Search

Here we search for hydrogen chloride (HCl) in Neptune’s stratosphere using observations of the 1876.22 GHz J=3–2 transition from the Heterodyne Instrument for the Far-Infrared (HIFI) on Herschel. Observations comprise a 7.2 hr disc-averaged integration, originally designed to investigate stratospheric methane. Significant HCl emission was not detected. Instead, we determine upper limits using step-type abundance profiles, defined by zero deep abundance and uniform volume mixing ratio for pressures less than a transition pressure (assumed to be 0.1 or 1 mbar). These profiles are a reasonable first-order approximation for an externally sourced species; at higher pressures HCl is expected to be removed by aerosol scavenging and reactions with ammonia. The 3 σ upper limits are 0.70 parts per billion (ppb) for a 0.1 mbar transition pressure and 0.076 ppb for a 1 mbar transition pressure. These upper limits are the most stringent to date and are consistent with current estimates of interplanetary dust particle flux and the hypothesis that Neptune experienced a large comet impact in the past 1000 years.

The martian atmosphere is host to many trace gases including water (H 2 O) and its isotopologues, methane (CH 4 ) and potentially sulphur dioxide (SO 2 ), nitrous oxide (N 2 O) and further organic compounds, which would serve as indirect tracers of geological, chemical and biological processes on Mars. With exception of the recent detection of CH 4 by Curiosity, previous detections of these species have been unsuccessful or considered tentative due to the low concentrations of these species in the atmosphere (∼10 −9 partial pressures), limited spectral resolving power and/or signal-to-noise and the challenge of discriminating between telluric and martian features when observing from the Earth. In this study, we present radiative transfer simulations of an alternative method for detection of trace gas species – the gas correlation radiometry method. Two potential observing scenarios were explored where a gas correlation filter radiometer (GCFR) instrument: (1) performs nadir and/or limb sounding of the martian atmosphere in the thermal infrared (200–2000 cm −1 from an orbiting spacecraft or (2) performs solar occultation measurements in the near-infrared (2000–5000 cm −1 ) from a lander on the martian surface. In both scenarios, simulations of a narrowband filter radiometer (without gas correlation) were also generated to serve as a comparison. From a spacecraft, we find that a gas correlation filter radiometer, in comparison to a filter radiometer (FR), offers a greater discrimination between temperature and dust, a greater discrimination between H 2 O and HDO, and would allow detection of N 2 O and CH 3 OH at concentrations of ∼10 ppbv and ∼2 ppbv, respectively, which are lower than previously-derived upper limits. However, the lowest retrievable concentration of SO 2 (approximately 2 ppbv) is comparable with previous upper limits and CH 4 is only detectable at concentrations of approximately 10 ppbv, which is an order of magnitude higher than the concentration recently measured by Curiosity. From a lander in low dust conditions, both a filter radiometer and gas correlation filter radiometer would provide measurement of H 2 O and HDO, which allows the D/H ratio in H 2 O to be determined. Detection of N 2 O, CH 4 , SO 2 , C 2 H 2 , C 2 H 6 at concentrations lower than previously-derived upper limits would be possible using a gas correlation filer radiometer in low dust conditions. However, either radiometer would be unable to detect these trace gases in high dust conditions, with the exception of H 2 O.

Jupiter's tropospheric composition and cloud structure are studied using Cassini VIMS 4.5-5.1 {\mu}m thermal emission spectra from the 2000-2001 flyby. We make use of both nadir and limb darkening observations on the planet's nightside, and compare these with dayside observations. Although there is significant spatial variability in the 5-{\mu}m brightness temperatures, the shape of the spectra remain very similar across the planet, suggesting the presence of a spectrally-flat, spatially inhomogeneous cloud deck. We find that a simple cloud model consisting of a single, compact cloud is able to reproduce both nightside and dayside spectra, subject to the following constraints: (i) the cloud base is located at pressures of 1.2 bar or lower; (ii) the cloud particles are highly scattering; (iii) the cloud is sufficiently spectrally flat. Using this cloud model, we search for global variability in the cloud opacity and the phosphine deep volume mixing ratio. We find that the vast majority of the 5-{\mu}m inhomogeneity can be accounted for by variations in the thickness of the cloud decks, with huge differences between the cloudy zones and the relatively cloud-free belts. The relatively low spectral resolution of VIMS limits reliable retrievals of gaseous species, but some evidence is found for an enhancement in the abundance of phosphine at high latitudes., Comment: 16 pages, 17 figures, manuscript accepted for publication in Icarus

