Publications by Patrick Irwin

Latitudinal variability in Jupiter's tropospheric disequilibrium species: GeH4, AsH3 and PH3

Icarus Elsevier 289 (2016) 254-269

R Giles, L Fletcher, PG Irwin

Jupiter's tropospheric composition is studied using high resolution spatially-resolved 5-mm 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 (AsH3) and phosphine (PH3) both show an enhancement at high latitudes, while the abundance of germane (GeH4) remains approximately constant. These observations contrast with the theoretical predictions from Wang et al. (2016) and we discuss the possible explanations for this difierence.

HST/WFC3 Observations of Uranus’ 2014 storm clouds and comparison with VLT/SINFONI and IRTF/SpeX observations

Icarus Elsevier 288 (2017) 99-119

P Irwin, MH Wong, A Simon, GS Orton, D Toledo

<p>In November 2014 Uranus was observed with the Wide Field Camera 3 (WFC3) instrument of the Hubble Space Telescope as part of the Hubble 2020: Outer Planet Atmospheres Legacy program, OPAL. OPAL annually maps Jupiter, Uranus and Neptune (and will also map Saturn from 2018) in several visible/near-infrared wavelength filters. The Uranus 2014 OPAL observations were made on the 8/9th November at a time when a huge cloud complex, first observed by de Pater et al. (2015) and subsequently tracked by professional and amateur astronomers (Sayanagi et al., 2016), was present at 30–40°N. We imaged the entire visible atmosphere, including the storm system, in seven filters spanning 467–924 nm, capturing variations in the coloration of Uranus’ clouds and also vertical distribution due to wavelength dependent changes in Rayleigh scattering and methane absorption optical depth. Here we analyse these new HST observations with the NEMESIS radiative-transfer and retrieval code in multiple-scattering mode to determine the vertical cloud structure in and around the storm cloud system.</p> <p>The same storm system was also observed in the H-band (1.4–1.8 μm) with the SINFONI Integral Field Unit Spectrometer on the Very Large Telescope (VLT) on 31st October and 11th November, reported by Irwin et al. (2016, 10.1016/j.icarus.2015.09.010). To constrain better the cloud particle sizes and scattering properties over a wide wavelength range we also conducted a limb-darkening analysis of the background cloud structure in the 30–40°N latitude band by simultaneously fitting: a) these HST/OPAL observations at a range of zenith angles; b) the VLT/SINFONI observations at a range of zenith angles; and c) IRTF/SpeX observations of this latitude band made in 2009 at a single zenith angle of 23°, spanning the wavelength range 0.8–1.8 µm (Irwin et al., 2015, 10.1016/j.icarus.2014.12.020).</p> <p>We find that the HST observations, and the combined HST/VLT/IRTF observations at all locations are well modelled with a three-component cloud comprised of: 1) a vertically thin, but optically thick ‘deep’ tropospheric cloud at a pressure of ∼ 2 bars; 2) a methane-ice cloud based at the methane-condensation level of 1.23 bar, with variable vertical extent; and 3) a vertically extended tropospheric haze, also based at the methane-condensation level of ∼ 1.23 bar. We find that modelling both haze and tropospheric cloud with particles having an effective radius of ∼ 0.1 µm provides a good fit the observations, although for the tropospheric cloud, particles with an effective radius as large as 1.0 µm provide a similarly good fit. We find that the particles in both the tropospheric cloud and haze are more scattering at short wavelengths, giving them a blue colour, but are more absorbing at longer wavelengths, especially for the tropospheric haze. We find that the spectra of the storm clouds are well modelled by localised thickening and vertical extension of the methane-ice cloud. For the particles in the storm clouds, which we assume to be composed of methane ice particles, we find that their mean radii must lie somewhere in the range View the MathML sourcem. We find that the high clouds have low integrated opacity, and that “streamers” reminiscent of convective thunderstorm anvils are confined to levels deeper than 1 bar. These results argue against vigorous moist convective origins for the cloud features.</p>

