arXiv subjects

Leigh N. Fletcher

Publications and source records attributed to Leigh N. Fletcher.

At least 73 records · Page 4Linked to original sources

arXiv2019

Latitudinal variation in the abundance of methane (CH4) above the clouds in Neptune's atmosphere from VLT/MUSE Narrow Field Mode Observations

Observations of Neptune, made in 2018 using the new Narrow Field Adaptive Optics mode of the Multi Unit Spectroscopic Explorer (MUSE) instrument at the Very Large Telescope (VLT) from 0.48 - 0.93 micron, are analysed here to determine the latitudinal and vertical distribution of cloud opacity and methane abundance in Neptune's observable troposphere (0.1 - ~3 bar). Previous observations at these wavelengths in 2003 by HST/STIS (Karkoschka and Tomasko 2011, Icarus 205, 674-694) found that the mole fraction of methane above the cloud tops (at ~2 bar) varied from ~4% at equatorial latitudes to ~2% at southern polar latitudes, by comparing the observed reflectivity at wavelengths near 825 nm controlled primarily by either methane absorption or H2-H2/H2-He collision-induced absorption. We find a similar variation in cloud-top methane abundance in 2018, which suggests that this depletion of methane towards Neptune's pole is potentially a long-lived feature, indicative of long-term upwelling at mid-equatorial latitudes and subsidence near the poles. By analysing these MUSE observations along the central meridian with a retrieval model, we demonstrate that a broad boundary between the nominal and depleted methane abundances occurs at between 20 - 40S. We also find a small depletion of methane near the equator, perhaps indicating subsidence there, and a local enhancement near 60 - 70S, which we suggest may be associated with South Polar Features (SPFs) seen in Neptune's atmosphere at these latitudes. Finally, by the use of both a reflectivity analysis and a principal component analysis, we demonstrate that this depletion of methane towards the pole is apparent at all locations on Neptune's disc, and not just along its central meridian.

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arXiv2019

Potential Vorticity of Saturn's Polar Regions: Seasonality and Instabilities

We analyse the potential vorticity of Saturn's polar regions, as it is a fundamental dynamical tracer that enables us to improve our understanding of the dynamics of these regions and their seasonal variability. In particular, we present zonally averaged quasi-geostrophic potential vorticity maps between 68° planetographic latitude and the poles at altitudes between 500 mbar and 1mbar for three different epochs: (i) June 2013 (early northern summer) for the north polar region, (ii) December 2008 (late northern winter) for both polar regions and (iii) October 2006 (southern summer) for the south, computed using temperature profiles retrieved from Cassini Composite Infrared Spectrometer (CIRS) data and wind profiles obtained from Cassini's Imaging Science Subsystem (ISS). The results show that quasi-geostrophic potential vorticity maps are very similar at all the studied epochs, showing positive vorticities at the north and negative at the south, indicative of the dominance of the Coriolis parameter 2$Ωsinϕ$ at all latitudes, except near the pole. The meridional gradients of the quasi-geostrophic potential vorticity show that dynamical instabilities, mainly due to the barotropic term, could develop at the flanks of the Hexagon at 78°N, the jet at 73.9°S and on the equatorward flank of both polar jets. There are no differences in potential vorticity gradients between the two hemispheres that could explain why a hexagon forms in the north and not in the south. No seasonal variability of the potential vorticity and its meridional gradient has been found, despite significant changes in the atmospheric temperatures over time.

