Publications by Patrick Irwin

A correlated-k model of radiative transfer in the near-infrared windows of Venus


CCC Tsang, PGJ Irwin, FW Taylor, CF Wilson

The NEMESIS planetary atmosphere radiative transfer and retrieval tool

Journal of Quantitative Spectroscopy and Radiative Transfer 109 (2008) 1136-1150

PGJ Irwin, NA Teanby, R de Kok, LN Fletcher, CJA Howett, CCC Tsang, CF Wilson, SB Calcutt, CA Nixon, PD Parrish

With the exception of in situ atmospheric probes, the most useful way to study the atmospheres of other planets is to observe their electromagnetic spectra through remote observations, either from ground-based telescopes or from spacecraft. Atmospheric properties most consistent with these observed spectra are then derived with retrieval models. All retrieval models attempt to extract the maximum amount of atmospheric information from finite sets of data, but while the problem to be solved is fundamentally the same for any planetary atmosphere, until now all such models have been assembled ad hoc to address data from individual missions. In this paper, we describe a new general-purpose retrieval model, Non-linear Optimal Estimator for MultivariatE Spectral analySIS (NEMESIS), which was originally developed to interpret observations of Saturn and Titan from the composite infrared spectrometer on board the NASA Cassini spacecraft. NEMESIS has been constructed to be generally applicable to any planetary atmosphere and can be applied from the visible/near-infrared right out to microwave wavelengths, modelling both reflected sunlight and thermal emission in either scattering or non-scattering conditions. NEMESIS has now been successfully applied to the analysis of data from many planetary missions and also ground-based observations. © 2007 Elsevier Ltd. All rights reserved.

Global and temporal variations in hydrocarbons and nitriles in Titan's stratosphere for northern winter observed by Cassini/CIRS

Icarus 193 (2008) 595-611

NA Teanby, PGJ Irwin, R de Kok, CA Nixon, A Coustenis, E Royer, SB Calcutt, NE Bowles, L Fletcher, C Howett, FW Taylor

Mid-infrared spectra measured by Cassini's Composite InfraRed Spectrometer (CIRS) between July 2004 and January 2007 (Ls = 293 ° - 328 °) have been used to determine stratospheric temperature and abundances of C2H2, C3H4, C4H2, HCN, and HC3N. Over 65,000 nadir spectra with spectral resolutions of 0.5 and 2.5 cm-1 were used to probe spatial and temporal composition variations in Titan's stratosphere. Cassini's 180° orbital transfer in mid-2006 allowed low emission angle observations of the north polar region for the first time in the mission and allowed us to probe the full latitude range. We present the first measurements of composition variations within the polar vortex, which display increasing abundances right up to 90° N. The lack of a homogeneous abundance-latitude variation within the vortex indicates limited horizontal mixing and suggests that subsidence is greatest at the vortex core. Contrary to numerical model predictions and tropospheric cloud observations, we do not see any evidence for a secondary circulation cell near the south pole, which suggests a single Hadley-type circulation in the stratosphere at this epoch. This difference can be reconciled if the secondary cell is restricted to altitudes below 100 km, where there is no sensitivity in our data. Temporal variations in composition were observed in the south, with volatile species becoming less abundant as the season progressed. The observed variations are compared to numerical model predictions and observations from Voyager. © 2007 Elsevier Inc. All rights reserved.

Isotopic ratios in titan's atmosphere from cassini CIRS limb sounding: CO<inf>2</inf> at low and midlatitudes

Astrophysical Journal 681 (2008)

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

This Letter reports on a search for infrared emissions of isotopologues of CO2 in the atmosphere of Titan using spectral data recorded by the Cassini Composite Infrared Spectrometer (CIRS). We have made a successful 6.5 σ detection of 13CO2 at a fraction CO 2/13CO2 = 84 ± 17, consistent with measurements of 12C/13C in other species, and also the terrestrial value (89). We also And a probable 3.5 σ detection of C 16O18O at a fraction CO2/ C16O 18O = 173 ± 55, slightly lower than the terrestrial value (253) and consistent with the twofold enhancement in 18O reported previously in CO, or with an intermediate value as suggested by chemistry. These isotopic ratios provide important constraints on models of the formation, evolution, and current processes in Titan's atmosphere. © 2008, The American Astronomical Society, All rights reserved.

