bluuebunny/crossref_metadata_2025 huggingface dataset:
Magellan and comparative planetology
Authors: Turcotte Donald L.
Published: February 2004

Magellan data have been widely used to provide explanations for the tectonics and volcanism on Venus. However, it is also important to compare our understanding of these processes on Venus with the Earth, Moon, and Mars. One essential question concerning the behavior of these bodies is, How does the heat get out? Internal heat generation and planetary cooling are the energy sources responsible for planetary tectonics and volcanism. On the Earth the plates of plate tectonics are the thermal boundary layers of mantle convection cells. The subduction of these cold plates at ocean trenches contributes about 75% of the mantle heat flux. It is now quite clear that active plate tectonics is not occurring on Venus. The surface of Venus is a single rigid plate, as is the case for the Moon, Mars, and Mercury. Prior to Magellan, the general consensus was that Venus was in a near‐uniformatarian state, with conductive heat losses through the lithosphere balancing internal heat generation and secular cooling of the planet. However, crater studies from Magellan images provide direct evidence for a catastrophic or episodic evolution of Venus. One explanation is that Venus died tectonically 500 m.y. ago; plate tectonics occurred during the early evolution of the planet but ceased as the radioactive heat production decreased. An alternative explanation is that Venus experiences episodic global subduction events. Two major questions concerning the Earth remain largely unanswered, particularly when compared with Venus. (1) Why does the Earth have continents? (2) Why does the Earth have plate tectonics? A major difference between the Earth and Venus is the presence of liquid water (oceans) during much of the evolution of the Earth. It has been hypothesized that water is an essential ingredient for the silicic volcanism that produces continental rocks on the Earth, and the absence of well‐defined continental masses on Venus supports this hypothesis. It has long been recognized that plate tectonics cannot evolve in time without internal deformation of plates. Plate evolution in time results in geometrical incompatibilities that require overlaps or gaps. On the Earth these incompatibilities lead to broad zones of continental deformation, i.e., the western United States and China. The absence of plate tectonics on Venus suggests that rheologically soft continents are necessary for plate tectonics. A reasonable conclusion is that the Earth is the remarkable planet in terms of tectonics and volcanism, not Venus.
Venus coronae, craters, and chasmata
Authors: Stefanick Michael , Jurdy Donna M.
Published: February 2004

The distributions of Venus coronae and craters are related to chasmata, which are thought to be extensional zones. Coronae are almost twice as dense near the chasmata as a random set of the same size. Of the various types of coronae, the radial‐concentric and multiple are even more highly concentrated near chasmata, whereas the concentric‐caldera type are absent near the chasmata. Craters, to the first order, are randomly distributed, although when distributions are compared with random sets there is a deficit of about 15–20 craters close to the chasmata. The tectonized and embayed craters tend to be near the rift zones, and their distribution closely resembles that of the coronae. The higher proportion of tectonized, and especially embayed, craters within 5–10° of the arcs, is about the fraction that would be expected from modification of the full set of craters over a width of 100–200 km. Kolmogorov‐Smirnov statistics are used to compare cumulative distributions for craters and coronae with a large random distribution and to compute probabilities. Craters and coronae fill disjoint regions that are more connected than regions of randomly assigned points. This suggests that the volcano‐tectonic process creating coronae may be the same one destroying craters.
Long‐term rotation and mantle dynamics of the Earth, Mars, and Venus
Authors: Spada G. , Sabadini R. , Boschi E.
Published: February 2004

In response to internal and surface tectonic processes the terrestrial planets have the ability to displace the axis of rotation with respect to the mantle. This behavior is quantified by means of a nondimensional number, defined here as the rotational number Ro, that allows classification of the planets into two categories, the first containing Mars and the Earth, where true polar wander is a feasible mechanism, and the second, to which Venus belongs, where rotation equilibrium is attained by means of mega‐wobbles driven by internal mass anomalies. The number Ro is related to the timescale characterizing the readjustment of the equatorial bulge during long‐term polar motion and to the length of the sidereal day. If these two timescales are well separated, the planet experiences true polar wander. Nonlinear Liouville equations for stratified viscoelastic bodies with linear Maxwell rheology are solved in order to assess the relevance of surface and mantle processes in driving long‐term rotation instabilities in the terrestrial planets. The amount of true polar wander estimated for the Earth and Mars is reproduced correctly by our modeling with mantle viscosities and lithospheric thickness consistent with other studies. The major difference between the Earth and Mars is the driving mechanism, subduction for the Earth and lithospheric loading for Mars. When mantle viscosities similar to those of the Earth are considered for Venus, the most updated estimate for the offset of about 0.5° between the rotation axis and the axis of maximum inertia is well reproduced during the mega‐wobbles induced by internal mass redistribution. We show that the degree 2 topography of these three planets can be affected by their rotation, which is responsible for the dominance of the sectorial component on the Earth and Mars and for the zonal component on Venus.
The equatorial boundary of the ultraviolet Jovian north aurora observed with multispectral Hubble Space Telescope images
Authors: Grodent D. , Dols V. , Gérard J. C. , Rego D.
Published: February 2004

