Search

Oxygen is a constituent of many of the most abundant molecules detected in exoplanetary atmospheres and a key ingredient for tracking how and where a planet formed1. In particular, the O I 777.4 nm triplet is used to probe airglow and aurora on the Earth2 and the oxygen abundance in stellar atmospheres3,4,5,6, but has not been detected in an exoplanet atmosphere before. We present a definite ground-based detection of the neutral oxygen 777.4 nm triplet lines in the transmission spectrum of the ultrahot Jupiter KELT-9b7, the hottest known giant planet. The synthetic spectrum computed employing novel non-local thermodynamic equilibrium radiative transfer calculations8 matches the data significantly better than that computed assuming local thermodynamic equilibrium. These non-local thermodynamic equilibrium radiative transfer calculations imply a mass-loss rate of 108–109 kg s−1, which exceeds the lower limit of 107–108 kg s−1 required to facilitate the escape of oxygen and iron from the atmosphere. Assuming a solar oxygen abundance, the non-local thermodynamic equilibrium model points towards the need for microturbulence and macroturbulence broadening of 3.0 ± 0.7 km s−1 and 13 ± 5 km s−1, respectively, indicative of the presence of fast winds in the middle and upper atmosphere. Present and upcoming high-resolution spectrographs will allow the detection in other exoplanets of the 777.4 nm O I triplet, which is a powerful tool to constrain the key characteristics of exoplanetary atmospheres when coupled with forward modelling accounting for non-local thermodynamic equilibrium effects. © 2022, The Author(s), under exclusive licence to Springer Nature Limited., F.B. acknowledges support from PLATO ASI-INAF agreement 2015-019-R.1-2018. This research has made use of the Spanish Virtual Observatory (http://svo.cab.inta-csic.es) supported by the MINECO/FEDER through grant AyA2017-84089.7. T.K. acknowledges support by the NASA Exoplanet Research Program grant 80NSSC18K0569. D.S. acknowledges financial support from the State Agency for Research of the Spanish MCIU through the Center of Excellence Severo Ochoa award to the Instituto de Astrofísica de Andalucía (SEV-2017-0709). M.E.Y. acknowledges funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme under grant agreement 805445.

The Cassini/UVIS observations of Titan’s atmosphere extend from 2005 to 2017 providing a broad spatial and temporal view of the upper atmosphere including the seasonal change from Southern summer at the Cassini-Huygens arrival in 2005, to equinox (August 2009), to Northern summer (summer solstice in May 2017). The general emission characteristics of the illuminated Titan side are common among the observations: Near the ionospheric peak (~1100 km) the atomic and molecular emissions dominate the observed signal (Fig. 1). Atomic emissions include Ly-α (1216.7 Å) scattering from atomic hydrogen and emissions from N and N+ excited during the photolysis of N2, with major contributions at 1085 Å (N+, 3D0 à 3P), 1200 Å (N, 4D0 à 4S0 ) and 1493 Å (N, 2P à 4S0). Molecular emissions are dominated by the Lyman-Birge-Hopfield (LBH, α 1Πg à X 1Σg+) and Vegard-Kaplan (VK, A3Σu+ à Χ 1Σg+) bands, both in the FUV. These observations provide valuable constraints for the atmospheric structure. We use a detailed forward model to simulate the observed emission, which relies on constraints from models of solar energy deposition (Lavvas et al. 2011) and N2 aiglow (Lavvas et al. 2015). Our simulations demonstrate that all observed emissions result from the excitation of atomic and molecular nitrogen and from scattering by atomic hydrogen (Fig. 1). Using such a detailed forward model for the inversion of the atmospheric properties is, however, inefficient due to the large computational times involved. Instead we propose a simplified retrieval focusing on specific atomic lines in the FUV range, which allows for an efficient atmospheric characterization with minimal computational effort, while preserving the benefits of the detailed forward modeling. Figure 1: Observed (black) and simulated (red) limb spectra on Titan’s dayside during the 2016, DOY-015 PRIME observations. The top panel presents the average spectrum at 1100 km (100 km wide bin) demonstrating the strong Ly-α emission and the emissions from atomic (dashed lines) and molecular transitions (thin blue lines for LBH and thin red lines for VK bands). References Lavvas P., Yelle R.V., Heays A.N., Campbell L., Brunger M.J., Galand M., Vuitton V., 2015. N2 state population in Titan’s atmosphere. Icarus, 260, p.29-59. Lavvas P., Galand M., Yelle R.V., Hayes A.N., Lewis B.R., Lewis G.,R. Coates A.J., 2011. Energy deposotion and primary chemical products in Titan’s atmosphere. Icarus, 213, 233-251.

Photochemical hazes have been suggested as candidate for the high-altitude aerosols observed in the transmission spectra of many hot Jupiters. We present 3D simulations of the hot Jupiter HD 189733b to study how photochemical hazes are transported by atmospheric circulation. The model includes spherical, constant-size hazes particles that gravitationally settle and are transported by the winds as passive tracers, with particle radii ranging from 1 nm to 300 $��$m. We identify two general types of haze distribution based on particle size: In the small-particle regime (30 nm), hazes settle out quickly on the nightside, resulting in more hazes at the evening terminator. For small particles, terminator differences in haze mass mixing ratio and temperature considered individually can result in significant differences in the transit spectra of the terminators. When combining both effects for HD189733b, however, they largely cancel out each other, resulting in very small terminator differences in the spectra. Transit spectra based on the GCM-derived haze distribution fail to reproduce the steep spectral slope at short wavelengths in the current transit observations of HD 189733b. Differing optical properties of hazes, hotter temperatures at low pressures because of heating by hazes, enhanced sub-grid-scale mixing, or star spots might explain the mismatch between the model and observations., Accepted at MNRAS. This is a pre-copyedited, author-produced version following peer review

Temperatures of the outer planet thermospheres exceed those predicted by solar heating alone by several hundred degrees. Enough energy is deposited at auroral regions to heat the entire thermosphere, but models predict that equatorward distribution is inhibited by strong Coriolis forces and ion drag. A better understanding of auroral energy deposition and circulation are critical to solving this so-called energy crisis. Stellar occultations observed by the Ultraviolet Imaging Spectrograph instrument during the Cassini Grand Finale were designed to map the thermosphere from pole to pole. We analyze these observations, together with earlier observations from 2016 and 2017, to create a two-dimensional map of densities and temperatures in Saturns thermosphere as a function of latitude and depth. The observed temperatures at auroral latitudes are cooler and peak at higher altitudes and lower latitudes than predicted by models, leading to a shallower meridional pressure gradient. Under modified geostrophy, we infer slower westward zonal winds that extend to lower latitudes than predicted, supporting equatorward flow from approximately 70 to 30 degrees latitude in both hemispheres. We also show evidence of atmospheric waves in the data that can contribute to equatorward redistribution of energy through zonal drag., 25 pages, 6 figures, including Supplementary Information

The Normal-incidence Extreme Ultraviolet Photometer (NExtUP) is a smallsat mission concept designed to measure the EUV radiation conditions of exoplanet host stars, and F-M type stars in general. EUV radiation is absorbed at high altitude in a planetary atmosphere, in the exosphere and upper thermosphere, where the gas can be readily heated to escape temperatures. EUV heating and ionization are the dominant atmospheric loss drivers during most of a planet’s life. There are only a handful of accurately measured EUV stellar fluxes, all dating from Extreme Ultraviolet Explorer (EUVE) observations in the ‘90s. Consequently, current models of stellar EUV emission are uncertain by more than an order of magnitude and dominate uncertainties in planetary atmospheric loss models. NExtUP will use periodic and aperiodic multilayers on off-axis parabolic mirrors and a prime focus microchannel plate detector to image stars in 5 bandpasses between 150 and 900°A down to flux limits two orders of magnitude lower than reached by EUVE. NExtUP may also accomplish a compelling array of secondary science goals, including using line-of-sight absorption measurements to understand the structure of the local interstellar medium, and imaging EUV emission from energetic processes on solar system objects at unprecedented spatial resolution. NExtUP is well within smallsat weight limits, requires no special orbital conditions, and would be flown on a spacecraft supplied by MOOG Industries. It draws on decades of mission heritage expertise at SAO and LASP, including similar instruments successfully launched and operated to observe the Sun.

