bluuebunny/crossref_metadata_2025 huggingface dataset:
Observations and large‐eddy simulation modeling of footprints in the lower convective boundary layer
Authors: Leclerc Monique Y. , Shen Shaohua , Lamb Brian
Published: February 2004

Scalar source footprint predictions inside and above the unstable surface layer are performed using a large‐eddy simulation (LES) model. LES footprint results were evaluated against a tracer field experiment at one level and against other models beyond the 10 m measurement level in the surface layer. This paper further reports on an extension of the range over which current footprint models can be used by presenting LES calculations of flux mapping to the downwind region from the source location beyond the surface layer. Our LES flux mapping results show a gradual decoupling with increasing distance from the surface. Our results also indicate that the footprints, cumulative fluxes, and concentration distributions calculated using LES in unstable conditions are in general agreement with field observations and with those of other models. The present work shows promise for the use of LES as a tool to model more complex footprint cases such as those over heterogeneous surfaces.
The timing of giant radiating dike swarm emplacement on Venus: Implications for resurfacing of the planet and its subsequent evolution
Authors: Grosfils Eric B. , Head James W.
Published: February 2004

Stratigraphic study of a distributed population of 118 giant radiating dike swarms on Venus reveals that within each dike‐intruded region, emplacement of the swarms occurred prior to formation of most impact craters and rifts but subsequent to that of tessera, regional plains, and most wrinkle ridges. The density of impact craters superimposed on the swarm population (1.80 ± 0.57 craters/106 km2), when compared with the average global density (2.01 ± 0.14 craters/106 km2) and the densities reported for other geologic units, is consistent with the observed stratigraphy. On the basis of these data, we conclude that the population of giant radiating swarms formed during or slightly after the waning phases of an interval of widespread volcanic resurfacing. The stresses recorded by the dike swarm population, when combined with its age and compared with the predictions of several proposed resurfacing models, best support the hypothesis that the current surface formed as a result of the catastrophic foundering of a shallow depleted mantle layer. Formation of the dike swarms through shallow magma stalling is closely linked to and sensitively dependent upon the modern configurations of both long‐wavelength gravity and topography. In addition, the surface stresses recorded by the dike swarm population are similarly correlated at a global level with these same long‐wavelength characteristics. We interpret the old age of the dike swarm population to mean that there has been minimal alteration of either the long‐wavelength topographic expression across most of the planet or the interior processes responsible for such changes since the cratering record was reset at least several hundred million years ago.
Multiresolution‐element imaging of extrasolar Earthlike planets
Authors: Bender Peter L. , Stebbins Robin T.
Published: February 2004

It has been suggested recently that imaging of extrasolar Earthlike planets should be considered as a possible future goal of the NASA space program. As an aid to discussing what would be required in order to undertake imaging, a partial design is described for a separated spacecraft interferometer which could achieve images in the visible with 10 resolution elements across the planet. Between 15 and 25 large collector telescopes or clusters of telescopes spread out over roughly a 200‐km baseline in solar orbit at 1 AU from the Sun or possibly in high Earth orbit are required. A very preliminary approach to the use of multispectral remote sensing techniques is discussed also. Finally, the generalization of this approach to larger numbers of resolution elements across the planet is considered. Clearly, imaging is possible only if the truly staggering problem of avoiding the 1010 times stronger scattered light from the nearby star can be overcome. However, even if this can be done, the amount and precision of the required hardware for even poor‐quality images appears to present an obstacle to such a program which would be extremely difficult to overcome.
The nature of comet Shoemaker‐Levy 9 subnuclei from analysis of preimpact Hubble Space Telescope images
Authors: Rettig Terrence W. , Mumma Michael J. , Sobczak Gregory J. , Hahn Joseph M. , DiSanti Michael
Published: February 2004

We obtained Hubble Space Telescope (HST) Wide Field Planetary Camera images of comet Shoemaker‐Levy 9 (S‐L 9) prior to impact with Jupiter in 1994 to observe the effects of the Jovian gravitational field on the approaching fragments. The HST images were used to monitor the absolute brightness in the central regions of the comae, changes in the surface brightness profiles, and general morphological changes in the fragments as they approached final impact with Jupiter. S‐L 9 most likely came apart under the influence of the Jovian tidal force near perijove in 1992 [Scotti and Melosh, 1993; Asphaug and Benz, 1994a; Solem, 1994; Sekanina et al., 1994] into approximately 20 fragments [Scotti, 1993; Luu and Jewitt, 1993; Weaver et al., 1994]. In contrast to condensed or solid fragments, we suggested that after perijove, the breakup debris condensed initially as swarms of particles [Rettig et al., 1994a; Olson and Mumma, 1994] and if interparticle collisions were infrequent, some of these swarm‐like subnuclei might have remained extended throughout the 2‐year orbit. In this paper we discuss a simple analytical model to demonstrate how after the original nucleus came apart, the debris cloud formed 14–20 fragments. We also present temporal magnitudes of all fragments obtained from the HST images from January through July 1994 and an analysis of the preimpact images for four S‐L 9 condensations T(4), Q2(7b), P1(8a), and K(12) which suggests that not all of the central subnuclei were consistent with solid objects. The temporal magnitudes and surface brightness profiles of several S‐L 9 fragments are consistent with tidal dispersal at distances much greater than the classical ∼2RJ Roche limit.
Detection and monitoring of H2O and CO2 ice clouds on Mars
Authors: Bell James F. , Calvin Wendy M. , Ockert‐Bell Maureen E. , Crisp David , Pollack James B. , Spencer John
Published: February 2004

