Publications by Patrick Irwin

Mapping the zonal structure of Titan’s northern polar vortex

Icarus Elsevier 337 (2019) 113441

J Sharkey, N Teanby, M Sylvestre, D Mitchell, W Seviour, C Nixon, P Irwin

Exoplanetary Monte Carlo radiative transfer with correlated-k I. Benchmarking transit and emission observables

Monthly Notices of the Royal Astronomical Society Oxford University Press 487 (2019) 2082-2096

G Lee, SL Grimm, J Taylor, J-L Baudino, R Garland, PGJ Irwin, K Wood

Current observational data of exoplanets are providing increasing detail of their 3D atmospheric structures. As characterisation efforts expand in scope, the need to develop consistent 3D radiative-transfer methods becomes more pertinent as the complex atmospheric properties of exoplanets are required to be modelled together consistently. We aim to compare the transmission and emission spectra results of a 3D Monte Carlo Radiative Transfer (MCRT) model to contemporary radiative-transfer suites. We perform several benchmarking tests of a MCRT code, Cloudy Monte Carlo Radiative Transfer (CMCRT), to transmission and emission spectra model output. We add flexibility to the model through the use of k-distribution tables as input opacities. We present a hybrid MCRT and ray tracing methodology for the calculation of transmission spectra with a multiple scattering component. CMCRT compares well to the transmission spectra benchmarks at the 10s of ppm level. Emission spectra benchmarks are consistent to within 10% of the 1D models. We suggest that differences in the benchmark results are likely caused by geometric effects between plane-parallel and spherical models. In a practical application, we post-process a cloudy 3DHD 189733b GCM model and compare to available observational data. Our results suggest the core methodology and algorithms of CMCRT produce consistent results to contemporary radiative transfer suites. 3D MCRT methods are highly suitable for detailed post-processing of cloudy and non-cloudy 1D and 3D exoplanet atmosphere simulations in instances where atmospheric inhomogeneities, significant limb effects/geometry or multiple scattering components are important considerations.

Seasonal Evolution of Titan's Stratosphere During the Cassini Mission


NA Teanby, M Sylvestre, J Sharkey, CA Nixon, S Vinatier, PGJ Irwin

Oxygen isotopic ratios in Martian water vapour observed by ACS MIR on board the ExoMars Trace Gas Orbiter

Astronomy and Astrophysics EDP Sciences 630 (2019) A91

J Alday, A Patrakeev, A Grigoriev, A Shakun, CF Wilson, PGJ Irwin, KS Olsen, F Montmessin, L Baggio, A Trokhimovskiy, AA Fedorova, O Korablev, DA Belyaev

Oxygen isotope ratios provide important constraints on the history of the Martian volatile system, revealing the impact of several processes that might fractionate them, such as atmospheric loss into space or interaction with the surface. We report infrared measurements of the Martian atmosphere obtained with the mid-infrared channel (MIR) of the Atmospheric Chemistry Suite (ACS), onboard the ExoMars Trace Gas Orbiter. Absorption lines of the three main oxygen isotopologues of water vapour (H216O, H218O, and H217O) observed in the transmission spectra allow, for the first time, the measurement of vertical profiles of the 18O/16O and 17O/16O ratios in atmospheric water vapour. The observed ratios are enriched with respect to Earth-like values (δ18O = 200 ± 80‰ and δ17O = 230 ± 110‰ corresponding to the Vienna Standard Mean Ocean Water). The vertical structure of these ratios does not appear to show significant evidence of altitudinal variations.

Wave Activity in Jupiter's North Equatorial Belt From Near-Infrared Reflectivity Observations

Geophysical Research Letters 46 (2019) 1232-1241

RS Giles, GS Orton, AW Stephens, MH Wong, PGJ Irwin, JA Sinclair, F Tabataba-Vakili

©2019. American Geophysical Union. All Rights Reserved. High spatial resolution images of Jupiter at 1.58–2.28 μm are used to track and characterize a wave pattern observed in 2017 at a planetocentric latitude of 14°N. The wave pattern has a wave number of 18 and spans ∼5° in latitude. One bright crest remains stationary in System III longitude, while the remaining crests move slowly westward. The bright and dark regions of the near-infrared wave pattern are caused by variations in the vertical location of the upper tropospheric haze layer. A comparison with thermal infrared observations shows a correlation with temperature anomalies in the upper troposphere. The results are consistent with a Rossby wave, generated by flow around a stationary vortex.