Photochemically produced hazes provide a possible explanation for aerosol features in the transmission spectra of many hot Jupiters, especially those with relatively low equilibrium temperatures. Recent simulations of photochemical hazes as passive tracers in a 3D general circulation model demonstrate that the distribution of hazes can be highly inhomogeneous, with horizontal abundance variations of over an order of magnitude. At the same time, one-dimensional radiative transfer models show that absorption and scattering by hazes can change the atmospheric temperature profile by several hundred Kelvin. The additional heating and cooling have the potential to significantly affect atmospheric circulation. In this talk, we present new GCM simulations of hot Jupiter HD 189733b that include radiative feedback from hazes. A focus will be on changes in the atmospheric temperature structure and circulation. We will then compare the 3D haze distribution from previous simulations with radiatively passive hazes to the new haze distribution in simulations that include haze radiative feedback. Finally, we predict transit spectra based on the simulations and compare them to observations.

The innovative Saturn Ring Skimmer mission concept enables a wide range of investigations that address fundamental questions about Saturn and its rings, as well as giant planets and astrophysical disk systems in general. This mission would provide new insights into the dynamical processes that operate in astrophysical disk systems by observing individual particles in Saturn's rings for the first time. The Ring Skimmer would also constrain the origin, history, and fate of Saturn's rings by determining their compositional evolution and material transport rates. In addition, the Ring Skimmer would reveal how the rings, magnetosphere, and planet operate as an inter-connected system by making direct measurements of the ring's atmosphere, Saturn's inner magnetosphere and the material owing from the rings into the planet. At the same time, this mission would clarify the dynamical processes operating in the planet's visible atmosphere and deep interior by making extensive high-resolution observations of cloud features and repeated measurements of the planet's extremely dynamic gravitational field. Given the scientific potential of this basic mission concept, we advocate that it be studied in depth as a potential option for the New Frontiers program., Comment: White paper submitted to the Planetary Science and Astrobiology Decadal Survey (submission #420)

We review the importance of recent UV observations of solar system targets and discuss the need for further measurements, instrumentation and laboratory work in the coming decade. In the past decade, numerous important advances have been made in solar system science using ultraviolet (UV) spectroscopic techniques. Formerly used nearly exclusively for studies of giant planet atmospheres, planetary exospheres and cometary emissions, UV imaging spectroscopy has recently been more widely applied. The geyser-like plume at Saturn's moon Enceladus was discovered in part as a result of UV stellar occultation observations, and this technique was used to characterize the plume and jets during the entire Cassini mission. Evidence for a similar style of activity has been found at Jupiter's moon Europa using Hubble Space Telescope (HST) UV emission and absorption imaging. At other moons and small bodies throughout the solar system, UV spectroscopy has been utilized to search for activity, probe surface composition, and delineate space weathering effects; UV photometric studies have been used to uncover regolith structure. Insights from UV imaging spectroscopy of solar system surfaces have been gained largely in the last 1-2 decades, including studies of surface composition, space weathering effects (e.g. radiolytic products) and volatiles on asteroids (e.g. [2][39][48][76][84]), the Moon (e.g. [30][46][49]), comet nuclei (e.g. [85]) and icy satellites (e.g. [38][41-44][45][47][65]). The UV is sensitive to some species, minor contaminants and grain sizes often not detected in other spectral regimes. In the coming decade, HST observations will likely come to an end. New infrastructure to bolster future UV studies is critically needed. These needs include both developmental work to help improve future UV observations and laboratory work to help interpret spacecraft data. UV instrumentation will be a critical tool on missions to a variety of targets in the coming decade, especially for the rapidly expanding application of UV reflectance investigations of atmosphereless bodies.

We construct Saturn equatorial neutral temperature and density profiles of H, H$_2$, He, and CH$_4$, between 10$^{-12}$ and 1 bar using measurements from Cassini's Ion Neutral Mass Spectrometer (INMS) taken during the spacecraft's final plunge into Saturn's atmosphere on 15 September 2017, combined with previous deeper atmospheric measurements from the Cassini Composite InfraRed Spectrometer (CIRS) and from the UltraViolet Imaging Spectrograph (UVIS). These neutral profiles are fed into an energy deposition model employing soft X-ray and Extreme UltraViolet (EUV) solar fluxes at a range of spectral resolutions ($\Delta\lambda=4\times10^{-3}$ nm to 1 nm) assembled from TIMED/SEE, from SOHO/SUMER, and from the Whole Heliosphere Interval (WHI) quiet Sun campaign. Our energy deposition model calculates ion production rate profiles through photo-ionisation and electron-impact ionisation processes, as well as rates of photo-dissociation of CH$_4$. The ion reaction rate profiles we determine are important to obtain accurate ion density profiles, meanwhile methane photo-dissociation is key to initiate complex organic chemical processes. We assess the importance of spectral resolution in the energy deposition model by using a high-resolution H$_2$ photo-absorption cross section, which has the effect of producing additional ionisation peaks near 800 km altitude. We find that these peaks are still formed when using low-resolution ($\Delta\lambda=1$ nm) or mid-resolution ($\Delta\lambda=0.1$ nm) solar spectra, as long as high-resolution cross sections are included in the model., Comment: 21 pages, 13 figures, 5 tables, accepted for publication in Icarus

High levels of X-ray and UV activity on young M dwarfs may drive rapid atmospheric escape on temperate, terrestrial planets orbiting within the liquid water habitable zone. However, secondary atmospheres on planets orbiting older, less active M dwarfs may be stable and present more promising candidates for biomarker searches. We present new HST and Chandra observations of Barnard's Star (GJ 699), a 10 Gyr old M3.5 dwarf, acquired as part of the Mega-MUSCLES program. Despite the old age and long rotation period of Barnard's star, we observe two FUV ($\delta_{130}$ $\approx$ 5000s; $E_{130}$ $\approx$ 10$^{29.5}$ erg each) and one X-ray ($E_{X}$ $\approx$ 10$^{29.2}$ erg) flares, and estimate a high-energy flare duty cycle (defined here as the fraction of the time the star is in a flare state) of $\sim$ 25\%. A 5 A - 10 $\mu$m SED of GJ 699 is created and used to evaluate the atmospheric stability of a hypothetical, unmagnetized terrestrial planet in the habitable zone ($r_{HZ}$ $\sim$ 0.1 AU). Both thermal and non-thermal escape modeling indicate (1) the $quiescent$ stellar XUV flux does not lead to strong atmospheric escape: atmospheric heating rates are comparable to periods of high solar activity on modern Earth, and (2) the $flare$ environment could drive the atmosphere into a hydrodynamic loss regime at the observed flare duty cycle: sustained exposure to the flare environment of GJ 699 results in the loss of $\approx$ 87 Earth atmospheres Gyr$^{-1}$ through thermal processes and $\approx$ 3 Earth atmospheres Gyr$^{-1}$ through ion loss processes, respectively. These results suggest that if rocky planet atmospheres can survive the initial $\sim$ 5 Gyr of high stellar activity, or if a second generation atmosphere can be formed or acquired, the flare duty cycle may be the controlling stellar parameter for the stability of Earth-like atmospheres around old M stars., Comment: Accepted to AJ

