Publications by Patrick Irwin

Oxygen compounds in Titan's stratosphere as observed by Cassini CIRS

Icarus 186 (2007) 354-363

FW Taylor, de Kok R, Irwin PGJ, Teanby NA

Latitudinal variations in Uranus' vertical cloud structure from ukirt UIST observations

Astrophysical Journal 665 (2007)

PGJ Irwin, NA Teanby, GR Davis

Prior to Uranus' northern spring equinox in 2007 December, near-infrared spectra and images of Uranus were obtained in 2006 August and September using the UIST instrument on the United Kingdom Infrared Telescope (UKIRT). Latitudinally resolved spectra were recorded between 1 and 2.5 μm at a resolving power varying between 550 and 2000 with the instrument in long slit mode and with the slit aligned with the planet's central meridian to determine the north-south variations of Uranus' cloud structure. Our observations appear to be the first latitudinally resolved complete (1-2.5 μm) near-IR spectra of Uranus ever recorded, and we present initial determinations of the latitudinal variation of Uranus' vertical cloud structure from these data. We find two main cloud decks of similar density, one based near the 2 bar level and one based in the 8-10 bar region. The upper cloud is found to extend from 50° south to 45° north, increasing in thickness and altitude toward the south, especially in the south circumpolar collar at 45° south, but clearing toward the poles. However, we find that the deeper cloud layer is thickest at the equator and thins symmetrically toward both poles. We also report the firstever observation that the bright south circumpolar collar is, at some wavelengths, actually darker than other latitudes and provide an explanation of this phenomenon in terms of the latitudinal variations in cloud structure. © 2007. The American Astronomical Society.

Meridional variations of C<inf>2</inf>H<inf>2</inf> and C<inf>2</inf>H<inf>6</inf> in Jupiter's atmosphere from Cassini CIRS infrared spectra

Icarus 188 (2007) 47-71

CA Nixon, RK Achterberg, BJ Conrath, PGJ Irwin, NA Teanby, T Fouchet, PD Parrish, PN Romani, M Abbas, A LeClair, D Strobel, AA Simon-Miller, DJ Jennings, FM Flasar, VG Kunde

Hydrocarbons such as acetylene (C2H2) and ethane (C2H6) are important tracers in Jupiter's atmosphere, constraining our models of the chemical and dynamical processes. However, our knowledge of the vertical and meridional variations of their abundances has remained sparse. During the flyby of the Cassini spacecraft in December 2000, the Composite Infrared Spectrometer (CIRS) instrument was used to map the spatial variation of emissions from 10 to 1400 cm-1 (1000-7 μm). In this paper we analyze a zonally averaged set of CIRS spectra taken at the highest (0.48 cm-1) resolution, firstly to infer atmospheric temperatures in the stratosphere at 0.5-20 mbar via the ν4 band of CH4, and in the troposphere at 150-400 mbar, via the H2 absorption at 600-800 cm-1. Stratospheric temperatures at 5 mbar are generally warmer in the north than the south by 7-8 K, while tropospheric temperatures show no such asymmetry. Both latitudinal temperature profiles however do show a pattern of maxima and minima which are largely anti-correlated between the two levels. We then use the derived temperature profiles to infer the vertical abundances of C2H2 and C2H6 by modeling tropospheric absorption (∼200 mbar) and stratospheric emission (∼5 mbar) in the C2H2ν5 and C2H6ν9 bands, and also emission of the acetylene (ν4 + ν5) - ν4 hotband (∼0.1 mbar). Acetylene shows a distinct north-south asymmetry in the stratosphere, with 5 mbar abundances greatest close to 20° N and decreasing from there towards both poles by a factor of ∼4. At 200 mbar in contrast, acetylene is nearly flat at a level of ∼ 3 × 10-9. Additionally, the abundance gradient of C2H2 between 10 and 0.1 mbar is derived, based on interpolated temperatures at 0.1 mbar, and is found to be positive and uniform with latitude to within errors. Ethane at both 5 and 200 mbar shows increasing VMR towards polar regions of ∼1.75 towards 70° N and ∼2.0 towards 70° S. An explanation for the meridional trends is proposed in terms of a combination of photochemistry and dynamics. Poleward, the decreasing UV flux is predicted to decrease the abundances of C2H2 and C2H6 by factors of 2.7 and 3.5, respectively, at latitude 70°. However, the lifetime of C2H6 in the stratosphere (3 × 1010   s at 5 mbar) is much longer than the dynamical timescale for meridional mixing inferred from Comet SL-9 debris (5 - 50 × 108   s), and therefore the rising abundance towards high latitudes likely indicates that meridional mixing dominates over photochemical effects. For C2H2, the opposite occurs, with the relatively short photochemical lifetime (3 × 107   s), compared to meridional mixing times, ensuring that the expected photochemical trends are visible. © 2006 Elsevier Inc. All rights reserved.

