Alkaline Exospheres of Exoplanet Systems: Evaporative Transmission Spectra

Hydrostatic equilibrium is an excellent approximation for the dense layers of planetary atmospheres where it has been canonically used to interpret transmission spectra of exoplanets. Here we exploit the ability of high-resolution spectrographs to probe tenuous layers of sodium and potassium gas due to their formidable absorption cross-sections. We present an atmosphere-exosphere degeneracy between optically thick and optically thin mediums, raising the question of whether hydrostatic equilibrium is appropriate for Na I lines observed at exoplanets. To this end we simulate three non-hydrostatic, evaporative, density profiles: (i) escaping, (ii) exomoon, and (iii) torus to examine their imprint on an alkaline exosphere in transmission. By analyzing an evaporative curve of growth we find that equivalent widths of $W_{\mathrm{Na D2}} \sim 1- 10$ mA are naturally driven by evaporation rates $\sim 10^3 - 10^5$ kg/s of pure atomic Na. To break the degeneracy between atmospheric and exospheric absorption, we suggest that if the line ratio is $\mathrm{D2/D1} \gtrsim 1.2$ the gas is optically thin on average and roughly indicating a non-hydrostatic structure of the atmosphere/exosphere. We show this is the case for Na I observations at hot Jupiters WASP-49b and HD189733b and also simulate their K I spectra. Lastly, motivated by the slew of metal detections at ultra-hot Jupiters, we suggest a toroidal atmosphere at WASP-76b and WASP-121b is consistent with the Na I data at present.Comment: 23 pages, 21 figures, accepted by MNRA

Linking the evolution of terrestrial interiors and an early outgassed atmosphere to astrophysical observations

A terrestrial planet is molten during formation and may remain so if subject to intense insolation or tidal forces. Observations continue to favour the detection and characterisation of hot planets, potentially with large outgassed atmospheres. We aim to determine the radius of hot Earth-like planets with large outgassed atmospheres and explore differences between molten and solid silicate planets and their influence on the mass-radius relationship and transmission and emission spectra. An interior-atmosphere model, combined with static structure calculations, tracks the evolving radius of a rocky mantle that is outgassing CO$_2$ and H$_2$O. Synthetic emission and transmission spectra are generated for CO$_2$ and H$_2$O dominated atmospheres. Atmospheres dominated by CO$_2$ suppress the outgassing of H$_2$O to a greater extent than previously realised, as previous studies have applied an erroneous relationship between volatile mass and partial pressure. We therefore predict more H$_2$O can be retained by the interior during the later stages of magma ocean crystallisation. Furthermore, formation of a lid at the surface can tie outgassing of H$_2$O to the efficiency of heat transport through the lid, rather than the atmosphere's radiative timescale. Contraction of the mantle as it solidifies gives $\sim5\%$ radius decrease, which can partly be offset by addition of a relatively light species to the atmosphere. We conclude that a molten silicate mantle can increase the radius of a terrestrial planet by around $5\%$ compared to its solid counterpart, or equivalently account for a $13\%$ decrease in bulk density. An outgassing atmosphere can perturb the total radius according to its speciation. Atmospheres of terrestrial planets around M-stars that are dominated by CO$_2$ or H$_2$O can be distinguished by observing facilities with extended wavelength coverage (e.g., JWST).Comment: 19 pages, published in A&A, abstract shortene

Planetary evolution with atmospheric photoevaporation II: Fitting the slope of the radius valley by combining boil-off and XUV-driven escape

The Kepler satellite has revealed a gap between sub-Neptunes and super-Earths that atmospheric escape models had predicted as an evaporation valley. We seek to contrast results from a simple XUV-driven energy-limited (ELIM) escape model against those from a direct hydrodynamic (HYDRO) model. Besides XUV-driven escape, the latter also includes the boil-off regime. We couple the two models to an internal structure model and follow the planets' temporal evolution over Gyr. To see the population-wide imprint of the two models, we first employ a rectangular grid in initial conditions. We then study the slope of the valley also for initial conditions derived from the Kepler planets. For the rectangular grid, we find that the power-law slope of the valley with respect to orbital period is -0.18 and -0.11 in the ELIM and HYDRO model, respectively. For the initial conditions derived from the Kepler planets, the results are similar (-0.16 and -0.10). While the slope found with the ELIM model is steeper than observed, the one of the HYDRO model is in excellent agreement with observations. The reason for the shallower slope is caused by the two regimes in which the ELIM model fails: First, puffy planets at low stellar irradiation. For them, boil-off dominates mass loss. However, boil-off is absent in the ELIM model, thus it underestimates escape relative to HYDRO. Second, massive compact planets at high XUV irradiation. For them, the ELIM approximation overestimates escape relative to the HYDRO case because of cooling by thermal conduction, neglected in the ELIM model. The two effects act together in concert to yield in the HYDRO model a shallower slope of the valley that agrees very well with observations. We conclude that an escape model that includes boil-off and a more realistic treatment of cooling mechanisms can reproduce one of the most important constraints, the valley slope.Comment: 20 pages, 11 figures, accepted to A&

