bluuebunny/crossref_metadata_2025 huggingface dataset:
Atmospheric effects on cratering on Venus
Authors: Takata T. , Ahrens T. J. , Phillips R. J.
Published: February 2004

A paraboloidal bow shock model is developed in order to estimate the surface distribution of gas shock‐induced modifications surrounding Venusian impact craters. We apply two‐dimensional oblique shock dynamics to describe a three‐dimensional paraboloidal‐shaped bow shock impinging upon an assumed incompressible Venusian surface. The effects of the hypersonic atmospheric shock acting on the Venusian surface are considered in terms of induced maximum gas pressure, density, particle velocity, and temperature, for varying angles and velocities of impact. The maximum boulder size that can be saltated by the shock wave induced gas flow and the degree of mutual collision of the surface materials are also considered. The present calculations quantitatively predict the areal extent of the gas shock perturbed surface for normal and oblique impact as a function of impact angle and velocity, and radii of impactors. For a 1‐km radius stony meteorite impacting normally at 20 km/s, the radius of the disturbed area extends ∼10–17 times the 3–5 km crater radius. The perturbed surface affects the surface radar properties, and the present results can provide an explanation of the wide “dark/bright halos” surrounding some of the Venusian impact craters observed via Magellan imagery. For example, a ∼50‐km radius bright halo surrounding a ∼20‐km dark halo is observed around the 3.1‐km radius crater located at 16.5° north latitude and 334.4° longitude. The average value of the radar backscatter cross section of the ∼20‐km radius dark halo indicates that ∼50‐cm‐thick layer of porous lithologic material is superimposed upon an assumed undisturbed basement rock surface. The bright halo indicates that the surface roughness in this region is ∼30 % greater than that of the surrounding original surface. These features can be induced by atmospheric shock waves. The present model can relate the observed crater halo radii to the impact parameters, such as projectile radius and density, and the impact velocity and angle.
Time delays in the solar wind flow past Venus: Galileo‐Pioneer Venus correlations
Authors: Huddleston D. E. , Russell C. T. , Kivelson M. G. , Luhmann J. G.
Published: February 2004

The Galileo and Pioneer Venus Orbiter (PVO) spacecraft obtained simultaneous measurements at Venus during the Galileo flyby on February 9 and 10, 1990. The magnetic field data sets from the two spacecraft show corresponding convected features, even when Pioneer Venus is near periapsis on the nightside of the planet. Comparison of the observation times of these corresponding features reveals substantial delays at PVO associated with the deceleration of the flow carrying the draped magnetic field past Venus. Convection lags of up to 10 min are identified near PVO periapsis at an altitude of 1/4 Venus radii (1500 km) above the planet surface. A three‐dimensional convected field gas dynamic model is used to calculate expected lag times by tracing field lines from PVO to the open solar wind. Expected lags from the model are relatively insensitive to changes in field orientation and Mach number. This simple gas dynamic model with the standard ionopause obstacle size gives convection lags too small to explain the Galileo‐PVO observations. A likely explanation is that mass loading of the solar wind by the Venus atmosphere contributes to the deceleration of the flow near the planet. The observation of similar magnetic structures in the solar wind and deep in the Venus nightside ionosphere suggests that the impenetrable obstacle model is inappropriate in this region and the solar wind magnetic field is convected into the nightside ionosphere on timescales of 10 min or more.
Constraints on the structure and composition of Mars from thermal evolution models
Authors: Weizman A. , Prialnik D. , Podolak M.
Published: February 2004

A numerical model of the thermal evolution of Mars was developed, solving simultaneously the equations for energy conservation and hydrostatic equilibrium throughout the planet, and using an equation of state that includes the computation of the thermal pressure according to the Debye theory. Based on the known values of the size and mass of Mars, and choosing the value of 0.365 for the inertia factor, we determine the initial mass fractions of different materials constituting the core and mantle of Mars. The abundance ratios of the components are described by three parameters. Two of these parameters are fixed by the radius and inertia factor of Mars, while the third remains free. Several models that match the present radius and inertia factor were found, distinguished by their initial composition. The thermal evolution and present internal state of these models were evaluated, in order to establish constraints on the composition and internal structure of Mars.
A harmonic analysis of Martian topography
Authors: Bills Bruce G. , Nerem R. Steven
Published: February 2004

The topography of Mars is represented by a spherical harmonic model complete to degree and order 50. The data source is a recently revised U.S. Geological Survey compilation which encompasses a diverse array of spacecraft measurements and Earth‐based radar ranges. Because of the disparate nature of the original data sources, it is difficult to obtain reliable estimates of the likely errors in Martian topography. However, the variance spectrum has a shape similar to that found for other terrestrial planets, and the topography correlates well with recent estimates of the gravitational field of Mars. Both of these considerations suggest that the existing data provide a reasonable approximation to the true topography of Mars. Nevertheless, a detailed topographic mapping of the planet, such as will be provided by the Mars Global Surveyor, will be required before significant advances are made in understanding the relationship between topography and gravity on Mars.
Magnetospheric effects on micrometeoroid fluxes
Authors: Colwell Joshua E. , Horányi Mihály
Published: February 2004

