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To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2009.01.001 Byline: C.C.C. Tsang (a), F.W. Taylor (a), C.F. Wilson (a), S.J. Liddell (a), P.G.J. Irwin (a), G. Piccioni (b), P. Drossart (c), S.B. Calcutt (a) Keywords: Venus; atmosphere; Abundances; atmospheres; Atmospheres; dynamics; Spectroscopy Abstract: A fast method is presented for deriving the tropospheric CO concentrations in the Venus atmosphere from near-infrared spectra using the night side 2.3 [mu]m window. This is validated using the spectral fitting techniques of Tsang et al. [Tsang, C.C.C., Irwin, P.G.J., Taylor, F.W., Wilson, C.F., Drossart, P., Piccioni, G., de Kok, R., Lee, C., Calcutt, S.B., and the Venus Express/VIRTIS Team, 2008a. Tropospheric carbon monoxide concentrations and variability on Venus with Venus Express/VIRTIS-M observations. J. Geophys. Res. 113, doi: 10.1029/2008JE003089. E00B08] to show that monitoring CO in the deep atmosphere can be done quickly using large numbers of observations, with minimal effect from cloud and temperature variations. The new method is applied to produce some 1450 zonal mean CO profiles using data from the first eighteen months of operation from the Visible and Infrared Thermal Imaging Spectrometer infrared mapping subsystem (VIRTIS-M-IR) on Venus Express. These results show many significant long- and short-term variations from the mean equator-to-pole increasing trend previously found from earlier Earth- and space-based observations, including a possible North-South dichotomy, with interesting implications for the dynamics and chemistry of the lower atmosphere of Venus. Author Affiliation: (a) Atmospheric, Oceanic and Planetary Physics, Department of Physics, University of Oxford, Parks Road, Oxford, Oxon, OX1 3PU, UK (b) INAF-IASF, Via del Fosso del Cavaliere, 100 00133 Rome, Italy (c) LESIA, Observatoire de Paris, CNRS, UPMC Universite Paris-Diderot, France Article History: Received 5 August 2008; Revised 4 December 2008; Accepted 8 January 2009

We have investigated the abundances of Titan's stratospheric oxygen compounds using 0.5 [cm.sup.-1] resolution spectra from the Composite Infrared Spectrometer on the Cassini orbiter. The CO abundance was derived for several observations of far-infrared nadir spectra, taken at a range of latitudes (75[degrees] S-35[degrees] N) and emission angles (0[degrees]-60[degrees]), using rotational lines that have not been analysed before the arrival of Cassini at Saturn. The derived volume mixing ratios for the different observations are mutually consistent regardless of latitude. The weighted mean CO volume mixing ratio is 47 [+ or -] 8 ppm if CO is assumed to be uniform with latitude. [H.sub.2]O could not be detected and an upper limit of 0.9 ppb was determined. C[O.sub.2] abundances derived from mid-infrared nadir spectra show no significant latitudinal variations, with typical values of 16 [+ or -] 2 ppb. Midinfrared limb spectra at 55[degrees] S were used to constrain the vertical profile of C[O.sub.2] for the first time. A vertical C[O.sub.2] profile that is constant above the condensation level at a volume mixing ratio of 15 ppb reproduces the limb spectra very well below 200 km. This is consistent with the long chemical lifetime of C[O.sub.2] in Titan's stratosphere. Above 200 km the C[O.sub.2] volume mixing ratio is not well constrained and an increase with altitude cannot be ruled out there. Keywords: Titan; Atmospheres, composition

