The Uranian satellites: Surface compositions and opposition brightness surges     

Robert Hamilton Brown  Dale P. Cruikshank 《Icarus》1983,55(1):83-92

High-precision spectrophotometry at 5% resolution has been obtained for the Uranian satellites Ariel, Umbriel, Titania, and Oberon. These spectra cover the wavelength region 1.43–2.57 μm and represent a substantial improvement in precision or wavelength coverage over previous studies. The presence of a spectrally dominant water-ice component in the surface of Ariel, Umbriel, Titania, and Oberon is confirmed. The 1.5- and 2.0-μm water absorption band depths and the continuum reflectance (as defined by the reflectance at 1.78 and 2.25 μm) indicate significant differences among the surface compositional properties of the four satellites. Comparisons of the new spectra to those of other solar system bodies, and to laboratory spectra of water ice of various degrees of purity, indicate that these satellites have a significant non-water-ice component on/in their surfaces. The lack of spectral absorptions at 5% resolution attributable to anything other than water ice suggests that the non-water-ice component is a roughly neutral reflector in the 1.5- to 2.5- μm region. The nature of the non-water-ice component cannot be uniquely determined from these data, but it is relatively dark and has spectral similarities to substances such as carbon black, the dark substance covering one face of Iapetus, or other neutral-color, low-reflectance materials. Finally, preliminary measurements of the near-infrared opposition brightness surges of Ariel, Titania, and Oberon show them to be among the largest in the solar system.  相似文献   

Latitude variations of photospheric activity areas with particular reference to solar faculae     

G. M. Brown  D. R. Evans 《Solar physics》1980,68(1):141-149

Detailed studies of the development of photospheric activity centres for two solar cycles show that Spörer's Law holds in a very similar form to that applying to sunspots for the faculae which inhabit the sunspot zones. Similar differences between the two solar hemispheres can arise, and it seems to be confirmed that the average latitude of faculae tends to be a few degrees poleward of that of sunspots throughout a given cycle. It is shown that the normal averaging process involved in deriving Spörer's Law obscures a detail which is revealed in a breakdown into the variations within successive narrow latitude strips. These show the existence within a cycle of three separate maxima of activity occurring at different epochs and with different preferred latitudes. The main properties of these maxima are discussed.Now at NOAA, Environmental Data Service, NGSTDC, Boulder, Colo. 80302, U.S.A.  相似文献   

Global trace metal cycles and predictions     

Fred T. Mackenzie  Ronald J. Lantzy  Virginia Paterson 《Mathematical Geology》1979,11(2):99-142

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New IGCP Project 235 Metamorphism and Geodynamics     

Dr M. Brown 《Journal of Metamorphic Geology》1985,3(2):201-201

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Towards High-Resolution Habitat Suitability Modeling of Vulnerable Marine Ecosystems in the Deep-Sea: Resolving Terrain Attribute Dependencies     

Anna M. Rengstorf  Anthony Grehan  Chris Yesson  Colin Brown 《Marine Geodesy》2013,36(4):343-361

Recent habitat suitability models used to predict the occurrence of vulnerable marine species, particularly framework building cold-water corals, have identified terrain attributes such as slope and bathymetric position index as important predictive parameters. Due to their scale-dependent nature, a realistic representation of terrain attributes is crucial for the development of reliable habitat suitability models. In this paper, three known coral areas and a noncoral control area off the west coast of Ireland were chosen to assess quantitative and distributional differences between terrain attributes derived from bathymetry grids of varying resolution and information content. Correlation analysis identified consistent changes of terrain attributes as grain size was altered. Response characteristics and dimensions depended on terrain attribute types and the dominant morphological length-scales within the study areas. The subsequent effect on habitat suitability maps was demonstrated by preliminary models generated at different grain sizes. This study demonstrates that high resolution habitat suitability models based on terrain parameters derived from multibeam generated bathymetry are required to detect many of the topographical features found in Irish waters that are associated with coral. This has implications for marine spatial planning in the deep sea. Supplemental materials are available for this article. Go to the publisher's online edition of Marine Geodesy to view the free supplemental file.  相似文献   

Light effects on Crassulacean acid metabolism in the submerged aquatic plant Isoetes kirkii A. Braun     

