Publications by Patrick Irwin


Potential vorticity structure of Titan’s polar vortices from Cassini CIRS observations

Icarus Elsevier BV (2020) 114030

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


Jupiter in the Ultraviolet: Acetylene and Ethane Abundances in the Stratosphere of Jupiter from Cassini Observations between 0.15 and 0.19 mu m

ASTRONOMICAL JOURNAL American Astronomical Society 159 (2020) ARTN 291

H Melin, L Fletcher, P Irwin, S Edgington

© 2020. The American Astronomical Society. All rights reserved. At wavelengths between 0.15 and 0.19 μm, the far-ultraviolet spectrum of Jupiter is dominated by the scattered solar spectrum, attenuated by molecular absorptions primarily by acetylene and ethane, and to a lesser extent ammonia and phosphine. We describe the development of our radiative transfer code that enables the retrieval of abundances of these molecular species from ultraviolet reflectance spectra. As a proof-of-concept we present an analysis of Cassini Ultraviolet Imaging Spectrograph (UVIS) observations of the disk of Jupiter during the 2000/2001 flyby. The ultraviolet-retrieved acetylene abundances in the upper stratosphere are lower than those predicted by models based solely on infrared thermal emission from the mid-stratosphere observed by the Composite Infrared Spectrometer (CIRS), requiring an adjustment to the vertical profiles above 1 mbar. We produce a vertical acetylene abundance profile that is compatible with both CIRS and UVIS, with reduced abundances at pressures <1 mbar: the 0.1 mbar abundances are 1.21 ± 0.07 ppm for acetylene and 20.8 ± 5.1 ppm for ethane. Finally, we perform a sensitivity study for the JUICE ultraviolet spectrograph, which has extended wavelength coverage out to 0.21 μm, enabling the retrieval of ammonia and phosphine abundances, in addition to acetylene and ethane.


Advanced Net Flux Radiometer for the Ice Giants

Space Science Reviews Springer Verlag 216 (2020)

S Aslam, RK Achterberg, SB Calcutt, V Cottini, NJ Gorius, T Hewagama, PG Irwin, CA Nixon, G Quilligan, M Roos-Serote, AA Simon, D Tran, G Villanueva

The design of an advanced Net Flux Radiometer (NFR), for inclusion as a payload on a future Ice Giants probe mission, is given. The Ice Giants NFR (IG-NFR) will measure the upward and downward radiation flux (hence net radiation flux), in seven spectral bands, spanning the range from solar to far infra-red wavelengths, each with a 5∘ Field-Of-View (FOV) and in five sequential view angles ( ±80∘ , ±45∘ , and 0∘ ) as a function of altitude. IG-NFR measurements within either Uranus or Neptune’s atmospheres, using dedicated spectral filter bands will help derive radiative heating and cooling profiles, and will significantly contribute to our understanding of the planet’s atmospheric heat balance and structure, tropospheric 3-D flow, and compositions and opacities of the cloud layers. The IG-NFR uses an array of non-imaging Winston cones integrated to a matched thermopile detector Focal Plane Assembly (FPA), with individual bandpass filters, housed in a diamond windowed vacuum micro-vessel. The FPA thermopile detector signals are read out in parallel mode, amplified and processed by a multi-channel digitizer application specific integrated circuit (MCD ASIC) under field programmable gate array (FPGA) control. The vacuum micro-vessel rotates providing chopping between FOV’s of upward and downward radiation fluxes. This unique design allows for small net flux measurements in the presence of large ambient fluxes and rapidly changing ambient temperatures during the probe descent to ≥10 bar pressure.


