Abundances of Jupiter's trace hydrocarbons from Voyager and Cassini     

C.A. Nixon  R.K. Achterberg  P.N. Romani  X. Zhang  P.G.J. Irwin 《Planetary and Space Science》2010,58(13):1667-1680

The flybys of Jupiter by the Voyager spacecraft in 1979, and over two decades later by Cassini in 2000, have provided us with unique datasets from two different epochs, allowing the investigation of seasonal change in the atmosphere. In this paper we model zonal averages of thermal infrared spectra from the two instruments, Voyager 1 IRIS and Cassini CIRS, to retrieve the vertical and meridional profiles of temperature, and the abundances of the two minor hydrocarbons, acetylene (C₂H₂) and ethane (C₂H₆). The spatial variation of these gases is controlled by both chemistry and dynamics, and therefore their observed distribution gives us an insight into both processes. We find that the two gases paint quite different pictures of seasonal change. Whilst the 2-D cross-section of C₂H₆ abundance is slightly increased and more symmetric in 2000 (northern summer solstice) compared to 1979 (northern fall equinox), the major trend of equator to pole increase remains. For C₂H₂ on the other hand, the Voyager epoch exhibits almost no latitudinal variation, whilst the Cassini era shows a marked decrease polewards in both hemispheres. At the present time, these experimental findings are in advance of interpretation, as there are no published models of 2-D Jovian seasonal chemical variation available for comparison.  相似文献   

Spatial and Temporal Variability of Benthic Oxygen Demand and Nutrient Regeneration in an Anthropogenically Impacted New England Estuary     

Robinson W. Fulweiler  Scott W. Nixon  Betty A. Buckley 《Estuaries and Coasts》2010,33(6):1377-1390

Strong benthic–pelagic coupling is an important characteristic of shallow coastal marine ecosystems. Building upon a rich history of benthic metabolism data, we measured oxygen uptake and nutrient fluxes across the sediment–water interface along a gradient of water column primary production in Narragansett Bay, RI (USA). Despite the strong gradients seen in water column production, sediment oxygen demand (SOD) and benthic nutrient fluxes did not exhibit a clear spatial pattern. Some of our sites had been studied in the 1970s and 1980s and thus allowed historical comparison. At these sites, we found that SOD and benthic fluxes have not changed uniformly throughout Narragansett Bay. In the uppermost portion of the bay, the Providence River Estuary, we observed a significant decrease in dissolved inorganic phosphorus fluxes which we attribute to management interventions. At another upper bay site, we observed significant declines in SOD and dissolved inorganic nitrogen fluxes which may be linked to climate-induced decreases in water column primary production and shifts in bloom phenology. In the 1970s, benthic nutrient regeneration supplied 50% to over 200% of the N and P needed to support primary production by phytoplankton. Summer nutrient regeneration in the Providence River Estuary and Upper bay now may only supply some 5–30% of the N and 3–20% of the P phytoplankton demand.  相似文献   

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

R. de Kok  P.G.J. Irwin  E. Lellouch  B. Bézard  S. Vinatier  C.A. Nixon  C. Howett  N.E. Bowles  F.W. Taylor 《Icarus》2007,186(2):354-363

We have investigated the abundances of Titan's stratospheric oxygen compounds using 0.5 cm⁻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° S-35° N) and emission angles (0°-60°), 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±8 ppm if CO is assumed to be uniform with latitude. H₂O could not be detected and an upper limit of 0.9 ppb was determined. CO₂ abundances derived from mid-infrared nadir spectra show no significant latitudinal variations, with typical values of 16±2 ppb. Mid-infrared limb spectra at 55° S were used to constrain the vertical profile of CO₂ for the first time. A vertical CO₂ 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 CO₂ in Titan's stratosphere. Above 200 km the CO₂ volume mixing ratio is not well constrained and an increase with altitude cannot be ruled out there.  相似文献   

Search for spatial variation in the jovian N/N ratio from Cassini/CIRS observations     

Thierry Fouchet  Patrick G.J. Irwin  Simon B. Calcutt  Conor A. Nixon 《Icarus》2004,172(1):50-58

We have used the spectra obtained by the Composite Infrared Spectrometer (CIRS) onboard the Cassini spacecraft to search for latitudinal variation in the ¹⁵N/¹⁴N ratio on Jupiter. We found no variations statistically significant given the observational and model uncertainties. The absence of latitudinal variations demonstrates that ¹⁵NH₃ is not fractionated in Jupiter's atmosphere, and that the measured ¹⁵N/¹⁴N represents Jupiter's global value. Our mean value for the global jovian ¹⁵N/¹⁴N ratio of (2.22±0.52)×10⁻³ agrees with previous measurements made by Fouchet et al. (2000, Icarus 143, 223-243) and Owen et al. (2001, Astrophys. J. 553, L77-L79). We argue that the jovian isotopic ¹⁵N/¹⁴N ratio must represent the solar nitrogen isotopic composition. The solar ¹⁵N/¹⁴N ratio hence significantly differs from the terrestrial value: (¹⁵N/¹⁴N)_⊕=3.68×10⁻³. This supports the proposition that terrestrial nitrogen originates from a nitrogen reservoir isolated from the main nitrogen reservoir in the proto-solar nebula. The origin and carrier of this isolated reservoir are still unknown.  相似文献   