The Texas Echelon cross Echelle Spectrograph (TEXES), mounted on NASA’s Infrared Telescope Facility (IRTF), was used to map mid-infrared ammonia absorption features on both Jupiter and Saturn in February 2013. Ammonia is the principle reservoir of nitrogen on the giant planets, and the ratio of isotopologues ( 15 N/ 14 N) can reveal insights into the molecular carrier (e.g., as N 2 or NH 3 ) of nitrogen to the forming protoplanets, and hence the source reservoirs from which these worlds accreted. We targeted two spectral intervals (900 and 960 cm −1 ) that were relatively clear of terrestrial atmospheric contamination and contained close features of 14 NH 3 and 15 NH 3 , allowing us to derive the ratio from a single spectrum without ambiguity due to radiometric calibration (the primary source of uncertainty in this study). We present the first ground-based determination of Jupiter’s 15 N/ 14 N ratio (in the range from 1.4 × 10 - 3 to 2.5 × 10 - 3 ), which is consistent with both previous space-based studies and with the primordial value of the protosolar nebula. On Saturn, we present the first upper limit on the 15 N/ 14 N ratio of no larger than 2.0 × 10 - 3 for the 900-cm −1 channel and a less stringent requirement that the ratio be no larger than 2.8 × 10 - 3 for the 960-cm −1 channel ( 1 σ confidence). Specifically, the data rule out strong 15 N-enrichments such as those observed in Titan’s atmosphere and in cometary nitrogen compounds. To the extent possible with ground-based radiometric uncertainties, the saturnian and jovian 15 N/ 14 N ratios appear indistinguishable, implying that 15 N-enriched ammonia ices could not have been a substantial contributor to the bulk nitrogen inventory of either planet. This result favours accretion of primordial N 2 on both planets, either in the gas phase from the solar nebula, or as ices formed at very low temperatures. Finally, spatially-resolved TEXES observations are used to derive zonal contrasts in tropospheric temperatures, phosphine and 14 NH 3 on both planets, allowing us to relate thermal conditions and chemical compositions to phenomena observed at visible wavelengths in 2013 (e.g., Jupiter’s faint equatorial red colouration event and wave activity in the equatorial belts, plus the remnant warm band on Saturn following the 2010–11 springtime storm).

