Publications by Patrick 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.

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

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


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-

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.

A precise optical transmission spectrum of the inflated exoplanet WASP-52b


T Louden, PJ Wheatley, PGJ Irwin, J Kirk, I Skillen

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

Icarus (2017)

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.

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


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

D/H Ratios on Saturn and Jupiter from Cassini CIRS


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


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

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

Disruption of Saturn's quasi-periodic equatorial oscillation by the great northern storm

NATURE ASTRONOMY 1 (2017) 765-770

LN Fletcher, S Guerlet, GS Orton, RG Cosentino, T Fouchet, PGJ Irwin, L Li, FM Flasar, N Gorius, R Morales-Juberias

Jupiter's auroral-related stratospheric heating and chemistry I: Analysis of Voyager-IRIS and Cassini-CIRS spectra

ICARUS 292 (2017) 182-207

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

Moist convection and the 2010-2011 revival of Jupiter's South Equatorial Belt

ICARUS 286 (2017) 94-117

LN Fletcher, GS Orton, JH Rogers, RS Giles, AV Payne, PGJ Irwin, M Vedovato

Independent evolution of stratospheric temperatures in Jupiter's northern and southern auroral regions from 2014 to 2016


JA Sinclair, GS Orton, TK Greathouse, LN Fletcher, C Tao, GR Gladstone, A Adriani, W Dunn, JI Moses, V Hue, PGJ Irwin, H Melin, RS Giles

ALMA detection and astrobiological potential of vinyl cyanide on Titan.

Science advances 3 (2017) e1700022-

MY Palmer, MA Cordiner, CA Nixon, SB Charnley, NA Teanby, Z Kisiel, PGJ Irwin, MJ Mumma

Recent simulations have indicated that vinyl cyanide is the best candidate molecule for the formation of cell membranes/vesicle structures in Titan's hydrocarbon-rich lakes and seas. Although the existence of vinyl cyanide (C2H3CN) on Titan was previously inferred using Cassini mass spectrometry, a definitive detection has been lacking until now. We report the first spectroscopic detection of vinyl cyanide in Titan's atmosphere, obtained using archival data from the Atacama Large Millimeter/submillimeter Array (ALMA), collected from February to May 2014. We detect the three strongest rotational lines of C2H3CN in the frequency range of 230 to 232 GHz, each with >4σ confidence. Radiative transfer modeling suggests that most of the C2H3CN emission originates at altitudes of ≳200 km, in agreement with recent photochemical models. The vertical column densities implied by our best-fitting models lie in the range of 3.7 × 1013 to 1.4 × 1014 cm-2. The corresponding production rate of vinyl cyanide and its saturation mole fraction imply the availability of sufficient dissolved material to form ~107 cell membranes/cm3 in Titan's sea Ligeia Mare.



JK Barstow, S Aigrain, PGJ Irwin, DK Sing

Rayleigh scattering in the transmission spectrum of HAT-P-18b


J Kirk, PJ Wheatley, T Louden, AP Doyle, I Skillen, J McCormac, PGJ Irwin, R Karjalainen