We have developed an observational scheme for the detection and discrimination of Mars atmospheric H2O and CO2 clouds using ground‐based instruments in the near infrared. We report the results of our cloud detection and characterization study using Mars near IR images obtained during the 1990 and 1993 oppositions. We focused on specific wavelengths that have the potential, based on previous laboratory studies of H2O and CO2 ices, of yielding the greatest degree of cloud detectability and compositional discriminability. We have detected and mapped absorption features at some of these wavelengths in both the northern and southern polar regions of Mars. Compositional information on the nature of these absorption features was derived from comparisons with laboratory ice spectra and with a simplified radiative transfer model of a CO2 ice cloud overlying a bright surface. Our results indicate that both H2O and CO2 ices can be detected and distinguished in the polar hood clouds. The region near 3.00 μm is most useful for the detection of water ice clouds because there is a strong H2O ice absorption at this wavelength but only a weak CO2 ice band. The region near 3.33 μm is most useful for the detection of CO2 ice clouds because there is a strong, relatively narrow CO2 ice band at this wavelength but only broad “continuum” H2O ice absorption. Weaker features near 2.30 μm could arise from CO2 ice at coarse grain sizes, or surface/dust minerals. Narrow features near 2.00 μm, which could potentially be very diagnostic of CO2 ice clouds, suffer from contamination by Mars atmospheric CO2 absorptions and are difficult to interpret because of the rather poor knowledge of surface elevation at high latitudes. These results indicate that future ground‐based, Earth‐orbital, and spacecraft studies over a more extended span of the seasonal cycle should yield substantial information on the style and timing of volatile transport on Mars, as well as a more detailed understanding of the role of CO2 condensation in the polar heat budget.
Ground‐based detectability of terrestrial and Jovian extrasolar planets: Observations of CM Draconis at Lick Observatory
Authors: Doyle Laurance R. , Dunham Edward T. , Deeg Hans‐Jörg , Blue J. Ellen , Jenkins Jon M.
Published: February 2004

The detection of terrestrial‐sized extrasolar planets from the ground has been thought to be virtually impossible due to atmospheric scintillation limits. However, we show that this is not the case for specially selected (but nevertheless main sequence) stars, namely small eclipsing binaries. For the smallest of these systems, CM Draconis, several months to a few years of photometric observations with 1‐m‐class telescopes will be sufficient to detect the transits of any short‐period planets of sizes ≥1.5 Earth radii (RE), using cross‐correlation analysis with moderately good photometry. Somewhat larger telescopes will be needed to extend this detectability to terrestrial planets in larger eclipsing binary systems. (We arbitrarily define “terrestrial planets” herein as those whose disc areas are closer to that of Earth's than Neptune's i.e., less than about 2.78 RE.) As a “spin‐off” of such observations, we will also be able to detect the presence of Jovian‐mass planets without transits using the timing of the eclipse minima. Eclipse minima will drift in time as the binary system is offset by a sufficiently massive planet (i.e., one Jupiter mass) about the binary/giant‐planet barycenter, causing a periodic variation in the light travel time to the observer. We present here an outline of present observations taking place at the University of California Lick Observatory using the Crossley 0.9‐m telescope in collaboration with other observatories (in South Korea, Crete, France, Canary Islands, and New York) to detect or constrain the existence of terrestrial planets around main sequence eclipsing binary star systems, starting with CM Draconis. We demonstrate the applicability of photometric data to the general detection of gas giant planets via eclipse minima timings in many other small‐mass eclipsing binary systems as well.
Detection and study of extrasolar terrestrial planets: Techniques and technology
Authors: Shull J. Michael , Stern S. Alan , Thronson Harley A.
Published: February 2004