Motivation The transmission spectra of many hot Jupiters show signatures of high-altitude aerosols [e.g., 1,2]. One hypothesized formation mechanism for these aerosols is that photochemical processes generate hazes on the dayside. All previous studies of photochemical hazes on tidally locked giant planets used one-dimensional models [e.g., 3,4]. However, one-dimensional models have to make strongly simplifying assumptions about the strength of vertical mixing. Furthermore, they ignore that the strong day-night contrast on hot Jupiters and atmospheric circulation can lead to inhomogeneous aerosol distributions. For condensate clouds, it has been shown that inhomogeneous aerosol distributions are to be expected [5-7] and can lead to biases in the interpretation of observations [e.g., 8]. The same is likely to be true for photochemical hazes. Further, it has been suggested differences between the morning and evening terminator in ingress and egress transmission spectra could provide a diagnostic for distinguishing between condensate clouds and photochemical hazes [9]. Three-dimensional general circulation models (GCMs) are needed to study how atmospheric circulation shapes the distribution of photochemical hazes to guide future observations and models. Methods We present simulations of hot Jupiter HD 189733b using the MITgcm [10]. We use passive tracers representing photochemical hazes to study how hazes are transported by atmospheric circulation. Haze particles in our model have a constant size and are spherical. Results The results show that the haze mass mixing ratio varies horizontally by at least an order of magnitude for all particle sizes considered and over the entire simulated pressure range. Depending on the particle size, the resulting 3D haze distribution falls into one of two regimes: small (30 nm) particles. For small particles (< 30 nm), the timescale for gravitational settling is longer than the timescale for horizontal and vertical advection. The 3D distribution is thus controlled by advection and looks similar for all particle sizes in this regime. At low pressures, small particle hazes accumulate on the night side in two large midlatitude vortices centered east of the antistellar point (Fig. 1). Because the night side vortices span across the morning terminator, there are higher mass mixing ratios at the morning terminator compared to the evening terminator. For large particles (>30 nm), the 3D haze distribution is strongly influenced by settling. At very low pressures, where the settling timescale is much shorter than the advection timescale, the horizontal pattern closely mirrors the haze production function. At somewhat higher pressures, where both timescales are within an order of magnitude from each other, hazes are concentrated on the dayside and the hemisphere east of the substellar point (Fig. 2). This results in higher mass mixing ratios at the evening terminator compared to the morning terminator. Because the distribution of hazes is dependent on where in the atmosphere these timescales become equal, the 3D size distributions look much less similar between different particle sizes within this regime compared to the small particle regime. At pressures > 1 mbar, the advection time scale is shorter than the settling time scale for all particle sizes considered in our simulations. In this region, the equatorial jet dominates the atmospheric circulation and hazes of all sizes develop a more banded pattern (Fig. 3). Differences between morning and evening terminator become smaller in this pressure range. Our model does not include particle growth and thus does not make predictions about the particle size distribution. To estimate whether terminator differences could be observable, we tried using a constant particle size as well as a size distribution from a 1D microphysics model [3]. In either case, one obtains a relatively small difference in transit depth between leading and trailing limb (Fig. 4). We note that the simulated spectral slope at short wavelengths is too flat to match the observational data. There could be multiple factors explaining why our model does not reproduce the slope of the data, including that a fully coupled three-dimensional microphysics and circulation model might be needed to reproduce the observational data. Furthermore, part of the slope could arise from unaccounted star spots. Another possibility is that mixing from small-scale turbulence not resolved by the GCM could be much more important than expected. In that case, the haze mass mixing ratio would decline more rapidly with increasing pressure, resulting in a steeper spectral slope. References: [1] Pont, F., Sing, D. K., Gibson, N. P., et al.: The prevalence of dust on the exoplanet HD 189733b from Hubble and Spitzer observations, Monthly Notices of the Royal Astronomical Society, 432, 4, 2917-2944, 2013. [2] Sing, D. K., Fortney, J. J., Nikolov, N., et al.: A continuum from clear to cloudy hot-Jupiter exoplanets without primordial water depletion, Nature, 529, 7584, 59-62, 2016. [3] Lavvas, P. and Koskinen, T.: Aerosol Properties of the Atmospheres of Extrasolar Giant Planets, The Astrophysical Journal, 847, 1, 32, 2017. [4] Ohno, K. and Kawashima, Y.: Super-Rayleigh Slopes in Transmission Spectra of Exoplanets Generated by Photochemical Haze, The Astrophysical Journal Letters, 895, 2, L47, 2020. [5] Parmentier, V., Showman, A. P. and Lian, Y.: 3D mixing in hot Jupiters atmospheres I . Application to the day / night cold trap in HD 209458b, Astronomy & Astrophysics, 558, A91, 2013. [6] Lee, G., Dobbs-Dixon, I., Helling, Ch., et al.: Dynamic mineral clouds on HD 189733b. I. 3D RHD with kinetic, non-equilibrium cloud formation, Astronomy & Astrophysics, 594, A48, 2016. [7] Lines, S., Mayne, N. J., Boutle, I. A., et al.: Simulating the cloudy atmospheres of HD 209458 b and HD 189733 b with the 3D Met Office Unified Model, Astronomy & Astrophysics, 615, A97, 2018. [8] Line, M. and Parmentier, V.: The Influence of Nonuniform Cloud Cover on Transit Transmission Spectra, The Astrophysical Journal, 820, 1, 78, 2016. [9] Kempton E. M. R., Bean J. L., Parmentier V.: An Observational Diagnostic for Distinguishing between Clouds and Haze in Hot Exoplanet Atmospheres, The Astrophysical Journal, 845, 2, L20, 2017. [10] Adcroft A., Campin J.-M., Hill C., Marshall J.: Implementation of an Atmosphere Ocean General Circulation Model on the Expanded Spherical Cube, Monthly WeatherReview, 132, 2845, 2004.

International audience

On 1 February 2016 Titan occulted the three stars in Orion’s belt, as seen from the Casssini spacecraft. The ultraviolet spectrometer on Cassini observed this event with the stars arranged along the instrument’s entrance slit. These three stellar occultations probed different latitudes in Titan’s northern hemisphere but similar longitudes. The occultations were also observed simultaneously in the extreme ultraviolet and far ultraviolet regions of the spectrum. Analysis of these measurements provides information on the atmospheric composition and thermal structure. We determine altitude profiles of N 2 , CH 4 , C 2 H 2 , C 2 H 4 , C 2 H 6 , C 4 H 2 , C 6 H 6 , HCN, and HC 3 N densities along with aerosol extinction. We infer significant variations in latitude for C 2 H 2 , C 2 H 4 , C 4 H 2 , aerosols, and temperature, providing clues to global circulations patterns. The inferred mole fractions of C 2 H 2 , C 2 H 4 , C 4 H 2 , C 6 H 6 , HCN, and HC 3 N are roughly consistent with photochemical model predictions, but C 2 H 2 and C 2 H 4 are over predicted by tens of percent at pressures greater than 10−5 hPa, C 4 H 2 is under predicted by ∼ x2 at most levels, and C 6 H 6 is under predicted by ∼ x2 at pressures less than 10−5 hPa.

