arXiv subjects

Mark Hammond

Publications and source records attributed to Mark Hammond.

At least 19 recordsLinked to original sources

arXiv2026

A Highly Reflective Atmosphere on the Lava World TOI-561b Revealed by JWST/NIRSpec Phase Curve

Ultra-short period (USP) rocky planets are expected to be depleted of any substantial gaseous envelope due to the intense irradiation they receive from their host star, making the recent detection of an atmosphere around TOI-561~b particularly surprising. That finding was based on the planet's bulk density and dayside emission spectrum, but full-orbit phase curve observations offer a more comprehensive way to constrain the presence and characteristics of a planet's atmosphere. In this paper, we map TOI-561~b's 3-5~$\mu$m emission using JWST/NIRSpec multi-orbit spectroscopic phase curves, explicitly accounting for the curvature of the out-of-eclipse baseline and possible hotspot offsets. From a simple energy balance argument as well as comparing to general circulation models, we find the phase curve observations are best explained with high Bond albedo and moderate global heat transfer. Our 37-hour continuous observation provides a long baseline that allows us to model stellar granulation, the planetary phase curve, and dayside emission simultaneously, effectively disentangling these signals and yielding a dayside emission spectrum that is more robust against stellar variability and consistent with previous eclipse-only fits. Using general circulation model outputs, we constrain where clouds can plausibly form and test candidate compositions, finding that silicate clouds, such as SiO$_2$ and MgSiO$_3$, can form on the dayside near the terminator and explain the observed albedo. Our findings confirm that TOI-561~b appears to have a global reflective atmosphere, suggesting exchange of volatiles with the interior to maintain it.

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arXiv2026

Magnetic field strengths of hot giant exoplanets consistent with Solar System values

Magnetic fields are ubiquitous in the universe. They play a key role in shaping the activity of stars, the habitability of rocky planets, and the long-term retention of planetary atmospheres. Theoretical scaling laws are largely constrained by the limited set of stars and Solar System planets, leading to a wide range of possible values for hot giant planets outside of the Solar System from fractions of the Jovian field to orders of magnitude larger. Ultra-hot Jupiters, with their highly ionised atmospheres, provide a new avenue to probe magnetic effects, as their atmospheric circulation could be directly sensitive to atmospheric magnetic field strength. Using high-spectral resolution observations targeting the iron lines of ultra-hot Jupiters we measure the Doppler shift and thus the wind speed of seven transiting ultra-hot Jupiters. We find a clear decrease of wind speed with increasing planetary temperature, a trend inconsistent with purely hydrodynamic mechanisms but naturally reproduced by magnetic drag. From this relation we estimate the possible strength of magnetic fields of hot giant planets to at most a few gauss - comparable to the Jovian equatorial field. Our results support the idea that magnetic fields affect the atmospheric circulation of ultra-hot Jupiters and could provide a crucial benchmark for scaling laws used to predict magnetic fields in exoplanets, from hot Jupiters to rocky Earths with additional implications for future direct observations.

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arXiv2026

Horizontal transport as a source of disequilibrium chemistry on the nightside of a hot exoplanet

Hot Jupiters have temperature gradients of several hundreds of degrees between their permanent day and nightsides. In equilibrium, the primary carbon reservoir is expected to transition from CO on the dayside to CH4 on the nightside. Theory predicts that the atmospheric circulation, characterised by km/s winds, can advect chemical species from the dayside to the nightside faster than the time needed for the CO-to-CH4 chemical reaction to reach equilibrium. However direct evidence of this process has, so far, remained elusive, partly because it is often degenerate with other processes, such as vertical mixing or non-stellar elemental abundances. Here, we present observational evidence for such day-to-night transport of chemical species by observing both the dayside and the nightside of the hot Jupiter NGTS-10A b with the JWST/NIRSpec instrument. We constrain the presence of H2O and CO with similar abundances on both the dayside and nightside. Our observations are compatible with a solar-composition atmosphere at chemical equilibrium on the dayside, but indicative of disequilibrium chemistry for the nightside as it is significantly depleted in CH4 compared to equilibrium chemistry predictions. We further show that the lack of CH4 on the planet's nightside cannot be attributed to non-solar elemental abundances or to vertical mixing mechanisms and must therefore be due to horizontal chemical quenching. Our study shows the fundamental role atmospheric transport plays in shaping the distribution of chemical species on exoplanet atmospheres.