The planets, and their moons and rings, are bombarded by a population of interplanetary and interstellar micrometeoroids. This bombardment affects the evolution of planetary rings and electron densities in ionospheres. It also generates and overturns regolith on planetary satellites and ring particles. The impacting flux is enhanced by the planet's gravity, but the magnetosphere can also affect the flux for micrometeoroids near 1 μm in radius. We present calculations for micrometeoroids hitting Jupiter which show the magnetosphere shields the planet from impactors smaller than about 0.1 μm in radius, and enhances the flux of impactors by a factor of up to about 4 for grains near 0.5 μm in radius. Thus, in addition to the gravitational focusing factor, there is a size‐dependent electromagnetic focusing factor active up to about 1.0 μm. In addition, we find that after 1 (Earth) year, significant numbers of small particles still orbit Jupiter. The fate of these particles may represent an additional enhancement of the flux of micrometeoroids onto the planet.
Structure of Jupiter's upper atmosphere: Predictions for Galileo
Authors: Yelle Roger V. , Young Leslie A. , Vervack Ronald J. , Young Richard , Pfister Leonard , Sandel Bill R.
Published: February 2004

The Voyager mission to the outer solar system discovered that the thermospheres of all the giant planets are remarkably hot. To date, no convincing explanation for this phenomenon has been offered; however, there are a number of recent observational results which provide new information on the thermal structure of Jupiter's upper atmosphere that bear on this outstanding problem. We present an analysis of Jupiter's thermal structure using constraints from H3+ emissions, Voyager UVS occultation data, ground‐based stellar occultation data, and the properties of the Jovian UV dayglow. Although the initial, separate analysis of these data sets produced contradictory results, our reanalysis shows that the observations are consistent and that the temperature profile in Jupiter's upper atmosphere is well constrained. We find that the data demand the presence of a large temperature gradient, of order 3–10 K/km, near a pressure of 0.3 μbar. Analysis of the temperature profile implies that an energy source of roughly 1 erg cm−2 s−1 is required to produce the high thermospheric temperature and that this energy must be deposited in the 0.1–1.0 μbar region. It is also necessary that this energy be deposited above the region where diffusive separation of CH4 occurs, so that the energy is not radiated away by CH4. We show that dissipation of gravity waves can supply the energy required and that this energy will be deposited in the proper region. Moreover, because the turbulent mixing caused by gravity waves determines the level at which diffusive separation of CH4 occurs, the location of the energy source (dissipation of waves) and the energy sink (radiation by CH4) are coupled. We show that the gravity waves will deposit their energy several scale heights above the CH4 layer; energy is carried downward by thermal conduction in the intervening region, causing the large temperature gradient. Thus dissipation of gravity waves appears to be a likely explanation for the high thermospheric temperature. Our arguments are general and should apply to Saturn, Uranus, and Neptune, as well as Jupiter. The model temperature profiles presented here and the relationship between the gravity wave flux and thermospheric temperature are directly testable by the Atmospheric Structure Instrument carried by the Galileo probe.
On the outflow of O2+ ions at Mars
Authors: Kar J. , Mahajan K. K. , Kohli R.
Published: February 2004

The ASPERA (automatic space plasma experiment with a rotating analyzer) instrument aboard Phobos 2 has detected a large outflow of molecular ions (possibly O2+) from the Martian environment. We suggest that this molecular outflow results from horizontal transport of O2+ ions from the dayside. We demonstrate that the in situ profiles of O2+ ions as observed by the Viking 1 and 2 landers at Mars are eroded by the solar wind interaction down to an altitude of about 150–160 km. The profiles are considerably depleted compared with the corresponding diffusive equilibrium profiles. The difference between the diffusive equilibrium and the observed profile is the likely amount of O2+ ions which flow over to the nightside, a part of which would escape the gravity of the planet. However, the estimated escape rate exceeds the observed total ionospheric escape rate by about an order of magnitude and that of the molecular ions by 1 to 2 orders of magnitude. This suggests that there is possibly a missing component in the observed molecular ion outflow from Mars.
Assessment of sea‐surface temperature at 42°N in the California Current over the last 30,000 years
Authors: Prahl Fredrick G. , Pisias Nicklas , Sparrow Margaret A. , Sabin Anne
Published: February 2004

