Publications by Patrick Irwin

Mapping the zonal structure of Titan's northern polar vortex

Icarus 337 (2020)

J Sharkey, NA Teanby, M Sylvestre, DM Mitchell, WJM Seviour, CA Nixon, PGJ Irwin

© 2019 Elsevier Inc. Saturn exhibits an obliquity of 26.7° such that the largest moon, Titan, experiences seasonal variations including the formation of a polar vortex in the winter hemisphere. Titan's polar vortex is characterised by cold stratospheric temperatures due to the lack of insolation over the winter pole, and an increase in trace gas abundance as a result of complex organic chemistry in the upper atmosphere combined with polar subsidence. Meridional variations in temperature and gas abundance across the vortex have previously been investigated, but there has not yet been any in-depth study of the zonal variations in the temperature or composition of the northern vortex. Here we present the first comprehensive two-dimensional seasonal mapping of Titan's northern winter vortex. Using 18 nadir mapping sequences observed by the Composite InfraRed Spectrometer (CIRS) instrument on-board Cassini, we investigate the evolution of the vortex over almost half a Titan year, from late winter through to mid summer (Ls = 326 − 86°, 2007–2017). We find the stratospheric symmetry axis to be tilted from the solid body rotation axis by around 3.5°, although our results for the azimuthal orientation of the tilt are inconclusive. We find that the northern vortex appears to remain zonally uniform in both temperature and composition at all times. A comparison with vortices observed on Earth, Mars, and Venus shows that large-scale wave mechanisms that are important on other terrestrial planets are not as significant in Titan's atmosphere. This allows the northern vortex to be more symmetrical and persist longer throughout the annual cycle compared to other terrestrial planets.

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


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

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


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

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.

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 7.8-mu m CH4 emission during a solar-wind compression

NATURE ASTRONOMY 3 (2019) 607-613

JA Sinclair, GS Orton, J Fernandes, Y Kasaba, TM Sato, T Fujiyoshi, C Tao, MF Vogt, D Grodent, B Bonfond, JI Moses, TK Greathouse, W Dunn, RS Giles, F Tabataba-Vakili, LN Fletcher, PGJ Irwin

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 333 (2019) 22-36

JF Sanz-Requena, S Perez-Hoyos, A Sanchez-Lavega, T Del Rio-Gaztelurrutia, PGJ 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 321 (2019) 550-563

PGJ Irwin, D Toledo, R Garland, NA Teanby, LN Fletcher, GS Orton, B Bezard

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

ICARUS 321 (2019) 572-582

PGJ Irwin, N Bowles, AS Braude, R Garland, S Calcutt, PA Coles, SN Yurchenko, J Tennyson

Neptune's carbon monoxide profile and phosphine upper limits from Herschel/SPIRE: Implications for interior structure and formation

ICARUS 319 (2019) 86-98

NA Teanby, PGJ Irwin, JI Moses

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

ICARUS 331 (2019) 69-82

PGJ Irwin, D Toledo, AS Braude, R Bacon, PM Weilbacher, NA Teanby, LN Fletcher, GS Orton

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>