Mid-infrared limb spectra in the range 600-1400 [cm.sup.-1] taken with the Composite InfraRed Spectrometer (CIRS) on-board the Cassini spacecraft were used to determine vertical profiles of HCN, H[C.sub.3] N, [C.sub.2][H.sub.2], and temperature in Titan's atmosphere. Both high (0.5 [cm.sup.-1]) and low (13.5 [cm.sup.-1]) spectral resolution data were used. The 0.5 [cm.sup.-1] data gave profiles at four latitudes and the 13.5 [cm.sup.-1] data gave almost complete latitudinal coverage of the atmosphere. Both datasets were found to be consistent with each other. High temperatures in the upper stratosphere and mesosphere were observed at Titan's northern winter pole and were attributed to adiabatic heating in the subsiding branch of a meridional circulation cell. On the other hand, the lower stratosphere was much colder in the north than at the equator, which can be explained by the lack of solar radiation and increased IR emission from volatile enriched air. H[C.sub.3]N had a vertical profile consistent with previous ground based observations at southern and equatorial latitudes, but was massively enriched near the north pole. This can also be explained in terms of subsidence at the winter pole. A boundary observed at 60[degrees] N between enriched and un-enriched air is consistent with a confining polar vortex at 60[degrees] N and H[C.sub.3]N's short lifetime. In the far north, layers were observed in the H[C.sub.3]N profile that were reminiscent of haze layers observed by Cassini's imaging cameras. HCN was also enriched over the north pole, which gives further evidence for subsidence. However, the atmospheric cross section obtained from 13.5 [cm.sup.-1] data indicated a HCN enriched layer at 200-250 km, extending into the southern hemisphere. This could be interpreted as advection of polar enriched air towards the south by a meridional circulation cell. This is observed for HCN but not for H[C.sub.3]N due to HCN's longer photochemical lifetime. [C.sub.2][H.sub.2] appears to have a uniform abundance with altitude and is not significantly enriched in the north. This is consistent with observations from previous CIRS analysis that show increased abundances of nitriles and hydrocarbons but not [C.sub.2][H.sub.2] towards the north pole. Keywords: Titan; Atmospheres, composition

Mid- and far-infrared spectra from the Composite InfraRed Spectrometer (CIRS) have been used to determine volume mixing ratios of nitriles in Titan's atmosphere. HCN, H[C.sub.3]N, [C.sub.2][H.sub.2], and temperature were derived from 2.5 [cm.sup.-1] spectral resolution mid-IR mapping sequences taken during three flybys, which provide almost complete global coverage of Titan for latitudes south of 60[degrees] N. Three 0.5 [cm.sup.-1] spectral resolution far-IR observations were used to retrieve [C.sub.2][N.sub.2] and act as a check on the mid-IR results for HCN. Contribution functions peak at around 0.5-5 mbar for temperature and 0.1-10 mbar for the chemical species, well into the stratosphere. The retrieved mixing ratios of HCN, H[C.sub.3]N, and [C.sub.2][N.sub.2] show a marked increase in abundance towards the north, whereas [C.sub.2][N.sub.2] remains relatively constant. Variations with longitude were much smaller and are consistent with high zonal wind speeds. For 90[degrees]-20[degrees] S the retrieved HCN abundance is fairly constant with a volume mixing ratio of around 1 x [10.sup.-7] at 3 mbar. More northerly latitudes indicate a steady increase, reaching around 4 x [10.sup.-7] at 60[degrees] N, where the data coverage stops. This variation is consistent with previous measurements and suggests subsidence over the northern (winter) pole at approximately 2 x [10.sup.-4] m [s.sup.-1]. H[C.sub.3]N displays a very sharp increase towards the north pole, where it has a mixing ratio of around 4 x [10.sup.-8] at 60[degrees] N at the 0.1-mbar level. The difference in gradient for the HCN and H[C.sub.3]N latitude variations can be explained by H[C.sub.3]N's much shorter photochemical lifetime, which prevents it from mixing with air at lower latitude. It is also consistent with a polar vortex which inhibits mixing of volatile rich air inside the vortex with that at lower latitudes. Only one observation was far enough north to detect significant amounts of [C.sub.2][N.sub.2], giving a value of around 9 x [10.sup.-10] at 50[degrees] N at the 3-mbar level. Keywords: Titan; Atmospheres, composition