Ice giant circulation patterns: Implications for atmospheric probes

Space Science Reviews Springer 216 (2020) 21

L Fletcher, DP Imke, G Orton, M Hofstadter, P Irwin, M Roman, D Toledo Carrasco

Atmospheric circulation patterns derived from multi-spectral remote sensing can serve as a guide for choosing a suitable entry location for a future in situ probe mission to the Ice Giants. Since the Voyager-2 flybys in the 1980s, three decades of observations from ground- and space-based observatories have generated a picture of Ice Giant circulation that is complex, perplexing, and altogether unlike that seen on the Gas Giants. This review seeks to reconcile the various competing circulation patterns from an observational perspective, accounting for spatially-resolved measurements of: zonal albedo contrasts and banded appearances; cloud-tracked zonal winds; temperature and para-H2 measurements above the condensate clouds; and equator-to-pole contrasts in condensable volatiles (methane, ammonia, and hydrogen sulphide) in the deeper troposphere. These observations identify three distinct latitude domains: an equatorial domain of deep upwelling and upper-tropospheric subsidence, potentially bounded by peaks in the retrograde zonal jet and analogous to Jovian cyclonic belts; a mid-latitude transitional domain of upper-tropospheric upwelling, vigorous cloud activity, analogous to Jovian anticyclonic zones; and a polar domain of strong subsidence, volatile depletion, and small-scale (and potentially seasonally-variable) convective activity. Taken together, the multi-wavelength observations suggest a tiered structure of stacked circulation cells (at least two in the troposphere and one in the stratosphere), potentially separated in the vertical by (i) strong molecular weight gradients associated with cloud condensation, and by (ii) transitions from a thermally-direct circulation regime at depth to a wave- and radiative-driven circulation regime at high altitude. The inferred circulation can be tested in the coming decade by 3D numerical simulations of the atmosphere, and by observations from future world-class facilities. The carrier spacecraft for any probe entry mission must ultimately carry a suite of remote-sensing instruments capable of fully constraining the atmospheric motions at the probe descent location.


Uranus in Northern Mid-spring: Persistent Atmospheric Temperatures and Circulations Inferred from Thermal Imaging (vol 159, 45, 2020)

ASTRONOMICAL JOURNAL American Astronomical Society 160 (2020) ARTN 56

MT Roman, LN Fletcher, GS Orton, N Rowe-Gurney, PGJ Irwin


Understanding and mitigating biases when studying inhomogeneous emission spectra with JWST

Monthly Notices of the Royal Astronomical Society Royal Astronomical Society (2020)

J Taylor, V Parmentier, P Irwin, S Aigrain, G Lee, J Krissansen-Totton

Exoplanet emission spectra are often modelled assuming that the hemisphere observed is well represented by a horizontally homogenised atmosphere. However this approximation will likely fail for planets with a large temperature contrast in the James Webb Space Telescope (JWST) era, potentially leading to erroneous interpretations of spectra. We first develop an analytic formulation to quantify the signal-to-noise ratio and wavelength coverage necessary to disentangle temperature inhomogeneities from a hemispherically averaged spectrum. We find that for a given signal-to-noise ratio, observations at shorter wavelengths are better at detecting the presence of inhomogeneities. We then determine why the presence of an inhomogeneous thermal structure can lead to spurious molecular detections when assuming a fully homogenised planet in the retrieval process. Finally, we quantify more precisely the potential biases by modelling a suite of hot Jupiter spectra, varying the spatial contributions of a hot and a cold region, as would be observed by the different instruments of JWST/NIRSpec. We then retrieve the abundances and temperature profiles from the synthetic observations. We find that in most cases, assuming a homogeneous thermal structure when retrieving the atmospheric chemistry leads to biased results, and spurious molecular detection. Explicitly modelling the data using two profiles avoids these biases, and is statistically supported provided the wavelength coverage is wide enough, and crucially also spanning shorter wavelengths. For the high contrast used here, a single profile with a dilution factor performs as well as the two-profile case, with only one additional parameter compared to the 1-D approach.