Vertical profiles of HCN, HC3N, and C2H2 in Titan's atmosphere derived from Cassini/CIRS data

Icarus 186 (2007) 364-384

NA Teanby, P.G.J. Irwin, R. de Kok, S. Vinatier

The composition of the atmosphere of Jupiter

in Jupiter, Cambridge Univ Pr (2007)

FW Taylor, SK Atreya, T Encrenaz, D Hunten, PG Irwin, TC Owen

Infrared observations of saturn and Titan from Cassini

Optics InfoBase Conference Papers (2007)

DE Jennings, RK Achterberg, B Bézard, GL Bjoraker, JC Brasunas, R Carlson, A Coustenis, FM Flasar, PGJ Irwin, VG Kunde, AA Mamoutkine, CA Nixon, GS Orton, JC Pearl, PN Romani, ME Segura, AA Simon-Miller, EH Wishnow, S Vinatier

The Composite Infrared Spectrometer (CIRS) aboard the Cassini spacecraft has been orbiting Saturn for 2-1/2 years. CIRS is a Fourier transform spectrometer that measures atmospheric thermal structure and dynamics, and atmospheric composition, of Saturn and Titan. CIRS also maps the temperatures and dynamical processes of the rings and icy moons. © 2007 Optical Society of America.

Optical constant of ammonium hydrosulfide ice and ammonia ice

Journal of the Optical Society of America B: Optical Physics 24 (2007) 126-136

CJA Howett, RW Carlson, PGJ Irwin, SB Calcutt

Thin-film transmission spectra of ammonium hydrosulfide (NH4SH) ice and ammonia (NH3) ice between 1300 and 12,000 cm-1 were used to determine the ice's optical constants. The films were grown on a sapphire substrate, and a Fourier-transform spectrometer and a grating spectrometer were used together to record the spectra. Lambert's law was used to directly determine the imaginary component of the complex refractive indices; from this, the real component was derived using the Kramers-Kronig algorithm. It is shown that, contrary to what is expected, the optical constants determined for NH3 ice at 80 K are in good agreement with those in the cubic phase, rather than the metastable one. The phase of the NH4SH ice was observed to change from amorphous to polycrystalline as the film was annealed to 160 K. © 2006 Optical Society of America.

New upper limits for hydrogen halides on Saturn derived from Cassini-CIRS data

Icarus 185 (2006) 466-475

NA Teanby, Fletcher, LN, Irwin, PGJ, Fouchet, T

Improved near-infrared methane band models and k-distribution parameters from 2000 to 9500 cm<sup>-1</sup> and implications for interpretation of outer planet spectra

Icarus 181 (2006) 309-319

PGJ Irwin, LA Sromovsky, EK Strong, K Sihra, NA Teanby, N Bowles, SB Calcutt, JJ Remedios

The band model fits of Sihra [1998. Ph.D. Thesis. University of Oxford], subsequently reported by Irwin et al. [2005. Icarus 176, 255-271], to new measurements of low-temperature near-infrared self-broadened methane absorption spectra combined with earlier warmer, longer path measurements of both self- and hydrogen-broadened methane spectra measured by Strong et al. [1993. J. Quant. Spectrosc. Radiat. Transfer 50, 363-429], have been found to contain severe artefacts at wavelengths of very low methane absorption. Although spectra calculated from these new band data appear to be reliable for paths with low to medium absorption, transmissions calculated for long paths of high methane absorption, such as for Uranus, Neptune and Titan are severely compromised. The recorded laboratory transmission spectra of Sihra [1998. Ph.D. Thesis. University of Oxford] and Strong et al. [1993. J. Quant. Spectrosc. Radiat. Transfer 50, 363-429] have thus been refitted with a more robust model and new k-distribution data for both self- and hydrogen-broadened methane absorption derived. In addition, a new model of the temperature dependence of the absorption has been employed that improves the quality of the fit and should also provide more accurate extrapolations to low temperatures. © 2005 Elsevier Inc. All rights reserved.