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arXiv2026

Mantle Convection and Nightside Volcanism on Lava World K2-141 b

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 ($1.54 R_{\oplus}$, $5.31 M_{\oplus}$) 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 CO$_{2}$ and H$_{2}$O. 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.

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arXiv2025

Eclipse Mapping with Ariel: Future Prospects for a Population-Level Mapping Survey

Eclipse mapping is a powerful tool for measuring 3D profiles of exoplanet atmospheres. To date, only JWST has been capable of widely applying this technique, but as a general observatory, it is too time-limited to conduct population-level mapping studies. Ariel, on the other hand, is a dedicated exoplanet mission set to observe 1000 transiting exoplanets, making it a natural candidate for this. To assess Ariel's mapping potential, we quantitatively benchmark its abilities against those of JWST using a simulation-and-retrieval framework with existing JWST eclipse maps as test cases. We find that for high-ranking targets, Ariel will be able to derive qualitatively similar maps to JWST using the same amount of observations; for mid-ranking targets, Ariel will be able to compete using as few as 3x as many observations; and for lower-ranking targets, the use of phase curves overcomes the need for an impractical number of repeated eclipse observations. We find that while Ariel is unlikely to have extensive latitudinal mapping abilities, it will have wide-ranging longitudinal abilities, from which the first-order atmospheric dynamics can be constrained. Using an analytically-derived metric, we determine the best eclipse mapping targets for Ariel, finding that it will be able to map nearly 100 targets using full phase curves in only quarter of its lifetime. This would be the largest mapping survey to date, and have enormous ramifications for our demographic understanding of exoplanet atmospheric dynamics. Finally, we rank all the best mapping targets for both JWST and Ariel in order to encourage future eclipse mapping studies.

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arXiv2025

A Thick Volatile Atmosphere on the Ultrahot Super-Earth TOI-561 b

Ultrashort-period (USP) exoplanets -- with $R_p \leq 2~$R$_{\oplus}$ and periods $\leq$1 day -- are expected to be stripped of volatile atmospheres by intense host star irradiation, which is corroborated by their nominal bulk densities and previous eclipse observations consistent with bare rock surfaces. However, a few USP planets appear anomalously under-dense relative to an Earth-like composition, suggesting an exotic interior structure (e.g., core-less) or a volatile-rich secondary atmosphere increasing their apparent radius. Here we present the first dayside emission spectrum of the low-density (4.3$\pm$0.4 g~cm$^{-3}$) USP planet TOI-561 b, which orbits an iron-poor, alpha-rich, $\sim$10 Gyr old thick disk star. Our 3-5 $\mu$m JWST/NIRSpec observations demonstrate the dayside of TOI-561 b is inconsistent with a bare-rock surface at high statistical significance, suggesting instead a thick volatile envelope that is cooling the dayside to well below the $\sim$3000 K expected in the bare-rock or thin-atmosphere case. These results reject the popular hypothesis of complete atmospheric desiccation for highly irradiated exoplanets and support predictions that planetary-scale magma oceans can retain substantial reservoirs of volatiles, opening the geophysical study of ultrahot super-Earths through the lenses of their atmospheres.

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arXiv2025

JWST NIRISS Transmission Spectroscopy of the Super-Earth GJ 357b, a Favourable Target for Atmospheric Retention

We present a JWST NIRISS/SOSS transmission spectrum of the super-Earth GJ 357 b: the first atmospheric observation of this exoplanet. Despite missing the first $\sim$40 % of the transit due to using an out-of-date ephemeris, we still recover a transmission spectrum that does not display any clear signs of atmospheric features. We perform a search for Gaussian-shaped absorption features within the data but find that this analysis yields comparable fits to the observations as a flat line. We compare the transmission spectrum to a grid of atmosphere models and reject, to 3-$\sigma$ confidence, atmospheres with metallicities $\lesssim$100$\times$ solar ($\sim$4 g/mol) with clouds at pressures down to 0.01 bar. We analyse how the retention of a secondary atmosphere on GJ 357 b may be possible due to its higher escape velocity compared to an Earth-sized planet and the exceptional inactivity of its host star relative to other M2.5V stars. The star's XUV luminosity decays below the threshold for rapid atmospheric escape early enough that the volcanic revival of an atmosphere of several bars of CO$_2$ is plausible, though subject to considerable uncertainty. Finally, we model the feasibility of detecting an atmosphere on GJ 357 b with MIRI/LRS, MIRI photometry, and NIRSpec/G395H. We find that, with two eclipses, it would be possible to detect features indicative of an atmosphere or surface. Further to this, with 3-4 transits, it would be possible to detect a 1 bar nitrogen-rich atmosphere with 1000 ppm of CO$_2$.