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arXiv2019

Rotational Light Curves of Jupiter from UV to Mid-Infrared and Implications for Brown Dwarfs and Exoplanets

Rotational modulations are observed on brown dwarfs and directly imaged exoplanets, but the underlying mechanism is not well understood. Here, we analyze Jupiter's rotational light curves at 12 wavelengths from the ultraviolet (UV) to the mid-infrared (mid-IR). Peak-to-peak amplitudes of Jupiter's light curves range from sub percent to 4% at most wavelengths, but the amplitude exceeds 20% at 5 $\rm μm$, a wavelength sensing Jupiter's deep troposphere. Jupiter's rotational modulations are primarily caused by discrete patterns in the cloudless belts instead of the cloudy zones. The light-curve amplitude is dominated by the sizes and brightness contrasts of the Great Red Spot (GRS), expansions of the North Equatorial Belt (NEB), patchy clouds in the North Temperate Belt (NTB) and a train of hot spots in the NEB. In reflection, the contrast is controlled by upper tropospheric and stratospheric hazes, clouds, and chromophores in the clouds. In thermal emission, the small rotational variability is caused by the spatial distribution of temperature and opacities of gas and aerosols; the large variation is caused by the $\rm NH_{3}$ cloud holes and thin-thick clouds. The methane-band light curves exhibit opposite-shape behavior compared with the UV and visible wavelengths, caused by wavelength-dependent brightness change of the GRS. Light-curve evolution is induced by periodic events in the belts and longitudinal drifting of the GRS and patchy clouds in the NTB. This study suggests several interesting mechanisms related to distributions of temperature, gas, hazes, and clouds for understanding the observed rotational modulations on brown dwarfs and exoplanets.

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arXiv2018

Probable detection of hydrogen sulphide (H$_2$S) in Neptune's atmosphere

Recent analysis of Gemini-North/NIFS H-band (1.45 - 1.8 $μ$m) observations of Uranus, recorded in 2010, with recently updated line data has revealed the spectral signature of hydrogen sulphide (H$_2$S) in Uranus's atmosphere (Irwin et al., 2018). Here, we extend this analysis to Gemini-North/NIFS observations of Neptune recorded in 2009 and find a similar detection of H$_2$S spectral absorption features in the 1.57 - 1.58 $μ$m range, albeit slightly less evident, and retrieve a mole fraction of $\sim1-3$ ppm at the cloud tops. We find a much clearer detection (and much higher retrieved column abundance above the clouds) at southern polar latitudes compared with equatorial latitudes, which suggests a higher relative humidity of H$_2$S here. We find our retrieved H$_2$S abundances are most consistent with atmospheric models that have reduced methane abundance near Neptune's south pole, consistent with HST/STIS determinations (Karkoschka and Tomasko, 2011). We also conducted a Principal Component Analysis (PCA) of the Neptune and Uranus data and found that in the 1.57 - 1.60 $μ$m range, some of the Empirical Orthogonal Functions (EOFs) mapped closely to physically significant quantities, with one being strongly correlated with the modelled H$_2$S signal and clearly mapping the spatial dependence of its spectral detectability. Just as for Uranus, the detection of H$_2$S at the cloud tops constrains the deep bulk sulphur/nitrogen abundance to exceed unity (i.e. $ > 4.4 - 5.0$ times the solar value) in Neptune's bulk atmosphere, provided that ammonia is not sequestered at great depths, and places a lower limit on its mole fraction below the observed cloud of (0.4 - 1.3) $\times 10^{-5}$. The detection of gaseous H$_2$S at these pressure levels adds to the weight of evidence that the principal constituent of the 2.5 - 3.5-bar cloud is likely to be H$_2$S ice.

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arXiv2018

A New, Long-Lived, Jupiter Mesoscale Wave Observed at Visible Wavelengths

Small-scale waves were observed along the boundary between Jupiter's North Equatorial Belt and North Tropical Zone, ~16.5° N planetographic latitude in Hubble Space Telescope data in 2012 and throughout 2015 to 2018, observable at all wavelengths from the UV to the near IR. At peak visibility, the waves have sufficient contrast (~10%) to be observed from ground-based telescopes. They have a typical wavelength of about 1.2° (1400 km), variable-length wave trains, and westward phase speeds of a few m/s or less. New analysis of Voyager 2 data shows similar wave trains over at least 300 hours. Some waves appear curved when over cyclones and anticyclones, but most are straight, but tilted, shifting in latitude as they pass vortices. Based on their wavelengths, phase speeds, and faint appearance at high-altitude sensitive passbands, the observed NEB waves are consistent with inertia-gravity waves at the 500-mbar pressure level, though formation altitude is not well constrained. Preliminary General Circulation Model simulations generate inertia-gravity waves from vortices interacting with the environment and can reproduce the observed wavelengths and orientations. Several mechanisms can generate these waves, and all may contribute: geostrophic adjustment of cyclones; cyclone/anticyclone interactions; wind interactions with obstructions or heat pulses from convection; or changing vertical wind shear. However, observations also show that the presence of vortices and/or regions of convection are not sufficient by themselves for wave formation, implying that a change in vertical structure may affect their stability, or that changes in haze properties may affect their visibility.