Assessment of changes in surface ocean conditions, in particular, sea‐surface temperature (SST), is essential to understand long‐term changes in climate especially in regions where continental climate is strongly influenced by oceanographic processes. To evaluate changes in SST in the northeast Pacific, we have analyzed long‐chain alkenones of prymnesiophyte origin at 38 depths in a piston and associated trigger core collected beneath the contemporary core of the California Current System at 42°N, ∼270 km off the coast of Oregon/California. The samples span 30,000 years of deposition at this location. Unsaturation patterns
in the alkenone series display a statistically significant difference (p ≪ 0.001) between interglacial (0.44 ± 0.02, n = 11) and glacial (0.29 ± 0.04, n = 20) intervals of the cores. Detailed examination of other compositional features of the C37, C38, C39 alkenone series and a related C36 alkenoate series measured downcore suggests the published
‐ temperature calibration
, defined for cultures of a strain of Emiliania huxleyi isolated from the subarctic Pacific, provides best estimates of winter SST at our study site. This inference is purely statistical and does not imply, however, that the phytoplankton source of these biomarkers is most productive in winter or at the ocean surface. The temperature record for
implies (1) an ∼4°C shift occurred in winter SST from ∼ 7.5 ± 1.1°C at the last glacial maximum to ∼ 11.7 ± 0.7°C in the present interglacial period, and (2) this warming trend was confined to the time frame 14–10 Ka within the glacial to interglacial transition period. These conclusions are corroborated entirely by results from an independent SST transformation of radiolarian species assemblage data obtained from the same core materials.
Comparing simulated glacial climate and paleodata: A reexamination
Authors: Broccoli A. J. , Marciniak E. P.
Published: February 2004

Glacial sea surface temperatures (SSTs) simulated by an atmosphere‐mixed layer ocean model are compared with those reconstructed by the Climate: Long‐Range Investigation, Mapping, and Prediction (CLIMAP) Project using planktonic microfossils. Two methods of comparison are employed. The first is global and uses the subjectively analyzed (i.e., hand contoured, then digitized) data set published by CLIMAP. The second is restricted to only those discrete locations where the CLIMAP sediment cores were taken. Both methods indicate that many aspects of the reconstructed glacial SST changes are simulated reasonably well by the model, although there are areas of disagreement. The extent of the disagreement appears smaller when the SSTs are sampled at discrete locations, because the largest discrepancies occur in the subtropical Pacific where data are sparse. When examined separately for each ocean, the magnitude of the disagreement in low latitudes roughly corresponds to the magnitude of the uncertainties in SST estimation using planktonic microfossils, being largest in the Pacific and smallest in the Atlantic. Because the largest discrepancies occur where uncertainties in estimation are large, no clear determination of whether the climate model exaggerates or underestimates low‐latitude climate sensitivity appears possible. Nonetheless, sampling at discrete locations may be the best procedure for evaluating climate model performance, because errors associated with extending analyses to data‐void areas can be avoided and uncertainties associated with inadequate spatial sampling made more evident.
Control of North Atlantic Deep Water Circulation by the Greenland‐Scotland Ridge
Authors: Wright James D. , Miller Kenneth G.
Published: February 2004

Coherent bathymetric features along the Reykjanes Ridge indicate that there were significant changes in the flux of buoyant material within the Icelandic Hot Spot during the Neogene. The radial extent of the topographic swell associated with this hot spot is of the order of 1000 to 2000 km, and therefore these changes affected the Greenland‐Scotland Ridge (GSR). At present, sill depths along the GSR are generally less than 500 m with the deepest passage being less than 1000 m, making the overflow water sensitive to even small changes in the ridge depths. Reconstructions of Neogene mantle plume activity correlate with the deepwater circulation patterns in the North Atlantic. Times of high mantle plume activity caused Northern Component Water (NCW) production to cease. NCW fluxes resumed once this phase of high mantle plume activity slowed. The long‐term climate change during the Neogene must be controlled by factors other than NCW production. However, climatic optima during the late early Miocene and early Pliocene appear to have been augmented by high NCW fluxes, while subsequent middle Miocene and “middle” Pliocene coolings correlate with uplifts on the GSR and reduced NCW flux. We suggest that reductions in NCW may have contributed to both of these cooling events.
A reexamination of northeast Atlantic sea surface temperature and salinity over the last 16 kyr
Authors: Sikes Elisabeth L. , Keigwin Lloyd D.
Published: February 2004

We present a 15 kyr sea surface temperature (SST) record for a high sedimentation rate core (KNR51‐29GGC) from the Feni Drift off of Ireland, based on an organic geochemical technique for paleotemperature estimation, U37K′. We compare the U37K′ temperature record to planktonic foraminiferal δ18O and foraminiferal assemblage SST estimates from the same sample horizons. U37K′ gives SST estimates of 13°C for the early deglacial and 18°C for the Holocene and Recent, whereas assemblages give estimates of 9°C and 13°C, respectively. As in nearby core V23‐81, we find Ash Zone 1, the Younger Dryas increase in Neogloboquadrina pachyderma sinistral abundance, and maximum abundance of this species during glaciation. N. pachyderma dextral oxygen isotopic analyses have a late glacial to interglacial range of 1.5‰. A reduction of about 1 ‰ in δ18O occurred at about 12 ka, whereas U37K′ and the foraminiferal fauna indicate a 2°C warming. This implies a 0.9‰ salinity effect on δ18O which we attribute to meltwater freshening. All three parameters indicate cooling during the Younger Dryas. U37K′ SST estimates show that the major shift from deglacial to interglacial temperatures occurred after the Younger Dryas in termination 1b, in contrast to the assemblage data, which show this jump in SST at the end of the glaciation during termination Ia. Differences between the two SST estimators, which may result from their different (floral versus faunal) sources, are more pronounced between transitions Ia and Ib. This may reflect different habitats under the unusual sea surface conditions of the deglaciation.
Hubble Space Telescope results in planetary science
Authors: Beebe Reta
Published: February 2004