Multispectral observations of the far ultraviolet Jovian aurora with the faint object camera (FOC) on board the Hubble Space Telescope are reported. They are used to describe and compare the morphology and the mean brightness of the H2 Lyman and Werner bands observed at 153, 125, and 130 nm. It is shown that most of the emissions are confined inside the 6 RJ O6‐GSFC auroral oval and fill a large fraction of the polar cap. During the ∼50 hours time span of the observations following a very strong aurora, no bright arc was observed along the oval mapping the 30 RJ O6 oval. Hydrogen emissions at 125 and 130 nm are observed down to 50°N and somewhat below, a region not accessible to previous FOC observations at longer wavelengths. Temporal variations are also observed on timescales of hours to days.
Spectral properties and rotational spectral heterogeneity of 433 Eros
Authors: Murchie Scott L. , Pieters Carle M.
Published: February 2004

Composite spectra of different rotational aspects of the S asteroid 433 Eros were assembled from previous studies, covering the wavelength range 0.33–2.5 μm. These data allow us to measure significant attributes of Eros's spectrum, its mafic mineral absorptions, and the degree and nature of compositional heterogeneity of its surface. We also evaluated previous S‐asteroid taxonomies using independent methods, and from this test determined that the taxonomy of Gaffey et al. [1993] provides a sound basis for relating Eros to other S asteroids. On average, Eros resembles the S(IV) asteroids, which bear the closest resemblance to ordinary chondrites in mafic mineralogy. However, Eros also exhibits distinct rotational spectral variations comparable to the greatest yet recognized on an S asteroid. One side of the asteroid has a shorter‐wavelength 1‐μm absorption and a stronger 2‐μm absorption than the global average, and is interpreted to be richer in pyroxene; the opposite face has a longer‐wavelength 1‐μm absorption and a weaker 2‐μm absorption, and is interpreted to be richer in olivine. The pyroxene‐rich side coincides with a facet‐like face identified in radar studies, and the olivine‐rich side coincides with a convex‐shaped face. Each side of the asteroid exhibits spectral affinities with a different spectral class of S asteroids, demonstrating that such subdivisions of S asteroids represent different parts of a compositional continuum. High spatial resolution measurement of the mineralogic and elemental compositions and especially the geologic settings of these different materials by the Near‐Earth Asteroid Rendezvous mission will provide a great advance in the understanding of the origin and evolution of a candidate ordinary chondrite parent body.
An inversion of gravity and topography for mantle and crustal structure on Mars
Authors: Kiefer Walter S. , Bills Bruce G. , Nerem R. Steven
Published: February 2004

Analysis of the gravity and topography of Mars presently provides our primary quantitative constraints on the internal structure of Mars. We present an inversion of the long‐wavelength (harmonic degree ≤ 10) gravity and topography of Mars for lateral variations of mantle temperature and crustal thickness. Our formulation incorporates both viscous mantle flow (which most prior studies have neglected) and isostatically compensated density anomalies in the crust and lithosphere. Our nominal model has a 150‐km‐thick high‐viscosity surface layer over an isoviscous mantle, with a core radius of 1840 km. It predicts lateral temperature variations of up to a few hundred degrees Kelvin relative to the mean mantle temperature, with high temperature under Tharsis and to a lesser extent under Elysium and cool temperatures elsewhere. Surprisingly, the model predicts crustal thinning beneath Tharsis. If correct, this implies that thinning of the crust by mantle shear stresses dominates over thickening of the crust by volcanism. The major impact basins (Hellas, Argyre, Isidis, Chryse, and Utopia) are regions of crustal thinning, as expected. Utopia is also predicted to be a region of hot mantle, which is hard to reconcile with the surface geology. An alternative model for Utopia treats it as a mascon basin. The Utopia gravity anomaly is consistent with the presence of a 1.2 to 1.6 km thick layer of uncompensated basalt, in good agreement with geologic arguments about the amount of volcanic fill in this area. The mantle thermal structure is the dominant contributor to the observed geoid in our inversion. The mantle also dominates the topography at the longest wavelengths, but shorter wavelengths (harmonic degrees ≥4) are dominated by the crustal structure. Because of the uncertainty about the appropriate numerical values for some of the model's input parameters, we have examined the sensitivity of the model results to the planetary structural model (core radius and core and mantle densities), the mantle's viscosity stratification, and the mean crustal thickness. The model results are insensitive to the specific thickness or viscosity contrast of the high‐viscosity surface layer and to the mean crustal thickness in the range 25 to 100 km. Models with a large core radius or with an upper mantle low‐viscosity zone require implausibly large lateral variations in mantle temperature.
The state of SO2 on Io's surface
Authors: Kerton C. R. , Fanale F. P. , Salvail J. R.
Published: February 2004