The Cassini spacecraft's last orbits directly sampled Saturn's thermosphere and revealed a much more chemically complex environment than previously believed. Observations from the Ion and Neutral Mass Spectrometer (INMS) aboard Cassini provided compositional measurements of this region and found an influx of material raining into Saturn's upper atmosphere from the rings. We present here an in-depth analysis of the CH$_4$, H$_2$O, and NH$_3$ signal from INMS and provide further evidence of external material entering Saturn's atmosphere from the rings. We use a new mass spectral deconvolution algorithm to determine the amount of each species observed in the spectrum and use these values to determine the influx and mass deposition rate for these species., Comment: 24 pages, 7 figures, 3 tables, accepted for publication in the Journal of Geophysical Research: Planets

International audience; We analyze Saturn Lyman α system scans observed by the Cassini/UVIS instrument probing non-auroral airglow emissions from Saturn's disk, the ring atmosphere and the Saturn hydrogen torus. We constrain variations in disk brightness with latitude and local time, relating them to models of Saturn's upper atmosphere. We compare these results with Voyager observations of similar viewing geometry. The Cassini disk brightness is almost four time lower than the Voyager brightness. This difference cannot be explained by changes in the solar Lyman α flux or atmospheric structure. The change in solar flux is modest and atmospheric structure retrieved from occultation data is relatively stable over time. The disk brightness observed by Cassini can be explained by resonant scattering of solar Lyman α radiation and the best-fit hydrogen abundance in the thermosphere is consistent with atmospheric structure retrieved from the occultation data as well as photochemical model predictions. Our results support the recently proposed recalibration of the Voyager/UVS instruments that would bring the Voyager disk brightness in line with our model predictions and provide a consistent interpretation of Saturn Lyman α emissions over time. We also explore the extended emission in the Saturn system, centered near the ring plane in edge-on viewing. The extended emission can be explained by a distribution of atomic hydrogen that is symmetric about the ring plane. We do not find evidence for a plume of hot hydrogen atoms escaping from Saturn's sunlit atmosphere that was proposed previously. Instead, the structure earlier interpreted as a plume is most simply explained by solar illumination from below the ring plane during southern summer combined with the edge-on viewing geometry.

The growth of spectroscopic observations of exoplanetary systems allows the possibility of testing theoretical models and studying the interaction that exoplanetary atmospheres have with the wind and the energetic photons from the star. In this work, we present a set of numerical 3D simulations of HD 209458b for which spectral lines observations of their evaporative atmosphere are available. The different simulations aim to reproduce different scenarios for the star-planet interaction. With our models, we reconstruct the Lyα line during transit and compare with observations. The results allows us to analyse the shape of the line profile under these different scenarios and the comparison with the observations suggest that HD209458b may have a magnetic field off less than 1 G. We also explore the behaviour of the magnesium lines for models with and without magnetic fields.

The long-term stability of exoplanetary atmospheres depends critically on the extreme-ultraviolet (EUV) flux from the host star. The EUV flux likely controls the demographics of the short-period planet population as well the ability for rocky planets to maintain habitable environments long enough for the emergence of life. We present the Extreme-ultraviolet Stellar Characterization for Atmospheric Physics and Evolution (ESCAPE) mission, an astrophysics Small Explorer proposed to NASA. ESCAPE employs extreme- and far-ultraviolet spectroscopy (70 - 1800 Α) to characterize the highenergy radiation environment in the habitable zones (HZs) around nearby stars. ESCAPE provides the first comprehensive study of the stellar EUV environments that control atmospheric mass-loss and determine the habitability of rocky exoplanets. The ESCAPE instrument comprises an EUV grazing incidence telescope feeding four diffraction gratings and a photon-counting detector. The telescope is 50 cm diameter with four nested parabolic primary mirrors and four nested elliptical secondary mirrors, fabricated and aligned by NASA Marshall Space Flight Center and the Smithsonian Astrophysical Observatory. The off-plane grating assemblies are fabricated at Pennsylvania State University and the ESCAPE detector system is a micro-channel plate (MCP; 125mm x 40mm active area) sensor developed by the University of California, Berkeley. ESCAPE employs the versatile and high-heritage Ball Aerospace BCP-100 spacecraft.

We study the properties of photochemical hazes in super-Earths/mini-Neptunes atmospheres with particular focus on GJ1214b. We evaluate photochemical haze properties at different metallicities between solar and 10000$\times$solar. Within the four orders of magnitude change in metallicity, we find that the haze precursor mass fluxes change only by a factor of $\sim$3. This small diversity occurs with a non-monotonic manner among the different metallicity cases, reflecting the interaction of the main atmospheric gases with the radiation field. Comparison with relative haze yields at different metallicities from laboratory experiments reveals a qualitative similarity with our theoretical calculations and highlights the contributions of different gas precursors. Our haze simulations demonstrate that higher metallicity results into smaller average particle sizes. Metallicities at and above 100$\times$solar with haze formation yields of $\sim$10$\%$ provide enough haze opacity to satisfy transit observation at visible wavelengths and obscure sufficiently the H$_2$O molecular absorption features between 1.1 $\mu$m and 1.7 $\mu$m. However, only the highest metallicity case considered (10000$\times$solar) brings the simulated spectra into closer agreement with transit depths at 3.6 $\mu$m and 4.5 $\mu$m indicating a high contribution of CO/CO$_2$ in GJ1214b's atmosphere. We also evaluate the impact of aggregate growth in our simulations, in contrast to spherical growth, and find that the two growth modes provide similar transit signatures (for D$_f$=2), but with different particle size distributions. Finally, we conclude that the simulated haze particles should have major implications for the atmospheric thermal structure and for the properties of condensation clouds.

We review the recent progress in understanding the jet structures on exoplanets as well as on and inside the Sun. The emphasis is on the more robust aspects of observation and numerical modeling that relate directly to jets. For the exoplanets, the primary focus is on hot-Jupiters since many more observations are available for them presently than other types of exoplanets. Because not much is known about the morphology and strength of the jets on exoplanets, there is currently not much agreement. In contrast, the picture is very different for the Sun. In fact, the jet structure of the Sun is arguably one of the best known jet structures of all the planets and stars, due to the fact that Sun's disk is resolved and its interior can be probed with helioseismology. A discussion of several critical issues pertaining to the modeling of jets on exoplanets and the Sun is presented, along with a brief outlook on the subject.

Atmospheric waves have been discovered for the first time in Saturn's neutral upper atmosphere (thermosphere). Waves may be generated from instabilities, convective storms or other atmospheric phenomena. The inferred wave amplitudes change little with height within the sampled region, raising the possibility of the waves being damped, which in turn may enhance the eddy friction within the thermosphere. Using our Saturn Thermosphere Ionosphere General Circulation Model, we explore the parameter space of how an enhanced Rayleigh drag in different latitude regimes would affect the global circulation pattern within the thermosphere and, in turn, its global thermal structure. We find that Rayleigh drag of sufficient magnitude at midlatitudes may reduce the otherwise dominant Coriolis forces and enhance equatorward winds to transport energy from poles toward the equator, raising the temperatures there to observed values. Without this Rayleigh drag, energy supplied into the polar upper atmosphere by magnetosphere‐atmosphere coupling processes remains trapped at high latitudes and causes low‐latitude thermosphere temperatures to remain well below the observed levels. Our simulations thus suggest that giant planet upper atmosphere global circulation models need to include additional Rayleigh drag in order to capture the effects of physical processes otherwise not resolved by the codes.