The planet-encircling springtime storm in Saturn's troposphere (December 2010-July 2011, Fletcher, L.N. et al. [2011]. Science 332, 1413-1414; Sánchez-Lavega, A. et al. [2011]. Nature 475, 71-74; Fischer, G. et al. [2011]. Nature 475, 75-77) produced dramatic perturbations to stratospheric temperatures, winds and composition at mbar pressures that persisted long after the tropospheric disturbance had abated. Thermal infrared (IR) spectroscopy from the Cassini Composite Infrared Spectrometer (CIRS), supported by ground-based IR imaging from the VISIR instrument on the Very Large Telescope and the MIRSI instrument on NASA's IRTF, is used to track the evolution of a large, hot stratospheric anticyclone between January 2011 and March 2012. The evolutionary sequence can be divided into three phases: (I) the formation and intensification of two distinct warm airmasses near 0.5. mbar between 25 and 35°N (B1 and B2) between January-April 2011, moving westward with different zonal velocities, B1 residing directly above the convective tropospheric storm head; (II) the merging of the warm airmasses to form the large single 'stratospheric beacon' near 40°N (B0) between April and June 2011, disassociated from the storm head and at a higher pressure (2 mbar) than the original beacons, a downward shift of 1.4 scale heights (approximately 85. km) post-merger; and (III) the mature phase characterised by slow cooling (0.11. ±. 0.01. K/day) and longitudinal shrinkage of the anticyclone since July 2011. Peak temperatures of 221.6. ±. 1.4. K at 2. mbar were measured on May 5th 2011 immediately after the merger, some 80. K warmer than the quiescent surroundings. From July 2011 to the time of writing, B0 remained as a long-lived stable stratospheric phenomenon at 2. mbar, moving west with a near-constant velocity of 2.70. ±. 0.04. deg/day (-24.5. ±. 0.4. m/s at 40°N relative to System III longitudes). No perturbations to visible clouds and hazes were detected during this period.With no direct tracers of motion in the stratosphere, we use thermal windshear calculations to estimate clockwise peripheral velocities of 200-400m/s at 2mbar around B0. The peripheral velocities of the two original airmasses were smaller (70-140m/s). In August 2011, the size of the vortex as defined by the peripheral collar was 65° longitude (50,000km in diameter) and 25° latitude. Stratospheric acetylene (C 2H 2) was uniformly enhanced by a factor of three within the vortex, whereas ethane (C 2H 6) remained unaffected. The passage of B0 generated a new band of warm stratospheric emission at 0.5mbar at its northern edge, and there are hints of warm stratospheric structures associated with the beacons at higher altitudes (p

Observations of Neptune were made in September 2009 with the Gemini-North Telescope in Hawaii, using the NIFS instrument in the H-band covering the wavelength range 1.477-1.803 μm. Observations were acquired in adaptive optics mode and have a spatial resolution of approximately 0.15-0.25″. The observations were analysed with a multiple-scattering retrieval algorithm to determine the opacity of clouds at different levels in Neptune's atmosphere. We find that the observed spectra at all locations are very well fit with a model that has two thin cloud layers, one at a pressure level of ∼2. bar all over the planet and an upper cloud whose pressure level varies from 0.02 to 0.08. bar in the bright mid-latitude region at 20-40°S to as deep as 0.2. bar near the equator. The opacity of the upper cloud is found to vary greatly with position, but the opacity of the lower cloud deck appears remarkably uniform, except for localised bright spots near 60°S and a possible slight clearing near the equator. A limb-darkening analysis of the observations suggests that the single-scattering albedo of the upper cloud particles varies from ∼0.4 in regions of low overall albedo to close to 1.0 in bright regions, while the lower cloud is consistent with particles that have a single-scattering albedo of ∼0.75 at this wavelength, similar to the value determined for the main cloud deck in Uranus' atmosphere. The Henyey-Greenstein scattering particle asymmetry of particles in the upper cloud deck are found to be in the range g∼ 0.6-0.7 (i.e. reasonably strongly forward scattering).Numerous bright clouds are seen near Neptune's south pole at a range of pressure levels and at latitudes between 60 and 70°S. Discrete clouds were seen at the pressure level of the main cloud deck (∼2. bar) at 60°S on three of the six nights observed. Assuming they are the same feature we estimate the rotation rate at this latitude and pressure to be 13.2 ± 0.1. h. However, the observations are not entirely consistent with a single non-evolving cloud feature, which suggests that the cloud opacity or albedo may vary very rapidly at this level at a rate not seen in any other giant-planet atmosphere. © 2011 Elsevier Inc.