Isotopic ratios in titan's atmosphere from cassini cirs limb sounding: HC<inf>3</inf>N in the north

Astrophysical Journal 681 (2008)

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

This Letter reports the first detection of the three 13C isotopologues of HC3N on Titan, from Cassini Composite Infrared Spectrometer (CIRS) infrared spectra. The data are limb spectra taken at latitudes N54°-N69° in 2006 and 2007 when HC3N was enhanced in the north. Using a new line list for the vs bands of all isotopologues, we have modeled the isolated emission of H13CCCN at 658.7 cm-1 and both HC13CCN and HCC13CN at 663.0 cm-1, which are blended with the Q-branch of HC3N at 663.3 cm-1 at the resolution of CIRS (0.5 cm-1) and detectable as an increase in the intensity of the low-frequency wing. Using the resolved pair H13CCCN /HC3N we find 12C/ 13C = 79 ± 17, in line with other measurements on Titan from Cassini and Huygens. © 2008, The American Astronomical Society, All rights reserved.

Temperature and composition of Saturn's polar hot spots and hexagon.

Science 319 (2008) 79-81

LN Fletcher, PGJ Irwin, GS Orton, NA Teanby, RK Achterberg, GL Bjoraker, PL Read, AA Simon-Miller, C Howett, R de Kok, N Bowles, SB Calcutt, B Hesman, FM Flasar

Saturn's poles exhibit an unexpected symmetry in hot, cyclonic polar vortices, despite huge seasonal differences in solar flux. The cores of both vortices are depleted in phosphine gas, probably resulting from subsidence of air into the troposphere. The warm cores are present throughout the upper troposphere and stratosphere at both poles. The thermal structure associated with the marked hexagonal polar jet at 77 degrees N has been observed for the first time. Both the warm cyclonic belt at 79 degrees N and the cold anticyclonic zone at 75 degrees N exhibit the hexagonal structure.

Detection methods and properties of known exoplanets

in Exoplanets, Springer Verlag (2008)

PG Irwin

Meridional variations in stratospheric acetylene and ethane in the southern hemisphere of the saturnian atmosphere as determined from Cassini/CIRS measurements

Icarus 190 (2007) 556-572

CJA Howett, PGJ Irwin, NA Teanby, A Simon-Miller, SB Calcutt, LN Fletcher, R de Kok

These are the first results from nadir studies of meridional variations in the abundance of stratospheric acetylene and ethane from Cassini/CIRS data in the southern hemisphere of Saturn. High resolution, 0.5 cm-1, CIRS data was used from three data sets taken in June-November 2004 and binned into 2° wide latitudinal strips to increase the signal-to-noise ratio. Tropospheric and stratospheric temperatures were initially retrieved to determine the temperature profile for each latitude bin. The stratospheric temperature at 2 mbar increased by 14 K from 9° to 68° S, including a steep 4 K rise between 60° and 68° S. The tropospheric temperatures showed significantly more meridional variation than the stratospheric ones, the locations of which are strongly correlated to that of the zonal jets. Stratospheric acetylene abundance decreases steadily from 30 to 68° S, by a factor of 1.8 at 2.0 mbar. Between 18° and 30° S the acetylene abundance increases at 2.0 mbar. Global values for acetylene have been calculated as (1.9 ± 0.19) × 10-7 at 2.0 mbar, (2.6 ± 0.27) × 10-7 at 1.6 mbar and (3.1 ± 0.32) × 10-7 at 1.4 mbar. Global values for ethane are also determined and found to be (1.6 ± 0.25) × 10-5 at 0.5 mbar and (1.4 ± 0.19) × 10-5 at 1.0 mbar. Ethane abundance in the stratosphere increases towards the south pole by a factor of 2.5 at 2.0 mbar. The increase in stratospheric ethane is especially pronounced polewards of 60° S at 2.0 mbar. The increase of stratospheric ethane towards the south pole supports the presence of a meridional wind system in the stratosphere of Saturn. © 2007 Elsevier Inc. All rights reserved.