The Hubble Space Telescope (HST) was placed into orbit by the Discovery shuttlecraft on April 25, 1990, and first light images were obtained on May 20, 1990 (Feinberg, 1990a; Palga, 1990). On June 26, NASA announced that the telescope suffered from spherical aberration and that , instead of focusing 70 percent of a star's light in an area 0.2 arcsec in diameter, the light was spread over an area with a diameter of 1.4 arcsec. The resulting point‐spread function (PSF) contained a 0.2 arc sec central spike that contained 15% of the light and, because the problem concerned two excellent yet mismatched mirrors, the resulting apron of the PSF could be represented by a predictable diffraction pattern, indicating that part of the resolution of the two cameras could be recovered by image deconvolution. As a part of the effort to assess the magnitude of the problem, the Wide Field/Planetary Camera (WFPC‐1) team obtained broad band blue, green and red images of Saturn on August 26, 1990 (Westphal et al, 1991). Using PSFs derived from a star for the 547 nm filter and theoretical PSFs for the 439 nm and 718 nm filters, the WFPC‐1 team used Lucy's algorithm (1974) to deconvolve the data and produced a composite three color image. This effort showed that a narrow gap, the Encke Division, in the outer ring could be resolved and produced an effective resolution of the deconvolved images that was better than the best groundbased results. Even with the focusing problems, this effective resolution ‐ combined with the fact that HST suffers less from scattered light, has UV sensitivity and yields the same resolution from observation to observation ‐ recommended it for planetary studies involving spatial and temporal variability. These initial observations revealed that Saturn's north polar hexagon, seen by Voyager (Allison et al, 1990), was visible, implying that it is not a transient feature. The latitudinal locations of belt‐zone interfaces differed from those recorded in 1981 Voyager images, indicating that seasonal or undetected sporadic convective activity had altered the reflective properties of the atmosphere.
Lithosphere dynamics and continental deformation
Authors: Bird Peter
Published: February 2004

The unifying theme in this section is the remarkable weakness of major faults. I will consider the diverse new evidence for weakness, and the evidence for high pore pressure localized in faults as a fundamental cause. With this background one can better understand why faults remain active even after large rotations with respect to stress: I will look at large Neogene (≤23.7 million year old) rotations about horizontal axes in the Basin and Range province, and about vertical axes along the Pacific margin. Recent developments will be summarized from studies of Neogene tectonics (neotectonics) in California, Alaska, and the Mississippi embayment, in the context of a weak North American stress field that results mainly from topographic forces. To close, I will present new geophysical studies relevant to the continuing controversy over whether the basic structure of the North American mantle lithosphere was altered by an early Tertiary episode of flat subduction.
Stimulating our piece of the universe: Active experiments in space
Authors: Raitt W. J.
Published: February 2004

“Life science is the only space science discipline in which space experiments are performed.” This comment was made recently at a meeting of space scientists, striking those of us in physical science disciplines as somewhat bizarre. However, I realized on further reflection that in a sense the remark was largely correct except for one area of physical science space research known as active experiments in space.

Since the beginning of the space age, physical science investigators have made measurements predominantly of the space environment using advanced experimental techniques. Space experiments in this discipline have been much rarer events. Specific experiments designed to study the response of the space environment to controlled disturbances are generally termed active space experiments. In recent years the distinction between making measurements of the space environment and performing controlled experiments in that environment has been emphasized in the space physics community by a distinction in funding considerations between “Space Physics” and “Physics in Space”, that is passive versus active experimentation. However, the distinction is not always clear since even passive experiments provide some disturbance to the space environment, and some experiments use active techniques with minimal disturbance to measure ambient properties of the local or remote space environment. In this report, my intention is to address primarily those experiments which set out to actively perturb the environment from space platforms and study the effects of that perturbation.
New ideas about the physics of earthquakes
Authors: Rundle John B. , Klein William
Published: February 2004