Observations of Uranus were made in September 2009 with the Gemini-North telescope in Hawaii, using both the NIFS and NIRI instruments. Observations were acquired in Adaptive Optics mode and have a spatial resolution of approximately 0.1″. NIRI images were recorded with three spectral filters to constrain the overall appearance of the planet: J, H-continuum and CH 4 (long), and long slit spectroscopy measurements were also made (1.49–1.79 μm) with the entrance slit aligned on Uranus’ central meridian. To acquire spectra from other points on the planet, the NIFS instrument was used and its 3″ × 3″ field of view stepped across Uranus’ disc. These observations were combined to yield complete images of Uranus at 2040 wavelengths between 1.476 and 1.803 μm. The observed spectra along Uranus central meridian were analysed with the NEMESIS retrieval tool and used to infer the vertical/latitudinal variation in cloud optical depth. We find that the 2009 Gemini data perfectly complement our observations/conclusions from UKIRT/UIST observations made in 2006–2008 and show that the north polar zone at 45°N has continued to steadily brighten while that at 45°S has continued to fade. The improved spatial resolution of the Gemini observations compared with the non-AO UKIRT/UIST data removes some of the earlier ambiguities with our previous analyses and shows that the opacity of clouds deeper than the 2-bar level does indeed diminish towards the poles and also reveals a darkening of the deeper cloud deck near the equator, perhaps coinciding with a region of subduction. We find that the clouds at 45°N,S lie at slightly lower pressures than the clouds at more equatorial latitudes, which suggests that they might possibly be composed of a different condensate, presumably CH 4 ice, rather than H 2 S or NH 3 ice, which is assumed for the deeper cloud. In addition, analysis of the centre-to-limb curves of both the Gemini/NIFS and earlier UKIRT/UIST IFU observations shows that the main cloud deck has a well-defined top, and also allows us to better constrain the particle scattering properties. Overall, Uranus appeared to be less convectively active in 2009 than in the previous 3 years, which suggests that now the northern spring equinox (which occurred in 2007) is passed the atmosphere is settling back into the quiescent state seen by Voyager 2 in 1986. However, a number of discrete clouds were still observed, with one at 15°N found to lie near the 500 mb level, while another at 30°N, was seen to be much higher at near the 200 mb level. Such high clouds are assumed to be composed of CH 4 ice.

Five years of Cassini CIRS infrared spectra have been used to determine the tilt of Titan's stratospheric symmetry axis with respect to the solid body rotation axis. Measurements of HCN abundance centred around 5 mbar (125 km altitude) at equatorial latitudes show the symmetry axis is tilted by 4.0 ± 1 . 5{ring operator} in a direction 70 ± 40{ring operator} W of the sub-solar point. This value is consistent with tilts determined from temperature and haze measurements by Achterberg et al. (2008a) and Roman et al. (2009). The consistency of results from three independent methods suggests that Titan's entire stratosphere is tilted and provides a powerful constraint on the underlying atmospheric dynamics. © 2010 Elsevier Ltd. All rights reserved.

Fine scale layering of haze and composition in Titan's stratosphere and mesosphere was investigated using visible/UV images from Cassini's Imaging Science Sub-system (ISS) and IR spectra from Cassini's Composite Infra-Red Spectrometer (CIRS). Both ISS and CIRS independently show fine layered structures in haze and composition, respectively, in the 150-450 km altitude range with a preferred vertical wavelength of around 50 km. Layers are most pronounced around the north polar winter vortex, although some weaker layers do exist at more southerly latitudes. The amplitude of composition layers in each trace gas profile is proportional to the relative enrichment of that species in the winter polar vortex compared to equatorial latitudes. As enrichment is caused by polar subsidence, this suggests a dynamical origin. We propose that the polar layers are caused by cross-latitude advection across the vortex boundary. This is analogous to processes that lead to ozone laminae formation around Earth's polar vortices. © 2009 Elsevier Inc. All rights reserved.