ALMA observations of Titan's atmospheric chemistry and seasonal variation

Proceedings of the International Astronomical Union 13 (2017) 95-102

MA Cordiner, JC Lai, NA Teanby, CA Nixon, MY Palmer, SB Charnley, AE Thelen, EM Molter, Z Kisiel, V Vuitton, PGJ Irwin, MJ Mumma

Copyright © International Astronomical Union 2018. Results are presented from our ongoing studies of Titan using ALMA during the period 2012-2015, including a confirmation of the previous detection of vinyl cyanide (C2H3CN), as well as the first spatial map for this species on Titan. Simultaneous mapping of HC3N, CH3CN and C2H5CN reveal characteristic abundance patterns for each species that provide insight into their individual photochemical lifetimes, and help inform our understanding of Titan's unique, time-variable atmospheric chemistry and global circulation. A time-sequence of HC3N maps covering 38 months reveals a dramatic change in the distribution of this gas consistent with high-altitude photochemical production followed by advection towards the southern (winter) pole, combined with rapid loss in the north after Titan's 2009 seasonal equinox. The 2015 C2H3CN and C2H5CN maps show abundance peaks in Titan's southern hemisphere, similar to those observed for the short-lived HC3N molecule. The longer-lived CH3CN, on the other hand, remains more concentrated in the north.

Jupiter's para-H2 distribution from SOFIA/FORCAST and Voyager/IRIS 17-37 μm spectroscopy

Icarus Elsevier 286 (2016) 223-240

LN Fletcher, I de Pater, WT Reach, MH Wong, GS Orton, P Irwin, RD Gehrz

Spatially resolved maps of Jupiter’s far-infrared 17-37 μm 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 μm) with spatial resolutions of 2-4”. The spectra were inverted to map the zonal-mean temperature and para-H2 distribution (fp, 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. Tropospheric temperature contrasts match those identified by Voyager in 1979, within the limits of temporal variability consistent with previous investigations. Para-H2 increases from equator to pole, with low- fp air at the equator representing sub-equilibrium conditions (i.e., less para-H2 than expected from thermal equilibration), and high- fp air and possible super-equilibrium at higher latitudes. In particular, we confirm the continued presence of a region of high-fp air at high northern latitudes discovered by Voyager/IRIS, and an asymmetry with generally higher fp in the north than in the south. Far-IR aerosol opacity is not required to fit the data, but cannot be completely ruled out. We note that existing collision-induced absorption databases lack opacity from (H2)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-H2 and tropospheric ammonia, phosphine and cloud opacity derived from Voyager/IRIS at mid-infrared wavelengths (7-15 μm). We note, however, that para-H2 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, with conditions closer to equilibrium at higher latitudes. This gradient is superimposed onto smaller-scale variations associated with regional advection of para-H2 at the equator and poles.

Jupiter's auroral-related stratospheric heating and chemistry I: Analysis of Voyager-IRIS and Cassini-CIRS spectra