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2005.02.004 Byline: P.G.J. Irwin (a), K. Sihra (b), N. Bowles (a), F.W. Taylor (a), S.B. Calcutt (a) Keywords: Jupiter; Atmosphere; Radiative transfer Abstract: New measurements of the low-temperature near-infrared absorption of methane (Sihra, 1998, Laboratory measurements of near-infrared methane bands for remote sensing of the jovian atmosphere, Ph.D. thesis, University of Oxford) have been combined with existing, longer path-length, higher-temperature data of Strong et al. (1993, Spectral parameters of self- and hydrogen-broadened methane from 2000 to 9500 cm.sup.-1 for remote sounding of the atmosphere of Jupiter, J. Quant. Spectrosc. Radiat. Trans. 50, 309-325) and fitted with band models. The combined data set is found to be more consistent with previous low-temperature methane absorption measurements than that of Strong et al. (1993, J. Quant. Spectrosc. Radiat. Trans. 50, 309-325) but covers the same wider wavelength range and accounts for both self- and hydrogen-broadening conditions. These data have been fitted with k-coefficients in the manner described by Irwin et al. (1996, Calculated k-distribution coefficients for hydrogen- and self-broadened methane in the range 2000-9500 cm.sup.-1 from exponential sum fitting to band modelled spectra, J. Geophys. Res. 101, 26,137-26,154) and have been used in multiple-scattering radiative transfer models to assess their impact on our previous estimates of the jovian cloud structure obtained from Galileo Near-Infrared Mapping Spectrometer (NIMS) observations (Irwin et al., 1998, Cloud structure and atmospheric composition of Jupiter retrieved from Galileo NIMS real-time spectra, J. Geophys. Res. 103, 23,001-23,021; Irwin et al., 2001, The origin of belt/zone contrasts in the atmosphere of Jupiter and their correlation with 5-[mu]m opacity, Icarus 149, 397-415; Irwin and Dyudina, 2002, The retrieval of cloud structure maps in the equatorial region of Jupiter using a principal component analysis of Galileo/NIMS data, Icarus 156, 52-63). Although significant differences in methane opacity are found at cooler temperatures, the difference in the optical depth of the atmosphere due to methane is found to diminish rapidly with increasing pressure and temperature and thus has negligible effect on the cloud structure inferred at deeper levels. Hence the main cloud opacity variation is still found to peak at around 1-2 bar using our previous analytical approach, and is thus still in disagreement with Galileo Solid State Imager (SSI) determinations (Banfield et al., 1998, Jupiter's cloud structure from Galileo imaging data, Icarus 135, 230-250; Simon-Miller et al., 2001, Color and the vertical structure in Jupiter's belts, zones and weather systems, Icarus 154, 459-474) which place the main cloud deck near 0.9 bar. Further analysis of our retrievals reveals that this discrepancy is probably due to the different assumptions of the two analyses. Our retrievals use a smooth vertically extended cloud profile while the SSI determinations assume a thin NH.sub.3 cloud below an extended haze. When the main opacity in our model is similarly assumed to be due to a thin cloud below an extended haze, we find the main level of cloud opacity variation to be near the 1 bar level -- close to that determined by SSI and moderately close to the expected condensation level of ammonia ice of 0.85 bar, assuming that the abundance of ammonia on Jupiter is (7[+ or -]1)x10.sup.-4 (Folkner et al., 1998, Ammonia abundance in Jupiter's atmosphere derived from the attenuation of the Galileo probe's radio signal, J. Geophys. Res. 103, 22,847-22,855; Atreya et al., 1999, A comparison of the atmospheres of Jupiter and Saturn: deep atmospheric composition, cloud structure, vertical mixing, and origin, Planet. Space Sci. 47, 1243-1262). However our data in the 1-2.5 [mu]m range have good height discrimination and our lowest estimate of the cloud base pressure of 1 bar is still too great to be consistent with the most recent estimates of the ammonia abundance of 3.5 x solar. Furthermore the observed limited spatial distribution of ammonia ice absorption features on Jupiter suggests that pure ammonia ice is only present in regions of localised vigorous uplift (Baines et al., 2002, Fresh ammonia ice clouds in Jupiter: spectroscopic identification, spatial distribution, and dynamical implications, Icarus 159, 74-94) and is subsequently rapidly modified in some way which masks its pure absorption features. Hence we conclude that the main cloud deck on Jupiter is unlikely to be composed of pure ammonia ice and instead find that it must be composed of either NH.sub.4SH or some other unknown combination of ammonia, water, and hydrogen sulphide and exists at pressures of between 1 and 2 bar. Author Affiliation: (a) Atmospheric, Oceanic, and Planetary Physics, University of Oxford, Clarendon Laboratory, Parks Rd, Oxford, OX1 3PU, United Kingdom (b) National Radiological Protection Board, Chilton, Didcot, Oxon, OX11 0RQ, United Kingdom Article History: Received 24 February 2004; Revised 9 February 2005

