C. D. Parkinson

New observations of the extended hydrogen exosphere of the extrasolar planet HD 209458b

Context: Atomic hydrogen escaping from the planet HD 209458b provides the largest observational signature ever detected for an extrasolar planet atmosphere. However, the Space Telescope Imaging Spectrograph (STIS) used in previous observational studies is no longer available, whereas additional observations are still needed to better constrain the mechanisms subtending the evaporation process and to determine the evaporation state of other “hot Jupiters”. Aims: Here, we aim to detect the extended hydrogen exosphere of HD 209458b with the Advanced Camera for Surveys (ACS) on board the Hubble Space Telescope (HST) and to find evidence of a hydrogen comet-like tail trailing the planet, whose size would depend on the escape rate and the amount of ionizing radiation emitted by the star. These observations also provide a benchmark for other transiting planets, in the frame of a comparative study of the evaporation state of close-in giant planets. Methods: Eight HST orbits were used to observe two transits of HD 209458b. Transit light curves were obtained by performing photometry of the unresolved stellar Lyman-α (Lyα) emission line during both transits. Absorption signatures of exospheric hydrogen during the transit were compared to light curve models predicting a hydrogen tail. Results: Transit depths of (9.6±7.0)% and (5.3±10.0)% were measured on the whole Lyα line in visits 1 and 2, respectively. Averaging data from both visits, we find an absorption depth of (8.0±5.7)%, in good agreement with previous studies. Conclusions: The extended size of the exosphere confirms that the planet is most likely losing hydrogen to space, yet, the photometric precision achieved does not allow us to better constrain the hydrogen mass-loss rate. Based on observations made with the Advanced Camera for Surveys on board the Hubble Space Telescope.

Enceladus: Cassini observations and implications for the search for life

Aims. The recent Cassini discovery of water vapor plumes ejected from the south pole of the Saturnian satellite, Enceladus, presents a unique window of opportunity for the detection of extant life in our solar system. Methods. With its significant geothermal energy source propelling these plumes >80 km from the surface of the moon and the ensuing large temperature gradient with the surrounding environment, it is possible to have the weathering of rocks by liquid water at the rock/liquid interface. For the cases of the putatively detected salt-water oceans beneath the ice crusts of Europa and Callisto, an isolated subsurface ocean without photosynthesis or contact with an oxidizing atmosphere will approach chemical equilibrium and annihilate any ecosystems dependent on redox gradients unless there is a substantial alternative energy source. This thermodynamic tendency imposes severe constraints on any biota that is based on chemical energy. On Enceladus, the weathering of rocks by liquid water and any concomitant radioactive emissions are possible incipient conditions for life. If there is CO, CO2 and NH3 present in the spectra obtained from the plume, then this is possible evidence that amino acids could be formed at the rock/liquid interface of Enceladus. The combination of a hydrological cycle, chemical redox gradient and geochemical cycle give favorable conditions for life. Results. We discuss the search for signatures of these species and organics in the Cassini UVIS spectra of the plume and implications for the possible detection of life.

Methanol on Enceladus

Near infrared spectra of the surface of Enceladus returned by Cassini show the presence of an absorption feature at 3.53 μm, ascribed by Brown et al. (2006) to “short chain organics,” and by Newman et al. (2007) to hydrogen peroxide. We assign this feature tentatively to methanol. Variations in the peak position of the feature suggest that methanol in the “tiger stripes” region may be segregated from the water ice, and not homogeneously distributed in the ice matrix. The photolytic destruction of methanol implies that methane or methanol itself must be continually deposited on the surface. On Enceladus, methanol may be generated photochemically from a mixed methane/water ice, or deposited from the plume itself. The variation in the concentration of methanol over the surface could be used to distinguish between these two processes.

Modelling the Lyman β dayglow in the Jovian atmosphere

The Lyman β dayglow of Jupiter provides an important source of information about its atmosphere. The H Lyman β and the H 2 6-0 P(1) line of the Lyman system overlap, resulting in a coupling effect between these two lines. In this paper, we evaluate the effect of this overlapping via radiative transfer modelling and show its effect both on the integrated intensity and the line profile. The intensity increases at about 1025 A at the centre of the disc, tends to slightly decrease at the limb, and creates an asymmetry in the profile. This asymmetry is also present in the jovian H-Lyman a bulge region.