Icarus Elsevier 292 (2017) 182-207

JA Sinclair, GS Orton, TK Greathouse, LN Fletcher, JI Moses, V Hue, P Irwin

Auroral processes are evident in Jupiter's polar atmosphere over a large range in wavelength (X-ray to radio). In particular, previous observations in the mid-infrared (5-15 μm) have shown enhanced emission from CH4, C2H2 and C2H4 and further stratospheric hydrocarbon species in spatial regions coincident with auroral processes observed at other wavelengths. These regions, described as auroral-related hotspots, observed at approximately 160°W to 200°W (System III) at high-northern latitudes and 330°W to 80°W at high-southern latitudes, indicate that auroral processes modify the thermal structure and composition of the neutral atmosphere. However, previous studies have struggled to differentiate whether the aforementioned enhanced emission is a result of either temperature changes and/or changes in the concentration of the emitting species. We attempt to address this degeneracy in this work by performing a retrieval analysis of Voyager 1-IRIS spectra (acquired in 1979) and Cassini-CIRS spectra (acquired in 2000/2001) of Jupiter. Retrievals of the vertical temperature profile in Cassini-CIRS spectra covering the auroral-related hotspots indicate the presence of two discrete vertical regions of heating at the 1-mbar level and at pressures of 10-μbar and lower. For example, in Cassini-CIRS 2.5 cm-1 'MIRMAP' spectra at 70°N (planetographic) 180°W (centred on the auroral oval), we find temperatures at the 1-mbar level and 10-μbar levels are enhanced by 15.3 ± 5.2 K and 29.6 ± 15.0 K respectively, in comparison to results at 70°N, 60°W in the same dataset. High temperatures at 10 μbar and lower pressures were considered indicative of joule heating, ion and/or electron precipitation, ion-drag and energy released form exothermic ion-chemistry. However, we conclude that the heating at the 1-mbar level is the result of either a layer of aurorally-produced haze particles, which are heated by incident sunlight and/or adiabatic heating by downwelling within the auroral hot-spot region. The former mechanism would be consistent with the vertical profiles of polycyclic aromatic hydrocarbons (PAHs) and haze particles predicted in auroral-chemistry models (Wong et al., 2000; 2003). Retrievals of C2H2 and C2H6 were also performed and indicate C2H2 is enriched but C2H6 is depleted in auroral regions relative to quiescent regions. For example, using CIRS δν ˜= 2.5 cm-1 spectra, we determined that C2H2 at 0.98 mbar increases by 175.3 ± 89.3 ppbv while C2H6 at 4.7 mbar decreases by 0.86 ± 0.59 ppmv in comparing results at 70°N, 180°W and 70°N, 60°W. These results represent a mean of values retrieved from different initial assumptions and thus we believe they are robust. We believe these contrasts in C2H2 and C2H6 between auroral and quiescent regions can be explained by a coupling of auroral-driven chemistry and horizontal advection. Ion-neutral and electron recombination chemistry in the auroral region enriches all C2 hydrocarbons but in particular, the unsaturated C2H2 and C2H4 hydrocarbons. Once advected outside of the auroral region, the unsaturated C2 hydrocarbons are converted into C2H6 by neutral photochemistry thereby enriching C2H6 in quiescent regions, which gives the impression it is depleted inside the auroral region.

A consistent retrieval analysis of 10 hot Jupiters observed in transmission

Astrophysical Journal American Astronomical Society 834 (2017) 50

JK Barstow, S Aigrain, P Irwin, DK Sing

We present a consistent optimal estimation retrieval analysis of 10 hot Jupiter exoplanets, each with transmission spectral data spanning the visible to near-infrared wavelength range. Using the NEMESIS radiative transfer and retrieval tool, we calculate a range of possible atmospheric states for WASP-6b, WASP-12b, WASP-17b, WASP-19b, WASP-31b, WASP-39b, HD 189733b, HD 209458b, HAT-P-1b, and HAT-P-12b. We find that the spectra of all 10 planets are consistent with the presence of some atmospheric aerosol; WASP-6b, WASP-12b, WASP-17b, WASP-19b, HD 189733b, and HAT-P-12b are all fit best by Rayleigh scattering aerosols, whereas WASP-31b, WASP-39b and HD 209458b are better represented by a gray cloud model. HAT-P-1b has solutions that fall into both categories. WASP-6b, HAT-P-12b, HD 189733b, and WASP-12b must have aerosol extending to low atmospheric pressures (below 0.1 mbar). In general, planets with equilibrium temperatures between 1300 and 1700 K are best represented by deeper, gray cloud layers, whereas cooler or hotter planets are better fit using high Rayleigh scattering aerosol. We find little evidence for the presence of molecular absorbers other than H2O. Retrieval methods can provide a consistent picture across a range of hot Jupiter atmospheres with existing data, and will be a powerful tool for the interpretation of James Webb Space Telescope observations.