We have modified an earlier model of Io's frozen surface SO2 and its influence on Io's atmosphere [Fanale et al., 1982]. In our new models, we consider four additional factors: (1) the latent heat of SO2 frost, (2) the rotation rate of Io, (3) thermal conduction and Io's internal heat flow, and (4) the deposition of some solar energy below the surface. The net result is a significant lowering of estimated surface temperatures and hence equilibrium SO2 pressures over the frost deposits. The atmospheric pressures are predicted to be much lower than those in the 1982 model of Fanale et al., and, in some cases, are low enough to be below the current upper limits set by recent Hubble Space Telescope observations. In the absence of local volcanic sources, even if Io were completely covered with frozen SO2, the atmosphere would be everywhere ballistic except very close to the subsolar point, where it would be marginally kinetic. Local kinetic atmospheres result only from SO2‐rich plumes. The results of these models suggest that the sometimes reported posteclipse brightening of Io could be a real and temporally variable phenomenon and not an observational aberration.
Paleohydrographic changes in the Somali Basin (5°N upwelling and equatorial areas) during the last 160 Kyr, based on correspondence analysis of foraminiferal and radiolarian assemblages
Authors: Vénec‐Peyré Marie‐Thérèse , Caulet Jean Pierre , Grazzini Colette Vergnaud
Published: February 2004

This study documents the biological signatures impressed upon the sedimentary record underlying both the 5°N upwelling system of the Somali Current and the equatorial area of the Somali Basin out of the upwelling influence. The evolution of these two distinct hydrographic systems is compared for the last 160 kyr. Correspondence and cluster analyses are performed on combined radiolarian and planktonic foraminiferal quantitative data in order to study the changes of the planktonic assemblages through time and space. The Upwelling Radiolarian Index (URI) is used as a productivity proxy. The water temperature and hydrographic structure of the upper water masses appear to be the major factors controlling the distribution patterns of the fauna. The relative abundances of three groups of foraminifera, cold water form (dextral N. pachyderma), mixed layer dwellers (G. trilobus, G. ruber, G. sacculifer, G. conglobatus, and G. glutinata), and thermocline dwellers (G. menardii, G. tumida, N. dutertrei, G. crassaformis, and P. obliquiloculata), follow distinct evolutionary patterns at the two sites during the last 160 kyr. At the equatorial site (core MD 85668), downcore fluctuations in the relative abundances of the three groups are closely related to the glacial/interglacial cyclicity and provide some insights into the interpretation of hydrographic changes. The dominance of the mixed layer foraminifera at the transition intervals between isotope stages 6/5 and 2/1, combined with weak URI values, is thought to reflect the reorganization of the oceanographic circulation. These short‐term events (with a duration of < 5000 year) could be related to the rapid inflow of oxygen‐depleted water through the Indonesian straits as a result of sea level rise during deglaciation. Underneath the 5°N gyre (core MD 85674), the response to global climatic changes is overprinted by the regional effect of the Somalian upwelling, which has been persistent over the last 160 kyr.
Is the Greenland Ice Sheet bistable?
Authors: Crowley Thomas J. , Baum Steven K.
Published: February 2004

Ice core work on Greenland has produced dramatic evidence for an instability of the climate system in the North Atlantic sector. In this paper, we provide climate modeling results indicating another possible example of a multiple equilibrium climate state, where such behavior might apply to the ice sheet itself. Baseline sensitivity experiments, in which the Greenland Ice Sheet was removed, simulate summer temperatures on Greenland of about 10°C. This number varied between about 6 and 14°C depending on specification of vegetation type, elevation of Greenland, and orbital forcing. The implied hysteresis indicates that the present geographic configuration may have been sufficient to maintain an ice‐free state prior to formation of the ice cap. Explanations for formation of the ice sheet therefore present a dilemma. Four possibilities involve undocumented Miocene‐Pliocene CO2 excursions to values lower than present, a high sensitivity of the climate system, a variable sensitivity of the climate system, or significant problems with the climate model. A variable sensitivity is consistent with our results indicating a multiple steady state climate for Greenland. Although most CO2 scenarios do not predict a collapse of the Greenland Ice Sheet in the future, our results suggest that if it did, present boundary conditions may be sufficient to maintain Greenland in an ice‐free environment even after the greenhouse effect has dissipated.
Could deep subantarctic convection feed the world deep basins during the Last Glacial Maximum?
Authors: Michel Elisabeth , Labeyrie Laurent D. , Duplessy Jean‐Claude , Gorfti Nabila , Labracherie Monique , Turon Jean‐Louis
Published: February 2004

Simple box model calculations are used to simulate the oceanic circulation during the last glacial maximum (LGM). These experiments show that the main features of the δ13C and Δ14C distributions and of the lysocline depth may be explained by a circulation pattern very different from the modem one. Intermediate and upper deep waters were produced in the North Atlantic Ocean, whereas deep waters of Subantarctic Mode type, forming at the northern edge of the Subantarctic convergence, invaded the main oceanic basins. The Southern Ocean, mainly self ventilated, had a reduced deep component that flew southward along the East Pacific Ridge and the Australian west cost. The thermodynamic fractionation that occurs during air‐sea exchange has only contributed slightly to the glacial deep δ13C distribution through surface water temperature variations.
Is there a component of Pleistocene CO2 change associated with carbonate dissolution cycles?
Authors: Keir Robin S.
Published: February 2004

