Publications by Patrick Irwin


Assessing the long-term variability of acetylene and ethane in the stratosphere of Jupiter

ICARUS 305 (2018) 301-313

H Melin, LN Fletcher, PT Donnelly, TK Greathouse, JH Lacy, GS Orton, RS Giles, JA Sinclair, PGJ Irwin


Haze and cloud structure of Saturn's North Pole and Hexagon Wave from Cassini/ISS imaging

Icarus (2018)

JF Sanz-Requena, S Pérez-Hoyos, A Sánchez-Lavega, A Antuñano, PGJ Irwin

© 2017 Elsevier Inc. In this paper we present a study of the vertical haze and cloud structure in the upper two bars of Saturn's Northern Polar atmosphere using the Imaging Science Subsystem (ISS) instrument onboard the Cassini spacecraft. We focus on the characterization of latitudes from 53° to 90° N. The observations were taken during June 2013 with five different filters (VIO, BL1, MT2, CB2 and MT3) covering spectral range from the 420 nm to 890 nm (in a deep methane absorption band). Absolute reflectivity measurements of seven selected regions at all wavelengths and several illumination and observation geometries are compared with the values produced by a radiative transfer model. The changes in reflectivity at these latitudes are mostly attributed to changes in the tropospheric haze. This includes the haze base height (from 600 ± 200 mbar at the lowest latitudes to 1000 ± 300 mbar in the pole), its particle number density (from 20 ± 2 particles/cm 3 to 2 ± 0.5 particles/cm 3 at the haze base) and its scale height (from 18 ± 0.1 km to 50 ± 0.1 km). We also report variability in the retrieved particle size distribution and refractive indices. We find that the Hexagonal Wave dichotomizes the studied stratospheric and tropospheric hazes between the outer, equatorward regions and the inner, Polar Regions. This suggests that the wave or the jet isolates the particle distribution at least at tropospheric levels.


The Origin of Titan's External Oxygen: Further Constraints from ALMA Upper Limits on CS and CH2NH

ASTRONOMICAL JOURNAL 155 (2018) ARTN 251

NA Teanby, MA Cordiner, CA Nixon, PGJ Irwin, SM Horst, M Sylvestre, J Serigano, AE Thelen, AMS Richards, SB Charnley


LRG-BEASTS III: ground-based transmission spectrum of the gas giant orbiting the cool dwarf WASP-80

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY 474 (2018) 876-885

J Kirk, PJ Wheatley, T Louden, I Skillen, GW King, J McCormac, PGJ Irwin


Spatial variations in Titan's atmospheric temperature: ALMA and Cassini comparisons from 2012 to 2015

Icarus (2018)

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

© 2017 Elsevier Inc. Submillimeter emission lines of carbon monoxide (CO) in Titan's atmosphere provide excellent probes of atmospheric temperature due to the molecule's long chemical lifetime and stable, well constrained volume mixing ratio. Here we present the analysis of 4 datasets obtained with the Atacama Large Millimeter/Submillimeter Array (ALMA) in 2012, 2013, 2014, and 2015 that contain strong CO rotational transitions. Utilizing ALMA's high spatial resolution in the 2012, 2014, and 2015 observations, we extract spectra from 3 separate regions on Titan's disk using datasets with beam sizes ranging from 0.35 × 0.28'' to 0.39 × 0.34''. Temperature profiles retrieved by the NEMESIS radiative transfer code are compared to Cassini Composite Infrared Spectrometer (CIRS) and radio occultation science results from similar latitude regions. Disk-averaged temperature profiles stay relatively constant from year to year, while small seasonal variations in atmospheric temperature are present from 2012 to 2015 in the stratosphere and mesosphere (~100-500 km) of spatially resolved regions. We measure the stratopause (320 km) to increase in temperature by 5 K in northern latitudes from 2012 to 2015, while temperatures rise throughout the stratosphere at lower latitudes. We observe generally cooler temperatures in the lower stratosphere (~100 km) than those obtained through Cassini radio occultation measurements, with the notable exception of warming in the northern latitudes and the absence of previous instabilities; both of these results are indicators that Titan's lower atmosphere responds to seasonal effects, particularly at higher latitudes. While retrieved temperature profiles cover a range of latitudes in these observations, deviations from CIRS nadir maps and radio occultation measurements convolved with the ALMA beam-footprint are not found to be statistically significant, and discrepancies are often found to be less than 5 K throughout the atmosphere. ALMA's excellent sensitivity in the lower stratosphere (60-300 km) provides a highly complementary dataset to contemporary CIRS and radio science observations, including altitude regions where both of those measurement sets contain large uncertainties. The demonstrated utility of CO emission lines in the submillimeter as a tracer of Titan's atmospheric temperature lays the groundwork for future studies of other molecular species - particularly those that exhibit strong polar abundance enhancements or are pressure-broadened in the lower atmosphere, as temperature profiles are found to consistently vary with latitude in all three years by up to 15 K.


