Further observations of 8-μm polar brightenings of Jupiter     

John Caldwell  Alan T. Tokunaga  Glenn S. Orton 《Icarus》1983,53(1):133-140

North to south scans of Jupiter at 7.8-μm wavelength in early 1981 confirm polar brightening events that correlate with LCM_III, such that a polar limb is bright when the corresponding magnetic pole is tilted eartward. The correlation with magnetic features of the planet suggests that the energy source for the brightenings is magnetospheric particles incident upon the polar regions of the atmosphere. The northern polar events are more prominent and more regular than the southern ones. The polar emission may be indirectly related to the ultraviolet absorber observed near the poles by Voyager 2.  相似文献   

The Jovian stratosphere in the ultraviolet     

Richard Wagener  John Caldwell  Tobias Owen  Therese Encrenaz  Michel Combes 《Icarus》1985,63(2):222-236

The center-of-disk reflectivity of Jupiter in the wavelength range from 1450 to 3150A?has been computed from 30 low-dispersion IUE spectra taken during solar maximum in 1978–1980. A vertically inhomogeneous radiative transfer program is used to compute model reflectivities of various stratospheric compositions for comparison. Ammonia and acetylene are well determined because they show narrow absorption bands in the ultraviolet. Above 1800A?, these two gases provide a good fit to the data, but not below. At shorter wavelengths the fit would be much improved by a small amount (5–15 ppb) of propadiene/allene (C₃H₄). Voyager IRIS spectra show that the IR bands of allene are not strong enough to be detected in such a small amount. Additional absorption around 1600A?can be reproduced best with the presence of cyclopropane (C₃H₆, <15ppb), although other absorbers (e.g., hydrocarbon molecules with more than three carbon atoms, oxygen- or nitrogen-containing molecules, or a high-attitude haze) could also explain the spectrum in this region. The data are too noisy to detect possible CO Cameron band absorption near 2000A?.  相似文献   

Photometry of Venus below 2200 Å from the orbiting astronomical observatory-2     

Edward Shaya  John Caldwell 《Icarus》1976,27(2):255-264

Spectrophotometry of Venus from 2170 to about 1950 Å has been obtained by OAO-2 at 10 Å resolution. The new data confirm and extend previous indications that the geometric albedo decreases continuously below 2500 Å. Secular changes in either the amount or distribution, or both, of absorbing constituents in the upper atmosphere are strongly suggested. A narrow absorption feature is found near 2145 Å, confirming an earlier report by Anderson et al. [J. Atmos. Sci.26 (1969), 874–888]. Absorption by trace amounts of nitrogen-bearing molecules, including N₂O, HNO₃ in aqeous solution, and possibly also NO, together with Rayleigh scattering from CO₂, can account for the variation in albedo below 3200 Å, but other explanations are not excluded. For example, H₂S may contribute to or be responsible for the decrease in albedo below 2500 Å.  相似文献   

<Superscript>40</Superscript>Ar/<Superscript>39</Superscript>Ar dating of the eruptive history of Mount Erebus,Antarctica: summit flows,tephra, and caldera collapse     

Christopher?J.?Harpel  Philip?R.?KyleEmail author  Richard?P.?Esser  William?C.?McIntosh  David?A.?Caldwell 《Bulletin of Volcanology》2004,66(8):687-702

Eruptive activity has occurred in the summit region of Mount Erebus over the last 95 ky, and has included numerous lava flows and small explosive eruptions, at least one plinian eruption, and at least one and probably two caldera-forming events. Furnace and laser step-heating ⁴⁰Ar/³⁹Ar ages have been determined for 16 summit lava flows and three englacial tephra layers erupted from Mount Erebus. The summit region is composed of at least one or possibly two superimposed calderas that have been filled by post-caldera lava flows ranging in age from 17 ± 8 to 1 ± 5 ka. Dated pre-caldera summit flows display two age populations at 95 ± 9 to 76 ± 4 ka and 27 ± 3 to 21 ± 4 ka of samples with tephriphonolite and phonolite compositions, respectively. A caldera-collapse event occurred between 25 and 11 ka. An older caldera-collapse event is likely to have occurred between 80 and 24 ka. Two englacial tephra layers from the flanks of Mount Erebus have been dated at 71 ± 5 and 15 ± 4 ka. These layers stratigraphically bracket 14 undated tephra layers, and predate 19 undated tephra layers, indicating that small-scale explosive activity has occurred throughout the late Pleistocene and Holocene eruptive history of Mount Erebus. A distal, englacial plinian-fall tephra sample has an age of 39 ± 6 ka and may have been associated with the older of the two caldera-collapse events. A shift in magma composition from tephriphonolite to phonolite occurred at around 36 ka.Editorial responsibility: Julie Donnelly-Nolan  相似文献   