M. R. Rattray  D. R. Webb  J. M. A. Brown 《新西兰海洋与淡水研究杂志》2013,47(3-4):465-470

In the submerged aquatic CAM plant, Isoetes kirkii A. Braun, light was shown to be the environmental factor having the major influence on daily did malic acid/titratable acid fluctuations. In Lake Taupo (New Zealand), a large oligotrophic lake, seasonal variation in the level of did fluctuations was demonstrated and this corresponded to changes in temperature, light, and daylength while Total Inorganic Carbon (TIC) remained relatively constant. However, in situ shading experiments demonstrated that, at any one time, the diel acid fluctuations were controlled by the ambient light levels as the other variables were constant. Laboratory experiments supported this and also showed that there was a close relationship between photosynthetic rates and de‐acidification when related to light intensity.  相似文献   

Ellipsoidal geometry in asteroid thermal models: The standard radiometric model     

Robert Hamilton Brown 《Icarus》1985,64(1):53-63

This paper reports results of the incorporation of ellipsoidal geometry into the standard radiometric model for asteroids. For small departures from spherical shape the standard model using spherical geometry predicts fluxes in good agreement with ellipsoidal models. Large departures from spherical shape, however, can produce substantial differences in the calculated flux depending on the subsolar temperature and the wavelength of interest. The results derived here suggest that radiometric measurements of highly nonspherical, low-obliquity asteroids interpreted with spherical models result in systematically smaller diameter and higher albedos. In addition, non-spherical shape can also result in a systematic difference in the diameter of a particular asteroid derived from separate 10- and 20-μm flux measurements interpreted with spherical models. Thermal-infrared diurnal lightcurves calculated for ellipsoids have amplitudes that depend on wavelength as well as projected area, and phase curves calculated for ellipsoids are indistinguishable from those calculated for spheres.  相似文献   

Implications for Future Survival of Delta Smelt from Four Climate Change Scenarios for the Sacramento–San Joaquin Delta, California     

Larry R. Brown  William A. Bennett  R. Wayne Wagner  Tara Morgan-King  Noah Knowles  Frederick Feyrer  David H. Schoellhamer  Mark T. Stacey  Michael Dettinger 《Estuaries and Coasts》2013,36(4):754-774

Changes in the position of the low salinity zone, a habitat suitability index, turbidity, and water temperature modeled from four 100-year scenarios of climate change were evaluated for possible effects on delta smelt Hypomesus transpacificus, which is endemic to the Sacramento–San Joaquin Delta. The persistence of delta smelt in much of its current habitat into the next century appears uncertain. By mid-century, the position of the low salinity zone in the fall and the habitat suitability index converged on values only observed during the worst droughts of the baseline period (1969–2000). Projected higher water temperatures would render waters historically inhabited by delta smelt near the confluence of the Sacramento and San Joaquin rivers largely uninhabitable. However, the scenarios of climate change are based on assumptions that require caution in the interpretation of the results. Projections like these provide managers with a useful tool for anticipating long-term challenges to managing fish populations and possibly adapting water management to ameliorate those challenges.  相似文献   

Osumilite–melt interactions in ultrahigh temperature granulites: phase equilibria modelling and implications for the P–T–t evolution of the Eastern Ghats Province,India     

F.J. Korhonen  M. Brown  C. Clark  S. Bhattacharya 《Journal of Metamorphic Geology》2013,31(8):881-907