We present an empirical model of H 2 densities and temperatures above the homopause for the Saturn thermosphere, based on stellar occultations observed by the Cassini/UVIS instrument. These observations span the whole Cassini mission and probe a wide range of latitudes, longitudes and local times. Although the model is based on Cassini stellar occultations only, the results agree well with Voyager occultations and reasonably well with other measurements of temperature and density in the thermosphere. The principal meridional trend that the model captures is an increase in temperature with latitude away from the equator, associated with significantly elevated density level altitudes at latitudes higher than about 40° in both hemispheres. These results confirm the importance of auroral heating in Saturn’s thermosphere and constrain models of global circulation. We estimate the uncertainty in the model based on the standard deviation of the data points around the mean state. We do not attempt to distinguish retrieval errors from spatial and temporal trends. We find that the most important source of uncertainty is an apparent semi-annual variation in H 2 density and temperature, associated with expansion and contraction of the pressure levels in the thermosphere. Similar variation occurs in the Earth’s thermosphere where it is likely driven by seasonal changes to global circulation. The model presented here is a useful tool that constrains physical models, helps to interpret existing observations, and supports the planning of future observations and missions.

Transit spectroscopy is a key tool for exoplanet atmospheric characterization. However, transit spectrum observations can be limited by aerosol extinction when gas opacities are weak. The ultraviolet wavelength range contains a variety of strong molecular and atomic features, potentially enabling gas species detection even when atmospheric hazes are present. To understand the interplay between aerosol extinction and ultraviolet molecular opacities, we investigate transmission through the atmosphere of Saturn’s moon Titan during an occultation observed with the Ultraviolet Imaging Spectrometer (UVIS) on board NASA’s Cassini orbiter. We analyze the derived ultraviolet transit spectrum of Titan using exoplanet-relevant atmospheric retrieval models that both include and exclude treatments for hazes. Our retrieved gas column densities are consistent with previous studies analyzing UVIS occultation data. Despite the apparent haze impact on the underlying occultation data, our treatments fail to correctly characterize the haze in fits derived from simulated transit observations. This suggests that oversimplified haze parameterizations can hinder detection of atmospheric hazes in transit. Our work indicates that continued characterization of exoplanets in the ultraviolet wavelength regime can provide novel atmospheric constraints even if transit spectra are dominated by haze extinction at longer wavelengths.

NASA Cassini Data Analysis and Participating Scientist grant [NNX14AD51G]; NASA Solar System Workings grant [NNX16AG10G]; Cassini Project; CNES

Electrodynamics plays an important role in controlling circulation and energy balance in Saturn's auroral thermosphere, but less is known about its effects at lower latitudes. Recent observations by the Cassini magnetometer instrument, taken around the equator during the Grand Finale tour, revealed azimuthal magnetic field perturbations of the order of 10–30 nT associated with electrodynamics in Saturn's upper atmosphere. In order to investigate the implications of these observations, we develop a steady-state, axisymmetric model adapted from terrestrial studies to simulate wind-driven electrodynamics at low to middle latitudes in Saturn's thermosphere. Our results demonstrate, based on rigorous theory, that the magnetic field observations can be reproduced by a wind dynamo generating electric currents in the ionosphere of the order 10−9 − 10−7 Am−2 provided that eastward zonal winds of the order of 100 ms−1 exist in the equatorial thermosphere. The resistive (Joule) heating rate based on the equatorial current system, however, is significantly lower than that required to explain the temperatures in Saturn's thermosphere. In spite of this, we find that resistive heating and ion drag due to a mid-latitude wind dynamo have the potential to alter the energy balance and general circulation in Saturn's thermosphere and should be treated self-consistently in future global circulation models.

International audience; Cassini's final orbits in 2016 and 2017 provided unprecedented spatial resolution of Saturn's polar regions from near-polar spacecraft viewing geometries. Long-wavelength channels of Cassini's Ultraviolet Imaging Spectrograph instrument detected Saturn's UV-dark north polar hexagon near 180 nm at planetocentric latitudes near 75°N. The dark polar hexagon is surrounded by a larger, less UV-dark collar poleward of planetocentric latitude 65°N associated with the dark north polar region seen in ground-based images. The hexagon is closely surrounded by the main arc of Saturn's UV aurora. The UV-dark material was locally darkest on one occasion (23 January 2017) at the boundary of the hexagon; in most Ultraviolet Imaging Spectrograph images the dark material more uniformly fills the hexagon. The observed UV-dark stratospheric material may be a hydrocarbon haze produced by auroral ion-neutral chemistry at submicrobar pressure levels. Ultraviolet Imaging Spectrograph polar observations are sensitive to UV-absorbing haze particles at pressures lower than about 10-20 mbar.

With a day-side temperature in excess of 4500K, comparable to a mid-K-type star, KELT-9b is the hottest planet known. Its extreme temperature makes KELT-9b a particularly interesting test bed for investigating the nature and diversity of gas giant planets. We observed the transit of KELT-9b at high spectral resolution (R$\sim$94,600) with the CARMENES instrument on the Calar Alto 3.5-m telescope. Using these data, we detect for the first time ionized calcium (CaII triplet) absorption in the atmosphere of KELT-9b; this is the second time that CaII has been observed in a hot Jupiter. Our observations also reveal prominent H$\alpha$ absorption, confirming the presence of an extended hydrogen envelope around KELT-9b. We compare our detections with an atmospheric model and find that all four lines form between atmospheric temperatures of 6100 K and 8000 K and that the CaII lines form at pressures between 10 and 50 nbar while the H$\alpha$ line forms at a lower pressure ($\sim$6 nbar), higher up in the atmosphere. The altitude that the core of H$\alpha$ line forms is found to be $\sim$1.4 R$_{p}$, well within the planetary Roche lobe ($\sim$1.9 R$_{p}$). Therefore, rather than probing the escaping upper atmosphere directly, the H$\alpha$ line and the other observed Balmer and metal lines serve as atmospheric thermometers enabling us to probe the planet's temperature profile, thus energy budget., Comment: Accepted to ApJL (December 11 2019), 11 pages, 3 figures, 2 tables, 2 Appendices (with 2 figures)

The inflated transiting hot Jupiter HD 209458b is one of the best studied objects since the beginning of exoplanet characterization. Transmission observations of this system between the mid infrared and the far ultraviolet have revealed the signature of atomic, molecular, and possibly aerosol species in the lower atmosphere of the planet, as well as escaping hydrogen and metals in the upper atmosphere. From a re-analysis of near-ultraviolet (NUV) transmission observations of HD 209458b, we detect ionized iron (Fe II) absorption in a 100 A-wide range around 2370 A, lying beyond the planetary Roche lobe. However, we do not detect absorption of equally strong Fe II lines expected to be around 2600 A. Further, we find no evidence for absorption by neutral magnesium (Mg I), ionized magnesium (Mg II), nor neutral iron (Fe I). These results avoid the conflict with theoretical models previously found by Vidal-Madjar et al. (2013), which detected Mg I but did not detect Mg II from this same data set. Our results indicate that hydrodynamic escape is strong enough to carry atoms as heavy as iron beyond the planetary Roche lobe, even for planets less irradiated than the extreme ultra-hot-Jupiters such as WASP-12b and KELT-9b. The detection of iron and non-detection of magnesium in the upper atmosphere of HD 209458b can be explained by a model in which the lower atmosphere forms (hence, sequesters) primarily magnesium-bearing condensates, rather than iron condensates. This is suggested by current microphysical models. The inextricable synergy between upper- and lower-atmosphere properties highlights the value of combining observations that probe both regions., Accepted for publication at AAS Journals

We combine measurements from stellar occultations observed by the Cassini Ultraviolet Imaging Spectrograph (UVIS) and limb scans observed by the Composite Infrared Spectrometer (CIRS) to create empirical atmospheric structure models for Saturn corresponding to the locations probed by the occultations. The results cover multiple locations at low to mid-latitudes between the spring of 2005 and the fall of 2015. We connect the temperature-pressure (T-P) profiles retrieved from the CIRS limb scans in the stratosphere to the T-P profiles in the thermosphere retrieved from the UVIS occultations. We calculate the altitudes corresponding to the pressure levels in each case based on our best fit composition model that includes H2, He, CH4 and upper limits on H. We match the altitude structure to the density profile in the thermosphere that is retrieved from the occultations. Our models depend on the abundance of helium and we derive a volume mixing ratio of 11 ± 2% for helium in the lower atmosphere based on a statistical analysis of the values derived for 32 different occultation locations. We also derive the mean temperature and methane profiles in the upper atmosphere and constrain their variability. Our results are consistent with enhanced heating at the polar auroral region and a dynamically active upper atmosphere.