South-polar features on Venus similar to those near the north pole

Nature 450 (2007) 637-640

G Piccioni, P Drossart, A Sanchez-Lavega, R Hueso, FW Taylor, CF Wilson, D Grassi, L Zasova, M Moriconi, A Adriani, S Lebonnois, A Coradini, B Bézard, F Angrilli, G Arnold, KH Baines, G Bellucci, J Benkhoff, JP Bibring, A Blanco, MI Blecka, RW Carlson, A Di Lellis, T Encrenaz, S Erard, S Fonti, V Formisano, T Fouchet, R Garcia, R Haus, J Helbert, NI Ignatiev, PGJ Irwin, Y Langevin, MA Lopez-Valverde, D Luz, L Marinangeli, V Orofino, AV Rodin, MC Roos-Serote, B Saggin, DM Stam, D Titov, G Visconti, M Zambelli, E Ammannito, A Barbis, R Berlin, C Bettanini, A Boccaccini, G Bonnello, M Bouye, F Capaccioni, A Cardesin Moinelo, F Carraro, G Cherubini, M Cosi, M Dami, M De Nino, D Del Vento, M Di Giampietro, A Donati, O Dupuis, S Espinasse, A Fabbri, A Fave, IF Veltroni, G Filacchione, K Garceran, Y Ghomchi, M Giustini, B Gondet, Y Hello, F Henry, S Hofer, G Huntzinger, J Kachlicki, R Knoll, K Driss, A Mazzoni, R Melchiorri, G Mondello, F Monti, C Neumann, F Nuccilli, J Parisot, C Pasqui, S Perferi, G Peter, A Piacentino, C Pompei, JM Reess, JP Rivet, A Romano, N Russ, M Santoni, A Scarpelli, A Semery, A Soufflot, D Stefanovitch

Venus has no seasons, slow rotation and a very massive atmosphere, which is mainly carbon dioxide with clouds primarily of sulphuric acid droplets. Infrared observations by previous missions to Venus revealed a bright 'dipole' feature surrounded by a cold 'collar' at its north pole. The polar dipole is a 'double-eye' feature at the centre of a vast vortex that rotates around the pole, and is possibly associated with rapid downwelling. The polar cold collar is a wide, shallow river of cold air that circulates around the polar vortex. One outstanding question has been whether the global circulation was symmetric, such that a dipole feature existed at the south pole. Here we report observations of Venus' south-polar region, where we have seen clouds with morphology much like those around the north pole, but rotating somewhat faster than the northern dipole. The vortex may extend down to the lower cloud layers that lie at about 50 km height and perhaps deeper. The spectroscopic properties of the clouds around the south pole are compatible with a sulphuric acid composition. ©2007 Nature Publishing Group.

Scientific goals for the observation of Venus by VIRTIS on ESA/Venus express mission

Planetary and Space Science 55 (2007) 1653-1672

P Drossart, G Piccioni, A Adriani, F Angrilli, G Arnold, KH Baines, G Bellucci, J Benkhoff, B Bézard, JP Bibring, A Blanco, MI Blecka, RW Carlson, A Coradini, A Di Lellis, T Encrenaz, S Erard, S Fonti, V Formisano, T Fouchet, R Garcia, R Haus, J Helbert, NI Ignatiev, PGJ Irwin, Y Langevin, S Lebonnois, MA Lopez-Valverde, D Luz, L Marinangeli, V Orofino, AV Rodin, MC Roos-Serote, B Saggin, A Sanchez-Lavega, DM Stam, FW Taylor, D Titov, G Visconti, M Zambelli, R Hueso, CCC Tsang, CF Wilson, TZ Afanasenko