On May 15–17, 1995, three Rocky Mountain research institutions hosted a conference to discuss the scientific basis, technological options, and programmatic implications of a large‐scale effort to find and study Earth‐like planets outside the Solar System. The workshop attracted scientists, engineers, space agency administrators, and the public media to discuss and debate the most promising technological options and opportunities. Major programs and proposals to search for and study exo‐planets were presented and discussed. This special issue of the Journal of Geophysical Research contains several of the most interesting poster papers, which were extended to greater length by the authors and submitted for peer review.
New laboratory measurements of mid‐IR emission spectra of simulated planetary surfaces
Authors: Henderson Bradley G. , Lucey Paul G. , Jakosky Bruce M.
Published: February 2004

We present emission spectra of particulate quartz measured in an environment chamber designed to simulate the conditions on actual planetary surfaces. The goal was to investigate near‐surface thermal gradients and their effects on emission spectra for other planetary environments. Our experiment parallels that of Logan et al. [1973] but is different, in that our samples were heated at the base by a temperature‐controlled hot plate rather than from above by a solar lamp in order to separate infrared surface cooling from solar heating effects. Our spectra show prominent emission peaks which are attributed to the presence of near‐surface thermal gradients created by infrared cooling of the uppermost layer of the material. The contrast of the emission peak is maximized under vacuum conditions, for which it is estimated that a temperature difference of at least 40 K existed within the top emission skin depth. The wavelength location of the emission peak occurs near the Christiansen wavelength at 7.35 μm but has been shifted by approximately 0.2 μm to shorter wavelengths. This result is in agreement with the earlier results of Logan et al. [1973] and points out that the existence of a thermal gradient violates the conditions required by Kirchoff's law, and therefore care should be taken when spectra of surfaces on airless bodies are interpreted using emissivity spectra converted from reflectance data. Increasing the atmospheric pressure in the chamber increased the conductivity of the soil, mitigating the thermal gradient and decreasing the contrast of the emission maxima. Although thermal gradients complicate the interpretation of emission spectra of airless bodies, they tend to enhance certain spectral features, and therefore emission spectroscopy should be useful for remote sensing of the surfaces of the Moon and Mercury.
The lunar crust: Global structure and signature of major basins
Authors: Neumann Gregory A. , Zuber Maria T. , Smith David E. , Lemoine Frank G.
Published: February 2004

New lunar gravity and topography data from the Clementine Mission provide a global Bouguer anomaly map corrected for the gravitational attraction of mare fill in mascon basins. Most of the gravity signal remaining after corrections for the attraction of topography and mare fill can be attributed to variations in depth to the lunar Moho and therefore crustal thickness. The large range of global crustal thickness (∼20–120 km) is indicative of major spatial variations in melting of the lunar exterior and/or significant impact‐related redistribution. The 61‐km average crustal thickness, constrained by a depth‐to‐Moho measured during the Apollo 12 and 14 missions, is preferentially distributed toward the farside, accounting for much of the offset in center‐of‐figure from the center‐of‐mass. While the average farside thickness is 12 km greater than the nearside, the distribution is nonuniform, with dramatic thinning beneath the farside, South Pole‐Aitken basin. With the global crustal thickness map as a constraint, regional inversions of gravity and topography resolve the crustal structure of major mascon basins to half wavelengths of 150 km. In order to yield crustal thickness maps with the maximum horizontal resolution permitted by the data, the downward continuation of the Bouguer gravity is stabilized by a three‐dimensional, minimum‐slope and curvature algorithm. Both mare and non‐mare basins are characterized by a central upwarped moho that is surrounded by rings of thickened crust lying mainly within the basin rims. The inferred relief at this density interface suggests a deep structural component to the surficial features of multiring lunar impact basins. For large (>300 km diameter) basins, moho relief appears uncorrelated with diameter, but is negatively correlated with basin age. In several cases, it appears that the multiring structures were out of isostatic equilibrium prior to mare emplacement, suggesting that the lithosphere was strong enough to maintain their state of stress to the present.
Surface roughness, radar backscatter, and visible and near‐infrared reflectance in Death Valley, California
Authors: Weeks Robin J. , Smith Milton , Pak Kyung , Li Wen‐Hao , Gillespie Alan , Gustafson Bill
Published: February 2004