Global energy budgets and 'Trenberth diagrams' for the climates of terrestrial and gas giant planets

Quarterly Journal of the Royal Meteorological Society Wiley 142 (2016) 703-720

PL Read, J Barstow, B Charnay, S Chelvaniththilan, PGJ Irwin, S Knight, S Lebonnois, SR Lewis, J Mendonça, L Montabone

The climate on Earth is generally determined by the amount and distribution of incoming solar radiation, which must be balanced in equilibrium by the emission of thermal radiation from the surface and atmosphere. The precise routes by which incoming energy is transferred from the surface and within the atmosphere and back out to space, however, are important features that characterize the current climate. This has been analysed in the past by several groups over the years,based on combinations of numerical model simulations and direct observations of theEarths climate system. The results are often presented in schematic form to show the main routes for the transfer of energy into, out of and within the climate system. Although relatively simple in concept, such diagrams convey a great deal of information about the climate system in a compact form. Such an approach has not so far been widely adopted in any systematic way for other planets of the Solar System, let alone beyond, although quite detailed climate models of several planets are now available, constrained bymany new observations and measurements. Here we present an analysis of the global transfers of energy within the climate systems of a range of planets within the Solar System,including Mars, Titan, Venus a nd Jupit er, a s mo delled by rela t ively co mprehens iveradiative transfer and (in some cases) numerical circulation models. These results are presented in schematic form for comparison with the classical global energy budget analyses (e.g.Trenberth et al. 2009; Stephenset al.2012; Wildet al.2013; IPCC 2013)for the Earth, highlighting important similarities and differences. We also take the first steps towards extending this approach to other Solar System and extra-solar planets,including Mars, Venus, Titan, Jupiter and the ‘hot Jupiter’ exoplanet HD189733b, presenting a synthesis of `both previously published and new calculations for all of these planets.

Exposed water ice on the nucleus of comet 67P/Churyumov-Gerasimenko.

Nature 529 (2016) 368-372

G Filacchione, MC De Sanctis, F Capaccioni, A Raponi, F Tosi, M Ciarniello, P Cerroni, G Piccioni, MT Capria, E Palomba, G Bellucci, S Erard, D Bockelee-Morvan, C Leyrat, G Arnold, MA Barucci, M Fulchignoni, B Schmitt, E Quirico, R Jaumann, K Stephan, A Longobardo, V Mennella, A Migliorini, E Ammannito, J Benkhoff, JP Bibring, A Blanco, MI Blecka, R Carlson, U Carsenty, L Colangeli, M Combes, M Combi, J Crovisier, P Drossart, T Encrenaz, C Federico, U Fink, S Fonti, WH Ip, P Irwin, E Kuehrt, Y Langevin, G Magni, T McCord, L Moroz, S Mottola, V Orofino, U Schade, F Taylor, D Tiphene, GP Tozzi, P Beck, N Biver, L Bonal, J-P Combe, D Despan, E Flamini, M Formisano, S Fornasier, A Frigeri, D Grassi, MS Gudipati, D Kappel, F Mancarella, K Markus, F Merlin, R Orosei, G Rinaldi, M Cartacci, A Cicchetti, S Giuppi, Y Hello, F Henry, S Jacquinod, JM Reess, R Noschese, R Politi, G Peter

Although water vapour is the main species observed in the coma of comet 67P/Churyumov-Gerasimenko and water is the major constituent of cometary nuclei, limited evidence for exposed water-ice regions on the surface of the nucleus has been found so far. The absence of large regions of exposed water ice seems a common finding on the surfaces of many of the comets observed so far. The nucleus of 67P/Churyumov-Gerasimenko appears to be fairly uniformly coated with dark, dehydrated, refractory and organic-rich material. Here we report the identification at infrared wavelengths of water ice on two debris falls in the Imhotep region of the nucleus. The ice has been exposed on the walls of elevated structures and at the base of the walls. A quantitative derivation of the abundance of ice in these regions indicates the presence of millimetre-sized pure water-ice grains, considerably larger than in all previous observations. Although micrometre-sized water-ice grains are the usual result of vapour recondensation in ice-free layers, the occurrence of millimetre-sized grains of pure ice as observed in the Imhotep debris falls is best explained by grain growth by vapour diffusion in ice-rich layers, or by sintering. As a consequence of these processes, the nucleus can develop an extended and complex coating in which the outer dehydrated crust is superimposed on layers enriched in water ice. The stratigraphy observed on 67P/Churyumov-Gerasimenko is therefore the result of evolutionary processes affecting the uppermost metres of the nucleus and does not necessarily require a global layering to have occurred at the time of the comet's formation.