Detection of Propadiene on Titan


NA Lombardo, CA Nixon, TK Greathouse, B Bezard, A Jolly, S Vinatier, NA Teanby, MJ Richter, PJG Irwm, A Coustenis, FM Flasar

Abundance measurements of Titan's stratospheric HCN, HC3N, C3H4, and CH3CN from ALMA observations

Icarus 319 (2019) 417-432

AE Thelen, CA Nixon, NJ Chanover, MA Cordiner, EM Molter, NA Teanby, PGJ Irwin, J Serigano, SB Charnley

© 2018 Elsevier Inc. Previous investigations have employed more than 100 close observations of Titan by the Cassini orbiter to elucidate connections between the production and distribution of Titan's vast, organic-rich chemical inventory and its atmospheric dynamics. However, as Titan transitions into northern summer, the lack of incoming data from the Cassini orbiter presents a potential barrier to the continued study of seasonal changes in Titan's atmosphere. In our previous work (Thelen et al., 2018), we demonstrated that the Atacama Large Millimeter/submillimeter Array (ALMA) is well suited for measurements of Titan's atmosphere in the stratosphere and lower mesosphere (∼100−500 km) through the use of spatially resolved (beam sizes < 1′′) flux calibration observations of Titan. Here, we derive vertical abundance profiles of four of Titan's trace atmospheric species from the same 3 independent spatial regions across Titan's disk during the same epoch (2012–2015): HCN, HC3N, C3H4, and CH3CN. We find that Titan's minor constituents exhibit large latitudinal variations, with enhanced abundances at high latitudes compared to equatorial measurements; this includes CH3CN, which eluded previous detection by Cassini in the stratosphere, and thus spatially resolved abundance measurements were unattainable. Even over the short 3-year period, vertical profiles and integrated emission maps of these molecules allow us to observe temporal changes in Titan's atmospheric circulation during northern spring. Our derived abundance profiles are comparable to contemporary measurements from Cassini infrared observations, and we find additional evidence for subsidence of enriched air onto Titan's south pole during this time period. Continued observations of Titan with ALMA beyond the summer solstice will enable further study of how Titan's atmospheric composition and dynamics respond to seasonal changes.

Ethane in Titan's Stratosphere from Cassini CIRS Far- and Mid-infrared Spectra


NA Lombardo, CA Nixon, M Sylvestre, DE Jennings, N Teanby, PJG Irwin, FM Flasar

Constraints on Uranus's haze structure, formation and transport

Icarus Elsevier BV 333 (2019) 1-11

D Toledo, PGJ Irwin, P Rannou, NA Teanby, AA Simon, MH Wong, GS Orton

Measurement of CH3D on Titan at Submillimeter Wavelengths


AE Thelen, CA Nixon, MA Cordiner, SB Charnley, PGJ Irwin, Z Kisiel

A brightening of Jupiter’s auroral 7.80-μm CH4 emission during a solar-wind compression

Nature Astronomy Nature Research 3 (2019) 607-613

J Sinclair, G Orton, J Fernandes, Y Kasaba, T Sato, T Fujiyoshi, C Tao, F Vogt, G Grodent, B Bonfond, J Moses, T Greathouse, W Dunn, F Tabataba-Vakili, R Giles, L Fletcher, P Irwin

Enhanced mid-infrared emission from CH4 and other stratospheric hydrocarbons has been observed coincident with Jupiter’s ultraviolet auroral emission1,2,3. This suggests that auroral processes and the neutral stratosphere of Jupiter are coupled; however, the exact nature of this coupling is unknown. Here we present a time series of Subaru-COMICS images of Jupiter measured at a wavelength of 7.80 μm on 11–14 January, 4–5 February and 17–20 May 2017. These data show that both the morphology and magnitude of the auroral CH4 emission vary on daily timescales in relation to external solar-wind conditions. The southern auroral CH4 emission increased in brightness temperature by about 3.8 K between 15:50 UT, 11 January and 12:57 UT, 12 January, during a predicted solar-wind compression. During the same compression, the northern auroral emission exhibited a duskside brightening, which mimics the morphology observed in the ultraviolet auroral emission during periods of enhanced solar-wind pressure4,5. These results suggest that changes in external solar-wind conditions perturb the Jovian magnetosphere in such a way that energetic particles are accelerated into the planet’s atmosphere, deposit their energy as deep as the neutral stratosphere, and modify the thermal structure, the abundance of CH4 or the population of energy states of CH4. We also find that the northern and southern auroral CH4 emission evolved independently between the January, February and May images, as has been observed at X-ray wavelengths over shorter timescales6 and at mid-infrared wavelengths over longer timescales7.

