Publications by Patrick Irwin

Dynamical implications of seasonal and spatial variations in Titan's stratospheric composition.

Philos Trans A Math Phys Eng Sci 367 (2009) 697-711

NA Teanby, PGJ Irwin, R de Kok, CA Nixon

Titan's diverse inventory of photochemically produced gases can be used as tracers to probe atmospheric circulation. Since the arrival of the Cassini-Huygens mission in July 2004 it has been possible to map the seasonal and spatial variations of these compounds in great detail. Here, we use 3.5 years of data measured by the Cassini Composite InfraRed Spectrometer instrument to determine spatial and seasonal composition trends, thus providing clues to underlying atmospheric motions. Titan's North Pole (currently in winter) displays enrichment of trace species, implying subsidence is occurring there. This is consistent with the descending branch of a single south-to-north stratospheric circulation cell and a polar vortex. Lack of enrichment in the south over most of the observed time period argues against the presence of any secondary circulation cell in the Southern Polar stratosphere. However, a residual cap of enriched gas was observed over the South Pole early in the mission, which has since completely dissipated. This cap was most probably due to residual build-up from southern winter. These observations provide new and important constraints for models of atmospheric photochemistry and circulation.

Retrievals of atmospheric variables on the gas giants from ground-based mid-infrared imaging

Icarus 200 (2009) 154-175

LN Fletcher, GS Orton, P Yanamandra-Fisher, BM Fisher, PD Parrish, PGJ Irwin

Thermal-infrared imaging of Jupiter and Saturn using the NASA/IRTF and Subaru observatories are quantitatively analyzed to assess the capabilities for reproducing and extending the zonal mean atmospheric results of the Cassini/CIRS experiment. We describe the development of a robust, systematic and reproducible approach to the acquisition and reduction of planetary images in the mid-infrared (7-25 μm), and perform an adaptation and validation of the optimal estimation, correlated-k retrieval algorithm described by Irwin et al. [Irwin, P., Teanby, N., de Kok, R., Fletcher, L., Howett, C., Tsang, C., Wilson, C., Calcutt, S., Nixon, C., Parrish, P., 2008. J. Quant. Spectrosc. Radiat. Trans. 109 (6), 1136-1150] for channel-integrated radiances. Synthetic spectral analyses and a comparison to Cassini results are used to verify our abilities to retrieve temperatures, haze opacities and gaseous abundances from filtered imaging. We find that ground-based imaging with a sufficiently high spatial resolution is able to reproduce the three-dimensional temperature and para-H2 fields measured by spacecraft visiting Jupiter and Saturn, allowing us to investigate vertical wind shear, pressure and, with measured cloud-top winds, Ertel potential vorticity on potential temperature surfaces. Furthermore, by scaling vertical profiles of NH3, PH3, haze opacity and hydrocarbons as free parameters during thermal retrievals, we can produce meridional results comparable with CIRS spectroscopic investigations. This paper demonstrates that mid-IR imaging instruments operating at ground-based observatories have access to several dynamical and chemical diagnostics of the atmospheric state of the gas giants, offering the prospect for quantitative studies over much longer baselines and often covering much wider areas than is possible from spaceborne platforms. © 2008 Elsevier Inc.

Spatial variability of carbon monoxide in venus' mesosphere from venus express/visible and infrared thermal imaging spectrometer measurements

Journal of Geophysical Research E: Planets 114 (2009)

PGJ Irwin, R De Kok, A Negrão, CCC Tsang, CF Wilson, P Drossart, G Piccioni, D Grassi, FW Taylor

[1] Observations of Venus' mesosphere by the Visible and Infrared Thermal Imaging Spectrometer (VIRTIS)-M instrument of Venus Express have been used to investigate the spatial distribution of CO above Venus' nightside cloud tops by fitting the CO absorption in the (1-0) CO band around 4.7 μm. We find little spatial variation in the abundance of CO at midlatitudes, with a retrieved abundance of approximately 40 ± 10 ppm just above the cloud tops between 65 and 70 km altitude. Unfortunately, we find it very difficult to constrain the abundance of CO in the cold polar collar, centered at about 70°S, as the retrieved temperature structure in the CO line-forming region masks the absorption lines. However, there is a possibility that CO increases toward the poles, as we detect a significant signature of high levels of CO over Venus' south polar dipole feature in all the observations analyzed so far. To constrain the abundance of CO more closely will require the analysis of higher-resolution VIRTIS-H observations. In addition, limb observations would greatly help to resolve any possible temperature/cloud ambiguities and allow us to assess vertical variations in the abundance of CO. Copyright 2008 by the American Geophysical Union.