Venus Upper Clouds and the UV Absorber From MESSENGER/MASCS Observations

JOURNAL OF GEOPHYSICAL RESEARCH-PLANETS 123 (2018) 145-162

S Perez-Hoyos, A Sanchez-Lavega, A Garcia-Munoz, PGJ Irwin, J Peralta, G Holsclaw, WM McClintock, JF Sanz-Requena


Retrieval of H<inf>2</inf>O abundance in Titan's stratosphere: A (re)analysis of CIRS/Cassini and PACS/Herschel observations

Icarus 311 (2018) 288-305

S Bauduin, PGJ Irwin, E Lellouch, V Cottini, R Moreno, CA Nixon, NA Teanby, T Ansty, FM Flasar

© 2018 Elsevier Inc. Since its first measurement 20 years ago by the Infrared Space Observatory (ISO), the water (H 2 O) mole fraction in Titan's stratosphere remains uncertain due to large differences between the determinations from available measurements. More particularly, the recent measurements made from the Herschel observatory (PACS and HIFI) estimated the H 2 O mole fraction to be 0.023 ppb at 12.1 mbar. A mixing ratio of 0.14 ppb at 10.7 mbar was, however, retrieved from nadir spatially-resolved observations of Cassini/CIRS. At the same pressure level (10.7 mbar), this makes a difference of a factor of 5.5 between PACS and CIRS measurements, and this has notably prevented current models from fully constraining the oxygen flux flowing into Titan's atmosphere. In this work, we try to understand the differences between the H 2 O mole fractions estimated from Herschel/PACS and Cassini/CIRS observations. The strategy for this is to 1) analyse recent disc-averaged observations of CIRS to investigate if the observation geometry could explain the previous observed differences, and 2) (re)analyse the three types of observation with the same retrieval scheme to assess if previous differences in retrieval codes/methodology could be responsible for the previous discrepancies. With this analysis, we show that using the same retrieval method better reconcile the previous measurements of these instruments. However, the addition of the disc-averaged CIRS observations, instead of confirming the consistency between the different datasets, reveals discrepancies between one of the CIRS disc-averaged set of observations and PACS measurements. This raises new questions regarding the possibility of latitudinal variations of H 2 O, which could be triggered by seasonal changes of the meridional circulation. As it has already been shown for nitriles and hydrocarbons, this circulation could potentially impact the latitudinal distribution of H 2 O through the subsidence or upwelling of air rich in H 2 O. The possible influence of spatial/time variations of the OH/H 2 O input flux in Titan's atmosphere is also discussed. The analysis of more observations will be needed in future work to address the questions arising from this work and to improve the understanding of the sources of H 2 O in Titan's atmosphere.


Jupiter's auroral-related stratospheric heating and chemistry II: Analysis of IRTF-TEXES spectra measured in December 2014

ICARUS 300 (2018) 305-326

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


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

ICARUS 302 (2018) 426-436

PGJ Irwin, N Bowles, AS Braude, R Garland, S Calcutt


Detection of hydrogen sulfide above the clouds in Uranus's atmosphere

Nature Astronomy 2 (2018) 420-427

PGJ Irwin, D Toledo, R Garland, NA Teanby, LN Fletcher, GA Orton, B Bézard

© 2018 The Author(s). Visible-to-near-infrared observations indicate that the cloud top of the main cloud deck on Uranus lies at a pressure level of between 1.2 bar and 3 bar. However, its composition has never been unambiguously identified, although it is widely assumed to be composed primarily of either ammonia or hydrogen sulfide (H 2 S) ice. Here, we present evidence of a clear detection of gaseous H 2 S above this cloud deck in the wavelength region 1.57-1.59 μm with a mole fraction of 0.4-0.8 ppm at the cloud top. Its detection constrains the deep bulk sulfur/nitrogen abundance to exceed unity ( > 4.4-5.0 times the solar value) in Uranus's bulk atmosphere, and places a lower limit on the mole fraction of H 2 S below the observed cloud of (1.0-2.5) × 10 -5 . The detection of gaseous H 2 S at these pressure levels adds to the weight of evidence that the principal constituent of 1.2-3-bar cloud is likely to be H 2 S ice.


