ABSTRACT
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 analysing an evaporative curve of growth, we find that equivalent widths of $W_{\mathrm{Na D2}} \sim 1{\!-\!} 10\, \mathrm{m\mathring{\rm A}}$ are naturally driven by evaporation rates ∼103−105 kg s−1 of pure atomic Na. To break the degeneracy between atmospheric and exospheric absorption, we find that if the line ratio is D2/D1 ≳ 1.2 the gas is optically thin on average 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.
Context. A terrestrial planet is molten during formation and may remain molten due to intense insolation or tidal forces. Observations favour the detection and characterisation of hot planets, ...potentially with large outgassed atmospheres. Aims. We aim to determine the radius of hot Earth-like planets with large outgassing atmospheres. Our goal is to explore the differences between molten and solid silicate planets on the mass–radius relationship and transmission and emission spectra. Methods. An interior–atmosphere model was combined with static structure calculations to track the evolving radius of a hot rocky planet that outgasses CO2 and H2O. We generated synthetic emission and transmission spectra for CO2 and H2O dominated atmospheres. Results. Atmospheres dominated by CO2 suppress the outgassing of H2O to a greater extent than previously realised since previous studies applied an erroneous relationship between volatile mass and partial pressure. We therefore predict that more H2O can be retained by the interior during the later stages of magma ocean crystallisation. Formation of a surface lid can tie the outgassing of H2O to the efficiency of heat transport through the lid, rather than the radiative timescale of the atmosphere. Contraction of the mantle, as it cools from molten to solid, reduces the radius by around 5%, which can partly be offset by the addition of a relatively light species (e.g. H2O versus CO2) to the atmosphere. Conclusions. 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 composition, notably the abundance of light versus heavy volatile species. Atmospheres of terrestrial planets around M-stars that are dominated by CO2 or H2O can be distinguished by observing facilities with extended wavelength coverage (e.g. JWST).
Extrasolar satellites are generally too small to be detected by nominal searches. By analogy to the most active body in the solar system, Io, we describe how sodium (Na i) and potassium (K i) gas ...could be a signature of the geological activity venting from an otherwise hidden exo-Io. Analyzing ∼a dozen close-in gas giants hosting robust alkaline detections, we show that an Io-sized satellite can be stable against orbital decay below a planetary tidal . This tidal energy is also focused into the satellite driving an ∼105 2 higher mass-loss rate than Io's supply to Jupiter's Na exosphere based on simple atmospheric loss estimates. The remarkable consequence is that several exo-Io column densities are, on average, more than sufficient to provide the ∼1010 1 Na cm−2 required by the equivalent width of exoplanet transmission spectra. Furthermore, the benchmark observations of both Jupiter's extended (∼1000 RJ) Na exosphere and Jupiter's atmosphere in transmission spectroscopy yield similar Na column densities that are purely exogenic in nature. As a proof of concept, we fit the "high-altitude" Na at WASP-49b with an ionization-limited cloud similar to the observed Na profile about Io. Moving forward, we strongly encourage time-dependent ingress and egress monitoring along with spectroscopic searches for other volcanic volatiles.
Prevailing models for the formation of the Moon invoke a giant impact between a planetary embryo and the proto-Earth (Canup, 2004; Ćuk et al., 2016). Despite similarities in the isotopic and chemical ...abundances of refractory elements compared to Earth's mantle, the Moon is depleted in volatiles (Wolf and Anders, 1980). Current models favour devolatilisation via incomplete condensation of the proto-Moon in an Earth-Moon debris-disk (Charnoz and Michaut, 2015; Canup et al., 2015; Lock et al., 2018). However the physics of this protolunar disk is poorly understood and thermal escape of gas is inhibited by the Earth's strong gravitational field (Nakajima and Stevenson, 2014). Here we investigate a simple process, wherein the Earth's tidal pull promotes intense hydrodynamic escape from the liquid surface of a molten proto-Moon assembling at 3–6 Earth radii. Such tidally-driven atmospheric escape persisting for less than 1 Kyr at temperatures ~1600 − 1700 K reproduces the measured lunar depletion in K and Na, assuming the escape starts just above the liquid surface. These results are also in accord with timescales for the rapid solidification of a plagioclase lid at the surface of a lunar magma ocean (Elkins-Tanton et al., 2011). We find that hydrodynamic escape, both in an adiabatic or isothermal regime, with or without condensation, induces advective transport of gas away from the lunar surface, causing a decrease in the partial pressures of gas species (Ps) with respect to their equilibrium values (Psat). The observed enrichment in heavy stable isotopes of Zn and K (Paniello et al., 2012; Wang and Jacobsen, 2016) constrain Ps/Psat > 0.99, favouring a scenario in which volatile loss occurred at low hydrodynamic wind velocities (<1% of the sound velocity) and thus low temperatures. We conclude that tidally-driven atmospheric escape is an unavoidable consequence of the Moon's assembly under the gravitational influence of the Earth, and provides new pathways toward understanding lunar formation.
•Moon sample shows strong depletion in volatile elements compared to the Bulk Silicated Earth•Origin of this depletion is still a mystery•It can be explained by the hydrodynamical escape of an outgased atmosphere when the moon was young and fully molten•This escape is facilitated by the Earth's tidal forces.