Radio-Loud Exoplanet-Exomoon Survey (RLEES): GMRT Search for Electron Cyclotron Maser Emission

We conducted the first dedicated search for signatures of exoplanet-exomoon interactions using the Giant Metrewave Radio Telescope (GMRT) as part of the radio-loud exoplanet-exomoon survey (RLEES). Due to stellar tidal heating, irradiation, and subsequent atmospheric escape, candidate `exo-Io' systems are expected to emit up to $10^6$ times more plasma flux than the Jupiter-Io DC circuit. This can induce detectable radio emission from the exoplanet-exomoon system. We analyze three `exo-Io' candidate stars: WASP-49, HAT-P 12, and HD 189733. We perform 12-hour phase-curve observations of WASP-49b at 400 MHz during primary $\&$ secondary transit, as well as first $\&$ third quadratures achieving a 3$\sigma$ upper-limit of 0.18 mJy/beam averaged over four days. HAT-P~12 was observed with GMRT at 150 and 325 MHz. We further analyzed the archival data of HD 189733 at 325 MHz. No emission was detected from the three systems. However, we place strong upper limits on radio flux density. Given that most exo-Io candidates orbit hot Saturns, we encourage more multiwavelength searches (in particular low frequencies) to span the lower range of exoplanet B-field strengths constrained here.Comment: 7 pages, 3 figures, accepted for publication in The Astronomical Journa

uGMRT observations of the hot-Saturn WASP 69b: Radio-Loud Exoplanet-Exomoon Survey II (RLEES II)

Exomoons have so far eluded ongoing searches. Several studies have exploited transit and transit timing variations and high-resolution spectroscopy to identify potential exomoon candidates. One method of detecting and confirming these exomoons is to search for signals of planet-moon interactions. In this work, we present the first radio observations of the exomoon candidate system WASP 69b. Based on the detection of alkali metals in the transmission spectra of WASP-69b, it was deduced that the system might be hosting an exomoon. WASP 69b is also one of the exoplanet systems that will be observed as part of JWST cycle-1 GTO. This makes the system an excellent target to observe and follow up. We observed the system for 32 hrs at 150 MHz and 218 MHz using the upgraded Giant Metrewave Radio Telescope (uGMRT). Though we do not detect radio emission from the systems, we place strong $3\sigma$ upper limits of 3.3 mJy at 150 MHz and 0.9 mJy at 218 MHz. We then use these upper limits to estimate the maximum mass loss from the exomoon candidate.Comment: Accepted in MNRAS, 8 pages, 4 Figure

Water Ice-Vapor Evolving over Europa and Ganymede's 85 and 172 hour Orbits

International audienceEuropa and Ganymede's near-surface atmospheres are sourced primarily by magnetospheric and thermal radiation of water ice. Radiolytic sputtering triggers the formation and evolution of predominantly O2 atmospheres observed and simulated to be at least ~3 x 1014 O2/cm2. Given the previous characterization of orbital variability in Europa and Ganymede's O2 atmospheres (Leblanc et al. 2017; Oza et al. 2019) we now seek to understand the behavior of their water ice-vapor atmospheres with exosphere general model (EGM) simulations. Our 3-D model includes satellite revolution about Jupiter for Europa (τorb~85 h) and Ganymede (τorb~172 h), so that one can explicitly study the orbital evolution of water product species. By sputtering and sublimating a water vapor atmosphere, with components of hydroxide (OH), hydrogen (H), peroxide (H2O2) and a recently identified thermally-driven molecular component (H2 and O2), we seek to determine the ambient exospheric state of a pure ice surface. EGM column densities are roughly consistent with recent ground-based and space-based observations. Despite the expectation of Europa being far more cryovolcanically active, Ganymede's global exosphere harbors ~10x more water exceeding 1015 H2O/cm2. This is primarily due to the fact that Europa is smaller, and far colder than Ganymede on average. We find that when water ice sublimation is significant compared to direct sputtering, a water vapor atmosphere builds up by noon, Europa local time, and collapses during eclipse/midnight, akin to Ganymede. Furthermore, we examine several physical processes on both satellites which appear to be critical during egress, upon exit from Jupiter's shadow. Our simulations provide specific orbital phases that would be amenable to distinguishing a transient, localized atmosphere by Europa Clipper, JUICE, and future landers. At Europa for instance, simulations suggest that if line-of-sight gas phase observations detect > ~1014 H2O cm-2 at orbital longitudes when the colder, leading hemisphere begins to be illuminated Φorb~0-90°, a geologic source may be favored over a sublimated or sputtered exosphere. Since the onset of the 4:2:1 Laplace Resonance with Io, Europa & Ganymede's ices have of course experienced a unique regime of tidal heating over geological timescales, whose contributions to the exosphere are poorly understood at present. The column density maps presented here serve as a background exosphere which enable the search for and characterization of trace species (Na, K, SO2) with both ground and in-situ spacecraft measurements