Seasonal evolution of temperatures in Titan's lower stratosphere

Icarus (2019)

M Sylvestre, NA Teanby, J Vatant d'Ollone, S Vinatier, B Bézard, S Lebonnois, PGJ Irwin

© 2019 Elsevier Inc. The Cassini mission offered us the opportunity to monitor the seasonal evolution of Titan's atmosphere from 2004 to 2017, i.e. half a Titan year. The lower part of the stratosphere (pressures greater than 10 mbar) is a region of particular interest as there are few available temperature measurements, and because its thermal response to the seasonal and meridional insolation variations undergone by Titan remain poorly known. In this study, we measure temperatures in Titan's lower stratosphere between 6 mbar and 25 mbar using Cassini/CIRS spectra covering the whole duration of the mission (from 2004 to 2017) and the whole latitude range. We can thus characterize the meridional distribution of temperatures in Titan's lower stratosphere, and how it evolves from northern winter (2004) to summer solstice (2017). Our measurements show that Titan's lower stratosphere undergoes significant seasonal changes, especially at the South pole, where temperature decreases by 19 K at 15 mbar in 4 years.

Corrigendum to “Neptune's carbon monoxide profile and phosphine upper limits from Herschel/SPIRE” (Icarus, vol 319, p86–98, 2019) (Icarus (2019) 319 (86–98), (S0019103518304457), (10.1016/j.icarus.2018.09.014))

Icarus 322 (2019) 261-261

NA Teanby, PGJ Irwin, JI Moses

© 2018 The authors would like to publish the below information which was incorrectly published in its original version. Page 90: The equation for saturation vapour pressure should be PSVP(T) =exp(a+b/T +cT). Page92: TheD/HratiomeasuredbyFeuchtgruberetal.(2013)fromHerschelPACSshouldbe 4.1±0.4×10−5. References Feuchtgruber, H., Lellouch, E., Orton, G., de Graauw, T., Vandenbussche, B., Swinyard, B., Moreno, R., Jarchow, C., Billebaud, F., Cavali´e, T., Sidher, S., Hartogh, P., 2013. The D/H ratio in the atmospheres of Uranus and Neptune from Herschel-PACS observations. Astron. Astrophys. 551, 1–9.

Jupiter's auroral-related stratospheric heating and chemistry III: Abundances of C <inf>2</inf> H <inf>4</inf> , CH <inf>3</inf> C <inf>2</inf> H, C <inf>4</inf> H <inf>2</inf> and C <inf>6</inf> H <inf>6</inf> from Voyager-IRIS and Cassini-CIRS