It may be no exaggeration to claim that this most recent quaddrenium has seen more controversy and thus more progress in understanding the physics of earthquakes than any in recent memory. The most interesting development has clearly been the emergence of a large community of condensed matter physicists around the world who have begun working on the problem of earthquake physics. These scientists bring to the study of earthquakes an entirely new viewpoint, grounded in the physics of nucleation and critical phenomena in thermal, magnetic, and other systems. Moreover, a surprising technology transfer from geophysics to other fields has been made possible by the realization that models originally proposed to explain self‐organization in earthquakes can also be used to explain similar processes in problems as disparate as brain dynamics in neurobiology (Hopfield, 1994), and charge density waves in solids (Brown and Gruner, 1994). An entirely new sub‐discipline is emerging that is focused around the development and analysis of large scale numerical simulations of the dynamics of faults. At the same time, intriguing new laboratory and field data, together with insightful physical reasoning, has led to significant advances in our understanding of earthquake source physics. As a consequence, we can anticipate substantial improvement in our ability to understand the nature of earthquake occurrence. Moreover, while much research in the area of earthquake physics is fundamental in character, the results have many potential applications (Cornell et al., 1993) in the areas of earthquake risk and hazard analysis, and seismic zonation.
Seismology of the core
Authors: Tromp Jeroen
Published: February 2004

The purpose of this report is to review seismological research related to the Earth's core published during the period 1991–1994. The review period 1987–1990 is covered in Masters [1991]. I have attempted to provide a complete overview of research conducted in the United States; research carried out in other parts of the world is only partially reviewed.

This report is divided into two main sections concerned with the inner and outer core, respectively. The section on the inner core discusses exciting recent developments related to its apparent anisotropy, presents the latest views on its attenuation, and considers implications for its growth and composition. The discussion of the outer core focuses on its velocity profile and core oscillations.
Deep‐crustal seismology of continental margins
Authors: Brocher Thomas M.
Published: February 2004

This review discusses results from recent seismic experiments at continental margins conducted by U.S. investigators as well as those by non‐U. S. investigators necessary to place the results of U.S. workers into context. Continental margins may be characterized as either rifted, transform, or convergent. Rifted (or passive) margins form where continents are torn apart during continental drift. Transform margins accommodate transform plate motions along strike‐slip faults such as the San Andreas fault. At convergent margins oceanic crust is being thrust beneath (or subducted under) the continental crust. All three types of margins are found in North America.
U.S. contributions to the physical oceanography of continental shelves in the early 1990's
Authors: Lentz S. J.
Published: February 2004

The objective of this paper is to provide an overview of progress by U.S. investigators, during the early 1990's, in our understanding of the physical oceanography of continental shelves. In contrast to previous IUGG review articles on continental shelf physical oceanography [Allen et al., 1983; Brink, 1987], no attempt is made to give a comprehensive overview of all the research which has been done over the previous four years. Instead, this overview focuses on a few areas where there has been significant progress, as examples of the types of research being done in coastal physical oceanography. For a more general overview of shelf dynamics, see the recent summaries by Huyer [1990] and Brink [1991].
Recent developments associated with decision support systems in water resources
Authors: Watkins David W. , McKinney Daene C.
Published: February 2004

In order to limit the scope of this review, a working definition of a decision support system is needed. L. Adelman has defined decision support systems (DSSs) as “interactive computer programs that utilize analytical methods, such as decision analysis, optimization algorithms, program scheduling routines, and so on, for developing models to help decision makers formulate alternatives, analyze their impacts, and interpret and select appropriate options for implementation” (Adelman [1992], p. 2). Another definition has been offered by S. J. Andriole, who defined decision support as consisting of “any and all data, information, expertise or activities that contribute to option selection“ (Andriole [1989], p. 3). A common idea explicit in each of these definitions is that DSSs integrate various technologies and aid in option selection. Implicit in each definition is that these are options for solving relatively large, unstructured problems. Thus, the following working definition of a DSS will be used in this review: A DSS is an integrated, interactive computer system, consisting of analytical tools and information management capabilities, designed to aid decision makers in solving relatively large, unstructured problems.
The ocean component of the global water cycle
Authors: Schmitt Raymond W.
Published: February 2004

The distribution of evaporation and precipitation over the ocean (its hydrologic cycle) is one of the least understood elements of the climate system. However, it is now considered one of the most important, especially for ocean circulation changes on decadal to millennial time‐scales. The ocean covers 70% of the Earth's surface and contains nearly all (97%) of its free water, thus, it plays a dominant role in the global water cycle. The atmosphere only holds a few centimeters of liquid water, or 0.001% of the total. However, most discussions of the water cycle focus on the rather small component associated with terrestrial processes [Chahine, 1992]. This is understandable, since the water cycle is so vital to agriculture and all of man's activities. Yet current estimates indicate that 86% of global evaporation and 78% of global precipitation occurs over the oceans [Baumgartner and Reichel, 1975]; (Figure 1 ). Since the oceans are the source of most rainwater, it behooves us to work toward a better understanding of the ocean hydrologic cycle; small changes in ocean evaporation and precipitation patterns may have dramatic consequences for the much smaller terrestrial water cycle. For example, if less than 1% of the rain falling on the Atlantic Ocean were to be concentrated in the central US, it would double the discharge of the Mississippi river!
Application of satellite altimetry to ocean circulation studies: 1987–1994
Authors: Fu Lee‐Lueng , Cheney Robert E.
Published: February 2004