Jupiter's moon Europa is thought to have an ocean beneath its ice shell and the habitability of the internal ocean depends on the availability of redox gradients. Downward transport of radiolytic ...materials produced at the surface through the ice shell sets the flux of oxidants into the ocean. Here, we propose that oxidants are transported through the ice shell by the drainage of near‐surface brines formed concurrently with chaotic terrains. We estimate that Europa's porous regolith contains 3.7 × 1014 to 5.6 × 1018 mol (1.2 × 1013 − 1.8 × 1017 kg) of trapped O2. Simulations of coupled melt‐migration and eutectic phase behavior show that brines drain before they refreeze, delivering ∼85% of the surface oxidants to the ocean on timescales of 2 × 104 years. From the distribution of chaotic terrains and from Europa's surface age we estimate that brine drainage could deliver O2 to the ocean at rates of 2.0 × 106 to 1.3 × 1010 mol/yr (0.002−13.2 kg/s).
Plain Language Summary
Some icy moons in the outer solar system likely contain an ocean beneath their ice shells. Jupiter's moon Europa may be especially suitable for life if radiolytic oxidants generated at its surface travel efficiently through the ice. We propose that oxidants can be transported through the ice by drainage of brines generated during the formation of chaotic terrains. These enigmatic surface features are thought to require the formation of large volumes of near‐surface brine. We show that the brines drain through the underlying ice before re‐freezing and transport oxidants in pulses of melt called “porosity waves.” These pulses may propagate through the ice on timescales of 20,000 years. The rate of oxidant delivery is thus linked to chaotic terrain formation and is correspondingly spatially localized and episodic.
Key Points
We synthesize available observations and show that Europa's near‐surface O2 abundance may be two times higher than previous estimates
Near‐surface brines generated during the formation of chaotic terrains drain downward through the ice shell in porosity waves
More than 80% of surface oxidants entrained into the brines may be delivered to the internal ocean
Abstract
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. ...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 hr phase-curve observations of WASP-49b at 400 MHz during primary & secondary transit, as well as first & third quadratures achieving a 3
σ
upper limit of 0.18 mJy beam
−1
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.
ABSTRACT
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 h at 150 and 218 MHz using the upgraded Giant Metrewave Radio Telescope (uGMRT). Though we do not detect radio emission from the systems, we place strong 3σ 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.
•A 1-D analytical model is presented to examine dusk/dawn asymmetries in tidally-locked satellites.•The atmospheric shift towards dusk for a thermally-dependent source depends on the ratio between ...the rotation rate and the atmospheric loss rate.•A simple thermally-dependent O2 source is identified at Europa and possibly Ganymede.•At Europa, the thermally-dependent O2 source implies a large O2 reservoir embedded in the porous ice.•If this large O2 reservoir exists it could be oxidizing the subsurface ocean as suggested by previous works.
We use a simple analytic model to examine the effect of the atmospheric source properties on the spatial distribution of a volatile in a surface-bounded atmosphere on a satellite that is tidally-locked to its planet. Spatial asymmetries in the O2 exosphere of Europa observed using the Hubble Space Telescope appear to reveal on average a dusk enhancement in the near-surface ultraviolet auroral emissions. Since the hop distances in these ballistic atmospheres are small, we use a 1-D mass conservation equation to estimate the latitudinally-averaged column densities produced by suggested O2 sources. Although spatial asymmetries in the plasma flow and in the surface properties certainly affect the spatial distribution of the near-surface aurora, the dusk enhancements at Europa can be understood using a relatively simple thermally-dependent source. Such a source is consistent with the fact that radiolytically produced O2 permeates their porous regoliths and is not so sensitive to the local production rate from ice. The size of the shift towards dusk is determined by the ratio of the rotation rate and atmospheric loss rate. A thermally-dependent source emanating from a large reservoir of O2 permeating Europa’s icy regolith is consistent with the suggestion that its subsurface ocean might be oxidized by subduction of such radiolytic products.
Abstract The signal from a transiting planet can be diluted by astrophysical contamination. In the case of circumstellar debris disks, this contamination could start in the mid-infrared and vary as a ...function of wavelength, which would then change the observed transmission spectrum for any planet in the system. The MIRI/Low Resolution Spectrometer WASP-39b transmission spectrum shows an unexplained dip starting at ∼10 μ m that could be caused by astrophysical contamination. The spectral energy distribution displays excess flux at similar levels to that which are needed to create the dip in the transmission spectrum. In this Letter, we show that this dip is consistent with the presence of a bright circumstellar debris disk, at a distance of >2 au. We discuss how a circumstellar debris disk like that could affect the atmosphere of WASP-39b. We also show that even faint debris disks can be a source of contamination in MIRI exoplanet spectra.
As gas giant planets and brown dwarfs radiate away the residual heat from their formation, they cool through a spectral type transition from L to T, which encompasses the dissipation of cloud opacity ...and the appearance of strong methane absorption. While there are hundreds of known T-type brown dwarfs, the first generation of directly imaged exoplanets were all L type. Recently, Kuzuhara et al. announced the discovery of GJ 504 b, the first T dwarf exoplanet. GJ 504 b provides a unique opportunity to study the atmosphere of a new type of exoplanet with a ~500 K temperature that bridges the gap between the first directly imaged planets (~1000 K) and our own solar system's Jupiter (~130 K). Of particular interest, our model fits suggest that GJ 504 b has a superstellar metallicity. Since planet formation can create objects with nonstellar metallicities, while binary star formation cannot, this result suggests that GJ 504 b formed like a planet, not like a binary companion.
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