Self-gravity wake parameters in Saturn’s A and B rings     

P.D. Nicholson  M.M. Hedman 《Icarus》2010,206(2):410-423

An increasing body of evidence shows that, at the sub-km level, Saturn’s main A and B rings are dominated by an ever-changing pattern of elongated, canted structures known as self-gravity wakes. Best known for causing azimuthal variations in the rings’ reflectivity, these structures also have a profound influence on how the transmission of the rings varies with both longitude and opening angle, B (Colwell et al. [2006] Geophys. Res. Lett. 33, 7201; Colwell et al. [2007] Icarus 190, 127-144; Hedman et al. [2007] Astron. J. 133, 2624-2629). We use data from three stellar occultations observed by Cassini’s Visual and Infrared Mapping Spectrometer (VIMS) to measure the transmission of the rings as a function of B, when viewed parallel to the wakes. These data are used to constrain properties of the self-gravity wakes as a function of radius across the A and B rings: specifically the fractional width of the gaps between the wakes, G/λ, and the average normal optical depth in the gaps, τ_G. We find that the overall normal optical depth of the rings, τ_n is primarily controlled by G/λ, which varies between <0.05 and ∼0.70 in the A and B rings. The gaps, however, are not completely empty, being filled by material — possibly cm-sized ring particles — with an average normal optical depth which varies from 0.12 to ∼0.4. In addition to regional variations, local variations in τ_G are seen in the regular structure which dominates the inner B ring, and in the environs of strong density waves in the A ring. The same model applied to the lower optical depth Cassini Division reveals very little evidence of self-gravity wakes, except where τ_n exceeds ∼0.25.  相似文献   

Slow degradation of ATP in simulated martian environments suggests long residence times for the biosignature molecule on spacecraft surfaces on Mars     

Andrew C. Schuerger  Patricia Fajardo-Cavazos  John E. Moores  Wayne L. Nicholson 《Icarus》2008,194(1):86-100

Prelaunch planetary protection protocols on spacecraft are designed to reduce the numbers and diversity of viable bioloads on surfaces in order to mitigate the forward contamination of planetary surfaces. In addition, there is a growing appreciation that prelaunch spacecraft cleaning protocols will be required to reduce the levels of biogenic signature molecules on spacecraft to levels that will not compromise life-detection experiments on landers. The biogenic molecule, adenosine triphosphate (ATP) was tested for long-term stability under simulated Mars surface conditions of high UV flux, low temperature, low pressure, Mars atmosphere, and clear-sky dust loading conditions. Data on UV-induced ATP degradation rates were then extrapolated to a diversity of global conditions using a radiative transfer model for UV on Mars. The UV-induced degradation of ATP tested at 4.1 W m⁻² UVC (200-280 nm), −10 °C, 7.1 mb, 95% CO₂ gas composition, and an atmospheric opacity of τ=0.1 yielded a half-life for ATP of 1342 kJ m⁻²; or extrapolated to approximately 22 sols on equatorial Mars with an atmospheric opacity of τ=0.5. Temperature was found to moderately affect ATP degradation rates under martian conditions; tests at −80 or 20 °C yielded ATP half-lives of 2594 or 1183 kJ m⁻², respectively. The ATP degradation rates reported here are over 10 orders of magnitude slower than the UV-induced biocidal rates reported in the literature on the inactivation of strongly UV-resistant bacterial spores from Bacillus pumilus SAFR-032 [Schuerger, A.C., Richards, J.T., Newcombe, D.A., Venkateswaran, K.J., 2006. Icarus 181, 52-62]. Extrapolating results to global Mars conditions, residence times for a 99% reduction of ATP on spacecraft surfaces ranged from 158 sols on Sun-exposed surfaces to approximately 32,000 sols for the undersides of landers similar to Viking. However, spacecraft materials greatly affected the survival times of ATP under martian conditions. Stainless steel was found to enhance the UV degradation of ATP by over 2 orders of magnitude compared to ATP-doped iridited aluminum, graphite, and astroquartz coupons. Extrapolating these results to global conditions, ATP on stainless steel might be expected to persist between 2 and 320 sols for upper and lower surfaces of landers. Liquid chromatography-mass spectrometry data supported the conclusion that UV irradiation acted to remove the γ-phosphate group from ATP, and no evidence was observed for the UV-degradation of d-ribose or adenine moieties. Long residence times for ATP on spacecraft materials under martian conditions suggest that prelaunch cleaning protocols may need to be strengthened to mitigate against possible ATP contamination of life-detection experiments on Mars landers.  相似文献   

A Semi-Quantitative, Regional Precipitation Data Set for Studying African Climates of the Nineteenth Century, Part I. Overview of the Data Set     