Tropospheric carbon monoxide concentrations and variability on Venus from Venus Express/VIRTIS-M observations

Journal of Geophysical Research E: Planets 114 (2009)

CCC Tsang, PGJ Irwin, CF Wilson, FW Taylor, C Lee, R De Kok, P Drossart, G Piccioni, B Bezard, S Calcutt

[1] We present nightside observations of tropospheric carbon monoxide in the southern hemisphere near the 35 km height level, the first from Venus Express/Visible and Infrared Thermal Imaging Spectrometer (VIRTIS)-M-IR. VIRTIS-M data from 2.18 to 2.50 μm, with a spectral resolution of 10 nm, were used in the analysis. Spectra were binned, with widths ranging from 5 to 30 spatial pixels, to increase the signal-to-noise ratio, while at the same time reducing the total number of retrievals required for complete spatial coverage. We calculate the mean abundance for carbon monoxide at the equator to be 23 ± 2 ppm. The CO concentration increases toward the poles, peaking at a latitude of approximately 60°S, with a mean value of 32 ± 2 ppm. This 40% equator-to-pole increase is consistent with the values found by Collard et al. (1993) from Galileo/NIMS observations. Observations suggest an overturning in this CO gradient past 60°S, declining to abundances seen in the midlatitudes. Zonal variability in this peak value has also been measured, varying on the order of 10% (∼3 ppm) at different longitudes on a latitude circle. The zonal variability of the CO abundance has possible implications for the lifetime of CO and its dynamics in the troposphere. This work has definitively established a distribution of tropospheric CO, which is consistent with a Hadley cell circulation, and placed limits on the latitudinal extent of the cell. Copyright 2008 by the American Geophysical Union.

Methane and its isotopologues on Saturn from Cassini/CIRS observations

Icarus 199 (2009) 351-367

LN Fletcher, GS Orton, NA Teanby, PGJ Irwin, GL Bjoraker

High spectral resolution observations from the Cassini Composite Infrared Spectrometer [Flasar, F.M., and 44 colleagues, 2004. Space Sci. Rev. 115, 169-297] are analysed to derive new estimates for the mole fractions of CH4, CH3D and 13CH4 of (4.7 ± 0.2) × 10-3, (3.0 ± 0.2) × 10-7 and (5.1 ± 0.2) × 10-5 respectively. The mole fractions show no hemispherical asymmetries or latitudinal variability. The analysis combines data from the far-IR methane rotational lines and the mid-IR features of methane and its isotopologues, using both the correlated-k retrieval algorithm of Irwin et al. [Irwin, P., and 9 colleagues, 2008. J. Quant. Spectrosc. Radiat. Trans. 109, 1136-1150] and a line-by-line approach to evaluate the reliability of the retrieved quantities. C/H was found to be enhanced by 10.9 ± 0.5 times the solar composition of Grevesse et al. [Grevesse, N., Asplund, M., Sauval, A., 2007. Space Sci. Rev. 130 (1), 105-114], 2.25 ± 0.55 times larger than the enrichment on Jupiter, and supporting the increasing fractional core mass with distance from the Sun predicted by the core accretion model of planetary formation. A comparison of the jovian and saturnian C/N, C/S and C/P ratios suggests different reservoirs of the trapped volatiles in a primordial solar nebula whose composition varies with distance from the Sun. This is supported by our derived D/H ratio in methane of (1.6 ± 0.2) × 10-5, which appears to be smaller than the jovian value of Lellouch et al. [Lellouch, E., Bézard, B., Fouchet, T., Feuchtgruber, H., Encrenaz, T., de Graauw, T., 2001. Astron. Astrophys. 370, 610-622]. Mid-IR emission features provided an estimate of 12C / 13C = 91.8-7.8+8.4, which is consistent with both the terrestrial ratio and jovian ratio, suggesting that carbon was accreted from a shared reservoir for all of the planets. © 2008 Elsevier Inc.

Photometric changes on Saturn's Titan: Evidence for active cryovolcanism


RM Nelson, LW Kamp, RMC Lopes, DL Matson, RL Kirk, BW Hapke, SD Wall, MD Boryta, FE Leader, WD Smythe, KL Mitchell, KH Baines, R Jaumann, C Sotin, RN Clark, DP Cruikshank, P Drossart, JI Lunine, M Combes, G Bellucci, J-P Bibring, F Capaccioni, P Cerroni, A Coradini, V Formisano, G Filacchione, Y Langevin, TB McCord, V Mennella, PD Nicholson, B Sicardy, PGJ Irwin, JC Pearl

Saturn's Titan: Surface change, ammonia, and implications for atmospheric and tectonic activity