The Visible and Infrared Thermal Imaging Spectrometer (VIRTIS) on board the ESA/Venus Express mission has technical specifications well suited for many science objectives of Venus exploration. VIRTIS will both comprehensively explore a plethora of atmospheric properties and processes and map optical properties of the surface through its three channels, VIRTIS-M-vis (imaging spectrometer in the 0.3-1 μm range), VIRTIS-M-IR (imaging spectrometer in the 1-5 μm range) and VIRTIS-H (aperture high-resolution spectrometer in the 2-5 μm range). The atmospheric composition below the clouds will be repeatedly measured in the night side infrared windows over a wide range of latitudes and longitudes, thereby providing information on Venus's chemical cycles. In particular, CO, H2O, OCS and SO2 can be studied. The cloud structure will be repeatedly mapped from the brightness contrasts in the near-infrared night side windows, providing new insights into Venusian meteorology. The global circulation and local dynamics of Venus will be extensively studied from infrared and visible spectral images. The thermal structure above the clouds will be retrieved in the night side using the 4.3 μm fundamental band of CO2. The surface of Venus is detectable in the short-wave infrared windows on the night side at 1.01, 1.10 and 1.18 μm, providing constraints on surface properties and the extent of active volcanism. Many more tentative studies are also possible, such as lightning detection, the composition of volcanic emissions, and mesospheric wave propagation. © 2007 Elsevier Ltd. All rights reserved.

Vertical abundance profiles of hydrocarbons in Titan's atmosphere at 15° S and 80° N retrieved from Cassini/CIRS spectra

Icarus 188 (2007) 120-138

S Vinatier, B Bézard, T Fouchet, NA Teanby, R de Kok, PGJ Irwin, BJ Conrath, CA Nixon, PN Romani, FM Flasar, A Coustenis

Limb spectra recorded by the Composite InfraRed Spectrometer (CIRS) on Cassini provide information on abundance vertical profiles of C2H2, C2H4, C2H6, CH3C2H, C3H8, C4H2, C6H6 and HCN, along with the temperature profiles in Titan's atmosphere. We analyzed two sets of spectra, one at 15° S (Tb flyby) and the other one at 80° N (T3 flyby). The spectral range 600-1400 cm-1, recorded at a resolution of 0.5 cm-1, was used to determine molecular abundances and temperatures in the stratosphere in the altitude range 100-460 km for Tb and 170-495 km for T3. Both temperature profiles show a well defined stratopause, at around 310 km (0.07 mbar) and 183 K at 13° S, and 380 km (0.01 mbar) with 207 K at 80° N. Near the north pole, stratospheric temperatures are colder and mesospheric temperatures are warmer than near the equator. C2H2, C2H6, C3H8 and HCN display vertical mixing ratio profiles that increase with height at 15° S and 80° N, consistent with their formation in the upper atmosphere, diffusion downwards and condensation in the lower stratosphere, as expected from photochemical models. The CH3C2H and C4H2 mixing ratios also increase with height at 15° S. But near the north pole, their profiles present an unexpected minimum around 300 km, observed for the first time thanks to the high vertical resolution of the CIRS limb data. C2H4 is the only molecule having a vertical abundance profile that decreases with height at 15° S. At 80° N, it also displays a minimum of its mixing ratio around the 0.1-mbar level. For C6H6, an upper limit of 1.1 ppb (in the 0.3-10 mbar range) is derived at 15° S, whereas a constant mixing ratio profile of 3-1.5+3   ppb is inferred near the north pole. At 15° S, the vertical profile of HCN exhibits a steeper gradient than other molecules, which suggests that a sink for this molecule exists in the stratosphere, possibly due to haze formation. All molecules display a more or less pronounced enrichment towards the north pole, probably due, in part, to subsidence of air at the north (winter) pole that brings air enriched in photochemical compounds from the upper atmosphere to lower levels. © 2006 Elsevier Inc. All rights reserved.