Latitudinal variations of HCN, HC3N, and C2N2 in Titan's stratosphere derived from cassini CIRS data

Icarus 181 (2006) 243-255

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

Improved near-infrared methane band models and k-distribution parameters from 2000 to 9500 cm-1 and implications for interpretation of outer planet spectra

Icarus 181 (2006) 309-319

PG Irwin, L.A. Sromovsky, E.K. Strong, K. Sihra

Scattering properties and location of the jovian 5-micron absorber from Galileo/NIMS limb-darkening observations

Journal of Quantitative Spectroscopy and Radiative Transfer 101 (2006) 448-461

M Roos-Serote, PGJ Irwin

The upper jovian atmosphere is particularly transparent at wavelengths near 5 μ m. Levels well below the cloud layers, which are situated between 0.5 and 2 bar, can be sounded. Large spatial variations of the brightness are observed, which are directly related to the opacity of the overlying cloud layer. Yet, the nature of the 5- μ m absorber in the jovian atmosphere has been subject of much debate. The cloud layer has been modelled many times as a thin, non-scattering layer, the opacity adjusted to fit the overall radiance level. This has proven to work well for individual spectra. Data from the Galileo near infrared mapping spectrometer (NIMS), covering the 0.7- 5.2 μ m range, include a number of observations of the same areas, separated by several hours, at different emission angles. Should the 5 μ m absorber be a thin absorbing layer then, apart from a change in radiance level, the overall shape of the 5- μ m spectrum is also expected to change significantly with emission angle. However, comparison of the 5- μ m spectra measured by NIMS of the same location but at different viewing angles reveals that while the overall radiance level decreases with increasing emission angle, the shape of the spectra remain unchanged. In this paper we present atmospheric models that include scattering to explain this effect. We show that the 5- μ m absorbing cloud particles must be significantly scattering ( ω = 0.9 ± 0.05 ) in order to explain these observations, and find that the base of the cloud layer must reside at pressures less than 2 bar. Furthermore, we show that the scattering within this cloud has important consequences on the retrieval of gas abundances from spectra in the 5- μ m region. © 2006 Elsevier Ltd. All rights reserved.

Near-IR methane absorption in outer planet atmospheres: Improved models of temperature dependence and implications for Uranus cloud structure

Icarus 182 (2006) 577-593

LA Sromovsky, PGJ Irwin, PM Fry

Near-IR absorption of methane in the 2000-9500 cm-1 spectral region plays a major role in outer planet atmospheres. However, the theoretical basis for modeling the observations of reflectivity and emission in these regions has had serious uncertainties at temperatures needed for interpreting observations of the colder outer planets. A lack of line parameter information, including ground-state energies and the absence of weak lines, limit the applicability of line-by-line calculations at low temperatures and for long path lengths, requiring the use of band models. However, prior band models have parameterized the temperature dependence in a way that cannot be accurately extrapolated to low temperatures. Here we use simulations to show how a new parameterization of temperature dependence can greatly improve band model accuracy and allow extension of band models to the much lower temperatures that are needed to interpret observations of Uranus, Neptune, Titan, and Saturn. Use of this new parameterization by Irwin et al. [Irwin, P.G.J., Sromovsky, L.A., Strong, E.K., Sihra, K., Bowles, N., Calcutt, S.B., 2005b. Icarus. In press] has verified improved fits to laboratory observations of Strong et al. [Strong, K., Taylor, F.W., Calcutt, S.B., Remedios, J.J., Ballard, J., 1993. J. Quant. Spectrosc. Radiat. Trans. 50, 363-429] and Sihra [1998. Ph.D. Thesis, Univ. of Oxford], which cover the temperature range from 100 to 340 K. Here we compare model predictions to 77 K laboratory observations and to Uranus spectra, which show much improved agreement between observed and modeled spectral features, allowing tighter constraints on pressure levels of Uranus cloud particles, implying that most scattering contributions arise from pressures near 2 bars and 6 bars rather than expected pressures near 1.25 and 3.1 bars. Between visible and near-IR wavelengths, both cloud layers exhibit strong decreases in reflectivity that are indicative of low opacity and submicron particle sizes. © 2006 Elsevier Inc. All rights reserved.