ICARUS 199 (2009) 429-441

RM Nelson, LW Kamp, DL Matson, PGJ Irwin, KH Baines, MD Boryta, FE Leader, R Jaumann, WD Smythe, C Sotin, RN Clark, DP Cruikshank, P Drossart, JC Pearl, BW Hapke, J Lunine, M Combes, G Bellucci, J-P Bibring, F Capaccioni, P Cerroni, A Coradini, V Formisano, G Filacchione, RY Langevin, TB McCord, V Mennella, PD Nicholson, B Sicardy

Photometric changes on Saturn's Titan: Evidence for active cryovolcanism

Geophysical Research Letters 36 (2009)

RM Nelson, LW Kamp, RMC Lopes, DL Matson, RL Kirk, BW Hapke, SD Wall, MD Boryta, FE Leader, WD Smythe, KL Mitchell, KH Baines, R Jaumann, C Sotin, RN Clark, DP Cruikshank, P Drossart, JI Lunine, M Combes, G Bellucci, JP Bibring, F Capaccioni, P Cerroni, A Coradini, V Formisano, G Filacchione, Y Langevin, TB McCord, V Mennella, PD Nicholson, B Sicardy, PGJ Irwin, JC Pearl

We report infrared spectrophotometric variability on the surface of Saturn's moon Titan detected in images returned by the Visual and Infrared Mapping Spectrometer (VIMS) onboard the Cassini Saturn Orbiter. The changes were observed at 7°S, 138°W and occurred between October 27, 2005 and January 15, 2006. After that date the surface was unchanged until the most recent observation, March 18, 2006. We previously reported spectrophotometric variability at another location (26°S, 78°W). Cassini Synthetic Aperture RADAR (SAR) images find that the surface morphology at both locations is consistent with surface flows possibly resulting from cryovolcanic activity (Wall et al., companion paper, this issue). The VIMS-reported time variability and SAR morphology results suggest that Titan currently exhibits intermittent surface changes consistent with present ongoing surface processes. We suggest that these processes involve material from Titan's interior being extruded or effiised and deposited on the surface, as might be expected from cryovolcanism. © 2009.

Infrared limb sounding of Titan with the Cassini Composite InfraRed Spectrometer: effects of the mid-IR detector spatial responses.

Appl Opt 48 (2009) 1912-1925

CA Nixon, NA Teanby, SB Calcutt, S Aslam, DE Jennings, VG Kunde, FM Flasar, PG Irwin, FW Taylor, DA Glenar, MD Smith

The composite infrared spectrometer (CIRS) instrument on board the Cassini Saturn orbiter employs two 1x10 HgCdTe detector arrays for mid-infrared remote sensing of Titan's and Saturn's atmospheres. In this paper we show that the real detector spatial response functions, as measured in ground testing before launch, differ significantly from idealized "boxcar" responses. We further show that neglecting this true spatial response function when modeling CIRS spectra can have a significant effect on interpretation of the data, especially in limb-sounding mode, which is frequently used for Titan science. This result has implications not just for CIRS data analysis but for other similar instrumental applications.

Giant Planets of Our Solar System

Springer Verlag, 2009

PGJ Irwin

This book reviews the current state of knowledge of the atmospheres of the giant gaseous planets: Jupiter, Saturn, Uranus, and Neptune.

Isotopic ratios in Titan's atmosphere from Cassini CIRS limb sounding: CO2 at low and midlatitudes


CA Nixon, DE Jennings, B Bezard, NA Teanby, RK Achterberg, A Coustenis, S Vinatier, PGJ Irwin, PN Romani, T Hewagama, FM Flasar

Evidence for anomalous cloud particles at the poles of Venus


CF Wilson, S Guerlet, PGJ Irwin, CCC Tsang, FW Taylor, RW Carlson, P Drossart, G Piccioni

Condensation in Titan's stratosphere during polar winter

ICARUS 197 (2008) 572-578

R de Kok, PGJ Irwin, NA Teanby

Band parameters for self-broadened ammonia gas in the range 0.74 to 5.24 mu m to support measurements of the atmosphere of the planet Jupiter

ICARUS 196 (2008) 612-624

N Bowles, S Calcutt, P Irwin, J Temple

Intense polar temperature inversion in the middle atmosphere on Mars

Nature Geoscience 1 (2008) 745-749

DJ McCleese, JT Schofield, FW Taylor, WA Abdou, O Aharonson, D Banfield, SB Calcutt, NG Heavens, PGJ Irwin, DM Kass, A Kleinböhl, WG Lawson, CB Leovy, SR Lewis, DA Paige, PL Read, MI Richardson, N Teanby, RW Zurek