Sharon E. Nicholson 《Climatic change》2001,50(3):317-353

This article describes a historical archive of proxy and actual precipitation data that extends the African climate record back to the early nineteenth century. The `proxy' archive includes verbal, documentary references which contain information related to rainfall conditions, such as references to famine, drought, agriculture or the nature of the rainy season. The precipitation archive includes all observations made in Africa during the nineteenth century. It consists of records for 60 stations in Algeria, 87 stations in South Africa and 304 stations scattered over the rest of Africa. Information is particularly plentiful for the 1880s and 1890s. The two parts have been be combined into a semi-quantitative regional data set indicating annual rainfall conditions in terms of anomaly classes (e.g., normal, dry, wet). This data set extends from the early nineteenth century to 1900 and distinguishes seven anomaly classes, using numbers ranging from –3 to +3 to represent very wet, wet, good rains, normal, dry, drought, and severe drought. The regionalization is based on 90 geographical regions shown via studies of the modern precipitation record to be climatically homogeneous with respect to the interannual variability of rainfall. The regional aggregation allows the voluminous fragmentary information available in historical sources to be used systematically to produce multi-year time series that can be directly integrated into the modern record for each region. The resultant time series can also be subjected to statistical analysis, in order to investigate nineteenth century climate over Africa. Spatial detail is added to the data set by utilizing a unique methodology based on climatic teleconnections established from studies of rainfall variability over Africa. The historical information and station records have been combined into a file containing a regional anomaly value for up to 90 geographic regions and the years 1801–1900. Gaps necessarily remain in the matrix, but as early as the 1820s over 40 regions are represented. By the 1880s generally around 70 regions or more are represented.  相似文献   

Petrogenesis of the Merensky Reef in the Rustenburg section of the Bushveld Complex     

D. M. Nicholson  E. A. Mathez 《Contributions to Mineralogy and Petrology》1991,107(3):293-309

A petrogenetic model for the Merensky Reef in the Rustenburg section of the Bushveld Complex has been developed based on detailed field and petrographic observations and electron microprobe data. The model maintains that the reef formed by reaction of hydrous melt and a partially molten cumulate assemblage. The model is devised to account for several key observations: (1) Although the dominant rock type in the Rusterburg sections is pegmatoidal feldspathic pyroxenite, there is a continous range of reef lithology from pyroxenite to pegmatoidal harzburgite and dunite, and small amounts of olivine are present in nearly all pegmatoids. (2) The pegmatoid is usually bounded above and below by chromitite seams and the basal chromitite separated from underlying norite by a centimeter-thick layer of anorthosite. The thicknesses of the two layers exhibit a well-defined, positive correlation. (3) Inclusion of pyroxenite identical to the hanging wall and of leuconorite identical to the footwall are present in the pegmatoid. The leuconorite inclusions are surrounded by thin anorthosite and chromitite layers in the same sequence as that at the base of the reef. (4) Chromite in seams adjacent to plagioclase-rich rocks is characterized by higher Mg/Mg+Fe and Al/R3 and lower Cr/R3 than that in seams adjacent to pyroxene-rich rocks. Similar variations in mineral compositions are observed across individual chromitite seams where the underlying and overlying rock types differ. The chromite compositional variations cannot be rationalized in terms of either fractional crystallization or reequilibration with surrounding silicates. It is proposed that the present reef was originally a melt-rich horizon in norite immediately overlain by relatively crystallized pyroxenite. Magmatic vapor generated by crystallization of intercumulus melt migrated upward through fractures in the cumulate pile below the protoreef. The melt-rich protoreef became hydrated because fractures were unable to propagate through it and because the melt itself was water-undersaturated. Hydration of the intercumulus melt was accompanied by melting, and the hydration/melting front migrated downward into the footwall and upward into the hanging wall. In the footwall melting resulted first in the dissolution of orthopyroxene and then of plagioclase. With continued hydration chromite was stabilized as melt alumina content increased. The regular variations in chromite compositions reflect the original gradients in melt composition at the hydration front. The stratigraphic sequence downward through the base of the reef or pegmatoid (melt)-chromitite-anorthosite-norite represents the sequence of stable mineral assemblages across the hydration/melting front. The sequence is shown to be consistent with knowledge gained from experiments on melting of hydrous mafic systems at crustal pressures. With cooling the hydrated mixture from partial melting of norite footwall and more mafic hanging wall crystallized in the sequence chromite-olivine-pyroxene-plagioclase, with peritectic loss of some olivine. Calculations of mass balance indicate that a significant proportion of the melt was lost from the melt-rich horizon. Variations in the development of the pegmatoid and associated lithologies and amount of modal olivine in the pegmatoids along the strike of the Merensky Reef resulted because the processes of hydration, melting and melt loss operated to varying extents.  相似文献   

Saturn's F Ring: Kinematics and Particle Sizes from Stellar Occultation Studies     

A.S. Bosh  C.B. OlkinR.G. French  P.D. Nicholson 《Icarus》2002,157(1):57-75

The occultation of GSC5249-01240 by Saturn's rings was observed in a spectrally resolved mode using the Faint Object Spectrograph on the Hubble Space Telescope. By combining these data with other occultation data, we have determined the inclination of the F ring to be 0.0065±0.0014 deg. Our inclined F ring orbit model explains an abrupt decrease in flux at the west ansa of the ring that was observed during the November 1995 ring-plane crossing as well as the nondetection of 1995 S5 during one set of observations in November 1995. The F ring's equivalent depth is found to have no significant dependence on wavelength between 0.27 and 0.74 μm, indicating the presence of a population of ring particles larger in size than ∼10 μm. This contrasts with the results from a previous analysis of Voyager images at various phase angles, in which the particles were determined to be predominantly submicron in size. The difference may be due to temporal or longitudinal variability within the ring.  相似文献   

Stratiform manganese mineralisation near Inverness,Scotland: A Devonian sublacustrine hot-spring deposit?     