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arXiv2018

Mapping of Jupiter's tropospheric NH$_3$ abundance using ground-based IRTF/TEXES observations at 5 $μ$m

We report on results of an observing campaign to support the Juno mission. At the beginning of 2016, using TEXES (Texas Echelon cross-dispersed Echelle Spectrograph), mounted on the NASA Infrared Telescope Facility (IRTF), we obtained data cubes of Jupiter in the 1930--1943 cm$^{-1}$ spectral ranges (around 5 $μ$m), which probe the atmosphere in the 1--4 bar region, with a spectral resolution of $\approx$ 0.15 cm$^{-1}$ and an angular resolution of $\approx$ 1.4". This dataset is analysed by a code that combines a line-by-line radiative transfer model with a non-linear optimal estimation inversion method. The inversion retrieves the vertical abundance profiles of NH$_3$ - which is the main contributor at these wavelengths - with a maximum sensitivity at $\approx$ 1--3 bar, as well as the cloud transmittance. This retrieval is performed on over more than one thousand pixels of our data cubes, producing maps of the disk, where all the major belts are visible. We present our retrieved NH$_3$ abundance maps which can be compared with the distribution observed by Juno's MWR (Bolton et al., 2017; Li et al., 2017) in the 2 bar region and discuss their significance for the understanding of Jupiter's atmospheric dynamics. We are able to show important latitudinal variations - such as in the North Equatorial Belt (NEB), where the NH$_3$ abundance is observed to drop down to 60 ppmv at 2 bar - as well as longitudinal variability. In the zones, we find the NH$_3$ abundance to increase with depth, from 100 $\pm$ 15 ppmv at 1 bar to 500 $\pm$ 30 ppmv at 3 bar. We also display the cloud transmittance--NH$_3$ abundance relationship, and find different behaviour for the NEB, the other belts and the zones. Using a simple cloud model, we are able to fit this relationship, at least in the NEB, including either NH$_3$-ice or NH$_4$SH particles with sizes between 10 and 100 $μ$m.

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arXiv2018

Seasonal stratospheric photochemistry on Uranus and Neptune

A time-variable 1D photochemical model is used to study the distribution of stratospheric hydrocarbons as a function of altitude, latitude, and season on Uranus and Neptune. The results for Neptune indicate that in the absence of stratospheric circulation or other meridional transport processes, the hydrocarbon abundances exhibit strong seasonal and meridional variations in the upper stratosphere, but that these variations become increasingly damped with depth due to increasing dynamical and chemical time scales. At high altitudes, hydrocarbon mixing ratios are typically largest where the solar insolation is the greatest, leading to strong hemispheric dichotomies between the summer-to-fall hemisphere and winter-to-spring hemisphere. At mbar pressures and deeper, slower chemistry and diffusion lead to latitude variations that become more symmetric about the equator. On Uranus, the stagnant, poorly mixed stratosphere confines methane and its photochemical products to higher pressures, where chemistry and diffusion time scales remain large. Seasonal variations in hydrocarbons are therefore predicted to be more muted on Uranus, despite the planet's very large obliquity. Radiative-transfer simulations demonstrate that latitude variations in hydrocarbons on both planets are potentially observable with future JWST mid-infrared spectral imaging. Our seasonal model predictions for Neptune compare well with retrieved C2H2 and C2H6 abundances from spatially resolved ground-based observations (no such observations currently exist for Uranus), suggesting that stratospheric circulation -- which was not included in these models -- may have little influence on the large-scale meridional hydrocarbon distributions on Neptune, unlike the situation on Jupiter and Saturn.