Current understanding of weather, climate and global atmospheric circulation on Mars is incomplete, in particular at altitudes above about 30 km. General circulation models for Mars are similar to those developed for weather and climate forecasting on Earth and require more martian observations to allow testing and model improvements. However, the available measurements of martian atmospheric temperatures, winds, water vapour and airborne dust are generally restricted to the region close to the surface and lack the vertical resolution and global coverage that is necessary to shed light on the dynamics of Mars middle atmosphere at altitudes between 30 and 80 km (ref.7). Here we report high-resolution observations from the Mars Climate Sounder instrument on the Mars Reconnaissance Orbiter. These observations show an intense warming of the middle atmosphere over the south polar region in winter that is at least 10-20 K warmer than predicted by current model simulations. To explain this finding, we suggest that the atmospheric downwelling circulation over the pole, which is part of the equator-to-pole Hadley circulation, may be as much as 50 more vigorous than expected, with consequences for the cycles of water, dust and CO"2 that regulate the present-day climate on Mars. © 2008 Macmillan Publishers Limited.

Retrieval of air temperature profiles in the Venusian mesosphere from VIRTIS-M data: Description and validation of algorithms


D Grassi, P Drossart, G Piccioni, NI Ignatiev, LV Zasova, A Adriani, ML Moriconi, PGJ Irwin, A Negrao, A Migliorini

The <sup>12</sup>C/<sup>13</sup>C isotopic ratio in Titan hydrocarbons from Cassini/CIRS infrared spectra

Icarus 195 (2008) 778-791

CA Nixon, RK Achterberg, S Vinatier, B Bézard, A Coustenis, PGJ Irwin, NA Teanby, R de Kok, PN Romani, DE Jennings, GL Bjoraker, FM Flasar

We have analyzed infrared spectra of Titan recorded by the Cassini Composite Infrared Spectrometer (CIRS) to measure the isotopic ratio 12C/13C in each of three chemical species in Titan's stratosphere: CH4, C2H2 and C2H6. This is the first measurement of 12C/13C in any C2 molecule on Titan, and the first measurement of 12CH4/13CH4 (non-deuterated) on Titan by remote sensing. Our spectra cover five widely-spaced latitudes, 65° S to 71° N and we have searched for both latitude variability of 12C/13C within a given species, and also for differences between the 12C/13C in the three gases. For CH4 alone, we find 12C / 13C = 76.6 ± 2.7 (1-σ), essentially in agreement with the 12CH4/13CH4 measured by the Huygens Gas Chromatograph/Mass Spectrometer instrument (GCMS) [Niemann, H.B., and 17 colleagues, 2005. Nature 438, 779-784]: 82.3 ± 1.0, and also with measured values in H13CN and 13CH3D by CIRS at lower precision [Bézard, B., Nixon, C., Kleiner, I., Jennings, D., 2007. Icarus 191, 397-400; Vinatier, S., Bézard, B., Nixon, C., 2007. Icarus 191, 712-721]. For the C2 species, we find 12C / 13C = 84.8 ± 3.2 in C2H2 and 89.8 ± 7.3 in C2H6, a possible trend of increasingly value with molecular mass, although these values are both compatible with the Huygens GCMS value to within error bars. There are no convincing trends in latitude. Combining all fifteen measurements, we obtain a value of 12C / 13C = 80.8 ± 2.0, also compatible with GCMS. Therefore, the evidence is mounting that 12C/13C is some 8% lower on Titan than on the Earth (88.9, inorganic standard), and lower than typical for the outer planets (88 ± 7 [Sada, P.V., McCabe, G.H., Bjoraker, G.L., Jennings, D.E., Reuter, D.C., 1996. Astrophys. J. 472, 903-907]). There is no current model for this enrichment, and we discuss several mechanisms that may be at work. © 2008 Elsevier Inc. All rights reserved.

Titan's winter polar vortex structure revealed by chemical tracers


NA Teanby, R de Kok, PGJ Irwin, S Osprey, S Vinatier, PJ Gierasch, PL Read, FM Flasar, BJ Conrath, RK Achterberg, B Bezard, CA Nixon, SB Calcutt

Isotopic ratios in Titan's atmosphere from Cassini CIRS limb sounding: HC3N in the north


DE Jennings, CA Nixon, A Jolly, B Bezard, A Coustenis, S Vinatier, PGJ Irwin, NA Teanby, PN Romani, RK Achterberg, FM Flasar

The C-12/C-13 isotopic ratio in Titan hydrocarbons from Cassini/CIRS infrared spectra

ICARUS 195 (2008) 778-791

CA Nixon, RK Achterberg, S Vinatier, B Bezard, A Coustenis, PGJ Irwin, NA Teanby, R de Kok, PN Romani, DE Jennings, GL Bjoraker, FM Flasar