The composition of Titan's stratosphere from Cassini/CIRS mid-infrared spectra

Icarus 189 (2007) 35-62

A Coustenis, RK Achterberg, BJ Conrath, DE Jennings, A Marten, D Gautier, CA Nixon, FM Flasar, NA Teanby, B Bézard, RE Samuelson, RC Carlson, E Lellouch, GL Bjoraker, PN Romani, FW Taylor, PGJ Irwin, T Fouchet, A Hubert, GS Orton, VG Kunde, S Vinatier, J Mondellini, MM Abbas, R Courtin

We have analyzed data recorded by the Composite Infrared Spectrometer (CIRS) aboard the Cassini spacecraft during the Titan flybys T0-T10 (July 2004-January 2006). The spectra characterize various regions on Titan from 70° S to 70° N with a variety of emission angles. We study the molecular signatures observed in the mid-infrared CIRS detector arrays (FP3 and FP4, covering roughly the 600-1500 cm-1 spectral range with apodized resolutions of 2.54 or 0.53 cm-1). The composite spectrum shows several molecular signatures: hydrocarbons, nitriles and CO2. A firm detection of benzene (C6H6) is provided by CIRS at levels of about 3.5 × 10-9 around 70° N. We have used temperature profiles retrieved from the inversion of the emission observed in the methane ν4 band at 1304 cm-1 and a line-by-line radiative transfer code to infer the abundances of the trace constituents and some of their isotopes in Titan's stratosphere. No longitudinal variations were found for these gases. Little or no change is observed generally in their abundances from the south to the equator. On the other hand, meridional variations retrieved for these trace constituents from the equator to the North ranged from almost zero (no or very little meridional variations) for C2H2, C2H6, C3H8, C2H4 and CO2 to a significant enhancement at high northern (early winter) latitudes for HCN, HC3N, C4H2, C3H4 and C6H6. For the more important increases in the northern latitudes, the transition occurs roughly between 30 and 50 degrees north latitude, depending on the molecule. Note however that the very high-northern latitude results from tours TB-T10 bear large uncertainties due to few available data and problems with latitude smearing effects. The observed variations are consistent with some, but not all, of the predictions from dynamical-photochemical models. Constraints are set on the vertical distribution of C2H2, found to be compatible with 2-D equatorial predictions by global circulation models. The D/H ratio in the methane on Titan has been determined from the CH3D band at 1156 cm-1 and found to be 1.17-0.28+0.23 × 10-4. Implications of this deuterium enrichment, with respect to the protosolar abundance on the origin of Titan, are discussed. We compare our results with values retrieved by Voyager IRIS observations taken in 1980, as well as with more recent (1997) disk-averaged Infrared Space Observatory (ISO) results and with the latest Cassini-Huygens inferences from other instruments in an attempt to better comprehend the physical phenomena on Titan. © 2007 Elsevier Inc. All rights reserved.

Characterising Saturn's vertical temperature structure from Cassini/CIRS

Icarus 189 (2007) 457-478

LN Fletcher, PGJ Irwin, NA Teanby, GS Orton, PD Parrish, R de Kok, C Howett, SB Calcutt, N Bowles, FW Taylor

Thermal infrared spectra of Saturn from 10-1400 cm-1 at 15 cm-1 spectral resolution and a spatial resolution of 1°-2° latitude have been obtained by the Cassini Composite Infrared Spectrometer [Flasar, F.M., and 44 colleagues, 2004. Space Sci. Rev. 115, 169-297]. Many thousands of spectra, acquired over eighteen-months of observations, are analysed using an optimal estimation retrieval code [Irwin, P.G.J., Parrish, P., Fouchet, T., Calcutt, S.B., Taylor, F.W., Simon-Miller, A.A., Nixon, C.A., 2004. Icarus 172, 37-49] to retrieve the temperature structure and para-hydrogen distribution over Saturn's northern (winter) and southern (summer) hemispheres. The vertical temperature structure is analysed in detail to study seasonal asymmetries in the tropopause height (65-90 mbar), the location of the radiative-convective boundary (350-500 mbar), and the variation with latitude of a temperature knee (between 150 and 300 mbar) which was first observed in inversions of Voyager/IRIS spectra [Hanel, R., and 15 colleagues, 1981. Science 212, 192-200; Hanel, R., Conrath, B., Flasar, F.M., Kunde, V., Maguire, W., Pearl, J.C., Pirraglia, J., Samuelson, R., Cruikshank, D.P., Gautier, D., Gierasch, P.J., Horn, L., Ponnamperuma, C., 1982. Science 215, 544-548]. Uncertainties due to both the modelling of spectral absorptions (collision-induced absorption coefficients, tropospheric hazes, helium abundance) and the nature of our retrieval algorithm are quantified. Temperatures in the stratosphere near 1 mbar show a 25-30 K temperature difference between the north pole and south pole. This asymmetry becomes less pronounced with depth as the radiative time constant for the atmospheric response increases at deeper pressure levels. Hemispherically-symmetric small-scale temperature structures associated with zonal winds are superimposed onto the temperature asymmetry for pressures greater than 100 mbar. The para-hydrogen fraction in the 100-400 mbar range is greater than equilibrium predictions for the southern hemisphere and parts of the northern hemisphere, and less than equilibrium predictions polewards of 40° N. The temperature knee between 150-300 mbar is larger in the summer hemisphere than in the winter, smaller and higher at the equator, deeper and larger in the equatorial belts and small at the poles. Solar heating on tropospheric haze is proposed as a possible mechanism for this effect; the increased efficiency of ortho- to para-hydrogen conversion in the southern hemisphere is consistent with the presence of larger aerosols in the summer hemisphere, which we demonstrate to be qualitatively consistent with previous studies of Saturn's tropospheric aerosol distribution. © 2007 Elsevier Inc. All rights reserved.

