T. T. Koskinen

The escape of heavy atoms from the ionosphere of HD209458b. I. A photochemical–dynamical model of the thermosphere

The detections of atomic hydrogen, heavy atoms and ions surrounding the extrasolar giant planet (EGP) HD209458b constrain the composition, temperature and density profiles in its upper atmosphere. Thus the observations provide guidance for models that have so far predicted a range of possible conditions. We present the first hydrodynamic escape model for the upper atmosphere that includes all of the detected species in order to explain their presence at high altitudes, and to further constrain the temperature and velocity profiles. This model calculates the stellar heating rates based on recent estimates of photoelectron heating efficiencies, and includes the photochemistry of heavy atoms and ions in addition to hydrogen and helium. The composition at the lower boundary of the escape model is constrained by a full photochemical model of the lower atmosphere. We confirm that molecules dissociate near the 1 μbar level, and find that complex molecular chemistry does not need to be included above this level. We also confirm that diffusive separation of the detected species does not occur because the heavy atoms and ions collide frequently with the rapidly escaping H and H+. This means that the abundance of the heavy atoms and ions in the thermosphere simply depends on the elemental abundances and ionization rates. We show that, as expected, H and O remain mostly neutral up to at least 3Rp, whereas both C and Si are mostly ionized at significantly lower altitudes. We also explore the temperature and velocity profiles, and find that the outflow speed and the temperature gradients depend strongly on the assumed heating efficiencies. Our models predict an upper limit of 8000 K for the mean (pressure averaged) temperature below 3Rp, with a typical value of 7000 K based on the average solar XUV flux at 0.047 AU. We use these temperature limits and the observations to evaluate the role of stellar energy in heating the upper atmosphere.

Aerosol growth in Titan’s ionosphere

Photochemically produced aerosols are common among the atmospheres of our solar system and beyond. Observations and models have shown that photochemical aerosols have direct consequences on atmospheric properties as well as important astrobiological ramifications, but the mechanisms involved in their formation remain unclear. Here we show that the formation of aerosols in Titan’s upper atmosphere is directly related to ion processes, and we provide a complete interpretation of observed mass spectra by the Cassini instruments from small to large masses. Because all planetary atmospheres possess ionospheres, we anticipate that the mechanisms identified here will be efficient in other environments as well, modulated by the chemical complexity of each atmosphere.

The escape of heavy atoms from the ionosphere of HD209458b. II. Interpretation of the observations

Transits in the H I 1216 A (Lyman α), 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 6 × 106 kg s−1. 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. Observations of the escaping atmosphere can potentially be used to constrain the strength of the planetary magnetic field. We find that a magnetic moment of m ≲ 0.04mJ, where mJ is the jovian magnetic moment, allows the ions to escape globally rather than only along open field lines. The detection of Si2+ in the thermosphere indicates that clouds of forsterite and enstatite do not form in the lower atmosphere. This has implications for the temperature and dynamics of the atmosphere that also affect the interpretation of transit and secondary eclipse observations in the visible and infrared wavelengths.

XUV-driven mass loss from extrasolar giant planets orbiting active stars

Upper atmospheres of Hot Jupiters are subject to extreme radiation conditions that can result in rapid atmospheric escape. The composition and structure of the upper atmospheres of these planets are affected by the high-energy spectrum of the host star. This emission depends on stellar type and age, which are thus important factors in understanding the behaviour of exoplanetary atmospheres. In this study, we focus on Extrasolar Giant Planets (EPGs) orbiting K and M dwarf stars. XUV spectra for three different stars – ∊ Eridani, AD Leonis and AU Microscopii – are constructed using a coronal model. Neutral density and temperature profiles in the upper atmosphere of hypothetical EGPs orbiting these stars are then obtained from a fluid model, incorporating atmospheric chemistry and taking atmospheric escape into account. We find that a simple scaling based solely on the host star’s X-ray emission gives large errors in mass loss rates from planetary atmospheres and so we have derived a new method to scale the EUV regions of the solar spectrum based upon stellar X-ray emission. This new method produces an outcome in terms of the planet’s neutral upper atmosphere very similar to that obtained using a detailed coronal model of the host star. Our results indicate that in planets subjected to radiation from active stars, the transition from Jeans escape to a regime of hydrodynamic escape at the top of the atmosphere occurs at larger orbital distances than for planets around low activity stars (such as the Sun).