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arXiv2018

Disruption of Saturn's Quasi-Periodic Equatorial Oscillation by the Great Northern Storm

Observations of planets throughout our Solar System have revealed that the Earth is not alone in possessing natural, inter-annual atmospheric cycles. The equatorial middle atmospheres of the Earth, Jupiter and Saturn all exhibit a remarkably similar phenomenon - a vertical, cyclic pattern of alternating temperatures and zonal (east-west) wind regimes that propagate slowly downwards with a well-defined multi-Earth-year period. Earth's Quasi-Biennial Oscillation (QBO, observed in the lower stratospheres with an average period of 28 months) is one of the most regular, repeatable cycles exhibited by our climate system, and yet recent work has shown that this regularity can be disrupted by events occurring far away from the equatorial region, an example of a phenomenon known as atmospheric teleconnection. Here we reveal that Saturn's equatorial Quasi-Periodic Oscillation (QPO, with a ~15-year period) can also be dramatically perturbed. An intense springtime storm erupted at Saturn's northern mid-latitudes in December 2010, spawning a gigantic hot vortex in the stratosphere at $40^\circ$N that persisted for 3 years. Far from the storm, the Cassini temperature measurements showed a dramatic $\sim10$-K cooling in the 0.5-5 mbar range across the entire equatorial region, disrupting the regular QPO pattern and significantly altering the middle-atmospheric wind structure, suggesting an injection of westward momentum into the equatorial wind system from waves generated by the northern storm. Hence, as on Earth, meteorological activity at mid-latitudes can have a profound effect on the regular atmospheric cycles in the tropics, demonstrating that waves can provide horizontal teleconnections between the phenomena shaping the middle atmospheres of giant planets.

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arXiv2017

Ammonia in Jupiter's troposphere from high-resolution 5-\textmu m spectroscopy

Jupiter's tropospheric ammonia (NH\textsubscript{3}) abundance is studied using spatially-resolved 5-\textmu m observations from CRIRES, a high resolution spectrometer at the European Southern Observatory's Very Large Telescope. The high resolving power (R=96,000) allows the line shapes of three NH\textsubscript{3} absorption features to be resolved. We find that within the 1--4 bar pressure range, the NH\textsubscript{3} abundance decreases with altitude. The instrument slit was aligned north-south along Jupiter's central meridian, allowing us to search for latitudinal variability. There is considerable uncertainty in the large-scale latitudinal variability, as the increase in cloud opacity in zones compared to belts can mask absorption features. However, we do find evidence for a strong NH\textsubscript{3} enhancement at 4--6$^{\circ}$N, consistent with a localised `ammonia plume' on the southern edge of Jupiter's North Equatorial Belt.

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arXiv2017

Jovian Temperature and Cloud Variability during the 2009-2010 Fade of the South Equatorial Belt

Mid-infrared 7-20 $μ$m imaging of Jupiter demonstrates that the increased albedo of Jupiter's South Equatorial Belt (SEB) during the `fade' (whitening) event of 2009-2010 was correlated with changes to atmospheric temperature and aerosol opacity. The opacity of the tropospheric condensation cloud deck at pressures less than 800 mbar increased by 80% between May 2008 and July 2010, making the SEB ($7-17^\circ$ S) as opaque in the thermal infrared as the adjacent equatorial zone. After the cessation of discrete convective activity within the SEB in May 2009, a cool quiescent band of high aerosol opacity (the SEB zone at $11-15^\circ$ S) was observed separating the cloud-free northern and southern SEB components. The cooling of the SEBZ (with peak-to-peak contrasts of $1.0\pm0.5$ K), as well as the increased aerosol opacity at 4.8 and 8.6 $μ$m, preceded the visible whitening of the belt by several months. A chain of five warm, cloud-free `brown barges' (subsiding airmasses) were observed regularly in the SEB between June 2009 and June 2010, by which time they too had been obscured by the enhanced aerosol opacity of the SEB, although the underlying warm circulation was still present in July 2010. The cool temperatures and enhanced aerosol opacity of the SEBZ after July 2009 are consistent with an upward flux of volatiles from deeper levels (e.g., ammonia-laden air) and enhanced condensation, obscuring the blue-absorbing chromophore and whitening the SEB by April 2010. Revival of the dark SEB coloration in the coming months will ultimately require sublimation of these ices by subsidence and warming of volatile-depleted air. [Abridged]