Impacts of interrelated biotic and abiotic processes during the past 125 000 years of landscape evolution in the Northern Mojave Desert, Nevada, USA     

D.S. Shafer  M.H. Young  S.F. Zitzer  T.G. Caldwell  E.V. McDonald 《Journal of Arid Environments》2007,69(4):633-657

Interrelated, biotic (flora and fauna) and abiotic (pedogenesis and hydrology) processes were examined at four sites (30, and approximately 1000–3000, 7000–12 000, and 125 000 years before present) in the northern Mojave Desert. Data collected at each included floral and faunal surveys; soil texture, structure, and morphology; and soil hydraulic properties. Separate measurements were made in shrub undercanopy and intercanopy microsites. At all sites, shrubs made up greater than 86 percent of total perennial cover, being least on the youngest site (4 percent) and most on the 7000–12 000-year-old site (31 percent). In the intercanopy, winter annual density was highest on the 1000- to 3000-year-old site (249 plants/m²) and lowest on the oldest site (4 plants/m²). Faunal activity, measured by burrow density, was highest on the 1000–3000- and 7000–12 000-year-old sites (0.21 burrows/m²) and density was twice as high in the undercanopy versus the intercanopy. Burrow density was lower at the two oldest sites, although density was not statistically greater in the undercanopy than intercanopy. At the older sites, the soil water balance was increasingly controlled by Av horizons in intercanopy soils in which saturated hydraulic conductivity (K_sat) decreased 95 percent from the youngest to the oldest site. No significant reduction in K_sat in undercanopy soils was observed. Decreases in the intercanopy sites correlated with decreases in annual plant density and bioturbation, suggesting these processes are interrelated with surface age.  相似文献   

Ultraviolet observations of small bodies in the solar system by OAO-2     

John Caldwell 《Icarus》1975,25(3):384-396

Broadband filter photometry from 2100 to 4300 Å has been obtained by OAO-2 for the following objects: The Galilean satellites; Titan; the rings of Saturn; and three asteroids. Agreement with independent ground-based photometry in the region of overlap is good. The previously known decrease in reflectivity from visual to ground-based ultraviolet wavelengths continues to 2590 Å for all these objects. Europa's reflectivity continues to decline towards 2110 Å, and the rings' reflectivity levels off from 2590 to 2110 Å. Other targets were too faint at 2110 Å to be measured reliably by OAO-2.The low ultraviolet albedo of Titan has important implications for that planet's atmospheric structure (Caldwell, Larach, and Danielson, 1973; Danielson, Caldwell, and Larach, 1973; Caldwell, 1974b). The ultraviolet reflectivity of Saturn's rings is suggestive of a two-component system, one being pure H₂O particles. The ultraviolet albedos of the Galilean satellites are consistent with existing upper limits for atmospheric abundances, but require either that former estimates of the fractional coverage of H₂O frost are too high, an unlikely circumstance, or that the frost has been darkened by some external agent in the space environment.  相似文献   

Exposure to 2,4-decadienal negatively impacts upon marine invertebrate larval fitness     

Caldwell GS  Lewis C  Olive PJ  Bentley MG 《Marine environmental research》2005,59(5):405-417

Diatoms liberate volatile, biologically active unsaturated aldehydes following cell damage, which negatively impact upon invertebrate reproductive processes such as fertilization, embryogenesis and larval survival. 2,4-Decadienal is frequently identified among the aldehydes produced and is one of the more biologically active. The majority of studies which have examined the toxic effects of diatom aldehydes to invertebrate reproduction have scored egg production and/or hatching success as indicators of biological impacts. There are very few studies which have dealt specifically with the impacts of diatom-derived aldehydes on larval fitness. Larval stages of the polychaetes Arenicola marina and Nereis virens and the echinoderms Asterias rubens and Psammechinus miliaris exposed to 2,4-decadienal at sub 1 microg ml(-1) concentrations suffered reduced survival over the incubation period (day 1-8 post fertilization) with detectable differences for the polychates at a concentration of 0.005 and 0.01-0.1 microg ml(-1) for the echinoderms. Susceptibility of larval N. virens was investigated using stage specific 24 h exposures at 2,4-decadienal concentrations up to 1.5 microg ml(-1). A clear stage specific effect was found, with earlier larval stages most vulnerable. Nectochaete larvae (9-10 d) showed no reduction in survival at the concentrations assayed. Fluctuating asymmetry (FA), defined as random deviations from perfect bilateral symmetry, was used to analyse fitness of larval P. miliaris exposed to 2,4-decadienal at concentrations of 0.1, 0.5 and 1 microg ml(-1). The degree and frequency of asymmetrical development increased with increasing 2,4-decadienal concentration. Equally, as FA increased larval survival decreased. These results provide further support for the teratogenic nature of 2,4-decadienal and its negative impact on invertebrate larval fitness.  相似文献   

