Search

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

[1] Isotopically dated corals from the central New Hebrides and New Georgia Island Group, Solomon Islands, indicate that both forearcs underwent rapid late Quaternary subsidence that was abruptly replaced by hundreds of meters of uplift at rates up to ~8 mm/yr, while total plate convergence was only a few kilometers. Two mechanisms that might account for these rapid reversals in vertical motion include (1) a 'displacement' mechanism in which the forearc is displaced upward by the volume of an object passing beneath on the subducting plate (as the object moves deeper and vacates the base of the forearc, the forearc subsides to near its original position) and (2) a 'crustal shortening' mechanism in which the forearc thickens and uplifts because of horizontal shortening when a large object impinges on the forearc and abruptly increases interplate coupling on the shallow end of the main thrust zone. Rapid subsidence follows when the impinging object is broken or otherwise decoupled, shallow interplate coupling becomes weak, and the uplifted forearc extends and subsides. The displacement mechanism surely plays a role on timescales over which plates converge tens of kilometers, but it fails to explain the geographic pattern, short time frame, and abruptness of the change from subsidence to uplift that we observe. The crustal shortening mechanism is preferred because it allows the observed abrupt uplift when an object impinges on a forearc and causes locking of a shallow segment of the interplate thrust zone. Citation: Taylor, F. W., et al. (2005), Rapid forearc uplift and subsidence caused by impinging bathymetric features: Examples from the New Hebrides and Solomon arcs, Tectonics, 24, TC6005, doi:l0.1029/2004TC001650.

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

Temperatures obtained from early Cassini infrared observations of Titan show a stratopause at an altitude of 310 kilometers (and 186 kelvin at 15°S). Stratospheric temperatures are coldest in the winter northern hemisphere, with zonal winds reaching 160 meters per second. The concentrations of several stratospheric organic compounds are enhanced at mid- and high northern latitudes, and the strong zonal winds may inhibit mixing between these latitudes and the rest of Titan. Above the south pole, temperatures in the stratosphere are 4 to 5 kelvin cooler than at the equator. The stratospheric mole fractions of methane and carbon monoxide are (1.6 ± 0.5) x [10.sup.-2] and (4.5 ± 1.5) x [10.sup.-5] respectively., Unlike other moons in the solar system, Titan has a substantial atmosphere and offers an interesting comparison with Earth and the other planets. Its pressure at the surface is 1.5 [...]

Stratospheric temperatures on Saturn imply a strong decay of the equatorial winds with altitude. If the decrease in winds reported from recent Hubble Space Telescope images is not a temporal change, then the features tracked must have been at least 130 kilometers higher than in earlier studies. Saturn's south polar stratosphere is warmer than predicted from simple radiative models. The C/H ratio on Saturn is seven times solar, twice Jupiter's. Saturn's ring temperatures have radial variations down to the smallest scale resolved (100 kilometers). Diurnal surface temperature variations on Phoebe suggest a more porous regolith than on the jovian satellites., Cassini observations of Saturn provide a detailed comparison with Jupiter, successfully studied by Galileo, which will sharpen our ideas about the formation of planetary systems. Each giant, fluid planet has [...]

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

The Composite Infrared Spectrometer observed Jupiter in the thermal infrared during the swing-by of the Cassini spacecraft. Results include the detection of two new stratospheric species, the methyl radical and diacetylene, gaseous species present in the north and south auroral infrared hot spots; determination of the variations with latitude of acetylene and ethane, the latter a tracer of atmospheric motion; observations of unexpected spatial distributions of carbon dioxide and hydrogen cyanide, both considered to be products of comet Shoemaker-Levy 9 impacts; characterization of the morphology of the auroral infrared hot spot acetylene emission; and a new evaluation of the energetics of the northern auroral infrared hot spot., Jupiter serves as an analog for the numerous extrasolar planets that have been detected; the formation and evolution of these are not well understood. The primary paradigm is Jupiter, which [...]

An outcrop within the last interglacial terrace on Barbados contains corals that grew during the penultimate deglaciation, or Termination II. We used combined [sup.230]Th and [sup.231]Pa dating to determine that they grew 135.8 ± 0.8 thousand years ago, indicating that sea level was 18 ± 3 meters below present sea level at the time. This suggests that sea level had risen to within 20% of its peak last-interglacial value by 136 thousand years ago, in conflict with Milankovitch theory predictions. Orbital forcing may have played a rote in the deglaciation, as may have isostatic adjustments due to large ice sheets. Other corals in the same outcrop grew during oxygen isotope (δ 18 O) substage 6e, indicating that sea level was 38 ± 5 meters below present sea level, about 168.0 thousand years ago. When compared to the [δ.sup.18]O signal in the benthic V19-30/V19-28 record at that time, the coral data extend to the previous glacial cycle the conclusion that deep-water temperatures were colder during glacial periods., The timing of the penultimate deglaciation (Termination II) has fundamental implications for the causes of climate change. Terminations, as distinct climatic events, provide an opportunity to test the Milankovitch theory [...]

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.

We describe the design, the development, and the calibration of a radiometer to measure water vapor abundance in the middle atmosphere, using the solar occultation technique from an orbiting platform. The use of gas cells containing water vapor and carbon dioxide leads to a relatively simple, but sensitive, design. This radiometer is shown to be capable of detecting water vapor in the mesosphere, where, because of dissociation by the Sun, the amounts are very small, of the order of 1 part per 10 to the sixth power or less. A flight version of the instrument will be launched in 1992 on board the European Retrievable Carrier space platform, and the data will be employed for a better understanding of the Earth's water budget.

The Septentrional fault zone, the major North American Caribbean plate-boundary fault in Hispaniola, is a likely source of large earthquakes in the Dominican Republic. An excavation into a Holocene alluvial fan deposited across the fault in the central Cibao Valley, Dominican Republic, provides evidence that it has been at least 430 yr and probably more than 730 yr since the last ground-rupturing earthquake along this segment of the fault. On the basis of these data and published estimates of the plate-tectonic slip rate, we propose that the Septentrional fault is a source of high seismic potential in the densely populated and rapidly developing Cibao Valley in the northern Dominican Republic.

THE FEBRUARY 1990 GALILEO FLYBY provided a unique opportunity to investigate the clouds, motions, and deep-atmosphere composition of Venus with infrared imaging spectrscopy. The spectral range and spatial resolution of [...]

Geodetic observations of crustal motions in the Tonga Lau system, using the Global Postioning System, suggest that the intense magnetic seismicity in the region can be attributed to high subduction and back-arc extension rates. The opening rate of the Lau basin increases northward up to nearly 60 mm per year. The northern Tonga ridge contains part of a rigid microplate, as suggested by the absence of straining in the region. Convergence rates across the Tonga increases northward up to nearly 240 mm per year.