Long-slit spectroscopy observations of Uranus by the United Kingdom Infrared Telescope UIST instrument in 2006, 2007 and 2008 have been used to monitor the change in Uranus' vertical and latitudinal cloud structure through the planet's northern spring equinox in December 2007. The observed reflectance spectra in the Long J (1.17-1.31 μm) and H (1.45-1.65 μm) bands, obtained with the slit aligned along Uranus' central meridian, have been fitted with an optimal estimation retrieval model to determine the vertical cloud profile from 0.1 to 6-8 bar over a wide range of latitudes. Context images in a number of spectral bands were used to discriminate general zonal cloud structural changes from passing discrete clouds. From 2006 to 2007 reflection from deep clouds at pressures between 2 and 6-8 bar increased at all latitudes, although there is some systematic uncertainty in the absolute pressure levels resulting from extrapolating the methane coefficients of Irwin et al. (Irwin, P.G.J., Sromovsky, L.A., Strong, E.K., Sihra, K., Teanby, N.A., Bowles, N., Calcutt, S.B., Remedios, J.J. [2006] Icarus, 181, 309-319) at pressures greater than 1 bar, as noted by Tomasko et al. and Karkoschka and Tomasko (Tomasko, M.G., Bezard, B., Doose, L., Engel, S., Karkoschka, E. [2008] Planet. Space Sci., 56, 624-647; Karkoschka, E., Tomasko, M. [2009] Icarus). However, from 2007 to 2008 reflection from these clouds throughout the southern hemisphere and from both northern and southern mid-latitudes (30° N,S) diminished. As a result, the southern polar collar at 45°S has diminished in brightness relative to mid-latitudes, a similar collar at 45°N has become more prominent (e.g. Rages, K.A., Hammel, H.B., Sromovsky, L. [2007] Bull. Am. Astron. Soc., 39, 425; Sromovsky, L.A., Fry, P.M., Ahue, W.M., Hammel, H.B., de Pater, I., Rages, K.A., Showalter, M.R., van Dam, M.A. [2008] vol. 40 of AAS/Division for Planetary Sciences Meeting Abstracts, pp. 488-489; Sromovsky, L.A., Ahue, W.K.M., Fry, P.M., Hammel, H.B., de Pater, I., Rages, K.A., Showalter, M.R. [2009] Icarus), and the lowering reflectivity from mid-latitudes has left a noticeable brighter cloud zone at the equator (e.g. Sromovsky, L.A., Fry, P.M. [2007] Icarus, 192, 527-557;Karkoschka, E., Tomasko, M. [2009] Icarus). For such substantial cloud changes to have occurred in just two years suggests that the circulation of Uranus' atmosphere is much more vigorous and/or efficient than is commonly thought. The composition of the main observed cloud decks between 2 and 6-8 bar is unclear, but the absence of the expected methane cloud at 1.2-1.3 bar (Lindal, G.F., Lyons, J.R., Sweetnam, D.N., Eshleman, V.R., Hinson, D.P. [1987] J. Geophys. Res., 92, 14987-15001) is striking (as previously noted by, among others, Sromovsky, L.A., Irwin, P.G.J., Fry, P.M. [2006] Icarus, 182, 577-593; Sromovsky, L.A., Fry, P.M. [2007] Icarus, 192, 527-557; Sromovsky, L.A., Fry, P.M. [2008] Icarus, 193, 252-266; Karkoschka, E., Tomasko, M. [2009] Icarus) and suggests that cloud particles may be considerably different from pure condensates and may be linked with stratospheric haze particles drizzling down from above, or that tropospheric hazes are generated near the methane condensation level and then drizzle down to deep pressures as suggested by Karkoschka and Tomasko (Karkoschka, E., Tomasko, M. [2009] Icarus). The retrieved cloud structures were also tested for different assumptions of the deep methane mole fraction, which Karkoschka and Tomasko (Karkoschka, E., Tomasko, M. [2009] Icarus) find may vary from ∼1-2% in polar regions to perhaps as much as 4% equatorwards of 45°N,S. We found that such variations did not significantly affect our conclusions. © 2009 Elsevier Inc. All rights reserved.