We retrieved the density and temperature profiles in Saturn’s thermosphere from 26 stellar occultations observed by the Cassini/UVIS instrument. These results expand upon and complement the previous analysis of 15 Cassini/UVIS solar occultations by Saturn’s upper thermosphere. We find that the exospheric temperatures based on the stellar occultations agree with the solar occultations and range from 380 K to 590 K. These temperatures are also consistent with the recent re-analysis of the Voyager/UVS occultations. The retrieved density profiles support our earlier inference that the shape of the atmosphere at low pressures is consistent with a meridional trend of increasing temperatures with absolute latitude. This implies a high-latitude heat source, such as auroral heating, although the existing circulation models that include auroral heating still underestimate the equatorial temperatures by overestimating the meridional temperature gradient. This suggests either that the circulation models are somehow incomplete or there is some other heat source at low to mid latitudes that is relatively less efficient than high-latitude heating. We also find evidence for the expansion of the exobase by about 500 km between 2006 and 2011 near the equator, followed by possible contraction after 2011. The expansion appears to be caused by significant warming of the lower thermosphere that anti-correlates with solar activity and may be connected to changes in global circulation. Lastly, we note that our density profiles are in good general agreement with the Voyager/UVS data. In particular, the Voyager density profiles are most consistent with the Cassini/UVIS stellar occultations from late 2008 and early 2009 that roughly coincide in season with the Voyager flybys.

Exoplanets orbiting close to their host star are expected to support a large ionosphere, which extends to larger pressures than witnessed in our Solar System. These ionospheres can be investigated with ground-based transit observations of the optical signatures of alkali metals, which are the source of the ions. Here, we present new observations and analyses of optical transit spectra that cover the Na doublet in the atmosphere of the exoplanet XO-2 b. To assess the consistency of our results, observations were obtained from two separate platforms: Gemini/GMOS and Mayall/KOSMOS. To mitigate the systematic errors, we chose XO-2, because it has a binary companion of the same brightness and stellar type, which provides an ideal reference star to model Earth's atmospheric effects. We find that interpretation of the data is highly sensitive to time-varying translations along the detector, which change according to wavelength and differ between the target and reference star. It was necessary to employ a time-dependent cross-correlation to align our wavelength bins and correct for atmospheric differential refraction. This approach allows us to resolve the wings of the Na line across 5 wavelength bins at a resolution of $\sim$1.6nm and limit the abundance of Na. We obtain consistent results from each telescope with a Na amplitude of 521$\pm$161 ppm and 403$\pm$186 ppm for GMOS and KOSMOS respectively. The results are analyzed with a radiative transfer model that includes the effects of ionization. The data are consistent with a clear atmosphere between $\sim$1--100 mbar which establish a lower limit on Na at 0.4$^{+2}_{-0.3}$ ppm ([Na/H]=-0.64$^{+0.78}_{-0.6}$), consistent with solar. However, we can not rule out the presence of clouds at $\sim$10 mbar which allow for higher Na abundances which would be consistent with stellar metallicity measured for the host star ([Na/H]=0.485$\pm$0.043)., Comment: AJ published, 20 pages, all code is available here: https://github.com/pearsonkyle

International audience; The Imaging UltraViolet Spectrograph (IUVS) instrument of the Mars Atmosphere and Volatile EvolutioN (MAVEN) mission has acquired data on Mars for more than one Martian year. During this time, beginning with March 2015, hundreds of stellar occultations have been observed, in 12 dedicated occultation campaigns, executed on average every two to three months. The occultations cover the latitudes from 80° S to 75° N and the full range longitude, and local times with relatively sparse sampling. From these measurements we retrieve CO2, O2, and O3 number densities as well as temperature profiles in the altitude range from 20 to 160 km, covering eight order of magnitudes in pressure from ∼2 × 101 to ∼4 × 10−7 Pa. These data constrain the composition and thermal structure of the atmosphere. The O2 mixing ratios retrieved during this study show a high variability from 1.5 × 10−3 to 6 × 10−3; however, the mean value seems to be constant with solar longitude. We detect ozone between 20 and 60 km. In many profiles there is a well defined peak between 30 and 40 km with a maximum density of 1 – 2 × 109 cm−3. Examination of the vertical temperature profiles reveals substantial disagreement with models, with observed temperatures both warmer and colder than predicted. Examination of the altitude profiles of density perturbations and their variation with longitude shows structured atmospheric perturbations at altitudes above 100 km that are likely non‐migrating tides. These perturbations are dominated by zonal wavenumber 2 and 3 with amplitudes greater than 45 %.

Past UV and optical observations of stars hosting hot Jupiters have shown that some of these stars present an anomalously low chromospheric activity, significantly below the basal level. For WASP-13, observations have shown that the apparent lack of activity is possibly caused by absorption from the intervening ISM. Inspired by this result, we study the effect of ISM absorption on activity measurements (S and logR'$_{\rm HK}$ indices) for main-sequence late-type stars. To this end, we employ synthetic stellar photospheric spectra combined with varying amounts of chromospheric emission and ISM absorption. We present the effect of ISM absorption on activity measurements by varying several instrumental, stellar, and ISM parameters. We find that for relative velocities between the stellar and ISM lines smaller than 30-40 km/s and for ISM CaII column densities logN$_{\rm CaII}$>12, the ISM absorption has a significant influence on activity measurements. Direct measurements and three dimensional maps of the Galactic ISM absorption indicate that an ISM CaII column density of logN$_{\rm CaII}$=12 is typically reached by a distance of about 100 pc along most sight lines. In particular, for a Sun-like star lying at a distance greater than 100 pc, we expect a depression (bias) in the logR'$_{\rm HK}$ value larger than 0.05-0.1 dex, about the same size as the typical measurement and calibration uncertainties on this parameter. This work shows that the bias introduced by ISM absorption must always be considered when measuring activity for stars lying beyond 100 pc. We also consider the effect of multiple ISM absorption components. We discuss the relevance of this result for exoplanet studies., Accepted for publication in A&A

In the Solar System giant planets, H Lyman α is the most intensive FUV thermospheric emission line. It is expected to be a very intense emission line in the FUV spectra of hot Jupiters. The detection and analysis of this line could be used to characterize the planetary environment. However, the emission of the star is much more intense and the contrast is not necessarily favorable for detecting the planetary signal. The aim of this study is to investigate conditions that would make the direct detection of a planetary Lyman α emission possible and to estimate the emission level. The cases of the two planets HD 209458b and HD 189733b are considered. The contrasts between the Lyman α emissions of the planets and their parent stars are calculated. Dayglow, thermal and auroral emissions are calculated. Scaling laws based on Solar System conditions are used to estimate the energy inputs into the atmospheres of the planets. A radiative transfer code is used, coupled with a kinetic code which describes the transport of electrons in the upper atmospheres of the planets. Absorption by the interstellar medium is taken into account in calculating the contrast between the stellar and planetary lines as observed from Earth. We find that the planetary emission is mainly due to dayside scattering of the stellar line: thermal emission contributes up to ∼6% of the total dayside planetary intensity and within our hypotheses, the auroral contribution is less than 10−3 of it. In the two planetary systems considered, the ratios between the emissions of the planets and their parent stars are ∼10−3. When the planets are at quadrature phase angle, the contrasts between the planetary and stellar fluxes reach values between ∼10−3 and 10−2 at Earth. The detectability of the emissions depends mainly on the signal to noise ratio and thus of both the total flux received from the system and the line profile.