Seasonal exposure of carbon dioxide ice on the nucleus of comet 67P/Churyumov-Gerasimenko.

Science American Association for the Advancement of Science 354 (2016) 1563-1566

G Filacchione, A Raponi, F Capaccioni, M Ciarniello, F Tosi, MT Capria, MC De Sanctis, A Migliorini, G Piccioni, P Cerroni, MA Barucci, S Fornasier, B Schmitt, E Quirico, S Erard, D Bockelee-Morvan, C Leyrat, G Arnold, V Mennella, E Ammannito, G Bellucci, J Benkhoff, JP Bibring, A Blanco, MI Blecka

Carbon dioxide is one of the most abundant species in cometary nuclei, but due to its high volatility CO2 ice is generally only found beneath the surface. We report the infrared spectroscopic identification of a CO2 ice-rich surface area, located in the Anhur region of comet 67P/Churyumov-Gerasimenko. Spectral modeling shows that about 0.1% of the 80×60 m area is CO2 ice. This exposed ice was observed a short time after exiting from local winter; following the increased illumination, the CO2 ice completely disappeared over about three weeks. We estimate the mass of the sublimated CO2 ice and the depth of the surface eroded layer. The presence of CO2 ice is interpreted as the result of the extreme seasonal changes induced by the rotation and orbit of the comet.

Seasonal variability of Saturn's tropospheric temperatures, winds and para-H-2 from Cassini far-IR spectroscopy

ICARUS 264 (2016) 137-159

LN Fletcher, PGJ Irwin, RK Achterberg, GS Orton, FM Flasar

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

Astronomy and Astrophysics EDP Sciences (2016)

R Giles, L Fletcher, P Irwin, E al.

<p>We use high-resolution ground-based observations from the VLT CRIRES instrument in November 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 ± 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 ± 40 K. Finally, we use the detection of an emission line from the 2ν2(2)-ν2 overtone to measure a vibrational temperature of 925 ± 25 K. We use these three independent temperature estimates to discuss the thermodynamic equilibrium of Jupiter’s ionosphere.</p>

Time variability of Neptune’s horizontal and vertical cloud structure revealed by VLT/SINFONI and Gemini/NIFS from 2009 to 2013

Icarus Elsevier (2016)

P Irwin, D Tice, LN Fletcher, SJ Owen, LN Fletcher, D Tice, SJ Owen, GS Orton, NA Teanby, GR Davis

<p>New observations of Neptune’s clouds in the near infrared were acquired in October 2013 with SINFONI on ESO’s Very Large Telescope (VLT) in Chile. SINFONI is an Integral Field Unit spectrometer returning a 64 × 64 pixel image with 2048 wavelengths. Image cubes in the J-band (1.09 – 1.41 μm) and H-band (1.43 – 1.87 μm) were obtained at spatial resolutions of 0.1″and 0.025″per pixel, while SINFONI’s adaptive optics provided an effective resolution of approximately 0.1″. Image cubes were obtained at the start and end of three successive nights to monitor the temporal development of discrete clouds both at short timescales (i.e. during a single night) as well as over the longer period of the three-day observing run. These observations were compared with similar H-band observations obtained in September 2009 with the NIFS Integral Field Unit spectrometer on the Gemini-North telescope in Hawaii, previously reported by Irwin et al., Icarus 216, 141-158, 2011, and previously unreported Gemini/NIFS observations at lower spatial resolution made in 2011.</p> <p>We find both similarities and differences between these observations, spaced over four years. The same overall cloud structure is seen with high, bright clouds visible at mid-latitudes (30 – 40°N,S), with slightly lower clouds observed at lower latitudes, together with small discrete clouds seen circling the pole at a latitude of approximately 60°S. However, while discrete clouds were visible at this latitude at both the main cloud deck level (at 2–3 bars) and in the upper troposphere (100–500mb) in 2009, no distinct deep (2–3 bar), discrete circumpolar clouds were visible in 2013, although some deep clouds were seen at the southern edge of the main cloud belt at 30–40°S, which have not been observed before. The nature of the deep sub-polar discrete clouds observed in 2009 is intriguing. While it is possible that in 2013 these deeper clouds were masked by faster moving, overlying features, we consider that it is unlikely that this should have happened in 2013, but not in 2009 when the upper-cloud activity was generally similar. Meanwhile, the deep clouds seen at the southern edge of the main cloud belt at 30 – 40°S in 2013, should also have been detectable in 2009, but were not seen. Hence, these observations may have detected a real temporal variation in the occurrence of Neptune’s deep clouds, pointing to underlying variability in the convective activity at the pressure of the main cloud deck at 2–3 bars near Neptune’s south pole and also in the main observable cloud belt at 30 – 40°S.</p>