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arXiv2017

Moist Convection and the 2010-2011 Revival of Jupiter's South Equatorial Belt

The transformation of Jupiter's South Equatorial Belt (SEB) from its faded, whitened state in 2009-2010 to its normal brown appearance is documented via comparisons of thermal-infrared (5-20 $μ$m) and visible-light imaging between November 2010 and November 2011. The SEB revival consisted of convective eruptions triggered over $\sim100$ days, potentially powered by the latent heat released by the condensation of water. The plumes rise from the water cloud base and ultimately diverge and cool in the stably-stratified upper troposphere. Thermal-IR images were acquired 2 days after the SEB disturbance was first detected by amateur observers on November 9th 2010. Subsequent images revealed the cold, putatively anticyclonic and cloudy plume tops surrounded by warm, cloud-free conditions at their peripheries. The majority of the plumes erupted from a single source near $140-160^\circ$W, coincident with the remnant cyclonic circulation of a brown barge that had formed during the fade. Additional plumes erupted from the leading edge of the central disturbance immediately east of the source. The tropospheric plumes excited stratospheric thermal waves over the SEB, showing a direct connection between moist convection and stratospheric wave activity. The subsidence of dry, unsaturated air warmed the troposphere and removed the white aerosols. The aerosol-free air was redistributed throughout the SEB by the zonal flow, following a westward-moving southern branch and an eastward-moving northern branch that revived the brown colouration over $\sim200$ days. The last stage of the revival was the re-establishment of normal convective activity northwest of the GRS in September 2011. Moist convection may therefore play an important role in controlling the timescale and atmospheric variability during the SEB life cycle. [Abridged]

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arXiv2016

The Great Saturn Storm of 2010-2011

In December 2010, a major storm erupted in Saturn's northern hemisphere near 37 degree planetographic latitude. This rather surprising event, occurring at an unexpected latitude and time, is the sixth "Great White Spot" (GWS) storm observed over the last century and a half. Such GWS events are planetary-scale atmospheric phenomena that dramatically change the typically bland appearance of the planet. Occurring while the Cassini mission was on-orbit at Saturn, the Great Storm of 2010-2011 was well-suited for intense scrutiny by the suite of sophisticated instruments onboard the Cassini spacecraft as well by modern instrumentation on ground-based telescopes and onboard the Hubble Space Telescope. This GWS erupted on December 5th and generated a major dynamical disturbance that affected the whole latitude band from 25 deg to 48 deg N. Lightning events were prominent and detected as outbursts and flashes at both optical and radio wavelengths. The activity of the head ceased after about seven months, leaving the cloud structure and ambient winds perturbed. The tops of the optically dense clouds of the storm's head reached the 300 mbar altitude level where a mixture of ices was detected. The energetics of the frequency and power of lightning, as well as the estimated power generated by the latent heat released in the water-based convection, both indicate that the power released for the storm was a significant fraction of Saturn's total radiated power. The effects of the storm propagated into the stratosphere forming two warm airmasses at the 0.5-5 mbar pressure level altitude. Related to the stratospheric disturbance, hydrocarbon composition excesses were found. The decades-long interval between storms is probably related to the insolation cycle and the long radiative time constant of Saturn's atmosphere, and several theories for temporarily storing energy have been proposed.