The only processes which could have decreased atmospheric CO2 during glacial climates without appreciably changing the carbon isotope distribution in the sea appear to be (1) dissolved calcium carbonate input to the ocean, e.g., coral reef buildup and erosion, (2) the ocean solubility pump, due to changes in surface temperature or air‐sea exchange, or (3) decreased biological production of calcium carbonate. It is assumed here that one of these mechanisms caused part of the atmospheric CO2 changes recorded in the 200‐kyr‐long Vostok ice core. Two residual CO2 records are generated by scaling the δ13C difference between planktonic and benthic foraminifera in marine sediment cores to −82 ppm CO2 per 1‰ increase in Δδ13C and subtracting from this the measured CO2 concentration in the ice core. Both residual CO2 records exhibit two broad maximums between about 20–50 ka and 140–200 ka, indicating that during these times, about 40 ppm of the CO2 decrease from interglacial levels cannot be explained by the interaction of the ocean's biological and vertical mixing cycles. The shape of the residual CO2 curve is similar qualitatively to the variation of calcium carbonate in central equatorial Pacific sediments during this time period, which would imply that changes in dissolved carbonate input to the ocean contributed the added component of CO2 change. However, recent models of atmospheric CO2 change in response to changing alkaline input to the ocean exhibit about a 25 to 35 ppm decrease per 1013 mol yr−1 increase in dissolved CaCO3 input. If compensation for the changing input is occurring mostly within an area of about 40 × 106 km² below the lysocline in the Indo‐Pacific, the change in carbonate accumulation rate corresponding to a −40 ppm CO2 change would be a minimum of 3 mg cm−2 yr−1. This can be compared to glacial increases of 0.5 to 1.0 mg cm−2 yr−1 during the last 200 kyr in central equatorial Pacific sediments. Thus, the added glacial accumulation of carbonate does not seem to match quantitatively with the 40 ppm amplitude of the residual CO2 signal, leading one to suspect that solubility plays a greater role than expected on the basis of a 2°C cooler surface ocean.
Quantitative resolution of eolian continental crustal material and volcanic detritus in North Pacific surface sediment
Authors: Weber Egon T. , Owen Robert M. , Dickens Gerald R. , Halliday Alex N. , Jones Charles E. , Rea David K.
Published: February 2004

Proxy records of continental climate and atmospheric circulation provided by analyses of eolian continental material extracted from marine sediment have resulted in significant new information concerning the behavior of these climate systems on various timescales. These studies, however, currently are limited to certain geographic areas because no chemical or physical extraction procedure provides an unambiguous separation of eolian continental crustal material from other contaminants like volcanic detritus. We employ a combined analytical and statistical procedure in an effort to extract a more refined eolian “signal” from areas that may be affected by volcanic detritus. Bulk surface sediment samples from 33 locations in the North Pacific were treated using a conventional sequential extraction procedure to remove the carbonate, silica and oxyhydroxide components, and the residue was analyzed by instrumental neutron activation analysis (INAA) for La, Ce, Sm, Eu, Yb, Lu, Hf, Sc, and Th. Q‐mode factor analysis of these data shows that > 99% of the variance is explained by two end‐members, which we interpret to be continental crustal material and volcanic detritus. Five least squares normative analysis models were evaluated to estimate the relative amount of these end‐members in each sample. The continental crustal component was approximated using a fine‐grained fraction of China loess and bulk loess. The volcanic component was approximated using the compositions of average Kurile‐Kamchatka volcanic material, a median Ocean Drilling Program (ODP) Leg 145 Ash, and a median Kurile basalt. The model based on average Kurile‐Kamchatka volcanic material and the fine loess fraction gives the most accurate results. Central and east Pacific samples typically contain up to 100% of the eolian continental crustal component, while samples near Japan, Kamchatka, and the Aleutians contain a majority of volcanic detritus. However, the highly variable composition of volcanic material can result in systematic errors up to 25% in samples dominated by volcanic detritus. The geographic distribution of the compositional end‐members is consistent with a continental dust source originating in Asia and being diluted by ash from the volcanic arcs of the Pacific rim. This improved identification and resolution of the eolian continental component realized in this approach should permit paleoclimatic reconstructions to be developed from sediments in significant portions of the world that were previously precluded from analysis because of limitations with chemical or physical extraction procedures.
Interocean exchange of glacial North Atlantic Intermediate Water: Evidence from subantarctic Cd/Ca and carbon isotope measurements
Authors: Lynch‐Stieglitz Jean , van Geen Alexander , Fairbanks Richard G.
Published: February 2004

New measurements of Cd/Ca ratios for glacial benthic foraminifera from the continental shelf and slope off Tasmania and South Australia are presented. When combined with glacial carbon isotope measurements [Lynch‐Stieglitz et al., 1994], these data from the glacial Subantarctic Zone suggest that a water mass with a high value of the conservative tracer δ13Cas occupied not only the shallow (1–2 km) portions of the Atlantic and Pacific Oceans but was present in the glacial Subantarctic as well. The oceanic Cd concentrations, Cdw, predicted for the glacial Subantarctic are intermediate between those from the glacial Atlantic and the glacial Pacific, supporting the idea that Glacial North Atlantic Intermediate Water may have reached the intermediate depth Pacific via the Antarctic Circumpolar Circulation [Lynch‐Stieglitz and Fairbanks, 1994].
Decoupling of δ13CΣCO 2 and phosphate in recent Weddell Sea deep and bottom water: Implications for glacial Southern Ocean paleoceanography
Authors: Mackensen A. , Hubberten H. ‐W. , Scheele N. , Schlitzer R.
Published: February 2004