Icarus 328 (2019) 176-193

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

© 2019 Elsevier Inc. We present an analysis of Voyager-1-IRIS and Cassini-CIRS spectra of Jupiter's high latitudes acquired during the spacecrafts' respective flybys in November 1979 and January 2001. We performed a forward-model analysis in order to derive the abundances of ethylene (C 2 H 4 ), methylacetylene (CH 3 C 2 H), diacetylene (C 4 H 2 ) and benzene (C 6 H 6 ) in Jupiter's northern and southern auroral regions. We also compared these abundances to: 1) lower-latitude abundances predicted by the Moses et al. (2005) ‘Model A’ photochemical model, henceforth ‘Moses 2005A’, and 2) abundances derived at non-auroral longitudes in the same latitude band. This paper serves as an extension of Sinclair et al. (2017b), where we retrieved the vertical profiles of temperature, C 2 H 2 and C 2 H 6 from similar datasets. We find that an enrichment of C 2 H 4 , CH 3 C 2 H and C 6 H 6 with respect to lower-latitude abundances is required to fit the spectra of Jupiter's northern and southern auroral regions. For example, for CIRS 0.5 cm −1 spectra of Jupiter's southern auroral region, scale factor enrichments of 6.40 −1.15+1.30 and 9.60 −3.67+3.98 are required with respect to the Moses 2005A vertical profiles of C 2 H 4 and C 6 H 6 , respectively, in order to fit the spectral emission features of these species at ∼950 and ∼674 cm −1 . Similarly, in order to fit the CIRS 2.5 cm −1 spectra of Jupiter's northern auroral region, scale factor enrichments of 1.60 −0.21+0.37 , 3.40 −1.69+1.89 and 15.00 −4.02+4.01 with respect to the Moses 2005A vertical profiles of C 2 H 4 , CH 3 C 2 H and C 6 H 6 were required, respectively. Outside of Jupiter's auroral region in the same latitude bands, only upper-limit abundances of C 2 H 4 , CH 3 C 2 H and C 6 H 6 could be determined due to the limited sensitivity of the measurements, the weaker emission features combined with cooler stratospheric temperatures (and therefore decreased thermal emission) of these regions. Nevertheless, for a subset of the observations, derived abundances of C 2 H 4 and C 6 H 6 in Jupiter's auroral regions were higher (by 1 σ) with respect to upper-limit abundances derived outside the auroral region in the same latitude band. This is suggestive that the influx of energetic ions and electrons from the Jovian magnetosphere and external solar-wind environment into the neutral atmosphere in Jupiter's auroral regions drives enhanced ion-related chemistry, as has also been inferred from Cassini observations of Saturn's high latitudes (Fletcher et al., 2018; Guerlet et al., 2015; Koskinen et al., 2016). We were not able to constrain the abundance of C 4 H 2 in either Jupiter's auroral regions or non-auroral regions due to its lower (predicted) abundance and weaker emission feature. Thus, only upper-limit abundances were derived in both locations. From CIRS 2.5 cm −1 spectra, the upper limit abundance of C 4 H 2 corresponds to a scale factor enhancement of 45.6 and 23.8 with respect to the Moses 2005A vertical profile in Jupiter's non-auroral and auroral regions.

Hazes and clouds in a singular triple vortex in Saturn's atmosphere from HST/WFC3 multispectral imaging

Icarus Elsevier 333 (2019) 22-36

JF Sanz-Requena, A Sanchez-Lavega, S Perez-Hoyos, T Del Rio-Gaztelurrutia, P Irwin

Spatial and seasonal variations in C_3/H_x hydrocarbon abundance in Titan's stratosphere from Cassini CIRS observations

Icarus 317 (2019) 454-469

NA Lombardo, CA Nixon, RK Achterberg, A Jolly, K Sung, PGJ Irwin, FM Flasar

© 2018 Of the C3Hxhydrocarbons, propane (C3H8) and propyne (methylacetylene, CH3C2H) were first detected in Titan's atmosphere during the Voyager 1 flyby in 1980. Propene (propylene, C3H6) was first detected in 2013 with data from the Composite InfraRed Spectrometer (CIRS) instrument on Cassini. We present the first measured abundance profiles of propene on Titan from radiative transfer modeling, and compare our measurements to predictions derived from several photochemical models. Near the equator, propene is observed to have a peak abundance of 10 ppbv at a pressure of 0.2 mbar. Several photochemical models predict the amount at this pressure to be in the range 0.3–1 ppbv and also show a local minimum near 0.2 mbar which we do not see in our measurements. We also see that propene follows a different latitudinal trend than the other C3molecules. While propane and propyne concentrate near the winter pole, transported via a global convective cell, propene is most abundant above the equator. We retrieve vertical abundances profiles between 125 km and 375 km for these gases for latitude averages between 60°S–20°S, 20°S–20°N, and 20°N–60°N over two time periods, 2004 through 2009 representing Titan's atmosphere before the 2009 equinox, and 2012 through 2015 representing time after the equinox. Additionally, using newly corrected line data, we determined an updated upper limit for allene (propadiene, CH2CCH2, the isomer of propyne). We claim a 3-σ upper limit mixing ratio of 2.5 × 10−9 within 30° of the equator. The measurements we present will further constrain photochemical models by refining reaction rates and the transport of these gases throughout Titan's atmosphere.

