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Juno Jupiter image

Prof. Patrick Irwin

Professor of Planetary Physics

Research theme

  • Exoplanets and planetary physics

Sub department

  • Atmospheric, Oceanic and Planetary Physics

Research groups

  • Exoplanet atmospheres
  • Planetary atmosphere observation analysis
  • Solar system
patrick.irwin@physics.ox.ac.uk
Telephone: 01865 (2)72083
Atmospheric Physics Clarendon Laboratory, room 306
Personal research page
NEMESIS
  • About
  • Publications

Seasonal Evolution of Titan’s Stratospheric Tilt and Temperature Field at High Resolution from Cassini/CIRS

The Planetary Science Journal IOP Publishing 6:5 (2025) 114

Authors:

Lucy Wright, Nicholas A Teanby, Patrick GJ Irwin, Conor A Nixon, Nicholas A Lombardo, Juan M Lora, Daniel Mitchell

Abstract:

The Cassini spacecraft observed Titan from 2004 to 2017, capturing key atmospheric features, including the tilt of the middle atmosphere and the formation and breakup of winter polar vortices. We analyze low spectral resolution infrared observations from Cassini’s Composite Infrared Spectrometer (CIRS), which provide excellent spatial and temporal coverage and the best horizontal spatial resolution of any of the CIRS observations. With approximately 4 times higher meridional resolution than previous studies, we map the stratospheric temperature for almost half a Titan year. We determine the evolution of Titan’s stratospheric tilt, finding that it is most constant in the inertial frame, directed 120° ± 6° west of the Titan–Sun vector at the northern spring equinox, with seasonal oscillations in the tilt magnitude between around 2 .° 5 and 8°. Using the high meridional resolution temperature field, we reveal finer details in the zonal wind and potential vorticity. In addition to the strong winter zonal jet, a weaker zonal jet in Titan’s summer hemisphere is observed, and there is a suggestion that the main winter hemisphere jet briefly splits into two. We also present the strongest evidence yet that Titan’s polar vortex is annular for part of its life cycle.
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The bolometric Bond albedo and energy balance of Uranus

Monthly Notices of the Royal Astronomical Society Oxford University Press (OUP) (2025)

Authors:

Patrick GJ Irwin, Daniel D Wenkert, Amy A Simon, Emma Dahl, Heidi B Hammel

Abstract:

<jats:title>Abstract</jats:title> <jats:p>Using a newly developed ‘holistic’ atmospheric model of the aerosol structure in Uranus’s atmosphere, based upon observations made by HST/STIS, Gemini/NIFS and IRTF/SpeX from 2000 – 2009, we make a new estimate the bolometric Bond albedo of Uranus during this time of A* = 0.338 ± 0.011, with a phase integral of q* = 1.36 ± 0.03. Then, using a simple seasonal model, developed to be consistent with the disc-integrated blue and green magnitude data from the Lowell Observatory from 1950 – 2016, we model how Uranus’s reflectivity and heat budget vary during its orbit and determine new orbital-mean average values for the bolometric Bond albedo of $\overline{A^*} = 0.349 \pm 0.016$ and for the absorbed solar flux of $\overline{P_\mathrm{in}}=0.604 \pm 0.027$ W m−2. Assuming the outgoing thermal flux to be $\overline{P_\mathrm{out}}=0.693 \pm 0.013$ W m−2, as previously determined from Voyager 2 observations, we arrive at a new estimate of Uranus’s average heat flux budget of Pout/Pin = 1.15 ± 0.06, finding considerable variation with time due to Uranus’s significant orbital eccentricity of 0.046. This leads the flux budget to vary from Pout/Pin = 1.03 near perihelion, to 1.24 near aphelion. We conclude that although Pout/Pin is considerably smaller than for the other giant planets, Uranus is not in thermal equilibrium with the Sun.</jats:p>
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The atmosphere of Titan in late northern summer from JWST and Keck observations

Nature Astronomy Springer Nature (2025)

Authors:

Conor A Nixon, Bruno Bézard, Thomas Cornet, Brandon Park Coy, Imke de Pater, Maël Es-Sayeh, Heidi B Hammel, Emmanuel Lellouch, Nicholas A Lombardo, Manuel López-Puertas, Juan M Lora, Pascal Rannou, Sébastien Rodriguez, Nicholas A Teanby, Elizabeth P Turtle, Richard K Achterberg, Carlos Alvarez, Ashley G Davies, Katherine de Kleer, Greg Doppmann, Leigh N Fletcher, Alexander G Hayes, Bryan J Holler, Patrick GJ Irwin, Carolyn Jordan, Oliver RT King, Nicholas W Kutsop, Theresa C Marlin, Henrik Melin, Stefanie N Milam, Edward M Molter, Luke Moore, Yaniss Nyffenegger-Péré, James O’Donoghue, John O’Meara, Scot CR Rafkin, Michael T Roman, Arina Rostopchina, Naomi Rowe-Gurney, Carl Schmidt, Judy Schmidt, Christophe Sotin, Tom S Stallard, John A Stansberry, Robert A West