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arXiv2024

Reliable Detections of Atmospheres on Rocky Exoplanets with Photometric JWST Phase Curves

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 day-side emission than would be expected for a black body planet. Another technique is to measure phase curves of thermal emission to search for night-side 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 thirty 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 black body planet in these filters. These results make it difficult to unambiguously detect or rule out atmospheres using their photometric day-side 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 night-side thermal emission.

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arXiv2024

Geodynamics of super-Earth GJ 486b

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 towards 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 ($R=1.34$ $R_{\oplus}$, $M=3.0$ $M_{\oplus}$, T$_\mathrm{eq}\approx700$ 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 favoured for strong temperature contrasts between the dayside and nightside and higher surface temperatures.

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arXiv2024

Global weather map reveals persistent top-of-atmosphere features on the nearest brown dwarfs

Brown dwarfs and planetary-mass companions display rotationally modulated photometric variability, especially those near the L/T transition. This variability is commonly attributed to top-of-atmosphere (TOA) inhomogeneities, with proposed models including patchy thick and thin clouds, planetary-scale jets, or chemical disequilibrium. Surface mapping techniques are powerful tools to probe their atmospheric structures and distinguish between models. One of the most successful methods for stellar surface mapping is Doppler imaging, where the existence of TOA inhomogeneities can be inferred from their varying Doppler shifts across the face of a rotating star. We applied Doppler imaging to the nearest brown dwarf binary WISE 1049AB (aka Luhman 16AB) using time-resolved, high-resolution spectroscopic observations from Gemini IGRINS, and obtained for the first time H and K band simultaneous global weather map for brown dwarfs. Compared to the only previous Doppler map for a brown dwarf in 2014 featuring a predominant mid-latitude cold spot on WISE 1049B and no feature on WISE 1049A, our observations detected persistent spot-like structures on WISE 1049B in the equatorial to mid-latitude regions on two nights, and revealed new polar spots on WISE 1049A. Our results suggest stability of atmospheric features over timescale of days and possible long-term stable or recurring structures. H and K band maps displayed similar structures in and out of CO bands, indicating the cold spots not solely due to chemical hotspots but must involve clouds. Upcoming 30-m extremely large telescopes (ELTs) will enable more sensitive Doppler imaging of dozens of brown dwarfs and even a small number of directly-imaged exoplanets.

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arXiv2024

Simultaneous retrieval of orbital phase resolved JWST/MIRI emission spectra of the hot Jupiter WASP-43b: evidence of water, ammonia and carbon monoxide

Spectroscopic phase curves of hot Jupiters measure their emission spectra at multiple orbital phases, thus enabling detailed characterisation of their atmospheres. Precise constraints on the atmospheric composition of these exoplanets offer insights into their formation and evolution. We analyse four phase-resolved emission spectra of the hot Jupiter WASP-43b, generated from a phase curve observed with the MIRI/LRS onboard the JWST, to retrieve its atmospheric properties. Using a parametric 2D temperature model and assuming a chemically homogeneous atmosphere within the observed pressure region, we simultaneously fit the four spectra to constrain the abundances of atmospheric constituents, thereby yielding more precise constraints than previous work that analysed each spectrum independently. Our analysis reveals statistically significant evidence of NH3 (4$\sigma$) in a hot Jupiter's emission spectra for the first time, along with evidence of H2O (6.5$\sigma$), CO (3.1$\sigma$), and a non-detection of CH4. With our abundance constraints, we tentatively estimate the metallicity of WASP-43b at 0.6-6.5$\times$solar and its C/O ratio at 0.6-0.9. Our findings offer vital insights into the atmospheric conditions and formation history of WASP-43b by simultaneously constraining the abundances of carbon, oxygen, and nitrogen-bearing species.