Determining a tilt in Titan’s north-south albedo asymmetry from Cassini images     

Michael T. Roman  Robert A. West  Peter J. Gierasch  Conor A. Nixon 《Icarus》2009,203(1):242-249

Analysis of Titan’s hemispheric brightness asymmetry from mapped Cassini images reveals an axis of symmetry that is tilted with respect to the rotational axis of the solid body. Twenty images taken from 2004 through 2007 show a mean axial offset of 3.8 ± 0.9° relative to the solid body’s pole, directed 79 ± 24° to the west of the sub-solar longitude. These values are consistent with recent measurements of an implied atmospheric spin axis determined from isothermal mapping by [Achterberg, R.K., Conrath, B.J., Gierasch, P.J., Flasar, F.M., Nixon, C.A., 2008. Icarus 197, 549-555].  相似文献   

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

N.A. Teanby  P.G.J. Irwin  C.A. Nixon  E. Royer  N.E. Bowles  C. Howett 《Icarus》2008,193(2):595-611

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

New marine ΔR values for Arctic Canada   总被引：1，自引：0，他引：1  

Roy D. Coulthard  Mark F.A. Furze  Anna J. Pieńkowski  F. Chantel Nixon  John H. England 《Quaternary Geochronology》2010,5(4):419-434

For more than four decades, the reporting of ¹⁴C dates on marine molluscs from Arctic Canada has been notable for the lack of consistently applied marine reservoir corrections. We propose that the common approach of reporting Canadian Arctic marine ¹⁴C dates using presumed time-invariant reservoir corrections be abandoned in favour of calibration of ¹⁴C dates, using the current standard protocol. This approach best facilitates inter- and intra-regional correlation, and correlation with other geochronometers. In order to enable the consistent calibration of marine ¹⁴C dates from Arctic Canada, we analysed a ¹⁴C database of 108 marine mollusc samples collected live between 1894 and 1956, and determined regional reservoir offset values (ΔR) for eight oceanographically distinct regions. The following new ΔR values should be used for ¹⁴C calibration: NW Canadian Arctic Archipelago, 335 ± 85 yrs; Foxe Basin, 310 ± 90 yrs; NE Baffin Island, 220 ± 20 yrs; SE Baffin Island, 150 ± 60 yrs; Hudson Strait, 65 ± 60 yrs; Ungava Bay, 145 ± 95 yrs; Hudson Bay, 110 ± 65 yrs; and James Bay, 365 ± 115 yrs.  相似文献   

Accumulation of sediments,trace metals (Pb,Cu) and total hydrocarbons in Narragansett Bay,Rhode Island     

Peter H. Santschi  Scott Nixon  Michael Pilson  Carlton Hunt 《Estuarine, Coastal and Shelf Science》1984,19(4):427-449

The accumulation of sediments, trace metals and hydrocarbons has been estimated from the analysis of the sediment from six coring sites in Narragansett Bay. Radionuclides (²³⁴Th_xs, ²¹⁰Pb_xs, ^239,240Pu) with known input functions and trace metals (Cu, Pb) were used. We estimate that 6·9 × 10⁴ tons of sediments, 51–90 tons of Pb, 72–100 tons of Cu and 400–1000 tons of total hydrocarbons accumulate annually under present conditions in the bay. This represents 64–117% (Pb), 89–123% (Cu) and 23–58% (hydrocarbons), respectively, of present day inputs to the bay. Furthermore, close to 100% of the particle-reactive radionuclides ²¹⁰Pb and ^239,240Pu accumulate in the bay. Present day inputs to the bay were calculated independently as 77–80 tons Pb and 81 tons of Cu. Sewage effluents were the dominant source of Cu, whereas atmospheric deposition and urban runoff were most important for Pb. Dredging activities by the U.S. Army Corps of Engineers between 1946 and 1971 removed more sediments from the bay than would have accumulated during the same time in the undredged areas of the bay. Copper smelting and coal mining on the shores of the upper bay during 1866–1880 left an imprint in the sediments which is still evident. Model derived accumulation rates of Pb, Cu and coal during that time were 3–4 times present-day inputs.  相似文献   

Toward a broader definition of estuaries?     

Scott W. Nixon 《Estuaries and Coasts》1990,13(4):492-492

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Electrical interferences observed in the Cassini CIRS spectrometer     

Cheong Chan  Shane Albright  Nicolas Gorius  John Brasunas  Don Jennings  F. Michael Flasar  Ronald Carlson  Ever Guandique  Conor Nixon 《Experimental Astronomy》2015,39(2):367-386

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