteria and the heating mode is either basal heating only or mixed heating (basal and internal heating). For LHS 3844b we find that the surface temperature dichotomy can lead to a hemispheric tectonic regime depending on the strength of the lithosphere and the heating mode in the mantle. In a hemispheric tectonic regime, downwellings occur preferentially on one side and upwellings rise on the other side (Fig. A). We compare these results to the case of 55 Cnc e, where large parts of the surface could be molten. At first order we expect that a magma ocean could homogenise the temperatures at the interface between the magma ocean and the underlying solid mantle and therefore reduce the likelihood of hemispheric tectonics operating on 55 Cnc e (Fig. B).For LHS 3844b, the contribution of the interior flux to the thermal phase curve is on the order of 15-30 K, and therefore below the detecting capabilities of current and near-future observations. However, for hemispheric tectonics, upwellings might lead to preferential melt generation and outgassing on one hemisphere that could manifest as a secondary signal in phase curve observations. Such signals could also be produced on hotter planets such as 55 Cnc e where parts of the surface are hot enough to melt.

Ultra-short period planets (USP) that are rocky in nature are plentiful in the exoplanet catalogue and may host tectonic regimes without analogues in the Solar System. Recently, the transit phase-curve of the super-Earth LHS 3844b (radius = 1.303 Earth radii) has been mapped by a 100 h Spitzer observing campaign and is consistent with a bare-rock model of a tidally-locked planet [3]. We use phase map estimates of the planet's surface temperature distribution to infer the interior dynamics of LHS 3844b using a simulation of interior flow. We infer that the strong dichotomy of surface temperature may lead to a dichotomy of interior dynamics, with implications for the interior-atmosphere exchange of volatiles in USP rocky planets. We use a mantle convection code with parameters from the literature suitable for LHS 3844b. The material properties of the interior, including viscosity, are constrained from previous theoretical calculations and laboratory experiments.For models with a weak near-surface layer (lithosphere), hot upwellings are evenly distributed between the dayside and nightside, albeit strong cold downwellings form dominantly on the nightside. Upwellings on the nightside have less lateral mobility than the dayside because they are confined by downwellings. In contrast, for models with a strong near-surface layer (lithosphere), the interior dynamics are mostly driven by a prominent downwelling on the dayside which flushes hot material from the lower thermal boundary layer around the core towards the nightside where upwellings preferentially arise. Therefore, current and near-future space missions could detect a signal from volatile outgassing or volcanism driven by interior upwellings that is either globally uniform or originating from the nightside only. A spatially-variable release of volatiles may produce corresponding asymmetries in astrophysical observations, such as secondary features in phase curves.1. MethodsThe geodynamic model is devised assuming an Earth-like composition with Perovskite (Pv) dominating the uppermost mantle and post-Perovskite (pPv) the lowermost mantle. A temperature- and pressure-dependent Arrhenius-type viscosity law is used with the lower-bound post-Perovskite parameters given in [2]. Mantle convection is modelled using the code StagYY [3] in a two-dimensional spherical annulus geometry. The effects of compressibility are modelled by employing the truncated anelastic liquid approximation (TALA) where a reference state density profile is assumed.We include plastic yielding to model the brittle nature of the lithosphere. Yielding occurs when the local stress state exceeds a prescribed yield stress criteria, thereby modelling plate-tectonic-like behaviour at the surface. 2. Results and DiscussionFor models with a weak near-surface layer (low yield stress criteria), we find that hot upwellings are evenly distributed around the CMB. Downwellings form on both sides, but they are stronger on the nightside (Figure 1). The high viscosity upwellings tend to dictate the long-wavelength pattern of flow, and therefore shepherd the upwellings along the CMB into position between the downwellings. On the dayside, the upwellings exhibit more lateral motion as the downwellings are less viscous and thus the shepherding effect is less pronounced. For a strong near-surface layer (large yield stress), an approximate degree-1 convection pattern is established: a prominent downwelling forms on the dayside and descends into the deep mantle. It flushes hot material from the lower thermal boundary layer around the CMB from the dayside to the nightside, thereby promoting the formation of upwellings on the nightside which rise to the surface. A return flow of cold material from the nightside to the dayside is established in the uppermost Perovskite layer (Figure 2).An explanation for this flow pattern is that yielding does not occur on the nightside and therefore the formation of downwellings is inhibited. On the dayside, high temperatures weaken the near-surface through the strong temperature dependance of viscosity and therefore enable downwellings to form.Previous work studying the interior dynamics of incompressible Earth-sized planets with asymmetric surface temperatures finds a flow pattern that is characterised by an upwelling near the substellar point and a near-surface flow from the dayside to the nightside [4]. Although this is also a degree-1 flow, it is reversed compared to our models. We surmise that the difference in flow direction is due to the strongly varying viscosity that we impose, and possibly the effects of compressibility and planet size.In summary, our models show that a strong dichotomy of the surface temperature can lead to an asymmetry in the distribution of upwellings and downwellings in the interior of rocky super-Earths, depending on the strength of the near-surface layer.An asymmetric flow pattern has not been inferred for any of the rocky Solar System planets, and therefore our work presents the first observation-constrained inference of a novel tectonic mode that is applicable to some rocky exoplanets. In this regime, upwellings could produce corresponding asymmetry in the production of near-surface melt and subsequent outgassing, which can serve as a future test to distinguish between different model predictions.AcknowledgementsTGM and DJB acknowledge Swiss National Science Foundation (SNSF) Ambizione Grant 173992.References[1] Tackley, Paul J.: Modelling compressible mantle convection with large viscosity contrasts in a three-dimensional spherical shell using the Yin-Yang grid, Physics of the Earth and Planetary Interiors, Vol. 171, pp. 7-18, 2008.[2] Tackley, Paul J., et al.: Mantle dynamics in super-Earths: Post-perovskite rheology and self-regulation of viscosity, Vol. 225, pp. 50-61, 2013.[3] Kreidberg, L., et al.: Absence of a thick atmosphere on the terrestrial exoplanet LHS 3844b, Vol. 573, pp. 87-90, 2019.[4] Van Summeren, J.,...