We analyzed 15 solar occultations observed by the Cassini UVIS instrument to constrain the density and temperature structure near the exobase of Saturn. We retrieved the density of H2 and thus the temperature at altitudes higher than 1900 km above the 1 bar level by analyzing the ionization continuum of H2 at wavelengths shorter than 804 A. We find that the exospheric temperature ranges from 370 K to 540 K, with a typical uncertainty of less than 20 K. According to our data the temperature increases with latitude from the equator to the poles by 100–150 K. At similar latitudes, the temperature varies by 20–50 K at different times with no evidence for any systematic diurnal trend so far. Based on our data, the exobase of Saturn is 2700–3000 km above the 1 bar level and the thermal escape parameter near the exobase ranges from 260 to 340, implying that thermal escape from Saturn is firmly in the Jeans regime. The mixing ratio of H2 is close to unity at all altitudes below the exobase. We find that the pressure levels in the thermosphere deviate significantly from a simple spheroid predicted by potential theory. This is consistent with significant meridional temperature variations in the lower thermosphere. A global analysis of the temperature structure at different depths in the atmosphere is required to constrain both the shape and the deposition and redistribution of energy in the upper atmosphere further.

Transits in the H I 1216 A (Lyman alpha), O I 1334 A, C II 1335 A, and Si III 1206.5 A lines constrain the properties of the upper atmosphere of HD209458b. In addition to probing the temperature and density profiles in the thermosphere, they have implications for the properties of the lower atmosphere. Fits to the observations with a simple empirical model and a direct comparison with a more complex hydrodynamic model constrain the mean temperature and ionization state of the atmosphere, and imply that the optical depth of the extended thermosphere of the planet in the atomic resonance lines is significant. In particular, it is sufficient to explain the observed transit depths in the H I 1216 A line. The detection of O at high altitudes implies that the minimum mass loss rate from the planet is approximately 6e6 kg/s. The mass loss rate based on our hydrodynamic model is higher than this and implies that diffusive separation is prevented for neutral species with a mass lower than about 130 amu by the escape of H. Heavy ions are transported to the upper atmosphere by Coulomb collisions with H+ and their presence does not provide as strong constraints on the mass loss rate as the detection of heavy neutral atoms. Models of the upper atmosphere with solar composition and heating based on the average solar X-ray and EUV flux agree broadly with the observations but tend to underestimate the transit depths in the O I, C II, and Si III lines. This suggests that the temperature and/or elemental abundances in the thermosphere may be higher than expected from such models...The detection of Si2+ in the thermosphere indicates that clouds of forsterite and enstatite do not form in the lower atmosphere..., Accepted for publication in Icarus, submitted version, abstract truncated to fit astro-ph limits (see manuscript for full version)

The upper atmospheres of the planets and their satellites are more directly exposed to sunlight and solar wind particles than the surface or the deeper atmospheric layers. At the altitudes where the associated energy is deposited, the atmospheres may become ionized and are referred to as ionospheres. The details of the photon and particle interactions with the upper atmosphere depend strongly on whether the object has anintrinsic magnetic field that may channel the precipitating particles into the atmosphere or drive the atmospheric gas out to space. Important implications of these interactions include atmospheric loss over diverse timescales, photochemistry and the formation of aerosols, which affect the evolution, composition and remote sensing of the planets (satellites). The upper atmosphere connects the planet (satellite) bulk composition to the near-planet (-satellite) environment. Understanding the relevant physics and chemistry provides insight to the past and future conditions of these objects, which is critical for understanding their evolution. This chapter introduces the basic concepts of upper atmospheres and ionospheres in our solar system, and discusses aspects of their neutral and ion composition, wind dynamics and energy budget. This knowledge is key to putting in context the observations of upper atmospheres and haze on exoplanets, and to devise a theory that explains exoplanet demographics.

This is a pre-print (pre-peer review) manuscript. It is moderately different from the accepted manuscript and from the published article. Citation of published article: Fernando J. Capalbo, Yves Bénilan, Nicolas Fray, Martin Schwell, Norbert Champion, Et-touhami Es-sebbar, Tommi T. Koskinen, Ivan Lehocki, Roger V. Yelle. Icarus, vol. 265, p. 95 - 109. February 2016. doi: 10.1016/j.icarus.2015.10.006.; International audience; Benzene is an important molecule in Titan’s atmosphere because it is a potential link between the gas phase and the organic solid phase. We measured photoabsorption in the ultraviolet by benzene gas at temperatures covering the range from room temperature to 215 K. We derived benzene absorption cross sections and analyzed them in terms of the transitions observed. No significant variation with measurement temperature was observed. We discuss the implications of our measurements for the derivation of benzene abundance profiles in Titan’s thermosphere, by the Cassini/Ultraviolet Imaging Spectrograph (UVIS). The use of absorption cross sections at low temperature is recommended to avoid small systematic uncertainties in the profiles. We used our measurements, together with absorption cross sections from other molecules, to analyze four stellar occultations by Titan, measured by UVIS during flybys T21, T41, T41_II, and T53. We derived and compared benzene abundance profiles in Titan’s thermosphere between approximately 530 and 1000 km, for different dates and geographical locations. The comparisons of our benzene profiles with each other, and with profiles from models of the upper atmosphere, point to a complex behavior that is not explained by current photochemical models.

Stellar occultations observed by the Cassini/UVIS instrument provide unique data that probe the mesosphere and lower thermosphere of Titan at altitudes between 400 and 1400 km. This region is a site of complex photochemistry that forms hydrocarbon and nitrile species, and plays a crucial role in the formation of the organic hazes observed in the stratosphere, but has yet to be adequately characterized. We analyzed publicly available data obtained between flybys Tb in December 2004 and T58 in July 2009, with an emphasis on two stable occultations obtained during flybys T41 and T53. We derived detailed density profiles for CH 4 , C 2 H 2 , C 2 H 4 , C 4 H 2 , HCN, HC 3 N and C 6 H 6 between ∼400 and 1200 km and extinction coefficients for aerosols between 400 and 900 km. Our analysis reveals the presence of extinction layers in the occultation data that are associated with large perturbations in the density profiles of the gaseous species and extinction profiles of the aerosols. These relatively stable features vary in appearance with location and change slowly over time. In particular, we identify a sharp extinction layer between 450 and 550 km that coincides with the detached haze layer. In line with recent images obtained by Cassini/ISS, the altitude of this layer changes rapidly around the equinox in 2009. Our results point to unexpected complexity that may have significant consequences for the dynamics and physical processes taking place in the upper atmosphere of Titan.