On a section between 72°S and 42°S and a transect between 60°E and 10°E through the Weddell Sea and the southernmost eastern Atlantic Ocean, the water column was sampled on 72 stations, and the stable carbon isotopic composition of total dissolved inorganic carbon (δ13CΣCO2) as well as the stable oxygen isotopic composition of seawater (δ18O) was determined. These data were compared with potential temperature, salinity, dissolved oxygen and phosphate data from the same stations. The observed δ13CΣCO2/PO43− relationship in the deep Weddell Sea strongly differs from the global Redfield‐driven deep water relationship. We attribute this to enhanced thermodynamic fractionation at sites of bottom water formation that decouples the nutrient signal from the δ13CΣCO2 signal not only in surface and intermediate water masses but also in deep and bottom water. Different, water‐mass specific thermodynamic imprints due to different modes of bottom water formation are assumed to cause the observed deviation from the global δ13CΣCO2/PO43− relationship in the deep Weddell Sea. The influence of increased photosynthetic fractionation, i.e., a more negative than low‐latitude isotopic organic carbon composition, is shown to be minor. As a result, Recent Weddell Sea deep and bottom water δ13CΣCO2 is by 0.4–0.5‰ higher than expected if solely biologic fractionation would occur. A discussion of simple hypotheses of Weddell Sea deep and bottom water formation during glacial times reveals that regardless of what scenario is considered, the thermodynamic imprint on Southern Ocean deep water would increase. This makes it difficult to explain low glacial δ13C values observed in benthic foraminifera from the subpolar Southern Ocean as being calcified in Antarctic source bottom water and thus is in support of hypotheses looking for additional sites of deep water formation.
Outer planets and their major satellites: Atmospheric and surface properties
Authors: Lunine Jonathan I.
Published: February 2004

The outer planets and their satellites constitute a broad area of investigation in planetary science, and one in which progress has been a result both of high visibility spacecraft explorations as well as novel opportunities and technological improvements in ground‐based studies. The time period from 1991–1994 was dominated by detailed analyses of data from the Voyager flyby, radar and sensitive infrared techniques applied to the Galilean satellites and Titan, reanalysis and interpretation of stellar occultations of Titan and Pluto, and increasingly sensitive spectroscopic measurements from the ground of all these objects at a range of wavelengths. Meanwhile, the July 1994 impacts of Comet Shoemaker‐Levy 9 fragments into Jupiter, as well as the arrival in 1995 of Galileo, has led to an increased interest in this largest planet.
Charting the heliosphere in three dimensions
Authors: Neugebauer M.
Published: February 2004

Space physicists continue their exploration of the solar system's home in the galaxy—that bubble of space, called the heliosphere, carved out of the interstellar medium by the solar wind. Studies of the heliospheric plasma are driven by the search for answers to questions such as: What mechanisms are responsible for the acceleration of the solar wind? How do the properties of the solar wind evolve with distance from the Sun? What is the nature of the interaction of the solar wind with the interstellar medium? The principal progress made in the last four years has come from data obtained by the Pioneer and Voyager spacecraft moving away from the Sun at roughly 3 AU/year, from the first measurements at high latitudes currently being made by the joint European‐NASA mission named Ulysses, and from continued remote sensing of the inner heliosphere. This review focuses on the average values of the plasma velocity, density, and temperature and the magnetic field in different regions of the heliosphere, leaving discussion of the details of many important structures and processes to other papers.
Solar wind composition
Authors: Ogilvie K. W. , Coplan M. A.
Published: February 2004

The composition of the solar wind is a tangible measure of the region of the sun from which it originates, the solar corona. The solar wind also sweeps up material from the regions of interplanetary space through which it passes. There are many sources of entrained material including comets, asteroids, the atmospheres of planets and satellites and material from the local interstellar medium. When the entrained material can be distinguished from solar material, solar wind composition measurements provide information on the extrasolar sources. This review is divided into two parts, one dealing with solar wind composition and the sun, the other dealing with some sources of solar wind material other than the sun.
Tracer‐based inferences of new and export primary productivity in the ocean
Authors: Jenkins W.J.
Published: February 2004

Obtaining regional estimates of seasonal to decadal average new productivity in the ocean is an important goal. Tracer studies have proven useful in making such determinations, although the nature of the estimates made are colored by the character and space/time scales of the tracer systems used. These differences provide important clues as to mechanisms and processes. Knowledge of new, export and particulate export fluxes are powerful constraints for biogeochemical models. We have made significant advances over the past four years, with different tracer systems yielding more or less consistent results in, for example, the subtropical North Atlantic. The general agreement between essentially independent approaches does much to build our confidence in the veracity of their results. Further, exciting new insights are provided by new information regarding the relative magnitude of particulate export flux.