The geochemistry and petrogenesis of the volcanic rocks of Carriacou,Grenadine Islands,West Indies     

G. Caldwell  T. E. Smith  C. H. Uang  T. A. Jackson 《Bulletin of Volcanology》1984,47(3):467-482

Carriacou is small volcanic island located near the southern end of the Lesser Antillean chain. Field relationships, petrography and geochemistry of the Tertiary lavas, outcropping in the southern half of the island, are used to identify the rocks present and to determine their petrogenesis and assess their significance within the island arc.Six main volcanic units are present. From oldest to youngest, these are the clinopyroxene-phyric basalt (CPB) sequence, the amphibole-phyric andesite (APA) sequence, the clinopyroxene-megaphyric basalt (CMB) sequence, the olivine-microphyric basalt (OMB) sequence, the clinopyroxene phyric andesite (CPA) sequence, and the amphibole-megaphyric andesite (AMA) sequence. Volcaniclastic deposits are associated with the APA, CMB, and AMA sequences. The APA sequence is calcalkaline, whereas the other five sequences are tholeiitic.Sr isotope and rare earth element (REE) data suggest that these volcanic rocks were derived from partial melts of garnet-peridotite generated deep within the mantle. The OMB lavas have the highest temperature assemblages of intratelluric minerals and the least evolved chemical characteristics, and are considered to be closest in composition to a parental melt. Phenocryst assemblages and chemical variation suggest that the andesite sequences have been derived from the mafic melts by low pressure fractional crystallization of approximately 20% clinopyroxene and 20% olivine, plus smaller amounts of plagioclase and amphibole. The high concentrations of incompatible and compatible elements and the high⁸⁷Sr/⁸⁶Sr ratios may indicate that subduction is slower in the southern part of the arc, and fluids released during slab dehydration rich in Incompatible trace elements, in Radiogenic strontium, and in Silica (IRS), have modified the parental melts.  相似文献   

On the applicability of a universal elastic trench profile     

J.G. Caldwell  W.F. Haxby  D.E. Karig  D.L. Turcotte 《Earth and Planetary Science Letters》1976,31(2):239-246

Using thin elastic plate theory and neglecting horizontal applied forces, a universal deflection profile applicable to many oceanic trenches is derived. This theoretical profile is compared with bathymetric profiles from the central Aleutian, Kuril, Bonin, and Mariana trench-outer rise regions. The profiles were corrected for sediment thickness and age variation of the lithosphere. Good agreement between theory and observation is found. The distance from the first point of zero deflection seaward of the trench to the point of maximum height of the outer rise is directly related to the flexural rigidity of the lithosphere. The thickness of the elastic lithosphere is found to vary between 20 and 29 km for the trench profiles considered. The good agreement obtained shows that horizontal forces may be neglected and that the bending lithosphere behaves elastically in the cases considered. The analysis shows that only unreasonably large horizontal forces would affect the universal deflection curve. It is concluded that although the near-surface lithosphere may be subject to brittle fracture, the deeper lithosphere is capable of transmitting elastic stresses as high as 9 kbar.  相似文献   

Ultraviolet observations of Mars made by the orbiting astronomical observatory     

John Caldwell 《Icarus》1973,18(3):489-496

Ultraviolet albedos of Mars in the region γγ2000–3600 Å are discussed. When the reflectivity due to the known amount of CO₂ on Mars is accounted for, the remaining reflectivity may be used to set an upper limit for the surface albedo. The result disagrees qualitatively with published ultraviolet reflectivities of limonite and carbon suboxide. An alternate interpretation of the observations leads to the conclusion that CO₂ comprises at least 60% of the molecular atmosphere of Mars, assuming the remainder to be argon. A comparison of the OAO results with 1969 Mariner ultraviolet data reveals some important areas of conflict.Attempts to detect Mars at wavelengths less than γ2000 Å were unsuccessful, with only very high upper limits being set.  相似文献