Quantifying the effect of finite field-of-view size on radiative transfer calculations of Titan's limb spectra measured by Cassini-CIRS

Astrophysics and Space Science 310 (2007) 293-305

NA Teanby, PGJ Irwin

The Composite InfraRed Spectrometer (CIRS) on-board the Cassini spacecraft has currently returned around three years worth of data from Saturn's largest moon Titan. One of the unique aspects of CIRS is to take high spatial resolution spectra of the limb of Titan, with sub-scale height (20-40 km) resolutions. This is made possible by the small field-of-view (FOV) of the mid-IR detectors. However, many limb spectra have moderate to large sized FOVs, which introduces bias into retrieved profiles of temperature and abundance unless the finite FOV size is taken into account. The bias can be reduced by calculating a FOV-averaged spectrum comprising a weighted sum of a small number of spectra with infinitesimal FOVs across the FOV. Here we introduce a scheme for incorporating FOV averaging into radiative transfer calculations of CIRS spectra and quantify the errors as a function of number of FOV averaging points, FOV size, tangent altitude, and wavenumber. The optimum number of FOV averaging points for a given observation can then be found by matching the calculated FOV averaging error with the measurement error. This allows for accurate analysis of a vast amount of Cassini-CIRS data. © 2007 Springer Science+Business Media B.V.

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.

The meridional phosphine distribution in Saturn's upper troposphere from Cassini/CIRS observations

Icarus 188 (2007) 72-88

NE Bowles, L N Fletcher, N A Teanby, P G J Irwin

The 2003 November 14 occultation by Titan of TYC 1343-1865-1. II. Analysis of light curves

Icarus 192 (2007) 503-518

A Zalucha, A Fitzsimmons, JL Elliot, J Thomas-Osip, HB Hammel, VS Dhillon, TR Marsh, FW Taylor, PGJ Irwin

We observed a stellar occultation by Titan on 2003 November 14 from La Palma Observatory using ULTRACAM with three Sloan filters: u′, g′, and i′ (358, 487, and 758 nm, respectively). The occultation probed latitudes 2° S and 1° N during immersion and emersion, respectively. A prominent central flash was present in only the i′ filter, indicating wavelength-dependent atmospheric extinction. We inverted the light curves to obtain six lower-limit temperature profiles between 335 and 485 km (0.04 and 0.003 mb) altitude. The i′ profiles agreed with the temperature measured by the Huygens Atmospheric Structure Instrument [Fulchignoni, M., and 43 colleagues, 2005. Nature 438, 785-791] above 415 km (0.01 mb). The profiles obtained from different wavelength filters systematically diverge as altitude decreases, which implies significant extinction in the light curves. Applying an extinction model [Elliot, J.L., Young, L.A., 1992. Astron. J. 103, 991-1015] gave the altitudes of line of sight optical depth equal to unity: 396 ± 7 and 401 ± 20  km (u′ immersion and emersion); 354 ± 7 and 387 ± 7  km (g′ immersion and emersion); and 336 ± 5 and 318 ± 4  km (i′ immersion and emersion). Further analysis showed that the optical depth follows a power law in wavelength with index 1.3 ± 0.2. We present a new method for determining temperature from scintillation spikes in the occulting body's atmosphere. Temperatures derived with this method are equal to or warmer than those measured by the Huygens Atmospheric Structure Instrument. Using the highly structured, three-peaked central flash, we confirmed the shape of Titan's middle atmosphere using a model originally derived for a previous Titan occultation [Hubbard, W.B., and 45 colleagues, 1993. Astron. Astrophys. 269, 541-563]. © 2007 Elsevier Inc. All rights reserved.