The Pioneer Venus Orbiter Infrared Radiometer and Venera 15 Fourier Transform Spectrometer observations of thermal emission from Venus' middle atmosphere between 10[degrees] S and 50[degrees] N have been independently re-analyzed using a common method to determine global maps of temperature, cloud optical depth, and water vapor abundance. The spectral regions observed include the strong 15 [am carbon dioxide band and the 45 [micro]m fundamental rotational water band. The different spatial and spectral resolutions of the two instruments have necessitated the development of flexible analysis tools. New radiative transfer and retrieval models have been developed for this purpose based on correlated-k absorption tables calculated with up-to-date spectral line data. The common analysis of these two sets of observations has hence been possible for the first time. From the PV OIR observations, the cloud-top unit optical depth pressure showed a minimum of ~110 [+ or -] 10 mbars in the evening equatorial region and a maximum of ~160 [+ or -] 12 mbars in the morning mid-latitude regions. From the Venera 15 FTS spectra, the cloud-top pressure was found to increase from morning values of ~ 120 [+ or -] 10 to 200 [+ or -] 30 mbars in the late afternoon/early evening region. The cloud-top water vapor abundances observed by the PV O1R instrument were found to fluctuate from 10 4 [+ or -] 5 ppm at night up to 90 [+ or -] 15 ppm in the equatorial cloud-top region shortly after the sub-solar point. The mean Venera 15 FTS water vapor abundances were found to be 12 [+ or -] 5 ppm with only a slight enhancement over the equatorial latitude bands and no clear day-night distinction. The common analysis of these two sets of observations broadly validates previously published individual findings. The differences in the retrieved atmospheric state can no longer be attributed to radiative transfer modeling bias and suggest significant temporal variability in the middle atmosphere of Venus. Keywords: Venus, atmosphere; Atmospheres, composition; Abundances, atmospheres; Radiative transfer

On December 30th, 2000, the Cassini-Huygens spacecraft reached the perijove milestone on its continuing journey to the Saturnian System. During an extended six-month encounter, the Composite Infrared Spectrometer (CIRS) returned spectra of the jovian atmosphere, rings and satellites from 10-1400 [cm.sup.-1] (1000-7 [micro]m) at a programmable spectral resolution of 0.5 to 15 [cm.sup.-1]. The improved spectral resolution of CIRS over previous IR instrument-missions to Jupiter, the extended spectral range, and higher signal-to-noise performance provide significant advantages over previous data sets. CIRS global observations of the mid-infrared spectrum of Jupiter at medium resolution (2.5 [cm.sup.-1]) have been analysed both with a radiance differencing scheme and an optimal estimation retrieval model to retrieve the spatial variation of phosphine and ammonia tractional scale height in the troposphere between 60[degrees] S and 60[degrees] N at a spatial resolution of 6[degrees]. The ammonia fractional scale height appears to be high over the Equatorial Zone (EZ) but low over the North Equatorial Belt (NEB) and South Equatorial Belt (SEB) indicating rapid uplift or strong vertical mixing in the EZ. The abundance of phosphine shows a similar strong latitudinal variation which generally matches that of the ammonia fractional scale height. However while the ammonia fractional scale height distribution is to a first order symmetric in latitude, the phosphine distribution shows a North/South asymmetry at mid latitudes with higher amounts detected at 40[degrees] N than 40[degrees] S. In addition the data show that while the ammonia fractional scale height at this spatial resolution appears to be low over the Great Red Spot (GRS), indicating reduced vertical mixing above the ~500 mb level, the abundance of phosphine at deeper levels may be enhanced at the northern edge of the GRS indicating upwelling. Keywords: Jupiter: Atmosphere: Atmospheres, composition; Radiative transfer: Infrared observations