Probable detection of hydrogen sulphide (H2S) in Neptune’s atmosphere

Icarus Elsevier 321 (2018) 550-563

P Irwin, R Garland, D Toledo, N Teanby, L Fletcher, B Bezard, G Orton

Recent analysis of Gemini-North/NIFS H-band (1.45–1.8 µm) observations of Uranus, recorded in 2010, with recently updated line data has revealed the spectral signature of hydrogen sulphide (H2S) 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 H2S spectral absorption features in the 1.57–1.58 µm range, albeit slightly less evident, and retrieve a mole fraction of -1 - 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 H2S here. We find our retrieved H2S 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 µm range, some of the Empirical Orthogonal Functions (EOFs) mapped closely to physically significant quantities, with one being strongly correlated with the modelled H2S signal and clearly mapping the spatial dependence of its spectral detectability. Just as for Uranus, the detection of H2S at the cloud tops constrains the deep bulk sulphur/nitrogen abundance to exceed unity (i.e. &gt;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) x10 -5 . The detection of gaseous H2S 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 H2S ice.

Analysis of gaseous ammonia (NH3) absorption in the visible spectrum of Jupiter - Update

Icarus Elsevier 321 (2018) 572-582

P Irwin, N Bowles, A Braude, S Calcutt, R Garland, PA Coles, J Tennyson

An analysis of currently available ammonia (NH3) visible-to-near-infrared gas absorption data was recently undertaken by Irwin et al. (2018) to help interpret Very Large Telescope (VLT) MUSE observations of Jupiter from 0.48–0.93 µm, made in support of the NASA/Juno mission. Since this analysis a newly revised set of ammonia line data, covering the previously poorly constrained range 0.5–0.833 µm, has been released by the ExoMol project, “C2018” (Coles et al., 2018), which demonstrates significant advantages over previously available data sets, and provides for the first time complete line data for the previously poorly constrained 5520- and 6475-Å bands of NH3. In this paper we compare spectra calculated using the ExoMol–C2018 data set (Coles et al., 2018) with spectra calculated from previous sources to demonstrate its advantages. We conclude that at the present time the ExoMol–C2018 dataset provides the most reliable ammonia absorption source for analysing low- to medium-resolution spectra of Jupiter in the visible/near-IR spectral range, but note that the data are less able to model high-resolution spectra owing to small, but significant inaccuracies in the line wavenumber estimates. This work is of significance not only for solar system planetary physics, but for future proposed observations of Jupiter-like planets orbiting other stars, such as with NASA’s planned Wide-Field Infrared Survey Telescope (WFIRST).

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

Icarus Elsevier 331 (2019) 69-82

P Irwin, D Toledo Carrasco, A Braude, R Bacon, P Weilbacher, N Teanby, L Fletcher, G Orton

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 to 0.93 μm, are analysed here to determine the latitudinal and vertical distribution of cloud opacity and methane abundance in Neptune's observable troposphere (0.1–∼ 3bar). 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 and 40°S. We also find a small depletion of methane near the equator, perhaps indicating subsidence there, and a local enhancement near 60–70°S, 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.

Towards the analysis of JWST exoplanet spectra: the effective temperature in the context of direct imaging

Monthly Notices of the Royal Astronomical Society Oxford University Press (OUP) 490 (2019) 2086-2090

J-L Baudino, J Taylor, PGJ Irwin, R Garland

<jats:title>ABSTRACT</jats:title> <jats:p>The current sparse wavelength range coverage of exoplanet direct imaging observations, and the fact that models are defined using a finite wavelength range, lead both to uncertainties on effective temperature determination. We study these effects using blackbodies and atmospheric models and we detail how to infer this parameter. Through highlighting the key wavelength coverage that allows for a more accurate representation of the effective temperature, our analysis can be used to mitigate or manage extra uncertainties being added in the analysis from the models. We find that the wavelength range coverage will soon no longer be a problem. An effective temperature computed by integrating the spectroscopic observations of the James Webb Space Telescope will give uncertainties similar to, or better than, the current state–of–the–art, which is to fit models to data. Accurately calculating the effective temperature will help to improve current modelling approaches. Obtaining an independent and precise estimation of this crucial parameter will help the benchmarking process to identify the best practice to model exoplanet atmospheres.</jats:p>