Estimates of atomic deuterium abundance and Lyman-alpha airglow in the thermosphere of Jupiter

We have made calculations of the atomic D distribution in the thermosphere of Jupiter. The principal reactions determining the D abundance appear to be generation by reaction of H with vibrationally hot HD and loss by reaction of D with H2(υ=0,1) and CH3. The H and CH3 distributions have been calculated using a 1-D photochemical-diffusion model with the column H constrained using the Lyman-α airglow. For H2 effective vibrational temperatures, Tυ, between 1 and 4 times kinetic we find D columns between 4×1011 and 2×1013 atoms cm−2. HD can be vibrationally excited due to VV energy transfer from H2(υ=1). Using a radiative transfer model with coupling of the H and D Lyman-α lines we have calculated line profiles and total intensities across the Jovian disk and on the limb. For the above D columns and a H column ∼3.5×1017 cm−2, compatible with equatorial Lyman-α airglow observations, the disk D intensity varies from 80 to 600 R for overhead Sun and viewing, whereas on the terminator the D maximum total intensity is ∼60 R at ∼860 km above the 1 bar level for the maximum D column.

Enhanced transport in the polar mesosphere of Jupiter: Evidence from Cassini UVIS helium 584 Å airglow

[1] The eddy diffusion profile (K) in the auroral regions of Jupiter is not well determined. However, because of the intense auroral energy input, eddy mixing is expected to be much more effective and may be responsible for the enhancement of heavy hydrocarbon production in the polar region. In this paper, we estimate the increased eddy mixing in the Jovian auroral regions by comparing the Cassini Ultraviolet Imaging Spectrograph (UVIS) observations during the 2000 Jupiter flyby with radiative transfer calculations

H2 vibrational temperatures in the upper atmosphere of Jupiter

The Jovian upper atmosphere has been extensively studied over the past few decades with many observations having been made. Typically, the H2 vibrational temperatures have always been considered as kinetic temperatures in modelling efforts to date. However, recent studies have shown that this assumption is not robust and that we can expect enhanced vibrational temperatures due to overlapping lines to play an important role in the thermosphere of Jupiter. In this paper, we use a radiative transfer code to compute the total integrated H Lyman α dayglow intensity taking into account the overlapping with H2 hot bands. We show that an atmosphere with H2 vibrational temperatures equal to about 1.4 to 1.5 times kinetic reproduce the Voyager observations.

A Monte Carlo Model of Crustal Field Influences on Solar Energetic Particle Precipitation into the Martian Atmosphere

Solar energetic particles (SEPs) can precipitate directly into the atmospheres of weakly magnetized planets, causing increased ionization, heating, and altered neutral chemistry. However, strong localized crustal magnetism at Mars can deflect energetic charged particles and reduce precipitation. In order to quantify these effects, we have developed a model of proton transport and energy deposition in spatially varying magnetic fields, called Atmospheric Scattering of Protons and Energetic Neutrals (ASPEN). We benchmark the models particle tracing algorithm, collisional physics, and heating rates, comparing against previously published work in the latter two cases. We find that energetic non-relativistic protons precipitating in proximity to a crustal field anomaly will primarily deposit energy at either their stopping altitude or magnetic reflection altitude. We compared atmospheric ionization in the presence and absence of crustal magnetic fields at 50° S and 182° E during the peak flux of the 29 October 2003 “Halloween storm” SEP event. The presence of crustal magnetic fields reduced total ionization by ~30% but caused ionization to occur over a wider geographic area.

On radar sounding applications for Enceladean ice

Due to the nature of observations taken by planetary spacecraft, many surface and atmospheric studies have been performed at the icy moons of the outer planets, which have left the many seemingly complex interior processes in these bodies left unexplored and unexplained. It is notably difficult to access the interior regions in which planetary formation and dynamics take place. This paper presents the possibility that radar measurements could contribute to the understanding of interior structure, particularly that of Enceladus, the small but notably dynamic icy moon of Saturn. The application of such radar may lead to discoveries concerning formation mechanisms and surface processes. Additionally, radar sounding will contribute measurements that aid in diagnosing the dynamics system at work in the subsurface - perhaps most notably, the source reservoir and/or dynamics of the observed water plume at the moons south pole, in addition the moons role as a whole in the Saturnian system.

The hydrogen exosphere of exoplanet HD 209458b detected with HST /ACS

Exospheric atomic hydrogen escaping from the planet HD 209458b provides the largest observational signature ever detected for an extrasolar planet atmosphere. We present observations of this transiting planets extended exosphere with the Advanced Camera for Surveys on board the Hubble Space Telescope . From the two transit light curves obtained at Lyman α, we find an in-transit absorption of (8.0±5.7)%, in good agreement with previous studies. These new constraints on the size of the exosphere strengthens the evaporation scenario. Full details are provided in Ehrenreich et al . (2008).