The ocean plays a key role in determining the global climate and its time evolution. To understand this role and subsequently develop techniques for predicting future climate, one must understand the dynamics of the global ocean circulation—the movement of water that transports mass, heat, salt, and other biogeochemical properties of the ocean that are closely linked to the processes of climate change. The only viable approach to observing the global ocean circulation with sufficient resolution and consistent sampling is the use of a satellite radar altimeter to measure the height of the sea surface—the sea level (Wunsch and Gaposchkin, 1980; Stewart, 1983; Wunsch, 1992). After removing the effects of the tides and atmospheric pressure from the observation, the deviation of the sea surface from the geoid, called the ocean dynamic topography, is readily related to the velocity of the surface geostrophic flow—a component of the surface flow on which the surface pressure force is balanced by the Coriolis force due to the Earth's rotation. Moreover, the ocean dynamic topography provides a strong constraint for determining the ocean circulation through the entire water column via the dynamic equations governing the fluid motion. Precise measurement of the shape of the global sea surface thus provides a powerful tool for studying the dynamics of the ocean circulation.
Seismology of the lower mantle and core‐mantle boundary
Authors: Lay Thorne
Published: February 2004

The thermal and chemical processes operating near the core‐mantle boundary (CMB) play a major role in the planet's evolution, influencing the magnetic field behavior, chemical cycling in the mantle, irregularities in the rotation and gravitation of the planet, and the configuration of mantle convection. The past decade has witnessed extensive investigation of the core‐mantle transition zone using seismological, geodynamical, geomagnetic, and mineral physics experimental techniques. The many multidisciplinary contributions on this region have been surveyed in several recent review articles (e.g. Loper and Lay, 1994; Jeanloz and Lay, 1993; Bloxham and Jackson, 1991) which provide a comprehensive survey of the recent literature. I confine my attention here to recent seismological evidence for laterally heterogeneous structure in the D″ region, the lowermost 200 km of the mantle. While seismic models for D″ are still low resolution, we have attained general constraints on the extent of heterogeneity which raise provocative questions as to the origins of the complex structures. The idea that chemical and thermal heterogeneity in D″ is a manifestation of boundary layer processes is a long‐standing one (Bullen, 1949), but we are far short of quantifying this notion. In particular, it is not known whether heterogeneities in D″ are relics of subducted slabs, in‐situ accumulations of core‐mantle chemical reaction products, primordial residuum from the core‐formation process, or thermal heterogeneities within a hot, unstable thermal boundary layer.
Magmatism at ocean ridges
Authors: Kinzler Rosamond J.
Published: February 2004

This report provides an overview of publications by scientists at U.S. institutions between 1991 and 1994 on magmatic processes at ocean ridges. Study of the geochemical variations of the rocks that comprise the oceanic crust has become an integral part of multi‐disciplinary approaches to the study of oceanic crustal genesis. These multi‐disciplinary approaches link studies of geochemical variations of ocean ridge basalts, crystalline rocks, and peridotites to studies of mantle flow, melt generation, segregation and transport, and the tectonic processes that occur at the ridge axis. All of these processes control the chemical variations observed in the rocks that constitute the oceanic crust, and thus the nature of scientific investigation of compositional variations observed in rocks from the oceanic crust has become increasingly more diverse. As the following discussion indicates, substantial progress has been made over the past 4 years in documenting and understanding the nature and origin of the compositional variations observed in oceanic crustal rocks generated at ocean ridge spreading centers.
Coupled physical‐biological models in oceanography
Authors: Franks Peter J. S.
Published: February 2004

In this paper, I review coupled physical‐biological models over the last four years. For the purposes of this report, I will restrict my review to work performed in the U.S. It should be recognized, however, that excellent work in this area is being done in Europe, South Africa and Japan. I do not claim this review to be exhaustive, and I apologize to those whose work I passed over. Rather, I try to show trends in the field: the types of model being employed, the ways in which they are used, and the prospects for the future.
Climate change: Does it matter?
Authors: Firor John
Published: February 2004

Ever since Svante Arrhenius, early in this century, calculated how much our use of fossil fuel might raise the surface temperature of the earth, there have been speculations concerning the impact of this and other human‐induced climate changes on society (Arrhenius, 1908). The conversion of this speculation about impacts into well established estimates has been hampered by uncertainties concerning the rate at which the climate forcing activities are changing, uncertainties in our knowledge of the nature of the climate changes that will follow the changes in forcing, uncertainties in the reaction of important ecosystems to the changing climate, and the lack of any widely adopted and tested methodology for estimating societal impacts of a climate change. Despite these difficulties, a number of studies in the United States in the period 1991–1994 have considered the impact of a human‐induced climate change, and other studies have discussed the methodological issues inherent in such work.