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arXiv2016

Latitudinal variability in Jupiter's tropospheric disequilibrium species: GeH$_4$, AsH$_3$ and PH$_3$

Jupiter's tropospheric composition is studied using high resolution spatially-resolved 5-micron observation from the CRIRES instrument at the Very Large Telescope. The high resolving power (R=96,000) allows us to spectrally resolve the line shapes of individual molecular species in Jupiter's troposphere and, by aligning the slit north-south along Jupiter's central meridian, we are able to search for any latitudinal variability. Despite the high spectral resolution, we find that there are significant degeneracies between the cloud structure and aerosol scattering properties that complicate the retrievals of tropospheric gaseous abundances and limit conclusions on any belt-zone variability. However, we do find evidence for variability between the equatorial regions of the planet and the polar regions. Arsine (AsH$_3$) and phosphine (PH$_3$) both show an enhancement at high latitudes, while the abundance of germane (GeH$_4$) remains approximately constant. These observations contrast with the theoretical predictions from Wang et al. (2016) and we discuss the possible explanations for this difference.

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arXiv2016

Jupiter's Para-H$_2$ Distribution from SOFIA/FORCAST and Voyager/IRIS 17-37 $μ$m Spectroscopy

Spatially resolved maps of Jupiter's far-infrared 17-37 $μ$m hydrogen-helium collision-induced spectrum were acquired by the FORCAST instrument on the Stratospheric Observatory for Infrared Astronomy (SOFIA) in May 2014. Spectral scans in two grisms covered the broad S(0) and S(1) absorption lines, in addition to contextual imaging in eight broad-band filters (5-37 $μ$m) with spatial resolutions of 2-4". The spectra were inverted to map the zonal-mean temperature and para-H$_2$ distribution ($f_p$, the fraction of the para spin isomer with respect to the ortho spin isomer) in Jupiter's upper troposphere (the 100-700 mbar range). We compared these to a reanalysis of Voyager-1 and -2 IRIS spectra covering the same spectral range. Para-H$_2$ increases from equator to pole, with low-$f_p$ air at the equator representing sub-equilibrium conditions (i.e., less para-H$_2$ than expected from thermal equilibration), and high-$f_p$ air and possible super-equilibrium at higher latitudes. In particular, we confirm the continued presence of a region of high-$f_p$ air at high northern latitudes discovered by Voyager/IRIS, and an asymmetry with generally higher $f_p$ in the north than in the south. We note that existing collision-induced absorption databases lack opacity from (H$_2$)$_2$ dimers, leading to under-prediction of the absorption near the S(0) and S(1) peaks. There appears to be no spatial correlation between para-H$_2$ and tropospheric ammonia, phosphine and cloud opacity derived from Voyager/IRIS at mid-infrared wavelengths (7-15 $μ$m). We note, however, that para-H$_2$ tracks the similar latitudinal distribution of aerosols within Jupiter's upper tropospheric and stratospheric hazes observed in reflected sunlight, suggesting that catalysis of hydrogen equilibration within the hazes (and not the main clouds) may govern the equator-to-pole gradient. [Abridged]

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arXiv2016

Saturn's Polar Atmosphere

This book chapter, Saturn's Polar Atmosphere, is to be published by Cambridge University Press as part of a multi-volume work edited by Kevin Baines, Michael Flasar, Norbert Krupp, and Thomas Stallard, entitled "Saturn in the 21st Century." This chapter reviews the state of our knowledge about Saturn's polar atmosphere that has been revealed through Earth- and space-based observation as well as theoretical and numerical modeling. In particular, the Cassini mission to Saturn, which has been in orbit around the ringed planet since 2004, has revolutionized our understanding of the planet. The current review updates a previous review by Del Genio et al (2009; Saturn Atmospheric Structure and Dynamics, Chapter 7 of "Saturn from Cassini-Huygens"), written after Cassini's primary mission phase that ended in 2008, by focusing on the north polar region of Saturn and comparing it to the southern high latitudes. Two prominent features in the northern high latitudes are the northern hexagon and the north polar vortex; we extensively review observational and theoretical investigations to date of both features. We also review the seasonal evolution of the polar regions using the observational data accumulated during the Cassini mission since 2004 (shortly after the northern winter solstice in 2002), through the equinox in 2009, and approaching the next solstice in 2017. We conclude the current review by listing unanswered questions and describing the observations of the polar regions planned for the Grand Finale phase of the Cassini mission between 2016 and 2017.