A recent study shows that the temperature of Venus, at an altitude of 91 km is 10 K higher, and is lower by 3.6 K at an altitude between 74 and 83 km, as compared to the observations of the Pioneer Venus mission. The study, using a near infrared mapping spectrometer during Galileo-Venus encounter in Feb 1990, of Venusian atmosphere is done using high spatial resolution infrared spectra of night side.

Abstract: Four broad spectral features were identified in far-infrared limb spectra from the Cassini Composite Infrared Spectrometer (CIRS), two of which have not been identified before. The features are broader than the spectral resolution, which suggests that they are caused by particulates in Titan''s stratosphere. We derive here the spectral properties and variations with altitude for these four features for six latitudes between 65° S and 85° N. Titan''s main aerosol is called Haze 0 here. It is present at all wavenumbers in the far-infrared and is found to have a fractional scale height (i.e., the aerosol density scale height divided by the atmospheric density scale height) between 1.5 and 1.7 with a small increase in opacity in the north. A second feature around 140 cm−1 (Haze A) has similar spatial properties to Haze 0, but has a smaller fractional scale height of 1.2–1.3. Both Haze 0 and Haze A show an increase in retrieved abundance below 100 km. Two other features (Haze B around 220 cm−1 and Haze C around 190 cm−1) have a large maximum in their density profiles at 140 and 90 km, respectively. Haze B is much more abundant in the northern hemisphere compared to the southern hemisphere. Haze C also shows a large increase towards the north, but then disappears at 85° N. [Copyright &y& Elsevier]

Abstract: 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. [Copyright &y& Elsevier]

Abstract: The Cassini Composite Infrared Spectrometer (CIRS) has been used to derive the vertical and meridional variation of temperature and phosphine (PH3) abundance in Saturn''s upper troposphere. PH3 has a significant effect on the measured radiances in the thermal infrared and between May 2004 and September 2005 CIRS recorded thousands of spectra in both the far (10–600 cm−1) and mid (600–1400 cm−1) infrared, at a variety of latitudes covering the southern hemisphere. Low spectral resolution (15 cm−1) data has been used to constrain the temperature structure of the troposphere between 100 and 500 mbar. The vertical distributions of phosphine and ammonia were retrieved from far-infrared spectra at the highest spectral resolution (0.5 cm−1), and lower resolution (2.5 cm−1) mid-infrared data were used to map the meridional variation in the abundance of phosphine in the 250–500 mbar range. Temperature variations at the 250 mbar level are shown to occur on the same scale as the prograde and retrograde jets in Saturn''s atmosphere [Porco, C.C., and 34 colleagues, 2005. Science 307, 1243–1247]. The PH3 abundance at 250 mbar is found to be enhanced at the equator when compared with mid-latitudes. At mid latitudes we see anti-correlation between temperature and PH3 abundance at 250 mbar, phosphine being enhanced at 45° S and depleted at 25 and 55° S. The vertical distribution is markedly different polewards of 60–65° S, with depleted PH3 at 500 mbar but a slower decline in abundance with altitude when compared with the mid-latitudes. This variation is similar to the variations of cloud and aerosol parameters observed in the visible and near infrared, and may indicate the subsidence of tropospheric air at polar latitudes, coupled with a diminished sunlight penetration depth reducing the rate of PH3 photolysis in the polar region. [Copyright &y& Elsevier]