In this review, changes in all the systems mainly involving physical processes, e.g., atmosphere, ocean, ice caps, glaciers, and the hydrological cycle, will be regarded as part of the climate and not discussed here. The effect of changes in any of these physical systems on the biosphere, including humans and human societies, will be called impacts, and studies of these impacts will be at least sampled in this review. Changes in stratospheric ozone are sometimes considered to be climate changes, but this review will be restricted to the climate changes caused by increases in the concentrations of infrared‐trapping gases in the atmosphere.
New evidence for long‐distance fluid migration within the Earth's crust
Authors: Person M. , Baumgartner L.
Published: February 2004

During the past decade, geologists have come to appreciate the interconnectedness of hydrologic, tectonic, thermal, and geochemical processes operating within the Earth's continental crust [Oliver, 1992]. This has led to a new geologically‐based conceptual model of hydrology which is crustal‐scale and is centered in plate tectonics theory (Fig. 1). From a geological perspective, the tectonic and thermal processes which drive plate motion are also responsible, either directly or indirectly, for inducing fluid motion across and through the continents. Supporting evidence for this emerging paradigm is based on observations of pervasive rock‐water interactions associated with geologic processes as diverse as the chemical alteration of crustal rocks [Shelton et al, 1992; Elliott and Aronson, 1993; McManus and Hanor, 1993; Ague, 1991, 1994], devolatilization of minerals during burial and consequent metamorphism [Cox and Etheridge, 1989], the formation of energy and mineral deposits [Garven et al, 1993; and Cathles et al, 1993], remagnitization of ancient sedimentary rocks [McCabe and Elmore, 1989], the tectonic deformation of sedimentary basins [Oliver 1992, Ge and Garven, 1992], and the regulation of global climate [Caldeira et al, 1993, Kerrick and Caldeira, 1993, 1994]. This paper summarizes the many recent lines of theoretical, laboratory, and field evidence from diverse disciplines within the Earth Sciences supporting this emerging view of crustal‐scale hydrology. Evidence for two types of long‐distance fluid migration are highlighted: vertical pore water movement through crystalline rocks to depths greater than six km and lateral groundwater movement through sedimentary basins over hundereds of km. Also emphasized are the many driving mechanisms on fluid motion which are not typically considered in water quality and water supply investigations. Some geologic terms used in this paper, which may be unfamiliar to the reader, are defined in geologic dictionaries [American Geologic Institute, 1976].
Geochemical analysis of small samples: Micro‐analytical techniques for the nineties and beyond
Authors: Neal Clive R. , Davidson Jon P. , McKeegan Kevin D.
Published: February 2004

As technology becomes more sophisticated, the number and type of tools available to the geochemist continue to grow and the amount of data collected from a single sample is dramatically increased. Micro‐analytical techniques developed over the last 20–30 years allow the geochemist to obtain an almost overwhelming wealth of data from single samples. Not only can bulk analysis be routinely conducted, but detailed major‐ and trace‐element determinations, as well as isotopic analysis of individual components from geological, metallic, ceramic, and plastic materials are opening up a whole new avenue of research and petrogenetic interpretation. In situ measurements have become an important aspect of many geoscience applications, following the realization that much of the information relating to the origin and evolution of rock systems is retained at the scale of individual mineral grains [e.g., Reed, 1989, 1990, 1993]. The new generation of high resolution ion probes has furthered our ability to perform trace element and isotope analyses at the micron (μm = micron = 10‐3 mm) scale. Laser ablation techniques, coupled with Inductively Coupled Plasma Mass Spectrometry (ICP‐MS) analyses have opened up a parallel capacity for trace element analysis, while laser fluorination oxygen extraction is fast becoming the tool of choice for silicate oxygen isotope geochemistry. With these analytical techniques available, a detailed data base on a single sample or suite of samples can be quickly built up. For major and minor element analyses, the electron microprobe is normally used and as this has generally become a routine analytical technique and has recently been extensively reviewed by Reed [1993], it will not be discussed here.
When the sky fell on our heads: Identification and interpretation of impact products in the sedimentary record
Authors: Claeys Philippe
Published: February 2004

Research on impact products, mechanisms and consequences has benefited greatly in the last ten years from the highly publicized Cretaceous‐Tertiary (KT) boundary debate. This debate has led to an increasing awareness among Earth scientists of the importance and potential effects of impact events in Earth history. With more and more researchers realizing the role played by impacts and becoming familiar with the characteristics of impact materials, it is likely that in the coming years, an increasing number of impact events will be identified, in agreement with the Earth cratering rate estimations (Grieve, 1984; Shoemaker et al., 1990).