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arXiv2024

Identifying and Fitting Eclipse Maps of Exoplanets with Cross-Validation

Eclipse mapping uses the shape of the eclipse of an exoplanet to measure its two-dimensional structure. Light curves are mostly composed of longitudinal information, with the latitudinal information only contained in the brief ingress and egress of the eclipse. This imbalance can lead to a spuriously confident map, where the longitudinal structure is constrained by out-of-eclipse data and the latitudinal structure is wrongly determined by the priors on the map. We present a new method to address this issue. The method tests for the presence of an eclipse mapping signal by using k-fold cross-validation to compare the performance of a simple mapping model to the null hypothesis of a uniform disk. If a signal is found, the method fits a map with more degrees of freedom, optimising its information content. The information content is varied by penalising the model likelihood by a factor proportional to the spatial entropy of the map, optimised by cross-validation. We demonstrate this method for simulated datasets then apply it to three observational datasets. The method identifies an eclipse mapping signal for JWST MIRI/LRS observations of WASP-43b but does not identify a signal for JWST NIRISS/SOSS observations of WASP-18b or Spitzer Space Telescope observations of HD 189733b. It is possible to fit eclipse maps to these datasets, but we suggest that these maps are overfitting the eclipse shape. We fit a new map with more spatial freedom to the WASP-43b dataset and show a flatter east-west structure than previously derived.

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arXiv2024

Two-Dimensional Eclipse Mapping of the Hot Jupiter WASP-43b with JWST MIRI/LRS

We present eclipse maps of the two-dimensional thermal emission from the dayside of the hot Jupiter WASP-43b, derived from an observation of a phase curve with the JWST MIRI/LRS instrument. The observed eclipse shapes deviate significantly from those expected for a planet emitting uniformly over its surface. We fit a map to this deviation, constructed from spherical harmonics up to order $\ell_{\rm max}=2$, alongside the planetary, orbital, stellar, and systematic parameters. This yields a map with a meridionally-averaged eastward hot-spot shift of $(7.75 \pm 0.36)^{\circ}$, with no significant degeneracy between the map and the additional parameters. We show the latitudinal and longitudinal contributions of the day-side emission structure to the eclipse shape, finding a latitudinal signal of $\sim$200 ppm and a longitudinal signal of $\sim$250 ppm. To investigate the sensitivity of the map to the method, we fix the non-mapping parameters and derive an "eigenmap" fitted with an optimised number of orthogonal phase curves, which yields a similar map to the $\ell_{\rm max}=2$ map. We also fit a map up to $\ell_{\rm max}=3$, which shows a smaller hot-spot shift, with a larger uncertainty. These maps are similar to those produced by atmospheric simulations. We conclude that there is a significant mapping signal which constrains the spherical harmonic components of our model up to $\ell_{\rm max}=2$. Alternative mapping models may derive different structures with smaller-scale features; we suggest that further observations of WASP-43b and other planets will drive the development of more robust methods and more accurate maps.

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arXiv2024

Hot Jupiter Diversity and the Onset of TiO/VO Revealed by a Large Grid of Non-Grey Global Circulation Models

The population of hot Jupiters is extremely diverse, with large variations in their irradiation, period, gravity and chemical composition. To understand the intrinsic planet diversity through the observed population level trends, we explore the a-priori scatter in the population created by the different responses of atmospheric circulation to planetary parameters. We use the SPARC/MITgcm 3D global circulation model to simulate 345 planets spanning a wide range of instellation, metallicity, gravity and rotation periods typical for hot Jupiters, while differentiating between models with and without TiO/VO in their atmosphere. We show that the combined effect of the planetary parameters leads to a large diversity in the ability of atmospheres to transport heat from day-side to night-side at a given equilibrium temperature. We further show that the hot-spot offset is a non-monotonic function of planetary rotation period and explain our findings by a competition between the rotational and divergent parts of the circulation. As a consequence, hot-spot offset and phase curve amplitude are not necessarily correlated. Finally, we compare the observables from our grid to the population of Spitzer and Hubble observations of hot Jupiters. We find that the sudden jump in brightness temperature observed in the Spitzer secondary eclipse measurements can be naturally explained by the cold-trapping of TiO/VO at approximately 1800K. The grid of modelled spectra, phase curves and thermal structures are made available to the community, together with a python code for visualization of the grid properties, at https://doi.org/10.5281/zenodo.10785321 and http://sim3d.oca.eu/.