Some nagging concerns remain, however, both due to the mismatch between these measurements and other more traditional approaches (e.g., sediment traps), and some unresolved implications of some of the observations. With improved measurement techniques, more sophisticated interpretive tools, and the large field programs in progress, we have high hopes that the future will bring a clearer view.
The ionosphere's wild ride in outer space
Authors: Horwitz J. L.
Published: February 2004

Plasma of terrestrial origin is produced chiefly by either solar photo‐ionization or electron bombardment ionization of the Earth's upper atmosphere, forming the ionosphere. Under many conditions, components of the higher‐altitude ionosphere acquire sufficient energy to escape the Earth's gravitational attraction and flow out along geomagnetic field lines into various regions of the magnetosphere, and perhaps beyond, into the solar wind. Over the past two decades, intense observational and modeling efforts by numerous researchers have sought to understand the processes driving these ionospheric outflows and the resulting presence and consequences of these ionospheric plasmas populating the major regions of the magnetosphere.

The matter of ionospheric outflows directly influences two significant thrusts of current magnetospheric research. First, the exciting concept of imaging magnetospheric plasmas is now believed to be feasible and is planned for future NASA missions. Some of the envisioned imaging approaches are based on detection of solar EUV radiation resonantly‐scattered from ionospheric‐origin He+ and O+ ions. Second, there is the major unresolved issue of whether the ionospheric outflows supply much of the plasma content of such regions as the ring current and the plasma sheet, and exactly what the geophysical consequences of the significant presence of heavy ionospheric He+ and O+ ions might be on general wave‐particle processes and substorm triggering and amplification in the tail plasma sheet.
Low grade metamorphism of mafic rocks
Authors: Schiffman Peter
Published: February 2004

Through most of this past century, metamorphic petrologists in the United States have paid their greatest attention to high grade rocks, especially those which constitute the core zones of exhumed, mountain belts. The pioneering studies of the 50's through the 80's, those which applied the principles of thermodynamics to metamorphic rocks, focused almost exclusively on high temperature systems, for which equilibrium processes could be demonstrated. By the 1980's, metamorphic petrologists had developed the methodologies for deciphering the thermal and baric histories of mountain belts through the study of high grade rocks.

Of course, low grade metamorphic rocks ‐ here defined as those which form at pressures and temperatures up to and including the greenschist facies ‐ had been well known and described as well, initially through the efforts of Alpine and Circum‐Pacific geologists who recognized that they constituted an integral and contiguous portion of mountain belts, and that they underlay large portions of accreted terranes, many of oceanic origins. But until the mid 80's, much of the effort in studying low grade rocks ‐ for a comprehensive review of the literature to that point see Frey (1987) ‐ had been concentrated on mudstones, volcanoclastic rocks, and associated lithologies common to continental mountain belts and arcs. In the mid 80's, results of the Deep Sea Drilling Project (DSDP) rather dramatically mitigated a shift in the study of low grade metamorphic rocks.
Anthropogenic CO2: The natural carbon cycle reclaims center stage
Authors: Toggweiler J. R.
Published: February 2004

Atmospheric CO2 concentrations are increasing by about 1.5 ppm every year. This 1.5 ppm increase integrated over the volume of the atmosphere accounts for about 50% of the CO2 released to the atmosphere from fossil fuel burning and deforestation (henceforth “anthropogenic” CO2). The rest of the anthropogenic release (the part that does not remain in the atmosphere) goes mainly into the oceans and the terrestrial biosphere and soils. Knowing how CO2 is partitioned between oceanic and terrestrial CO2 sinks is critical for making CO2 forecasts into the future. This paper provides a brief review concerning the oceanic uptake of anthropogenic CO2. (A more detailed review is provided by Siengenthaler and Sarmiento, 1993.)
Do the dayside cusps blink?
Authors: Newell Patrick T.
Published: February 2004

The Earth's magnetic field forms a bubble which largely excludes the solar wind. Near noon, however, a narrow funnel of entry called the polar cusp exists, through which shocked solar wind (magnetosheath plasma) can reach the ionosphere. The low‐altitude particle cusp is that region of the ionosphere for which magnetosheath plasma can be observed. Nominally, the particle cusp equatorward boundary lies at 77° magnetic latitude, the latitudinal width is 1°, and the local time extent is about 2.5 h, centered about noon [Newell et al, 1989]. The interplanetary magnetic field (IMF), dipole tilt angle, solar wind pressure and direction, and, possibly, substorm activity, can all modify these nominal values (in approximately that order of importance).
Demarcating the magnetopause boundary
Authors: Sibeck David G.
Published: February 2004

Determining the shape of the magnetopause is one of the oldest problems in space physics and one of its earliest success stories. Chapman and Ferraro [1931] were the first to recognize that the Earth's magnetic field carves out a cavity in the oncoming solar wind. Theoretical studies based on gas dynamics which predicted the shape of the magnetopause bounding this cavity [e.g., Ferraro, 1960; Spreiter and Hyett, 1963] were followed by studies demonstrating that magnetopause crossings generally occur in the expected locations [Ness, 1965]. Some years passed before it was realized that the orientation of the interplanetary magnetic field (IMF) also has an important effect on the size and shape of the magnetosphere [Aubry et al, 1970].