Mid-infrared mapping of Jupiter’s temperatures, aerosol opacity and chemical distributions with IRTF/TEXES

Icarus Elsevier (2016)

LN Fletcher, TK Greathouse, GS Orton, JA Sinclair, RS Giles, PGJ Irwin, T Encrenaz

Global maps of Jupiter's atmospheric temperatures, gaseous composition and aerosol opacity are derived from a programme of 5–20 µm mid-infrared spectroscopic observations using the Texas Echelon Cross Echelle Spectrograph (TEXES) on NASA's Infrared Telescope Facility (IRTF). Image cubes from December 2014 in eight spectral channels, with spectral resolutions of R ∼2000−12,000 and spatial resolutions of 2–4° latitude, are inverted to generate 3D maps of tropospheric and stratospheric temperatures, 2D maps of upper tropospheric aerosols, phosphine and ammonia, and 2D maps of stratospheric ethane and acetylene. The results are compared to a re-analysis of Cassini Composite Infrared Spectrometer (CIRS) observations acquired during Cassini's closest approach to Jupiter in December 2000, demonstrating that this new archive of ground-based mapping spectroscopy can match and surpass the quality of previous investigations, and will permit future studies of Jupiter's evolving atmosphere. The visibility of cool zones and warm belts varies from channel to channel, suggesting complex vertical variations from the radiatively-controlled upper troposphere to the convective mid-troposphere. We identify mid-infrared signatures of Jupiter's 5-µm hotspots via simultaneous M, N and Q-band observations, which are interpreted as temperature and ammonia variations in the northern Equatorial Zone and on the edge of the North Equatorial Belt (NEB). Equatorial plumes enriched in NH3 gas are located south-east of NH3-desiccated ‘hotspots’ on the edge of the NEB. Comparison of the hotspot locations in several channels across the 5–20 µm range indicate that these anomalous regions tilt westward with altitude. Aerosols and PH3 are both enriched at the equator but are not co-located with the NH3 plumes. The equatorial temperature minimum and PH3/aerosol maxima have varied in amplitude over time, possibly as a result of periodic equatorial brightenings and the fresh updrafts of disequilibrium material. Temperate mid-latitudes display a correlation between mid-IR aerosol opacity and the white albedo features in visible light (i.e., zones). We find hemispheric asymmetries in the distribution of tropospheric PH3, stratospheric hydrocarbons and the 2D wind field (estimated via the thermal-wind equation) that suggest a differing efficiency of mechanical forcing (e.g., vertical mixing and wave propagation) between the two hemispheres that we argue is driven by dynamics rather than Jupiter's small seasonal cycle. Jupiter's stratosphere is notably warmer at northern mid-latitudes than in the south in both 2000 and 2014, although the latter can be largely attributed to strong thermal wave activity near 30°N that dominates the 2014 stratospheric maps and may be responsible for elevated C2H2 in the northern hemisphere. A vertically-variable pattern of temperature and windshear minima and maxima associated with Jupiter's Quasi Quadrennial Oscillation (QQO) is observed at the equator in both datasets, although the contrasts were more subdued in 2014. Large-scale equator-to-pole gradients in ethane and acetylene are superimposed on top of the mid-latitude mechanically-driven maxima, with C2H2 decreasing from equator to pole and C2H6 showing a polar enhancement, consistent with a radiatively-controlled circulation from low to high latitudes. Cold polar vortices beyond ∼60° latitude can be identified in the upper tropospheric and lower stratospheric temperature maps, suggesting enhanced radiative cooling from polar aerosols. Finally, compositional mapping of the Great Red Spot confirms the local enhancements in PH3 and aerosols, the north–south asymmetry in NH3 gas and the presence of a warm southern periphery that have been noted by previous authors.