Jupiter-sized planets in the Solar System and elsewhere

in Solar system update, Springer Verlag (2006)

PG Irwin

Exploring the Saturn system in the thermal infrared: The composite infrared spectrometer

Space Science Reviews 115 (2005) 169-297

FM Flasar, VG Kunde, MM Abbas, RK Achterberg, P Ade, A Barucci, B Bézard, GL Bjoraker, JC Brasunas, S Calcutt, R Carlson, CJ Césarsky, BJ Conrath, A Coradini, R Courtin, A Coustenis, S Edberg, S Edgington, C Ferrari, T Fouchet, D Gautier, PJ Gierasch, K Grossman, P Irwin, DE Jennings, E Lellouch, AA Mamoutkine, A Marten, JP Meyer, CA Nixon, GS Orton, TC Owen, JC Pearl, R Prangé, F Raulin, PL Read, PN Romani, RE Samuelson, ME Segura, MR Showalter, AA Simon-Miller, MD Smith, JR Spencer, LJ Spilker, FW Taylor

The Composite Infrared Spectrometer (CIRS) is a remote-sensing Fourier Transform Spectrometer (FTS) on the Cassini orbiter that measures thermal radiation over two decades in wavenumber, from 10 to 1400 cm- 1 (1 mm to 7μ m), with a spectral resolution that can be set from 0.5 to 15.5 cm- 1. The far infrared portion of the spectrum (10-600 cm - 1) is measured with a polarizing interferometer having thermopile detectors with a common 4-mrad field of view (FOV). The middle infrared portion is measured with a traditional Michelson interferometer having two focal planes (600-1100 cm- 1, 1100-1400 cm- 1). Each focal plane is composed of a 1× 10 array of HgCdTe detectors, each detector having a 0.3-mrad FOV. CIRS observations will provide three-dimensional maps of temperature, gas composition, and aerosols/condensates of the atmospheres of Titan and Saturn with good vertical and horizontal resolution, from deep in their tropospheres to high in their mesospheres. CIRS's ability to observe atmospheres in the limb-viewing mode (in addition to nadir) offers the opportunity to provide accurate and highly resolved vertical profiles of these atmospheric variables. The ability to observe with high-spectral resolution should facilitate the identification of new constituents. CIRS will also map the thermal and compositional properties of the surfaces of Saturn's icy satellites. It will similarly map Saturn's rings, characterizing their dynamical and spatial structure and constraining theories of their formation and evolution. The combination of broad spectral range, programmable spectral resolution, the small detector fields of view, and an orbiting spacecraft platform will allow CIRS to observe the Saturnian system in the thermal infrared at a level of detail not previously achieved. © 2004 Kluwer Academic Publishers.

Water vapour abundance in Venus' middle atmosphere from Pioneer Venus OIR and Venera 15 FTS measurements.

Icarus 173 (2005) 84-99

FW Taylor, Koukouli, M., P.G.J. Irwin

Methane absorption in the atmosphere of Jupiter from 1800 to 9500 cm-1 and implications for vertical cloud structure

Icarus 176 (2005) 255-271

PGJ Irwin, K Sihra, NE Bowles, FW Taylor, SB Calcutt

Temperatures, Winds, and Composition in the Saturnian System

Science 307 (2005) 1247-1251

FM Flasar, PGJ Irwin, SB Calcutt, R Achterberg, FW Taylor

Titan's Atmospheric Temperatures, Winds, and Composition.

Science 308 (2005) 975-978

FW Taylor, Flasar F.M., Achterberg, R.K., Conrath, B.J.

Retrievals of jovian tropospheric phosphine from Cassini/CIRS

ICARUS 172 (2004) 37-49

PGJ Irwin, P Parrish, T Fouchet, SB Calcutt, FW Taylor, AA Simon-Miller, CA Nixon