Abstract: Mid-infrared limb spectra in the range 600–1400 cm−1 taken with the Composite InfraRed Spectrometer (CIRS) on-board the Cassini spacecraft were used to determine vertical profiles of HCN, HC3N, C2H2, and temperature in Titan''s atmosphere. Both high (0.5 cm−1) and low (13.5 cm−1) spectral resolution data were used. The 0.5 cm−1 data gave profiles at four latitudes and the 13.5 cm−1 data gave almost complete latitudinal coverage of the atmosphere. Both datasets were found to be consistent with each other. High temperatures in the upper stratosphere and mesosphere were observed at Titan''s northern winter pole and were attributed to adiabatic heating in the subsiding branch of a meridional circulation cell. On the other hand, the lower stratosphere was much colder in the north than at the equator, which can be explained by the lack of solar radiation and increased IR emission from volatile enriched air. HC3N had a vertical profile consistent with previous ground based observations at southern and equatorial latitudes, but was massively enriched near the north pole. This can also be explained in terms of subsidence at the winter pole. A boundary observed at 60° N between enriched and un-enriched air is consistent with a confining polar vortex at 60° N and HC3N''s short lifetime. In the far north, layers were observed in the HC3N profile that were reminiscent of haze layers observed by Cassini''s imaging cameras. HCN was also enriched over the north pole, which gives further evidence for subsidence. However, the atmospheric cross section obtained from 13.5 cm−1 data indicated a HCN enriched layer at 200–250 km, extending into the southern hemisphere. This could be interpreted as advection of polar enriched air towards the south by a meridional circulation cell. This is observed for HCN but not for HC3N due to HCN''s longer photochemical lifetime. C2H2 appears to have a uniform abundance with altitude and is not significantly enriched in the north. This is consistent with observations from previous CIRS analysis that show increased abundances of nitriles and hydrocarbons but not C2H2 towards the north pole. [Copyright &y& Elsevier]

Abstract: Mid- and far-infrared spectra from the Composite InfraRed Spectrometer (CIRS) have been used to determine volume mixing ratios of nitriles in Titan''s atmosphere. HCN, HC3N, C2H2, and temperature were derived from 2.5 cm−1 spectral resolution mid-IR mapping sequences taken during three flybys, which provide almost complete global coverage of Titan for latitudes south of 60° N. Three 0.5 cm−1 spectral resolution far-IR observations were used to retrieve C2N2 and act as a check on the mid-IR results for HCN. Contribution functions peak at around 0.5–5 mbar for temperature and 0.1–10 mbar for the chemical species, well into the stratosphere. The retrieved mixing ratios of HCN, HC3N, and C2N2 show a marked increase in abundance towards the north, whereas C2H2 remains relatively constant. Variations with longitude were much smaller and are consistent with high zonal wind speeds. For 90°–20° S the retrieved HCN abundance is fairly constant with a volume mixing ratio of around 1×10−7 at 3 mbar. More northerly latitudes indicate a steady increase, reaching around 4×10−7 at 60° N, where the data coverage stops. This variation is consistent with previous measurements and suggests subsidence over the northern (winter) pole at approximately 2×10−4 m s−1. HC3N displays a very sharp increase towards the north pole, where it has a mixing ratio of around 4×10−8 at 60° N at the 0.1-mbar level. The difference in gradient for the HCN and HC3N latitude variations can be explained by HC3N''s much shorter photochemical lifetime, which prevents it from mixing with air at lower latitude. It is also consistent with a polar vortex which inhibits mixing of volatile rich air inside the vortex with that at lower latitudes. Only one observation was far enough north to detect significant amounts of C2N2, giving a value of around 9×10−10 at 50° N at the 3-mbar level. [Copyright &y& Elsevier]