Determining the location and shape of the magnetopause is useful for both practical and theoretical reasons. The magnetic field rotates from magnetosheath to magnetospheric orientations at the magnetopause. Since some satellites in geosynchronous orbit make attitude adjustments by applying magnetic torques using the ambient field, it is important to know whether the magnetopause is Earthward or Sunward of the satellite. An accurate knowledge of the magnetopause location may help in planning data acquisition during forthcoming missions which lack the resources to take continuous observations. Analytical modellers of the magnetospheric magnetic field wish to confine‐that field within a well‐determined magnetopause boundary. Finally, changes in the magnetopause location and shape reveal important information about solar wind and magnetospheric conditions.
Modeling of multiphase multicontaminant transport in the subsurface
Authors: Russell Thomas F.
Published: February 2004

This review addresses modeling of subsurface hydrological systems in which contaminants are transported in more than one fluid phase. The primary focus is on problems involving nonaqueous‐phase liquids (NAPLs), a dynamic gas phase, or both, so that the unsaturated and saturated zones are both discussed. Basic research in simpler settings is included if it develops concepts that could be applied in modeling of such problems.

Recent developments in practical models are reviewed, along with experimental work and theoretical issues related to the formulation of mathematical models. It will be seen that the extent of validity of the local equilibrium assumption (LEA) for component partitioning among phases is a major question influencing the direction of these formulations. Also emphasized are upscaling to macroscopic and grid lengths, and the choice and coupling of primary variables in multicomponent systems. Relations to and possible use of developments in petroleum reservoir simulation are discussed.
Advances in ocean modeling for climate change research
Authors: Holland William R. , Capotondi Antonietta , Holland Marika M.
Published: February 2004

An adequate understanding of climate variability and the eventual prediction of climate change are among the most urgent and far‐reaching efforts of the scientific community. The climate system is in an ever‐changing state with vast impact on mankind in all his activities. Both short and long‐term aspects of climate variability are of concern, and the unravelling of “natural” variability from “man‐induced” climate change is required to prepare for and ameliorate, if possible, the potentially devastating aspects of such change.

In terms of scientific effort, the climate community can be thought of as the union of the disciplinary sciences of meteorology, oceanography, sea ice and glaciology, and land surface processes. Since models are based upon mathematical and numerical constructs, mathematics and computer sciences are also directly involved. In addition, some of the problems of man‐induced climate change (release of greenhouse gases, the ozone‐hole problem, etc.) are basically chemical in nature, and the expertise of the atmospheric and oceanic chemist is also required. In addition, some part of the response to climate perturbations will arise in the biological world, due to upsetting the balance in the great food web that binds communities together on both the land and the sea. Thus, the problems to be solved are extraordinarily complex and require the efforts of many kinds of scientist.
Hydrothermal processes at mid‐ocean ridges
Authors: Humphris Susan E.
Published: February 2004

Submarine hydrothermal systems are an integral component of crustal construction along the global system of mid‐ocean ridges. Thermally‐induced circulation of seawater through the permeable parts of the crust and upper mantle has been estimated to account for 34% of the predicted global oceanic heat flux, which in turn comprises close to 25% of the total heat flux of the Earth [Stein and Stein, 1994, and references therein]. Discharge of hydrothermal fluids is manifest along mid‐ocean ridges as high temperature (∼200–400°C) focused and lower temperature (<200°C) diffuse fluid flow. Off‐axis fluid flow may extend out to the crustal sealing age of 65±10 million years, and may be responsible for more than 70% of the hydrothermal heat flux [Stein and Stein, 1994].
Some advances in understanding of the general circulation of the Pacific Ocean, with emphasis on recent U.S. contributions
Authors: Talley Lynne D.
Published: February 2004

Advances in observing and understanding the general circulation of the ocean have been made continuously through the past two centuries. A recent burst of activity in the Pacific has been due to an interest and ability to understand and model the El Niño phenomenon, primarily through the Tropical Ocean Global Atmosphere (TOGA) program, and due to the interest in larger scale phenomena such as heat transport and basics of the general circulation, through activities of the World Ocean Circulation Experiment (WOCE). These experiments have built on the extensive knowledge of the Pacific Ocean derived over the previous decades; a perusal of knowledge at this point shows it to be incomplete in both observations and theory and it is hoped that the greatly increased focus on climate issues, which very much include the ocean's general circulation and its variability, will go far to improve our understanding.