Abstract:

Saturn’s moon Titan undergoes a long annual cycle of 29.45 Earth years. Titan’s northern winter and spring were investigated in detail by the Cassini–Huygens spacecraft (2004–2017), but the northern summer season remains sparsely studied. Here we present new observations from the James Webb Space Telescope (JWST) and Keck II telescope made in 2022 and 2023 during Titan’s late northern summer. Using JWST’s mid-infrared instrument, we spectroscopically detected the methyl radical, the primary product of methane break-up and key to the formation of ethane and heavier molecules. Using the near-infrared spectrograph onboard JWST, we detected several non-local thermodynamic equilibrium CO and CO2 emission bands, which allowed us to measure these species over a wide altitude range. Lastly, using the near-infrared camera onboard JWST and Keck II, we imaged northern hemisphere tropospheric clouds evolving in altitude, which provided new insights and constraints on seasonal convection patterns. These observations pave the way for new observations and modelling of Titan’s climate and meteorology as it progresses through the northern fall equinox, when its atmosphere is expected to show notable seasonal changes.
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Improved Carbon and Nitrogen Isotopic Ratios for CH 3 CN in Titan’s Atmosphere Using ALMA

The Planetary Science Journal IOP Publishing 6:5 (2025) 107

Authors:

Jonathon Nosowitz, Martin A Cordiner, Conor A Nixon, Alexander E Thelen, Zbigniew Kisiel, Nicholas A Teanby, Patrick GJ Irwin, Steven B Charnley, Véronique Vuitton

Abstract:

Titan, Saturn’s largest satellite, maintains an atmosphere composed primarily of nitrogen (N2) and methane (CH4) that leads to complex organic chemistry. Some of the nitriles (CN-bearing organics) on Titan are known to have substantially enhanced 15N abundances compared to Earth and Titan’s dominant nitrogen (N2) reservoir. The 14N/15N isotopic ratio in Titan’s nitriles can provide better constraints on the synthesis of nitrogen-bearing organics in planetary atmospheres as well as insights into the origin of Titan’s large nitrogen abundance. Using high signal-to-noise ratio (>13), disk-integrated observations obtained with the Atacama Large Millimeter/submillimeter Array Band 6 receiver (211–275 GHz), we measure the 14N/15N and 12C/13C isotopic ratios of acetonitrile (CH3CN) in Titan’s stratosphere. Using the NEMESIS, we derived the CH3CN/13CH3CN ratio to be 89.2 ± 7.0 and the CH3CN/CH313CN ratio to be 91.2 ± 6.0, in agreement with the 12C/13C ratio in Titan’s methane and other solar system species. We found the 14N/15N isotopic ratio to be 68.9 ± 4.2, consistent with previously derived values for HCN and HC3N, confirming an enhanced 15N abundance in Titan’s nitriles compared with the bulk atmospheric N2 value of 14N/15N = 168, in agreement with chemical models incorporating isotope-selective photodissociation of N2 at high altitudes.
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A Search for the Near‐Surface Particulate Layer Using Venera 13 In Situ Spectroscopic Observations

Journal of Geophysical Research: Planets American Geophysical Union 130:4 (2025) e2024JE008728

Authors:

Shubham V Kulkarni, Patrick GJ Irwin, Colin F Wilson, Nikolai I Ignatiev

Abstract:

Whether or not there is a particulate layer in the lowest 10 km of the Venusian atmosphere is still an open question. Some of the past in situ experiments showed the presence of a detached particulate layer, and a few suggested the existence of finely dispersed aerosols, while other instruments supported the idea of no particulate matter in the deep atmosphere. In this work, we investigate the presence of a near‐surface particulate layer (NSPL) using in situ data from the Venera 13 mission. While the original spectrophotometric data from Venera 13 were lost, we have reconstructed a part of this data by digitizing the old graphic material and selected the eight most reliable Venera 13 downward radiance profiles from 0.48 to 0.8 μ ${\upmu }$ m for our retrievals. The retrievals suggest the existence of the particulate layer with a peak in the altitude range of 3.5–5 km. They further indicate a log‐normal particle size distribution with a mean radius between 0.6 and 0.85 μ ${\upmu }$ m. The retrievals constrain the real refractive index of the particles to lie around the range of 1.4–1.6, with the imaginary refractive index of a magnitude of 10 − 3 ${10}^{-3}$ . Based on refractive index retrievals, uplifted basalt particles or volcanic ash could be responsible for near‐surface particulates. In comparison, volatile condensates appear less likely to be behind the formation of NSPL.
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