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arXiv2024

Nightside clouds and disequilibrium chemistry on the hot Jupiter WASP-43b

Hot Jupiters are among the best-studied exoplanets, but it is still poorly understood how their chemical composition and cloud properties vary with longitude. Theoretical models predict that clouds may condense on the nightside and that molecular abundances can be driven out of equilibrium by zonal winds. Here we report a phase-resolved emission spectrum of the hot Jupiter WASP-43b measured from 5-12 $\mu$m with JWST's Mid-Infrared Instrument (MIRI). The spectra reveal a large day-night temperature contrast (with average brightness temperatures of 1524$\pm$35 and 863$\pm$23 Kelvin, respectively) and evidence for water absorption at all orbital phases. Comparisons with three-dimensional atmospheric models show that both the phase curve shape and emission spectra strongly suggest the presence of nightside clouds which become optically thick to thermal emission at pressures greater than ~100 mbar. The dayside is consistent with a cloudless atmosphere above the mid-infrared photosphere. Contrary to expectations from equilibrium chemistry but consistent with disequilibrium kinetics models, methane is not detected on the nightside (2$\sigma$ upper limit of 1-6 parts per million, depending on model assumptions).

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arXiv2023

Global Chemical Transport on Hot Jupiters: Insights from 2D VULCAN photochemical model

The atmospheric dynamics of tidally-locked hot Jupiters is characterized by strong equatorial winds. Understanding the interaction between global circulation and chemistry is crucial in atmospheric studies and interpreting observations. Two-dimensional (2D) photochemical transport models shed light on how the atmospheric composition depends on circulation. In this paper, we introduce the 2D photochemical (horizontal and vertical) transport model, VULCAN 2D, which improves on the pseudo-2D approaches by allowing for non-uniform zonal winds. We extensively validate our VULCAN 2D with analytical solutions and benchmark comparisons. Applications to HD 189733 b and HD 209458 b reveal a transition in mixing regimes: horizontal transport predominates below $\sim$0.1 mbar while vertical mixing is more important at higher altitudes above 0.1 mbar. Motivated by the previously inferred carbon-rich atmosphere, we find that HD 209458 b with super-solar carbon-to-oxygen ratio (C/O) exhibits pronounced C$_2$H$_4$ absorption on the morning limb but not on the evening limb, owing to horizontal transport from the nightside. We discuss when a pseudo-2D approach is a valid assumption and its inherent limitations. Finally, we demonstrate the effect of horizontal transport in transmission observations and its impact on the morning-evening limb asymmetry with synthetic spectra, highlighting the need to consider global transport when interpreting exoplanet atmospheres.

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arXiv2023

An analytical theory for the resolution attainable using eclipse mapping of exoplanets

We present an analytic theory for the resolution attainable via eclipse mapping of exoplanets, based on the Fourier components of the brightness distribution on the planetary disk. We find that the impact parameter determines which features can and cannot be seen, via the angle of the stellar edge relative to the axis of the orbit during the eclipse. We estimate the signal-to-noise ratio as a function of mapping resolution, and use this to determine the attainable resolution for a given star-planet system. We test this theory against numerical simulations and find good agreement; in particular, our predictions for the resolution as a function of stellar edge angle are accurate to the simulated data to within 10% over a wide range of angles. Our prediction for the number of spatial modes that can be constrained given a light curve error is similarly accurate. Finally, we give a list of exoplanets with the best expected resolution for observations with the NIRISS SOSS, NIRSpec G395H, and MIRI LRS instruments on JWST.

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arXiv2023

Interior dynamics of super-Earth 55 Cancri e

The ultra-short-period super-Earth 55 Cancri e has a measured radius of 1.8 Earth radii. Previous thermal phase curve observations suggest a strong temperature contrast between the dayside and nightside of around 1000 K with the hottest point shifted $41\pm12$ degrees east from the substellar point, indicating some degree of heat circulation. The dayside (and potentially even the nightside) is hot enough to harbour a magma ocean. We use results from general circulation models (GCMs) of atmospheres to constrain the surface temperature contrasts. There is still a large uncertainty on the vigour and style of mantle convection in super-Earths, especially those that experience stellar irradiation large enough to harbour a magma ocean. In this work, we aim to constrain the mantle dynamics of the tidally locked lava world 55 Cancri e. Using the surface temperature contrasts as boundary condition, we model the mantle flow of 55 Cancri e using 2D mantle convection simulations and investigate how the convection regimes are affected by the different climate models. We find that large super-plumes form on the dayside if that hemisphere is covered by a magma ocean and the nightside remains solid or only partially molten. Cold material descends into the deep interior on the nightside, but no strong downwellings form. In some cases, the super-plume also moves several tens of degrees towards the terminator. A convective regime where the upwelling is preferentially on the dayside might lead to preferential outgassing on that hemisphere which could lead to the build-up of atmospheric species that could be chemically distinct from the nightside.

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