There have been equal or possibly greater advances in understanding the circulation of the Atlantic, and the observational base arid theory of the circulation of the Indian and Arctic Oceans have been advanced substantially. Two other reports in this volume [Fine, 1995; Hogg and Johns, 1995] survey general‐circulation related issues in the Atlantic. A review of Indian Ocean circulation at this time would be premature given the expansion of WOCE programs to the Indian Ocean in the next year. Thus in order to limit the scope of this review, the principal focus is the Pacific Ocean, emphasizing the intermediate and deep circulation and long time scales of variability. Observations are reviewed as they relate to simple theories of the circulation.
The magnetosphere in the machine: Large scale theoretical models of the magnetosphere
Authors: Walker Raymond J. , Ashour‐Abdalla Maha
Published: February 2004

It has long been recognized that many of the macroscopic properties of the interaction of the solar wind with the magnetosphere can be understood by treating the solar wind as a fluid. For over thirty years gas dynamic models of the magnetosheath have been used successfully to model the position and shape of the bow shock and the plasma moments within the magnetosheath [Spreiter and Alksne, 1969]. In 1978 Leboeuf et al. [1978] developed the first global magnetohydrodynamic (MHD) simulation which modeled the magnetosphere as well as the magnetosheath self‐consistently. Since then advances in computers and improved numerical techniques have helped us to build high resolution three dimensional models with which to investigate the configuration and dynamics of the entire solar wind, magnetosphere and ionosphere system.
Advances in ionospheric physics: Roles, relevance and predictions in the system of solar‐terrestrial plasmas
Authors: Szuszczewicz Edward P.
Published: February 2004

The ionosphere can be viewed as the culmination of all coupling processes in the system of solar‐terrestrial plasmas, including such diverse phenomena as coronal mass ejections, magnetospherically‐imposed electric fields, and solar‐driven thermospheric tides. It follows therefore that the accurate and self‐consistent specification of ionospheric plasma distributions represents a critical test of our understanding of the ionosphere itself and the myriad of controls and coupling mechanisms from the Sun to the Earth.

There is, however, neither a code nor a model that can accurately specify the quiet‐ or storm‐time characteristics of ionospheric plasma distributions. Nor is there an adequate database to constrain the models and enforce unique quantitative solutions. Among the challenges are the ionospheric structures themselves which span eight orders of magnitude in size. Their theoretical and experimental investigation requires a broad computational grid and unique combinations of “in situ” and remote sensing techniques to define the hierarchy of sizes and cause‐effect terms. These structures range from tens‐of‐thousands of kilometers to tens‐of‐centimeters, with scale lengths that can be horizontal or vertical in the Earth's gravitational field, or parallel or perpendicular to the geomagnetic field depending on the causal mechanisms.
Toxic red tides and harmful algal blooms: A practical challenge in coastal oceanography
Authors: Anderson Donald M.
Published: February 2004

The debate over the relative value of practical or applied versus fundamental research has heated up considerably in recent years, and oceanography has not been spared this re‐evaluation of science funding policy. Some federal agencies with marine interests have always focused their resources on practical problems, but those with a traditional commitment to basic research such as the National Science Foundation have increasingly had to fight to maintain their freedom to fund quality science without regard to practical or commercial applications.

Within this context, it is instructive to highlight the extent to which certain scientific programs can satisfy both sides of this policy dilemma—i.e. address important societal issues through advances in fundamental or basic research. One clear oceanographic example of such a program involves the phenomena called “red tides” or “harmful algal blooms”. This paper describes the nature and extent of the problems caused by these outbreaks, emphasizing the alarming expansion in their incidence and their impacts in recent years, both in the U.S. and worldwide. The objective is to highlight fundamental physical, biological, and chemical oceanographic question that must be addressed if we are to achieve the practical goal of scientifically based management of fisheries resources, public health, and ecosystem health in regions threatened by toxic and harmful algae.
Volcano seismology, hazards assessment
Authors: Mori Jim
Published: February 2004

The last few years have seen several impressive examples of seismic monitoring at volcanoes to provide warnings that have saved lives and property. Changes in the rates and character of volcanic earthquakes provided precursory signs to explosive eruptions of moderate size at Redoubt volcano (Alaska), Mount Unzen (Japan), Mount Spurr (Alaska), and Rabaul caldera (Papua New Guinea), as well as the large eruption at Mount Pinatubo (Philippines). In all these cases, information was successfully communicated and put to practical use through broadcasts of public warnings prior to the eruptions and evacuations were initiated for the volcanoes located in populated areas. At Mount Pinatubo, probably thousands and possibly tens of thousands of lives were saved.
Assessing the magnetosphere's nonlinear behavior: Its dimension is low, its predictability, high
Authors: Surjalal Sharma A.
Published: February 2004

The complexity of magnetospheric dynamics arises from a number of physical processes extending from the sun to the ionosphere and encompasses many space and time scales. Ground‐based and spacecraft observations have shown irregularity in magnetospheric dynamics, and their interpretation using the conventional tools such as the linear prediction filters has led to many interesting results. Recent developments in the theory of nonlinear dynamical systems that exhibit such irregular behavior have led to new techniques of reconstructing the intrinsic phase space structure using time series data. The key advantage of these techniques is their ability to yield the inherent dynamical characteristics of the system as represented in the observational data, independent of particular modeling assumptions. The basis for this approach is the realization that seemingly irregular behavior does not necessarily require a complex dynamical or statistical description, but can be understood from simple models containing nonlinearities. Thus a physical system that exhibits complex behavior may be a deterministic dynamical system with a few significant degrees of freedom. Such a possibility, among others, have led to a variety of techniques in the study of nonlinear phenomena.