A dynamic upper atmosphere of Venus as revealed by VIRTIS on Venus Express.

Nature 450 (2007) 641-645

P Drossart, G Piccioni, JC Gérard, MA Lopez-Valverde, A Sanchez-Lavega, A Sanchez-Lavega, L Zasova, R Hueso, FW Taylor, B Bézard, A Adriani, F Angrilli, G Arnold, KH Baines, G Bellucci, J Benkhoff, JP Bibring, A Blanco, MI Blecka, RW Carlson, A Coradini, A Di Lellis, T Encrenaz, S Erard, S Fonti, V Formisano, T Fouchet, R Garcia, R Haus, J Helbert, NI Ignatiev, P Irwin, Y Langevin, S Lebonnois, D Luz, L Marinangeli, V Orofino, AV Rodin, MC Roos-Serote, B Saggin, DM Stam, D Titov, G Visconti, M Zambelli, C Tsang, VIRTIS-Venus Express Technical Team, E Ammannito, A Barbis, R Berlin, C Bettanini, A Boccaccini, G Bonnello, M Bouyé, F Capaccioni, A Cardesin, F Carraro, G Cherubini, M Cosi, M Dami, M De Nino, D Del Vento, M Di Giampietro, A Donati, O Dupuis, S Espinasse, A Fabbri, A Fave, IF Veltroni, G Filacchione, K Garceran, Y Ghomchi, M Giustizi, B Gondet, Y Hello, F Henry, S Hofer, G Huntzinger, J Kachlicki, R Knoll, D Kouach, A Mazzoni, R Melchiorri, G Mondello, F Monti, C Neumann, F Nuccilli, J Parisot, C Pasqui, S Perferi, G Peter, A Piacentino, C Pompei, J-M Réess, J-P Rivet, A Romano, N Russ, M Santoni, A Scarpelli, A Sémery, A Soufflot, D Stefanovitch, E Suetta, F Tarchi, N Tonetti, F Tosi, B Ulmer

The upper atmosphere of a planet is a transition region in which energy is transferred between the deeper atmosphere and outer space. Molecular emissions from the upper atmosphere (90-120 km altitude) of Venus can be used to investigate the energetics and to trace the circulation of this hitherto little-studied region. Previous spacecraft and ground-based observations of infrared emission from CO2, O2 and NO have established that photochemical and dynamic activity controls the structure of the upper atmosphere of Venus. These data, however, have left unresolved the precise altitude of the emission owing to a lack of data and of an adequate observing geometry. Here we report measurements of day-side CO2 non-local thermodynamic equilibrium emission at 4.3 microm, extending from 90 to 120 km altitude, and of night-side O2 emission extending from 95 to 100 km. The CO2 emission peak occurs at approximately 115 km and varies with solar zenith angle over a range of approximately 10 km. This confirms previous modelling, and permits the beginning of a systematic study of the variability of the emission. The O2 peak emission happens at 96 km +/- 1 km, which is consistent with three-body recombination of oxygen atoms transported from the day side by a global thermospheric sub-solar to anti-solar circulation, as previously predicted.

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


PGJ Irwin, NA Teanby, GR Davis

Characteristics of Titan's stratospheric aerosols and condensate clouds from Cassini CIRS far-infrared spectra

Icarus 191 (2007) 223-235

NE Bowles, N A Teanby, P G J Irwin, R de Kok