Atmospheric Bulges on Tidally-Locked Satellites

International audienceWe use a simple analytic model to examine the spatial distribution of a volatile species in a surface-bounded atmosphere on a rotating object that is tidally-locked to its parent body. Spatial asymmetries in such atmospheres have recently been observed via ultraviolet auroral emissions from the exospheres of the icy satellites Europa and Ganymede. The Hubble Space Telescope observations indicate that these satellites host unique, surface-bounded O2 exospheres which bulge towards dusk. Using a simple 1-D mass conservation balance we examine the nature of the volatile source, the surface temperature profile, the spatial morphology of the loss process, and the adsorption and desorption properties of the surfaces to understand the spatial distribution of the surface-bounded atmosphere for a number of objects. Since the ballistic hop distances are much smaller than the satellite radii, we show that the observed asymmetries at Europa and Ganymede can be simply due to a strongly thermally-dependent source, although asymmetries in the plasma-induced loss could contribute. A key condition for these atmospheric bulges that are shifted towards dusk is the relationship between the rotation rate and the atmospheric loss rate

Dusk/Dawn Atmospheric Asymmetries on Tidally-Locked Satellites II: Thermal Tides and Outgassing at the Galilean Satellites

International audienceIf a resonance exists between the atmospheric lifetime and rotation rate, a dusk-over-dawn atmospheric asymmetry appears on tidally-locked satellites

Dusk over dawn O₂ asymmetry in Europa's near-surface atmosphere

International audienceThe evolution of Europa's water-product exosphere over its 85-h day, based on current models, has not been shown to exhibit any diurnal asymmetries. Here we simulate Europa's exosphere using a 3-D Monte Carlo routine including, for the first time, the role of Europa's rotation on the evolution of exospheric molecules tracked throughout the orbit. In this work we focus on understanding the behavior of a single atmospheric constituent, O2, sputtered by a trailing hemisphere source with a temperature-dependence under isotropic plasma conditions as also modeled by previous works. Under rotation, the O2 is also subject to the centrifugal and Coriolis forces in addition to the standard gravitational forces by Jupiter and Europa in our model. We find that the O2 component, while global, is not homogeneous in Europa local time. Rather, the O2 consistently accumulates along the direction of Europa's rotation at the dusk hemisphere. When rotation is explicitly excluded in our simulations, no diurnal asymmetries exist, and any accumulation is due to the prescribed geometry of the sputtering source. We find that the assumed thermal-dependence on the O2 source is critical for a diurnal asymmetry: the diurnal surface temperature profile is imprinted on to the near-surface O2 atmosphere, due to small hop times for the non-adsorbing O2, which then effectively rotates with Europa. Simulation tests demonstrate that the diurnal asymmetry is not driven by the thermal inertia of the ice, found to have only a weak dependence (<7%) Altogether, the various test cases presented in this work conclude that the dusk-over-dawn asymmetry is driven by Europa's day-night O2 cycle synchronized with Europa's orbital period based on our model assumptions on O2 production and loss. This conclusion is in agreement with the recent understanding that a non-adsorbing, rotating O2 source peaking at noon will naturally accumulate from dawn-to-dusk, should the O2 lifetime be sufficiently long compared to the orbital period. Lastly we compare hemispherically-averaged dusk-over-dawn ratios to the recently observed oxygen emission data by the Hubble Space Telescope. We find that while the simulations are globally consistent with brighter oxygen emission at dusk than at dawn, the orbital evolution of the asymmetries in our simulations can be improved by ameliorating the O2 source & loss rates, and possibly adsorption onto the regolith

On the orbital variability of Ganymede's atmosphere

International audienceGanymede's atmosphere is produced by radiative interactions with its surface, sourced by the Sun and the Jovian plasma. The sputtered and thermally desorbed molecules are tracked in our Exospheric Global Model (EGM), a 3-D parallelized collisional model. This program was developed to reconstruct the formation of the upper atmosphere/exosphere of planetary bodies interacting with solar photon flux and magnetospheric and/or the solar wind plasmas. Here, we describe the spatial distribution of the H2O and O2 components of Ganymede's atmosphere, and their temporal variability along Ganymede's rotation around Jupiter. In particular, we show that Ganymede's O2 atmosphere is characterized by time scales of the order of Ganymede's rotational period with Jupiter's gravity being a significant driver of the spatial distribution of the heaviest exospheric components. Both the sourcing and the Jovian gravity are needed to explain some of the characteristics of the observed aurora emissions. As an example, the O2 exosphere should peak at the equator with systematic maximum at the dusk equator terminator. The sputtering rate of the H2O exosphere should be maximum on the leading hemisphere because of the shape of the open/close field lines boundary and displays some significant variability with longitude