K. Nicholson 《Mineralium Deposita》1990,25(2):126-131

A stratiform manganiferous horizon, up to 4 m thick but localised in aerial extent, occurs 10 km south-east of Inverness in north-east Scotland. It overlies Precambrian Grampian Group schists and is itself overlain by Middle Old Red Sandstone (Devonian) conglomerate, which elsewhere rests unconformably on the schists. The deposit was mined briefly in the early 1920s and is composed principally of fine-grained braunite with minor bustamite. Lower sections of the deposit contain cavities, infilled by late baryte, calcite and rhodochrosite, which resemble stromatactis. This horizon is underlain by a hematite-quartz layer, thought to represent a laterite formed by pre-MORS weathering of the underlying schists. The mineralogy and geochemistry of the manganese ore (enriched in Ag, As, Ba, Bi, Pb, Sb, Sr and Zn) are characteristic of a hydrothermal origin. Such an origin, when considered in conjunction with the outcrop, textures, mineralogy and stratigraphy of the deposit together with the palaeogeography of the region, can only be explained by deposition from hot-springs in a sublacustrine environment. Nearby baryte mineralisation may also have been deposited from the same geothermal system, which had the Caledonian 407 Ma Moy granitoid as heat source. The managanese ore is enriched in gold pathfinder elements and minor detrital gold has been found in the area. Gold mineralisation may therefore have also been deposited in the upper sections of this Devonian geothermal system.  相似文献   

Application of 3D empirical travel times to routine event location     

T. Nicholson 《Physics of the Earth and Planetary Interiors》2006,158(1):67-74

The unmodeled effects of lateral heterogeneity are a primary cause of event mislocation in routine methods. Approaches that account for the effect of lateral heterogeneity on travel times are commonly used in specialized studies, but some methods have recently been developed that are suitable for routine location. A variety of changes to the routine procedures have been suggested by other authors. These proposed changes are shown to be compatible to with the use of 3D travel time predictions. 3D empirical travel times are discussed in a routine context. This one step approach, based on direct use of a database of the most reliable previous observations, is shown to be suitable as an additional final step in routine procedures to give a supplementary high accuracy location. The empirical travel time approach is tested on a reference dataset of 155 ground truth events and shown to reduce epicentral mislocation by 45%. The approach is found to be particularly effective at reducing large mislocations. Improvement is demonstrated for every region represented in the dataset but particularly large improvements are shown for events in Europe, USA and Japan.  相似文献   

Modelling the hydrodynamics of offshore sandbanks     

S. Pan  N. MacDonald  J. Williams  B.A. O’Connor  J. Nicholson  A.M. Davies 《Continental Shelf Research》2007

This paper describes the details of a quasi-three-dimensional model (3DBANK), which has been developed to investigate medium and long-term morphological evolution and development of offshore sandbanks. The model is based on a three-dimensional tidal module using the Galerkin-eigenfunction method, but also includes four sub-modules to compute: the instantaneous bedform characteristics from which the temporal and spatial variations of the shear stresses at the sea bed can be derived; the suspended sediment concentration through the water column; the bed-load and suspended sediment fluxes at a point-in-plan; and the resulting morphological changes, respectively. The model also includes the effects of the wind and waves at the sea surface, as well as the wave–current interaction (WCI), and operates with full hydrodynamic and morphodynamic interaction. The components of the model were tested against laboratory and field data, and the complete model was then applied to Middlekerke Bank off the Flemish coast where extensive field measurements were taken during the European Community (EC) funded Circulation and Sediment Transport Around Banks (CSTAB) Project using various advanced instrumentation including STABLE and HF OSCR. Comparisons of the model results with the field measurements and observations show that the model is capable of reproducing the current and wave-induced bedforms, bed roughness, tidal currents and tidal residuals around the sandbank satisfactorily, and can be used to study the long-term sandbank evolution under various offshore conditions. This paper, however, focuses on the hydrodynamic aspects of the model, while the details of the morphological components will be given in a companion paper.  相似文献   

The analysis of fluorine in geochemical exploration — discussion     

K. Nicholson 《Journal of Geochemical Exploration》1982,16(3):239-242

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Multivariate trends in phytoplankton species groups in the southern North Sea     

Nicholson  Mike; Fryer  Rob; Maxwell  David 《ICES Journal of Marine Science》1998,55(4):581-586

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