Seasonal evolution of C2N2, C3H4, and C4H2 abundances in Titan's lower stratosphere

ASTRONOMY & ASTROPHYSICS 609 (2018) ARTN A64

M Sylvestre, NA Teanby, S Vinatier, S Lebonnois, PGJ Irwin


The formation and evolution of Titan's winter polar vortex.

Nature communications 8 (2017) 1586-1586

NA Teanby, B Bézard, S Vinatier, M Sylvestre, CA Nixon, PGJ Irwin, RJ de Kok, SB Calcutt, FM Flasar

Saturn's largest moon Titan has a substantial nitrogen-methane atmosphere, with strong seasonal effects, including formation of winter polar vortices. Following Titan's 2009 northern spring equinox, peak solar heating moved to the northern hemisphere, initiating south-polar subsidence and winter polar vortex formation. Throughout 2010-2011, strengthening subsidence produced a mesospheric hot-spot and caused extreme enrichment of photochemically produced trace gases. However, in 2012 unexpected and rapid mesospheric cooling was observed. Here we show extreme trace gas enrichment within the polar vortex dramatically increases mesospheric long-wave radiative cooling efficiency, causing unusually cold temperatures 2-6 years post-equinox. The long time-frame to reach a stable vortex configuration results from the high infrared opacity of Titan's trace gases and the relatively long atmospheric radiative time constant. Winter polar hot-spots have been observed on other planets, but detection of post-equinox cooling is so far unique to Titan.


Ammonia in Jupiter's Troposphere From High-Resolution 5 mu m Spectroscopy

GEOPHYSICAL RESEARCH LETTERS 44 (2017) 10838-10844

RS Giles, LN Fletcher, PGJ Irwin, GS Orton, JA Sinclair


D/H Ratios on Saturn and Jupiter from Cassini CIRS

ASTRONOMICAL JOURNAL 154 (2017) ARTN 178

JDR Pierel, CA Nixon, E Lellouch, LN Fletcher, GL Bjoraker, RK Achterberg, B Bezard, BE Hesman, PGJ Irwin, FM Flasar


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

ICARUS 289 (2017) 254-269

RS Giles, LN Fletcher, PGJ Irwin


HST/WFC3 observations of Uranus' 2014 storm clouds and comparison with VLT/SINFONI and IRTF/Spex observations

ICARUS 288 (2017) 99-119

PGJ Irwin, MH Wong, AA Simon, GS Orton, D Toledo


Mapping Vinyl Cyanide and Other Nitriles in Titan's Atmosphere Using ALMA

ASTRONOMICAL JOURNAL 154 (2017) ARTN 206

JC-Y Lai, MA Cordiner, CA Nixon, RK Achterberg, EM Molter, NA Teanby, MY Palmer, SB Chamley, JE Lindberg, Z Kisiel, MJ Mumma, PGJ Irwin


Climatological variations in Titan's atmospheric chemistry mapped using ALMA

ABSTRACTS OF PAPERS OF THE AMERICAN CHEMICAL SOCIETY 254 (2017)

M Cordiner, C Nixon, S Charnley, N Teanby, Z Kisiel, P Irwin, M Palmer, J Lai, X Thelen, V Vuitton


Jupiter's para-H-2 distribution from SOFIA/FORCAST and Voyager/IRIS 17-37 mu m spectroscopy

ICARUS 286 (2017) 223-240

LN Fletcher, I de Pater, WT Reach, M Wong, GS Orton, PGJ Irwin, RD Gehrz


A CONSISTENT RETRIEVAL ANALYSIS OF 10 HOT JUPITERS OBSERVED IN TRANSMISSION

ASTROPHYSICAL JOURNAL 834 (2017) ARTN 50

JK Barstow, S Aigrain, PGJ Irwin, DK Sing

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