The increasing recognition of impact event is demonstrated by the discovery or identification in recent years of several impact craters, some of large diameter: Chicxulub, >180 km in Yucatan; Mjølnir, 40 km in the Barents Sea; Ames, 16 km in Oklahoma; Avak 12 km in Alaska; Pretoria Saltpan, 1.1 km and Kalkkop, 640 m both in South Africa. Sediments record impact events in the form of impact glass, shocked minerals, Ir anomalies, and perhaps also polymic‐chaotic breccias and/or tsunami deposits. Identification of impact products, particularly in ancient rocks, is hampered by poor preservation, diagenetic alteration, metamorphism, misinterpretation, or even lack of interest or awareness of their significance. Impact products represent very short term events, most likely geographically localized and stratigraphically concentrated over a thin interval of sediments and often diluted by normal background deposition. Finding these impact signatures requires very detailed petrographic and geochemical investigations with closely spaced sampling (a few cm). Such detail is not commonly used when studying ancient Mesozoic and Paleozoic sediments.
VLBI data, acquisition, environmental effects
Authors: Herring Thomas A.
Published: February 2004

During this quadrennium, very long baseline interferometry (VLBI) data acquisition and system development has focused on improving the accuracy of the system sufficiently to allow the determination of reliable estimates of height variations. Associated with this aim has been improvements in the determinations of horizontal velocity fields, monitoring water vapor delay using interferometric methods, and improvements to Earth rotation measurements. The primary aims of the improvements to height measurement accuracy have been to directly measure the contemporary magnitudes of post glacial rebound, and to determine a height reference system for measuring global sea level rise. High frequency Earth rotation studies have been carried out to better define the transformation parameters from an inertial coordinate system to an Earth fixed one, and to better understand the coupling between the components of the atmosphere‐ocean‐solid Earth system. Two major VLBI campaigns were carried out in support of these studies: (1) Epoch‐92 in July 1992 and (2) Cont‐94 in January 1994. Each of these campaigns lasted approximately two‐weeks and involved multiple VLBI networks operating simultaneously in addition to other space geodetic systems operating during these periods.
Interslab earthquakes and phase changes in subducting lithosphere
Authors: Kirby Stephen
Published: February 2004

At 0033 UT on 9 June 1994, a great deep earthquake occurred 635 km beneath the Amazonian rain forest of northern Bolivia. The deep rupture of this record‐setting magnitude 8.3 shock lasted about a minute [Harvard University Centroid Moment Tensor Catalogue]. The seismic strain release of this single event, as indicated by its seismic moment, was greater than the total strain release for all deep earthquakes in the Harvard CMT catalogue during the previous 17 years of its coverage of global seismicity. Incredibly, ground motion was felt as far north as Seattle, Washington and Toronto, Canada. Evidently, it occurred within the subducting Nazca plate that is descending beneath South America. This earthquake reminds us of the sometimes grand scale of deep seismic failure and of the need to consider more closely the physical processes that accompany slab descent and, in particular, how such earthquakes may reflect the inner workings of slabs.
Tropospheric chemistry research in the U.S.: 1991–1994
Authors: Penner Joyce E. , Atherton Cynthia S. , Dignon Jane
Published: February 2004

The troposphere is chemically complex. Many of the important species in the troposphere are short‐lived, with lifetimes less than or of order of a month. Hence, the composition of the troposphere is regionally diverse, leading to regionally diverse chemical processes which control that composition. With the advent of three‐dimensional models and regionally‐specific estimates of emissions, however, it has become clear that human activity has perturbed the composition of even short‐lived species over vast regions of the globe. The list of short‐lived species of concern includes the reactive nitrogen oxides, reactive sulfur, ozone, nonmethane hydrocarbons and carbon monoxide. Observations have established that increases in the long‐lived species such as carbon dioxide, methane, nitrous oxide, and the chlorofluorocarbons are taking place with important resulting impacts on stratospheric chemistry. Further, there is some indication that carbonyl sulfide (a major precursor to stratospheric aerosols) may have an important anthropogenic source. Growth of pollutants such as these are of concern because they act as greenhouse gases or aerosol precursors (CO2, CH4, O3, N2O, chlorofluorocarbons, carbonyl sufide, reactive sulfur), as agents for depletion of stratospheric ozone (N2O, chlorofluorocarbons), are harmful to vegetation (O3, acids) or act as nutrients (nitrate, sulfate, trace metals). The chemical interactions are important to understand, because the build up of pollutants depends not only on the rates of their release into the troposphere but on their rate of removal in the troposphere. Removal rates depend on processes which determine the rate of oxidation of the component (which for most species proceeds mainly by reaction with the hydroxyl radical) or through precipitation scavenging or dissolution in the ocean (which requires that the pollutant or its oxidation product(s) be soluble in water).
Earthquake geodesy and hazard monitoring
Authors: Hudnut Kenneth W.
Published: February 2004

Catastrophic events such as major earthquakes and volcanic eruptions result when the earth's crust fails in response to accumulated deformation. The accumulation of deformation results from ongoing processes such as aseismic deformation of subcrustal rock associated with relative plate motions, or the ascent of magma through a volcanic plumbing system. Geodetic measurements document the crustal deformation leading to these failures and the deformation resulting from them, providing unique insight into the physical processes involved. By virtue of the space and time domains of ground deformation detected with geodetic measurements, geodesy is a useful tool in monitoring active tectonic and volcanic regimes. For a wide range of natural events including earthquakes, aseismic fault motions, and volcanic events, geodetic measurements constrain physical models of the processes that cause such events. Because these geodetic studies probe the underlying processes leading to and culminating in catastrophic natural events, many such studies are directly applicable to earthquake and volcano hazard assessment.