Extremely irradiated hot Jupiters, exoplanets reaching dayside temperatures ${>}$2000 K, stretch our understanding of planetary atmospheres and the models we use to interpret observations. While these objects are planets in every other sense, their atmospheres reach temperatures at low pressures comparable only to stellar atmospheres. In order to understand our \textit{a priori} theoretical expectations for the nature of these objects, we self-consistently model a number of extreme hot Jupiter scenarios with the PHOENIX model atmosphere code. PHOENIX is well-tested on objects from cool brown dwarfs to expanding supernovae shells and its expansive opacity database from the UV to far-IR make PHOENIX well-suited for understanding extremely irradiated hot Jupiters. We find several fundamental differences between hot Jupiters at temperatures ${>}$2500 K and their cooler counterparts. First, absorption by atomic metals like Fe and Mg, molecules including SiO and metal hydrides, and continuous opacity sources like H$^-$ all combined with the short-wavelength output of early-type host stars result in strong thermal inversions, without the need for TiO or VO. Second, many molecular species, including H$_2$O, TiO, and VO are thermally dissociated at pressures probed by eclipse observations, biasing retrieval algorithms that assume uniform vertical abundances. We discuss other interesting properties of these objects, as well as future prospects and predictions for observing and characterizing this unique class of astrophysical object, including the first self-consistent model of the hottest known jovian planet, KELT-9b., 23 pages, 16 figures, 1 table. Submitted to ApJ

Many extrasolar planets orbit close in and are subject to intense ionizing radiation from their host stars. Therefore, we expect them to have strong, and extended, ionospheres. Ionospheres are important because they modulate escape in the upper atmosphere and can modify circulation, as well as leave their signatures, in the lower atmosphere. In this paper, we evaluate the vertical location and extent of the EUV ionization peak layer. We find that -10 nbar—for a wide range of orbital distances (a = 0.047-1 AU) from the host star—and , where Hp is the pressure scale height. At , the plasma frequency is ~80-450 MHz, depending on a. We also study global ion transport, and its dependence on a, using a three-dimensional thermosphere-ionosphere model. On tidally synchronized planets with weak intrinsic magnetic fields, our model shows only a small, but discernible, difference in electron density from the dayside to the nightside (~9 × 1013 m–3 to ~2 × 1012 m–3, respectively) at . On asynchronous planets, the distribution is essentially uniform. These results have consequences for hydrodynamic modeling of the atmospheres of close-in extrasolar giant planets.

National Science Foundation of China (NSFC) [41525015, 41774186, 41504133]; Science and Technology Development Fund of Macau SAR (FDCT) [039/2013/A2, 082/2015/A3]

The extrasolar 'hot Jupiter' HD209458b is orbiting close to a solar-type star and is subject to intense heating as a result. It is surrounded by an expanded atmosphere of atomic hydrogen that is escaping from the planet. Such escape is theoretically possible at least inside an orbit of 0.1 AU (one AU is the distance between Earth and the Sun). But at 5 AU from the Sun, Jupiter has a stable atmosphere. So somewhere between those extremes there must be a crossover between stability and instability. Numerical modelling now suggests that crossover occurs between 0.14 and 0.16 AU for a Jupiter-like planet. Recent observations of the planet HD209458b indicate that it is surrounded by an expanded atmosphere of atomic hydrogen that is escaping hydrodynamically1,2,3. Theoretically, it has been shown that such escape is possible at least inside an orbit of 0.1 au (refs 4 and 5), and also that H3+ ions play a crucial role in cooling the upper atmosphere5,6. Jupiter’s atmosphere is stable7, so somewhere between 5 and 0.1 au there must be a crossover between stability and instability. Here we show that there is a sharp breakdown in atmospheric stability between 0.14 and 0.16 au for a Jupiter-like planet orbiting a solar-type star. These results are in contrast to earlier modelling4,8 that implied much higher thermospheric temperatures and more significant evaporation farther from the star. (We use a three-dimensional, time-dependent coupled thermosphere–ionosphere model6 and properly include cooling by H3+ ions, allowing us to model globally the redistribution of heat and changes in molecular composition.) Between 0.2 and 0.16 au cooling by H3+ ions balances heating by the star, but inside 0.16 au molecular hydrogen dissociates thermally, suppressing the formation of H3+ and effectively shutting down that mode of cooling.

Several models of extrasolar giant planet (EGP) atmospheres have been developed recently. Many of them are one-dimensional or concentrate on the lower or middle atmosphere. Three-dimensional hydrodynamic models are needed to study the horizontal variations in temperature and composition of EGP atmospheres. Circulation models for the upper atmosphere are particularly important as they can be used to study the thermal structure due to stellar irradiation, radiative cooling, and atmospheric circulation in the thermospheres of close-in EGPs and hence the rate of evaporation of their atmospheres. We present a generic gas giant model that is capable of generating three-dimensional, self-consistent global simulations of stable EGP thermospheres at different orbital distances. Calculations performed by this model indicate that IR emissions from H ions may play a significant role in cooling the thermospheres of EGPs at least in the range of 0.2-1 AU from a solar-type host star. In this range thermal dissociation of H2 is negligible and ion densities are small compared to the overall neutral density. Inside 0.2 AU thermal dissociation and dissociative photoionization of H2 may prevent the effective formation of H. In the absence of radiative cooling from H the upper atmospheres reach temperatures well above 10,000 K within ~0.5 AU. In this case the upper thermospheres are entirely converted into atomic hydrogen and the temperatures are high enough for significant atmospheric loss to take place. Our model is capable of calculating the IR signal strengths for various vibrational transitions of H based on the thermal state and the composition of the atmosphere. Potential detection of such signals would thus provide a validation of some of our results.

Several transiting hot Jupiters orbit relatively inactive main-sequence stars. For some of those, the logR'HK activity parameter lies below the basal level (-5.1). Two explanations have been proposed so far: (i) the planet affects the stellar dynamo, (ii) the logR'HK measurements are biased by extrinsic absorption, either by the interstellar medium (ISM) or by material local to the system. We present here Hubble Space Telescope/COS far-UV spectra of WASP-13, which hosts an inflated hot Jupiter and has a measured logR'HK value (-5.26), well below the basal level. From the star's spectral energy distribution we obtain an extinction E(B-V) = 0.045+/-0.025 mag and a distance d = 232+/-8 pc. We detect at >4 sigma lines belonging to three different ionization states of carbon (C1, C2, and C4) and the Si4 doublet at ~3 sigma. Using far-UV spectra of nearby early G-type stars of known age, we derive a C4/C1 flux ratio-age relation, from which we estimate WASP-13's age to be 5.1+/-2.0 Gyr. We rescale the solar irradiance reference spectrum to match the flux of the C4 1548 doublet. By integrating the rescaled solar spectrum, we obtain an XUV flux at 1 AU of 5.4 erg s^-1 cm^-2. We use a detailed model of the planet's upper atmosphere, deriving a mass-loss rate of 1.5x10^11 g s^-1. Despite the low logR'HK value, the star shows a far-UV spectrum typical of middle-aged solar-type stars, pointing toward the presence of significant extrinsic absorption. The analysis of a high-resolution spectrum of the Ca2H&K lines indicates that the ISM absorption could be the origin of the low logR'HK value. Nevertheless, the large uncertainty in the Ca2 ISM abundance does not allow us to firmly exclude the presence of circumstellar gas., Accepted for publication on ApJ

We adapted a multi-species escape model, developed for close-in extrasolar planets, to calculate the escape rates of CH4 and N2 from Pluto. In the absence of escape, CH4 should overtake N2 as the dominant species below the exobase. The CH4 profile depends strongly on the escape rate, however, and the typical escape rates predicted for Pluto lead to a nearly constant mixing ratio of less than 1 % below the exobase. In this case the CH4 escape rate is only 5-10 % of the N2 escape rate. Observations of the CH4 profile by the New Horizons/ALICE spectrograph can constrain the CH4 escape rate and provide a unique test for escape models., Accepted to Geophysical Research Letters