Abstract: New measurements of the low-temperature near-infrared absorption of methane (Sihra, 1998, Laboratory measurements of near-infrared methane bands for remote sensing of the jovian atmosphere, Ph.D. thesis, University of Oxford) have been combined with existing, longer path-length, higher-temperature data of Strong et al. (1993, Spectral parameters of self- and hydrogen-broadened methane from 2000 to 9500 cm−1 for remote sounding of the atmosphere of Jupiter, J. Quant. Spectrosc. Radiat. Trans. 50, 309–325) and fitted with band models. The combined data set is found to be more consistent with previous low-temperature methane absorption measurements than that of Strong et al. (1993, J. Quant. Spectrosc. Radiat. Trans. 50, 309–325) but covers the same wider wavelength range and accounts for both self- and hydrogen-broadening conditions. These data have been fitted with k-coefficients in the manner described by Irwin et al. (1996, Calculated k-distribution coefficients for hydrogen- and self-broadened methane in the range 2000–9500 cm−1 from exponential sum fitting to band modelled spectra, J. Geophys. Res. 101, 26,137–26,154) and have been used in multiple-scattering radiative transfer models to assess their impact on our previous estimates of the jovian cloud structure obtained from Galileo Near-Infrared Mapping Spectrometer (NIMS) observations (Irwin et al., 1998, Cloud structure and atmospheric composition of Jupiter retrieved from Galileo NIMS real-time spectra, J. Geophys. Res. 103, 23,001–23,021; Irwin et al., 2001, The origin of belt/zone contrasts in the atmosphere of Jupiter and their correlation with 5-μm opacity, Icarus 149, 397–415; Irwin and Dyudina, 2002, The retrieval of cloud structure maps in the equatorial region of Jupiter using a principal component analysis of Galileo/NIMS data, Icarus 156, 52–63). Although significant differences in methane opacity are found at cooler temperatures, the difference in the optical depth of the atmosphere due to methane is found to diminish rapidly with increasing pressure and temperature and thus has negligible effect on the cloud structure inferred at deeper levels. Hence the main cloud opacity variation is still found to peak at around 1–2 bar using our previous analytical approach, and is thus still in disagreement with Galileo Solid State Imager (SSI) determinations (Banfield et al., 1998, Jupiter''s cloud structure from Galileo imaging data, Icarus 135, 230–250; Simon-Miller et al., 2001, Color and the vertical structure in Jupiter''s belts, zones and weather systems, Icarus 154, 459–474) which place the main cloud deck near 0.9 bar. Further analysis of our retrievals reveals that this discrepancy is probably due to the different assumptions of the two analyses. Our retrievals use a smooth vertically extended cloud profile while the SSI determinations assume a thin NH3 cloud below an extended haze. When the main opacity in our model is similarly assumed to be due to a thin cloud below an extended haze, we find the main level of cloud opacity variation to be near the 1 bar level—close to that determined by SSI and moderately close to the expected condensation level of ammonia ice of 0.85 bar, assuming that the abundance of ammonia on Jupiter is (Folkner et al., 1998, Ammonia abundance in Jupiter''s atmosphere derived from the attenuation of the Galileo probe''s radio signal, J. Geophys. Res. 103, 22,847–22,855; Atreya et al., 1999, A comparison of the atmospheres of Jupiter and Saturn: deep atmospheric composition, cloud structure, vertical mixing, and origin, Planet. Space Sci. 47, 1243–1262). However our data in the 1–2.5 μm range have good height discrimination and our lowest estimate of the cloud base pressure of 1 bar is still too great to be consistent with the most recent estimates of the ammonia abundance of 3.5 × solar. Furthermore the observed limited spatial distribution of ammonia ice absorption features on Jupiter suggests that pure ammonia ice is only present in regions of localised vigorous uplift (Baines et al., 2002, Fresh ammonia ice clouds in Jupiter: spectroscopic identification, spatial distribution, and dynamical implications, Icarus 159, 74–94) and is subsequently rapidly modified in some way which masks its pure absorption features. Hence we conclude that the main cloud deck on Jupiter is unlikely to be composed of pure ammonia ice and instead find that it must be composed of either NH4SH or some other unknown combination of ammonia, water, and hydrogen sulphide and exists at pressures of between 1 and 2 bar. [Copyright &y& Elsevier]