Habitable worlds with JWST : transit spectroscopy of the TRAPPIST-1 system?

Monthly Notices of the Royal Astronomical Society: Letters Oxford University Press 461 (2016) L92-L96

JK Barstow, P Irwin

The recent discovery of three Earth-sized, potentially habitable planets around a nearby cool star, TRAPPIST-1, has provided three key targets for the upcoming James Webb Space Telescope (JWST). Depending on their atmospheric characteristics and precise orbit configurations, it is possible that any of the three planets may be in the liquid water habitable zone, meaning that they may be capable of supporting life. We find that present-day Earth levels of ozone, if present, would be detectable if JWST observes 60 transits for innermost planet 1b and 30 transits for 1c and 1d.


Astronomical Journal American Astronomial Society 152 (2016) 1-7

EM Molter, CA Nixon, MA Cordiner, J Serigano, P Irwin, NA Teanby, SB Charnley, JE Lindberg

All rights reserved.We present sub-millimeter spectra of HCN isotopologues on Titan, derived from publicly available ALMA flux calibration observations of Titan taken in early 2014. We report the detection of a new HCN isotopologue on Titan, H13C15N, and confirm an earlier report of detection of DCN. We model high signal-to-noise observations of HCN, H13CN, HC15N, DCN, and H13C15N to derive abundances and infer the following isotopic ratios: 12C/13C = 89.8 ±2.8, 14N/15N = 72.3 ±2.2, D/H = (2.5 ± 0.2) ×10-4, and HCN/H13C15N = 5800 ±270 (1σ errors). The carbon and nitrogen ratios are consistent with and improve on the precision of previous results, confirming a factor of ∼2.3 elevation in 14N/15N in HCN compared to N2 and a lack of fractionation in 12C/13C from the protosolar value. This is the first published measurement of D/H in a nitrile species on Titan, and we find evidence for a factor of ∼2 deuterium enrichment in hydrogen cyanide compared to methane. The isotopic ratios we derive may be used as constraints for future models to better understand the fractionation processes occurring in Titan's atmosphere.

Giant Planet Observations with the James Webb Space Telescope


J Norwood, J Moses, LN Fletcher, G Orton, PGJ Irwin, S Atreya, K Rages, T Cavalie, A Sanchez-Lavega, R Hueso, N Chanover

Probing Saturn's tropospheric cloud with Cassini/VIMS

Icarus Elsevier 271 (2016) 400-417

J Eberhardt, P Irwin, L Fletcher, R Giles, C Merlet, J Barstow

In its decade of operation the Cassini mission has allowed us to look deep into Saturn’s atmosphere and investigate the processes occurring below its enshrouding haze. We use Visual and Infrared Mapping Spectrometer (VIMS) 4.6—5.2 µm data from early in the mission to investigate the location and properties of Saturn’s cloud structure between 0.6 and 5 bars. We average nightside spectra from 2006 over latitude circles and model the spectral limb darkening using the NEMESIS radiative transfer and retrieval tool. We present our best-fit deep cloud model for latitudes −40◦ &lt; λ &lt; 50◦ , along with retrieved abundances for NH3, PH3 and AsH3. We find an increase in NH3 abundance at the equator, a cloud base at ∼2.3 bar and no evidence for cloud particles with strong absorption features in the 4.6—5.2 µm wavelength range, all of which are consistent with previous work. Non-scattering cloud models assuming a composition of either NH3 or NH4SH, with a scattering haze overlying, fit limb darkening curves and spectra at all latitudes well; the retrieved optical depth for the tropospheric haze is decreased in the northern (winter) hemisphere, implying that the haze has a photochemical origin. Our ability to test this hypothesis by examining spectra at different seasons is restricted by the varying geometry of VIMS observations over the life of the mission, and the appearance of the Saturn storm towards the end of 2010.