Electron densities and alkali atoms in exoplanet atmospheres

We describe a detailed study on the properties of alkali atoms in extrasolar giant planets, and specifically focus on their role in generating the atmospheric free electron densities, as well as their impact on the transit depth observations. We focus our study on the case of HD 209458 b, and we show that photoionization produces a large electron density in the middle atmosphere that is about two orders of magnitude larger than the density anticipated from thermal ionization. Our purely photochemical calculations though result in a much larger transit depth for K than observed for this planet. This result does not change even if the roles of molecular chemistry and excited state chemistry are considered for the alkali atoms. In contrast, the model results for the case of exoplanet XO-2 b are in good agreement with the available observations. Given these results we discuss other possible scenarios, such as changes in the elemental abundances, changes in the temperature profiles, and the possible presence of clouds, which could potentially explain the observed HD 209458 b alkali properties. We find that most of these scenarios can not explain the observations, with the exception of a heterogeneous source (i.e. clouds or aerosols) under specific conditions, but we also note the discrepancies among the available observations.

Thermal escape from extrasolar giant planets

The detection of hot atomic hydrogen and heavy atoms and ions at high altitudes around close-in extrasolar giant planets (EGPs) such as HD209458b implies that these planets have hot and rapidly escaping atmospheres that extend to several planetary radii. These characteristics, however, cannot be generalized to all close-in EGPs. The thermal escape mechanism and mass loss rate from EGPs depend on a complex interplay between photochemistry and radiative transfer driven by the stellar UV radiation. In this study, we explore how these processes change under different levels of irradiation on giant planets with different characteristics. We confirm that there are two distinct regimes of thermal escape from EGPs, and that the transition between these regimes is relatively sharp. Our results have implications for thermal mass loss rates from different EGPs that we discuss in the context of currently known planets and the detectability of their upper atmospheres.

ELECTRODYNAMICS ON EXTRASOLAR GIANT PLANETS

Strong ionization on close-in extrasolar giant planets suggests that their atmospheres may be affected by ion drag and resistive heating arising from wind-driven electrodynamics. Recent models of ion drag on these planets, however, are based on thermal ionization only and do not include the upper atmosphere above the 1 mbar level. These models are also based on simplified equations of resistive MHD that are not always valid in extrasolar planet atmospheres. We show that photoionization dominates over thermal ionization over much of the dayside atmosphere above the 100 mbar level, creating an upper ionosphere dominated by ionization of H and He and a lower ionosphere dominated by ionization of metals such as Na, K, and Mg. The resulting dayside electron densities on close-in exoplanets are higher than those encountered in any planetary ionosphere of the solar system, and the conductivities are comparable to the chromosphere of the Sun. Based on these results and assumed magnetic fields, we constrain the conductivity regimes on close-in EGPs and use a generalized Ohms law to study the basic effects of electrodynamics in their atmospheres. We find that ion drag is important above the 10 mbar level where it can also significantly alter the energy balance through resistive heating. Due to frequent collisions of the electrons and ions with the neutral atmosphere, however, ion drag is largely negligible in the lower atmosphere below the 10 mbar level for a reasonable range of planetary magnetic moments [abridged].

Ultraviolet C ii and Si iii Transit Spectroscopy and Modeling of the Evaporating Atmosphere of GJ436b

Hydrogen gas evaporating from the atmosphere of the hot-Neptune GJ436b absorbs over 50% of the stellar Lyα emission during transit. Given the planets atmospheric composition and energy-limited escape rate, this hydrogen outflow is expected to entrain heavier atoms such as C and O. We searched for C and Si in the escaping atmosphere of GJ436b using far-ultraviolet Hubble Space Telescope COS G130M observations made during the planets extended H i transit. These observations show no transit absorption in the C ii 1334,1335 A and Si iii 1206 A lines integrated over [−100, 100] km s−1, imposing 95% (2σ) upper limits of 14% (C ii) and 60% (Si iii) depth on the transit of an opaque disk and 22% (C ii) and 49% (Si iii) depth on an extended highly asymmetric transit similar to that of H i Lyα. C+ is likely present in the outflow according to a simulation we carried out using a spherically symmetric photochemical-hydrodynamical model. This simulation predicts an ~2% transit over the integrated bandpass, consistent with the data. At line center, we predict the C ii transit depth to be as high as 19%. Our model predicts a neutral hydrogen escape rate of g s−1 ( g s−1 for all species) for an upper atmosphere composed of hydrogen and helium.

The detection of benzene in Saturn's upper atmosphere

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

Probing the Martian Atmosphere with MAVEN/IUVS Stellar Occultations

The first campaign of stellar occultations with the Imaging Ultraviolet Spectrograph (IUVS) instrument on board of Mars Atmosphere and Volatile EvolutioN (MAVEN) mission was executed between 24 and 26 March 2015. From this campaign 13 occultations are used to retrieve CO2 and O2 number densities in the altitude range between 100 and 150 km. Observations probe primarily the low-latitude regions on the nightside of the planet, just past the dawn and dusk terminator. Calculation of temperature from the CO2 density profiles reveals that the lower thermosphere is significantly cooler than predicted by the models in the Mars Climate Database. A systematically cold layer with temperatures of 105–120 K is seen in the occultations at a pressure level around 7 × 10−6 Pa.