Publications by Patrick Irwin


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.


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.


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

© 2018 On Neptune, carbon monoxide and phosphine are disequilibrium species, and their abundance profiles can provide insights into interior processes and the external space environment. Here we use Herschel/SPIRE (Spectral and Photometric Imaging REceiver) observations from 14.9–51.5 cm-1to obtain abundances from multiple CO and PH3spectral features. For CO, we find that nine CO bands can be simultaneously fitted using a step profile with a 0.22 ppm tropospheric abundance, a 1.03 ppm stratospheric abundance, and a step transition pressure of 0.11 bar near the tropopause. This is in broad agreement with previous studies. However, we also find that the CO spectral features could be fitted, to well within measurement errors, with a profile that contains no tropospheric CO for pressure levels deeper than 0.5 bar, which is our preferred interpretation. This differs from previous studies that have assumed CO is well mixed throughout the troposphere, which would require an internal CO source to explain and a high O/H enrichment. Our interpretation removes the requirement for extreme interior O/H enrichment in thermochemical models and can finally reconcile D/H and CO measurements. If true, the lack of lower tropospheric CO would imply a decrease in Neptune's interior water content, favouring a silicate-rich instead of an ice-rich interior. This would be consistent with a protoplanetary ice source with a similar D/H ratio to the current solar system comet population. The upper tropospheric and stratospheric CO at pressures less than 0.5 bar could then be entirely externally sourced from a giant impact as suggested by Lellouch et al.(2005). We also derive a 3-σ upper limit for PH3of 1.1 ppb at 0.4–0.8 bar. This is the most stringent upper limit to-date and is entirely consistent with predictions from a simple photochemical model.


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 Bezard


A chemical survey of exoplanets with ARIEL

EXPERIMENTAL ASTRONOMY 46 (2018) 135-209

G Tinetti, P Drossart, P Eccleston, P Hartogh, A Heske, J Leconte, G Micela, M Ollivier, G Pilbratt, L Puig, D Turrini, B Vandenbussche, P Wolkenberg, J-P Beaulieu, LA Buchave, M Ferus, M Griffin, M Guedel, K Justtanont, P-O Lagage, P Machado, G Malaguti, M Min, HU Norgaard-Nielsen, M Rataj, T Ray, I Ribas, M Swain, R Szabo, S Werner, J Barstow, M Burleigh, J Cho, VC du Foresto, A Coustenis, L Decin, T Encrenaz, M Galand, M Gillon, R Helled, J Carlos Morales, AG Munoz, A Moneti, I Pagano, E Pascale, G Piccioni, D Pinfield, S Sarkar, F Selsis, J Tennyson, A Triaud, O Venot, I Waldmann, D Waltham, G Wright, J Amiaux, J-L Augueres, M Berthe, N Bezawada, G Bishop, N Bowles, D Coffey, J Colome, M Crook, P-E Crouzet, V Da Peppo, IE Sanz, M Focardi, M Frericks, T Hunt, R Kohley, K Middleton, G Morgante, R Ottensamer, E Pace, C Pearson, R Stamper, K Symonds, M Rengel, E Renotte, P Ade, L Affer, C Alard, N Allard, F Altieri, Y Andre, C Arena, I Argyriou, A Aylward, C Baccani, G Bakos, M Banaszkiewicz, M Barlow, V Batista, G Bellucci, S Benatti, P Bernardi, B Bezard, M Blecka, E Bolmont, B Bonfond, R Bonito, AS Bonomo, JR Brucato, AS Brun, I Bryson, W Bujwan, S Casewell, B Charnay, CC Pestellini, G Chen, A Ciaravella, R Claudi, R Cledassou, M Damasso, M Damiano, C Danielski, P Deroo, AM Di Giorgio, C Dominik, V Doublier, S Doyle, R Doyon, B Drummond, B Duong, S Eales, B Edwards, M Farina, E Flaccomio, L Fletcher, F Forget, S Fossey, M Fraenz, Y Fujii, A Garcia-Piquer, W Gear, H Geoffray, JC Gerard, L Gesa, H Gomez, R Graczyk, C Griffith, D Grodent, MG Guarcello, J Gustin, K Hamano, P Hargrave, Y Hello, K Heng, E Herrero, A Hornstrup, B Hubert, S Ida, M Ikoma, N Iro, P Irwin, C Jarchow, J Jaubert, H Jones, Q Julien, S Kameda, F Kerschbaum, P Kervella, T Koskinen, M Krijger, N Krupp, M Lafarga, F Landini, E Lellouch, G Leto, A Luntzer, T Rank-Luftinger, A Maggio, J Maldonado, J-P Maillard, U Mall, J-B Marquette, S Mathis, P Maxted, T Matsuo, A Medvedev, Y Miguel, V Minier, G Morello, A Mura, N Narita, V Nascimbeni, N Nguyen Tong, V Noce, F Oliva, E Palle, P Palmer, M Pancrazzi, A Papageorgiou, V Parmentier, M Perger, A Petralia, S Pezzuto, R Pierrehumbert, I Pillitteri, G Piotto, G Pisano, L Prisinzano, A Radioti, J-M Reess, L Rezac, M Rocchetto, A Rosich, N Sanna, A Santerne, G Savini, G Scandariato, B Sicardy, C Sierra, G Sindoni, K Skup, I Snellen, M Sobiecki, L Soret, A Sozzetti, A Stiepen, A Strugarek, J Taylor, W Taylor, L Terenzi, M Tessenyi, A Tsiaras, C Tucker, D Valencia, G Vasisht, A Vazan, F Vilardell, S Vinatier, S Viti, R Waters, P Wawer, A Wawrzaszek, A Whitworth, YL Yung, SN Yurchenko, MR Zapatero Osorio, R Zellem, T Zingales, F Zwart