Dual-telescope multi-channel thermal-infrared radiometer for outer planet fly-by missions

Acta Astronautica Elsevier 128 (2016) 628-639

S Aslam, M Amato, N Bowles, S Calcutt, T Hewagama, J Howard, C Howett, W-T Hsieh, T Hurford, J Hurley, P Irwin, DE Jennings, E Kessler, B Lakew, M Loeffler, M Mellon, A Nicoletti, CA Nixon, N Putzig, G Quilligan, J Rathbun, M Segura, J Spencer, J Spitale, G West

The design of a versatile dual-telescope thermal-infrared radiometer spanning the spectral wavelength range 8–200 µm, in five spectral pass bands, for outer planet fly-by missions is described. The dual-telescope design switches between a narrow-field-of-view and a wide-field-of-view to provide optimal spatial resolution images within a range of spacecraft encounters to the target. The switchable dual-field-of-view system uses an optical configuration based on the axial rotation of a source-select mirror along the optical axis. The optical design, spectral performance, radiometric accuracy, and retrieval estimates of the instrument are discussed. This is followed by an assessment of the surface coverage performance at various spatial resolutions by using the planned NASA Europa Mission 13-F7 fly-by trajectories as a case study.

Saturn’s tropospheric particles phase function and spatial distribution from Cassini ISS 2010–11 observations

Icarus Elsevier 277 (2016) 1-18

S Pérez-Hoyos, JF Sanz-Requena, A Sánchez-Lavega, P Irwin, A Smith

The phase function describes the way particles scatter the incoming radiation. This is a fundamental piece of knowledge in order to understand how a planetary atmosphere scatters sunlight and so it has a profound influence in the retrieved atmospheric properties such as cloud height, particle density distribution and radiative forcing by aerosols. In this work we analyze data from the Imaging Science Subsystem (ISS) instrument onboard Cassini spacecraft to determine the particle phase function at blue (451 nm) and near infrared wavelengths (727-890 nm) of particles in the upper troposphere, where most of the incoming visible sunlight is scattered. In order to do so, we use observations taken in later 2010 and 2011 covering a broad range of phase angles from ~10° to ~160° in the blue (BL1) and near infrared filters associated with intermediate and deep methane absorption bands (MT2, CB2, MT3). Particles at all latitudes are found to be strongly forward scattering. The equatorial particles are in good agreement with laboratory measurements of 10 μm ammonia ice crystals, while mid- and sub-polar latitude particles may be similar to the equatorial particles, but they may also be consistent with 1 μm ellipsoids with moderate aspect ratios. Uncertainties due to limited phase coverage and parameter degeneracy prevent strong constraints of the particle shapes and sizes at these locations. Results for the particle phase function are also used to describe the spatial distribution of tropospheric particles both vertically and latitudinally in the Northern hemisphere.

Isotopic ratios of carbon and oxygen in Titan's co using ALMA

Astrophysical Journal Letters IOP Publishing 821 (2016) L8-L8

J Serigano, CA Nixon, MA Cordiner, P Irwin, NA Teanby, SB Charnley, JE Lindberg

We report interferometric observations of carbon monoxide (CO) and its isotopologues in Titan's atmosphere using the Atacama Large Millimeter/submillimeter Array (ALMA). The following transitions were detected: CO (J = 1-0, 2-1, 3-2, 6-5), 13CO (J = 2-1, 3-2, 6-5), C18O (J = 2-1, 3-2), and C17O (J = 3-2). Molecular abundances and the vertical atmospheric temperature profile were derived by modeling the observed emission line profiles using NEMESIS, a line-by-line radiative transfer code. We present the first spectroscopic detection of 17O in the outer solar system with C17O detected at &gt;8σ confidence. The abundance of CO was determined to be 49.6 ± 1.8 ppm, assumed to be constant with altitude, with isotopic ratios 12C/13C = 89.9 ± 3.4, 16O/18O = 486 ± 22, and 16O/17O = 2917 ± 359. The measurements of 12C/13C and 16O/18O ratios are the most precise values obtained in Titan's atmospheric CO to date. Our results are in good agreement with previous studies and suggest no significant deviations from standard terrestrial isotopic ratios.