Color and aerosol changes in Jupiter after a North Temperate Belt disturbance

Icarus Elsevier BV 352 (2020) 114031

S Pérez-Hoyos, A Sánchez-Lavega, J Sanz-Requena, N Barrado-Izagirre, O Carrión-González, A Anguiano-Arteaga, P Irwin, A Braude


Colour and tropospheric cloud structure of Jupiter from MUSE/VLT: retrieving a universal chromophore

Icarus Elsevier 338 (2020) 113589

AS Braude, P Irwin, GS Orton, LN Fletcher

Recent work by Sromovsky et al. (2017, Icarus 291, 232-244) suggested that all red colour in Jupiter’s atmosphere could be explained by a single colour-carrying compound, a so-called ‘universal chromophore’. We tested this hypothesis on ground-based spectroscopic observations in the visible and near-infrared (480- 930 nm) from the VLT/MUSE instrument between 2014 and 2018, retrieving a chromophore absorption spectrum directly from the North Equatorial Belt, and applying it to model spatial variations in colour, tropospheric cloud and haze structure on Jupiter. We found that we could model both the belts and the Great Red Spot of Jupiter using the same chromophore compound, but that this chromophore must exhibit a steeper blue-absorption gradient than the proposed chromophore of Carlson et al. (2016, Icarus 274, 106–115). We retrieved this chromophore to be located no deeper than 0.2±0.1 bars in the Great Red Spot and 0.7±0.1 bars elsewhere on Jupiter. However, we also identified some spectral variability between 510 nm and 540 nm that could not be accounted for by a universal chromophore. In addition, we retrieved a thick, global cloud layer at 1.4 ± 0.3 bars that was relatively spatially invariant in altitude across Jupiter. We found that this cloud layer was best characterised by a real refractive index close to that of ammonia ice in the belts and the Great Red Spot, and poorly characterised by a real refractive index of 1.6 or greater. This may be the result of ammonia cloud at higher altitude obscuring a deeper cloud layer of unknown composition.


Spatial structure in Neptune’s 7.90-m stratospheric CH emission, as measured by VLT-VISIR

Icarus Elsevier 345 (2020) 113748

J Sinclair, G Orton, L Fletcher, M Roman, I de Pater, T Encrenaz, H Hammel, R Giles, T Velusamy, J Moses, P Irwin, T Momary, N Rowe-Gurney, F Tabataba-Vakili

We present a comparison of VLT-VISIR images and Keck-NIRC2 images of Neptune, which highlight the coupling between its troposphere and stratosphere. VLT-VISIR images were obtained on September 16th 2008 (UT) at 7.90 μm and 12.27 μm, which are primarily sensitive to 1-mbar CH4 and C2H6 emission, respectively. NIRC2 images in the H band were obtained on October 5th, 6th and 9th 2008 (UT) and sense clouds and haze in the upper troposphere and lower stratosphere (from approximately 600 to 20 mbar). At 7.90 μm, we observe enhancements of CH4 emission in latitude bands centered at approximately 25∘S and 48∘S (planetocentric). Within these zonal bands, tentative detections (<2σ) of discrete hotspots of CH4 emission are also evident at 24∘S, 181∘W and 42∘S, 170∘W. The longitudinal-mean enhancements in the CH4 emission are also latitudinally-coincident with bands of bright (presumably CH4 ice) clouds in the upper troposphere and lower stratosphere evidenced in the H-band images. This suggests the Neptunian troposphere and stratosphere are coupled in these specific regions. This could be in the form of (1) ‘overshoot’ of strong, upwelling plumes and advection of CH4 ice into the lower stratosphere, which subsequently sublimates into CH4 gas and/or (2) generation of waves by plumes impinging from the tropopause below, which impart their energy and heat the lower stratosphere. We favor the former process since there is no evidence of similar smaller-scale morphology in the C2H6 emission, which probes a similar atmospheric level. However, we cannot exclude temperature variations as the source of the morphology observed in CH4 emission. Future, near-infrared imaging of Neptune performed near-simultaneously with future mid-infrared spectral observations of Neptune by the James Webb Space Telescope would allow the coupling of Neptune's troposphere and stratosphere to be confirmed and studied in greater detail.