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arXiv2016

Stratospheric aftermath of the 2010 Storm on Saturn as observed by the TEXES instrument. I. Temperature structure

We report on spectroscopic observations of Saturn's stratosphere in July 2011 with the Texas Echelon Cross Echelle Spectrograph (TEXES) mounted on the NASA InfraRed Telescope Facility (IRTF). The observations, targeting several lines of the CH$_4$ $ν_4$ band and the H$_2$ S(1) quadrupolar line, were designed to determine how Saturn's stratospheric thermal structure was disturbed by the 2010 Great White Spot. A study of Cassini Composite Infrared Spectrometer (CIRS) spectra had already shown the presence of a large stratospheric disturbance centered at a pressure of 2~hPa, nicknamed the beacon B0, and a tail of warm air at lower pressures (Fletcher et al. 2012. Icarus 221, 560--586). Our observations confirm that the beacon B0 vertical structure determined by CIRS, with a maximum temperature of $180\pm1$K at 2~hPa, is overlain by a temperature decrease up to the 0.2-hPa pressure level. Our retrieved maximum temperature of $180\pm1$K is colder than that derived by CIRS ($200\pm1$K), a difference that may be quantitatively explained by terrestrial atmospheric smearing. We propose a scenario for the formation of the beacon based on the saturation of gravity waves emitted by the GWS. Our observations also reveal that the tail is a planet-encircling disturbance in Saturn's upper stratosphere, oscillating between 0.2 and 0.02~hPa, showing a distinct wavenumber-2 pattern. We propose that this pattern in the upper stratosphere is either the signature of thermal tides generated by the presence of the warm beacon in the mid-stratosphere, or the signature of Rossby wave activity.

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arXiv2016

Detection of H3+ auroral emission in Jupiter's 5-micron window

We use high-resolution ground-based observations from the VLT CRIRES instrument in December 2012 to identify sixteen previously undetected H3+ emission lines from Jupiter's ionosphere. These emission lines are located in Jupiter's 5-micron window (4.5-5.2 μm), an optically-thin region of the planet's spectrum where the radiation mostly originates from the deep troposphere. The H3+ emission lines are so strong that they are visible even against this bright background. We measure the Doppler broadening of the H3+ emission lines in order to evaluate the kinetic temperature of the molecules, and we obtain a value of 1390$\pm$160 K. We also measure the relative intensities of lines in the ν2 fundamental in order to calculate the rotational temperature, obtaining a value of 960$\pm$40 K. Finally, we use the detection of an emission line from the 2ν2(2)-ν2 overtone to measure a vibrational temperature of 925$\pm$25 K. We use these three independent temperature estimates to discuss the thermodynamic equilibrium of Jupiter's ionosphere.

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arXiv2016

Telling twins apart: Exo-Earths and Venuses with transit spectroscopy

The planned launch of the James Webb Space Telescope in 2018 will herald a new era of exoplanet spectroscopy. JWST will be the first telescope sensitive enough to potentially characterize terrestrial planets from their transmission spectra. In this work, we explore the possibility that terrestrial planets with Venus-type and Earth-type atmospheres could be distinguished from each other using spectra obtained by JWST. If we find a terrestrial planet close to the liquid water habitable zone of an M5 star within a distance of 10 parsecs, it would be possible to detect atmospheric ozone if present in large enough quantities, which would enable an oxygen-rich atmosphere to be identified. However, the cloudiness of a Venus-type atmosphere would inhibit our ability to draw firm conclusions about the atmospheric composition, making any result ambiguous. Observing small, temperate planets with JWST requires significant investment of resources, with single targets requiring of order 100 transits to achieve sufficient signal to noise. The possibility of detecting a crucial feature such as the ozone signature would need to be carefully weighed against the likelihood of clouds obscuring gas absorption in the spectrum.

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