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., Bezard, B., Fouchet, T., Feuchtgruber, H., Encrenaz, T., de Graauw, T., 2001. Astron. Astrophys. 370, 610–622]. Mid-IR emission features provided an estimate of C 12 / C 13 = 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.

In Titan's north polar region stratospheric clouds are expected to form due to a combination of low temperatures and downward motion of volatile-enriched air. Here we investigate possible sources of stratospheric clouds at Titan's pole using data from the Cassini Composite Infrared Spectrometer and a simple condensation model. An upper limit for C 4 N 2 gas was determined to be 9 × 10 −9 , which is less than required to make the C 4 N 2 cloud at the Voyager epoch. Hence, the presence of this cloud after equinox remains a mystery. The largest cloud seen in far-infrared spectra has a feature around 220 cm −1 and is located around an altitude of 140 km. The upper limit for propionitrile (C 2 H 5 CN) gas shows that the feature around 220 cm −1 is probably not due to pure propionitrile ice. Instead, our model calculations show that HCN should cause by far the largest cloud around 140 km. We therefore propose that HCN ice plays an important role in the formation of the massive polar cloud, because of the unavailability of sufficient condensable gas other than HCN to produce a strong enough condensate feature. However, the signature at 220 cm −1 is not consistent with that of pure HCN ice at 172 cm −1 and mixing of HCN ice with other ices, or chemical alteration of HCN ice might mask the HCN ice signature.

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.

International audience; 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, HC 3N, C 2H 2, 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. HC 3N 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 HC 3N's short lifetime. In the far north, layers were observed in the HC 3N 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 HC 3N due to HCN's longer photochemical lifetime. C 2H 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 2H 2 towards the north pole.

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 ( 7 ± 1 ) × 10 −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 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.

NIMS is the Near-Infrared Mapping Spectrometer on board the Galileo spacecraft in jovian orbit. We have selected four maps of warm-to-hot regions of the North Equatorial Belt (NEB) for study, analyzing the spectra emerging in the low-opacity 5-μm window. Two methods for calculating the spectrum have been used. The first is a full-scattering radiative transfer forward model that is slow but accurate. The second method calculates spectra by interpolating on a grid of spectra precalculated using the first method for a range of model atmospheres. This method of forward calculation is more suited to analysis of large data sets where application of the full radiative transfer in every instance would be computationally prohibitive. The faster method is verified against the first before being used alone. A retrieval (inversion) algorithm is then used to match model spectra to data and obtain values for cloud opacities and gas mixing ratios. We first sum spectra with similar peak radiances to produce mean spectra representative of brighter and darker (at 5 μm) regions of the maps. These coadded spectra are then analyzed with the fast retrieval code to obtain the average variations in atmospheric parameters from the center to the edges of the hotspots. These analyses confirm that 5-μm hotspots are relatively cloud free, and that a medium level (1.5-bar) cloud layer of large NH4SH particles is the main absorber at these wavelengths. Variations in water vapor relative humidity and high (0.5-bar) ammonia cloud opacity are also derived. We then analyze single spectra over wide areas to produce spatial maps of parameter variations. We find that models that do not include a deep water cloud (~4 bar) do not match all the spectra to within the noise level. A deep water cloud therefore seems to be present in localized areas, toward the edges of the hotspot regions. We examine these findings in the light of results from other Galileo instruments, concluding that the deep cloud observed by the SSI instrument at several locations is likely to be the deep water cloud required by the NIMS data. © 2001 Academic Press.

This paper considers the rationale for the exploration of Saturn's atmosphere by the Cassini mission, taking account of the key scientific questions posed by the earlier investigation by Voyager, and the capabilities of the instrumentation making up the Cassini payload. While by no means all objectives can be addressed by this particular configuration, in particular without a Saturn entry probe, if everything goes according to plan important progress should be possible on a number of key objectives. (C) 1998 Elsevier Science Ltd. All rights reserved.