Abundance Measurements of Titan’s Stratospheric HCN, HC3N, C3H4, and CH3CN from ALMA Observations

Icarus (2018)

A Thelen, C Nixon, C Nancy, M Cordiner, E Molter, N Teanby, PG IRWIN, J Serigano, S Charnley


A hexagon in Saturn's northern stratosphere surrounding the emerging summertime polar vortex.

Nature communications 9 (2018) 3564-

LN Fletcher, GS Orton, JA Sinclair, S Guerlet, PL Read, A Antuñano, RK Achterberg, FM Flasar, PGJ Irwin, GL Bjoraker, J Hurley, BE Hesman, M Segura, N Gorius, A Mamoutkine, SB Calcutt

Saturn's polar stratosphere exhibits the seasonal growth and dissipation of broad, warm vortices poleward of ~75° latitude, which are strongest in the summer and absent in winter. The longevity of the exploration of the Saturn system by Cassini allows the use of infrared spectroscopy to trace the formation of the North Polar Stratospheric Vortex (NPSV), a region of enhanced temperatures and elevated hydrocarbon abundances at millibar pressures. We constrain the timescales of stratospheric vortex formation and dissipation in both hemispheres. Although the NPSV formed during late northern spring, by the end of Cassini's reconnaissance (shortly after northern summer solstice), it still did not display the contrasts in temperature and composition that were evident at the south pole during southern summer. The newly formed NPSV was bounded by a strengthening stratospheric thermal gradient near 78°N. The emergent boundary was hexagonal, suggesting that the Rossby wave responsible for Saturn's long-lived polar hexagon-which was previously expected to be trapped in the troposphere-can influence the stratospheric temperatures some 300 km above Saturn's clouds.


Detectability of Biosignatures in Anoxic Atmospheres with the James Webb Space Telescope: A TRAPPIST-1e Case Study

ASTRONOMICAL JOURNAL 156 (2018) ARTN 114

J Krissansen-Totton, R Garland, P Irwin, DC Catling


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


Uranus's Northern Polar Cap in 2014

GEOPHYSICAL RESEARCH LETTERS 45 (2018) 5329-5335

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


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

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY 470 (2017) 742-754

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

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