The exposed residual crust in the Eastern Ghats Province records ultrahigh temperature (UHT) metamorphic conditions involving extensive crustal anatexis and melt loss. However, there is disagreement about the tectonic evolution of this late Mesoproterozoic–early Neoproterozoic orogen due to conflicting petrological, structural and geochronological interpretations. One of the petrological disputes in residual high Mg–Al granulites concerns the origin of fine‐grained mineral intergrowths comprising cordierite + K‐feldspar ± quartz ± biotite ± sillimanite ± plagioclase. These intergrowths wrap around porphyroblast phases and are interpreted to have formed by the breakdown of primary osumilite in the presence of melt trapped in the equilibration volume by the melt percolation threshold. The pressure (P)–temperature (T) evolution of four samples from three localities across the central Eastern Ghats Province is constrained using phase equilibria modelling in the chemical system Na₂O–CaO–K₂O–FeO–MgO–Al₂O₃–SiO₂–H₂O–TiO₂–Fe₂O₃ (NCKFMASHTO). Results of the modelling are integrated with published geochronological results for these samples to show that the central Eastern Ghats Province followed a common P–T–t history. This history is characterized by peak UHT metamorphic conditions of 945–955 °C and 7.8–8.2 kbar followed by a slight increase in pressure and close‐to‐isobaric cooling to the conditions of the elevated solidus at 940–900 °C and 8.5–8.3 kbar. In common with other localities from the Eastern Ghats Province, the early development of cordierite before osumilite and the peak to immediate post‐peak retrograde reaction between osumilite and melt to produce the intergrowth features requires that the prograde evolution was one of contemporaneous increasing pressure with increasing temperature. This counter‐clockwise (CCW) evolution is evaluated for one sample using inverse phase equilibria modelling along a schematic P–T path of 150 °C kbar^?1 starting from the low P–T end of the prograde P–T path as constrained by the phase equilibria modelling. The inverse modelling is executed by step‐wise down temperature reintegration of sufficient melt into the residual bulk chemical composition at the P–T point of the 1 mol.% melt isopleth at each step, representing the melt remaining on grain boundaries after each prograde drainage event, to reach the melt connectivity transition (MCT) of 7 mol.%. The procedure is repeated until a plausible protolith composition is recovered. The result demonstrates that clastic sedimentary rocks that followed a CCW P–T evolution could have produced the observed mineral assemblages and microstructures preserved in the central Eastern Ghats Province. This study also highlights the role of melt during UHT metamorphism, particularly its importance to both chemical and physical processes along the prograde and retrograde segments of the P–T path. These processes include: (i) an increase in diffusive length scales during the late prograde to peak evolution, creating equilibration volumes larger than a standard thin section; (ii) the development of retrograde mineral assemblages, which is facilitated if some melt is retained post‐peak; (iii) the presence of melt as a weakening mechanism and the advection of heat by melt, allowing the crust to thicken; and (iv) the effect of melt loss, which makes the deep crust both denser and stronger, and reduces heat production at depth, limiting crustal thickening and facilitating the transition to close‐to‐isobaric cooling.  相似文献   

Constraints on the composition of ore fluids and implications for mineralising events at the Cleo gold deposit,Eastern Goldfields Province,Western Australia     

S. M. Brown  C. A. Johnson  R. J. Watling  W. R. Premo 《Australian Journal of Earth Sciences》2013,60(1):19-38

The Cleo gold deposit, 55 km south of Laverton in the Eastern Goldfields Province of Western Australia, is characterised by banded iron‐formation (BIF)‐hosted ore zones in the gently dipping Sunrise Shear Zone and high‐grade vein‐hosted ore in the Western Lodes.There is evidence that gold mineralisation in the Western Lodes (which occurred at ca 2655 Ma) post‐dates the majority of displacement along the Sunrise Shear Zone, but it remains uncertain if the ore in both structures formed simultaneously or separately. Overall, the Pb, Nd, Sr, C, O and S isotopic compositions of ore‐related minerals from both the Western Lodes and ore zones in the Sunrise Shear Zone are similar. Early low‐salinity aqueous‐carbonic fluids and late high‐salinity fluids with similar characteristics are trapped in inclusions in quartz veins from both the Sunrise Shear Zone and the Western Lodes. The early CO₂, CO₂–H₂O, and H₂O‐dominant inclusions are interpreted as being related to ore formation, and to have formed from a single low‐salinity aqueous‐carbonic fluid as a result of intermittent fluid immiscibility. Homogenisation temperatures indicate that these inclusions were trapped at approximately 280°C and at approximately 4 km depth, in the deeper epizonal range. Differences between the ore zones are detected in the trace‐element composition of gold samples, with gold from the Sunrise Shear Zone enriched in Ni, Pb, Sn, Te and Zn, and depleted in As, Bi, Cd, Cu and Sb, relative to gold from the Western Lodes. Although there are differences in gold composition between the Sunrise Shear Zone and Western Lodes, and hence the metal content of ore fluids may have varied slightly between the different ore zones, no other systematic fluid or solute differences are detected between the ore zones. Given the fact that the ore fluids in each zone have very similar bulk properties, the considerable differences in gold grade, sulfide mineral abundance, and ore textures between the two ore zones most likely result from different gold‐deposition mechanisms. The association of ore zones in the Sunrise Shear Zone with pyrite‐replaced BIF suggests that wall‐rock sulfidation was the most significant mechanism of gold precipitation, through the destabilisation of gold‐bisulfide complexes. The Western Lodes, however, do not exhibit any host‐rock preference and multistage veins commonly contain coarse‐grained gold. Fluid‐inclusion characteristics and breccia textures in veins in the Western Lodes suggest that rapid pressure changes, brought about by intermittent release of overpressured fluids and concomitant phase separation, are likely to have caused the destabilisation of gold‐thiocomplexes, leading to formation of higher‐grade gold ore zones.  相似文献