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


Constraints on Neptune’s haze structure and formation from VLT observations in the H-band

Icarus Elsevier (2020)

D Toledo Carrasco, P Irwin, P Rannou, L Fletcher, N Teanby, M Wong, G Orton

A 1-dimensional microphysics model has been used to constrain the structure and formation of haze in Neptune's atmosphere. These simulations were coupled to a radiative-transfer and retrieval code (NEMESIS) to model spectral observations of Neptune in the H-band performed by the SINFONI Integral Field Unit Spectrometer on the Very Large Telescope (VLT) in 2013. It was found that observations in the H-band and with emission angles ≤60° are largely unaffected by the imaginary refractive index of haze particles, allowing a notable reduction of the free parameters required to fit the observations. Our analysis shows a total haze production rate of (2.61 ± 0.18) × 10−14 kg m−2 s−1, about 10 times larger than that found in Uranus's atmosphere, and a particle electric charge of q = 8.6 ± 1.1 electrons per μm radius at latitudes between 5 and 15° S. This haze production rate in Neptune results in haze optical depths about 10 times greater than those in Uranus. The effective radius reff was found to be 0.22 ± 0.01 and 0.26 ± 0.02 μm at the 0.1 and 1-bar levels, respectively, with haze number densities of 8.48+1.78−1.31 and 9.31+2.52−1.91 particles per cm3. The fit at weak methane-absorbing wavelengths reveals also the presence of a tropospheric cloud with a total optical depth >10 at 1.46 μm. The tropospheric cloud base altitude was found near the 2.5-bar level, although this estimation may be only representative of the top of a thicker and deeper cloud. Our analysis leads to haze opacities about 3.5 times larger than that derived from Voyager-2 observations (Moses et al., 1995). This larger opacity indicates a haze production rate 2 times larger at least. To study this difference haze opacity or production rate, we performed a timescale analysis with our microphysical model to estimate the time required for haze particles to grow and settle out. Although this analysis shows haze timescales (∼15 years) shorter than the time lapsed between Voyager-2 observations and 2013, the solar illumination at the top of the atmosphere has not varied significantly during this period (at the studied latitudes) to explain the increase in haze production. This difference in haze production rate derived for these two periods may arise from: a) the fact that in our analysis we employed spectral observations in the infrared (H-band), while Moses et al. (1995) used photometric images taken at 5 different filters in the visible. While high-phase-angle Voyager observations are more sensitive to small haze particles and at altitudes above the 0.1-bar level, the haze constraints derived from VLT spectra in H-band are limited to pressures greater than 0.1 bar. As a result of the different phase angles of the two set of observations, differences in the estimation of M0 may arise from the use of Mie phase functions as well. b) our 1-dimensional model does not account for latitudinal redistributions of the haze by dynamics. A possible meridional transport of haze with wind velocities greater than ∼0.03 m s−1 would result in dynamics timescales shorter than 15 years and thus might explain the observed variations in the haze production rate during this period. Compared with our estimations, photochemical models point to even larger production rates on Neptune (by a factor of 2.4). Assuming that the photochemical simulations are correct, we found that this discrepancy can be explained if haze particles evaporate before reaching the tropospheric-cloud levels. This scenario would decrease the cumulative haze opacity above the 1-bar level, and thus a larger haze production rate would be required to fit our observations. However, to validate this haze vertical structure future microphysical simulations that include the evaporation rates of haze particles are required.


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, J Taylor, SL Grimm, 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.


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

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.


Detection of Propadiene on Titan

ASTROPHYSICAL JOURNAL LETTERS 881 (2019) ARTN L33

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

ASTRONOMICAL JOURNAL 157 (2019) ARTN 219

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, R Giles, F Tabataba-Vakili, 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.


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

In this paper we present a study of the vertical haze and cloud structure over a triple vortex in Saturn's atmosphere in the planetographic latitude range 55°N-69°N (del Río-Gaztelurrutia et al., 2018) using HST/WFC3 multispectral imaging. The observations were taken during 29–30 June and 1 July 2015 at ten different filters covering spectral range from the 225 nm to 937 nm, including the deep methane band at 889 nm. Absolute reflectivity measurements of this region at all wavelengths and under a number of illumination and observation geometries are fitted with the values produced by a radiative transfer model. Most of the reflectivity variations in this wavelength range can be attributed to changes in the tropospheric haze. The anticyclones are optically thicker (τ ~25 vs ~10), more vertically extended (~3 gas scale heights vs ~2) and their bases are located deeper in the atmosphere (550 mbar vs 500 mbar) than the cyclone.


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>

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