The vast alluvial fans of Death Valley, California, provide an ideal environment to examine the remote sensing measurement of geologic surfaces. One of the objectives of the shuttle imaging radar C (SIR‐C) program in Death Valley is detection of the variation in surface microtopography with age of the surface. We present results of extensive field measurements of surface roughness together with an analysis of the effects of the surface microtopography on radar backscatter and visible and near‐infrared (VNIR) reflectance as measured by aircraft and satellite sensors. This subject is addressed in both the forward and inverse sense: surface simulation and forward modeling are used to determine expected roughness effects, while a method of inverse analysis that uses finite impulse response (FIR) filters is used to assess the potential for inversion of multifrequency, polarimetric, synthetic aperture radar (SAR), and multispectral VNIR imagery for surface roughness. The interaction of radar and VNIR radiation with the Death Valley surfaces is complicated. Simple roughness parameters such as rms height, and slope and offset of surface power spectra, do not represent a sufficiently complete description of surface roughness to predict the radar or VNIR signature uniquely. Multiple scattering, which is controlled to a large extent by aspects of the phase of the surface Fourier transform, also exerts a controlling influence on the observed signal. The phase aspect of surface roughness has not been considered in existing roughness characterization. Our inversions demonstrate retrieval of roughness parameters with almost equal success from both SAR data and Landsat thematic mapper (TM) data and indicate much potential for joint SAR/VNIR data analysis. The solutions are not, however, very stable and include effects of additional parameters such as intermediate‐scale topography and vegetation cover which masquerade as roughness variation. In designing a stable inversion of more general applicability, the multifrequency and polarimetric aspect of SIR C data is important. Nevertheless, high‐resolution roughness recovery will probably require hierarchical analysis of radar and optical images, and also SAR acquisition at multiple look angles and directions.
Emplacement of penetrators into planetary surfaces
Authors: Anderson William W. , Ahrens Thomas J. , Gibson Andrew , Scott Ronald , Suzuki Kojiro
Published: February 2004

We present experimental data and a model for the low‐velocity (subsonic, 0–1000 m/s) penetration of brittle materials by both solid and hollow (i.e., coring) penetrators. The experiments show that penetration is proportional to momentum/frontal area of the penetrator. Because of the buildup of a cap in front of blunt penetrators, the presence or absence of a streamlined or sharp front end usually has a negligible effect for impact into targets with strength. The model accurately predicts the dependence of penetration depth on the various parameters of the target‐penetrator system, as well as the qualitative condition of the target material ingested by a corer. In particular, penetration depth is approximately inversely proportional to the static bearing strength of the target. The bulk density of the target material has only a small effect on penetration, whereas friction can be significant, especially at higher impact velocities, for consolidated materials. This trend is reversed for impacts into unconsolidated materials. The present results suggest that the depth of penetration is a good measure of the strength, but not the density, of a consolidated target. Both experiments and model results show that, if passage through the mouth of a coring penetrator requires initially porous target material to be compressed to <26% porosity, the sample collected by the corer will be highly fragmented. If the final porosity remains above 26%, then most materials, except cohesionless materials, such as dry sand, will be collected as a compressed slug of material.
Noble gases and H chondrite meteoroid streams: No confirmation
Authors: Schultz Ludolf , Weber Hartwig W.
Published: February 2004

Eleven meteorites suggested to belong to a meteoroid stream of 17 H chondrites (cluster 1) have been analyzed for their noble gases. Together with literature data, the noble gas record of all these stones is discussed. Cluster 1 chondrites belong to different chemical‐petrologic types, and three are regolith breccias, as proven by solar type gases. The thermal history of the parent body of cluster 1 stones is not the same for all (as seen in the radiogenic 40Ar), and diffusive loss of helium in four of them suggests small perihelion distances for these specimens and thus different orbits from the rest of this group. Exposure ages vary between about 4 and 70 Ma. Thus the orbital elements of the stream must have been undisturbed for about 70 Ma. The exposure age distribution of cluster 1 chondrites is not distinct from that of “normal” H chondrites. These data appear inconsistent with the suggestion that cluster 1 chondrites belong to a specific meteoroid stream with very similar orbits.
Calculated k distribution coefficients for hydrogen‐ and self‐broadened methane in the range 2000–9500 cm−1 from exponential sum fitting to band‐modelled spectra
Authors: Irwin P. G. J. , Calcutt S. B. , Taylor F. W. , Weir A. L.
Published: February 2004

The spectral band data derived by Strong et al. [1993] for laboratory‐measured transmission spectra of hydrogen‐broadened methane at 10 cm−1 resolution have been fitted with k coefficients over a wide range of pressures and temperatures representing those likely to be encountered in the atmosphere of Jupiter. The mean fitting error is found to be only 2.0 × 10−3 in transmission. These data are essential for the scattering calculations likely to be necessary for analysis of the data from the Near Infrared Mapping Spectrometer aboard the NASA Galileo spacecraft. The new data have significant advantages over those previously derived by Baines et al. [1993] in that they cover a wider spectral range, are applicable to longer paths, and also apply to the hydrogen‐broadened case, which is the dominant broadening mechanism in this atmosphere. A similar table has also been calculated for the self‐broadening case for comparison.
Colored lava flows on the Earth: A warning to Io volcanologists
Authors: Rothery David A. , Babbs Tanya L. , Harris Andrew J. L. , Wooster Martin J.
Published: February 2004

Pale coatings of amorphous silica, sulfur, and sulfur‐bearing compounds can form on terrestrial basalt flows while they are still active and subsequently on timescales ranging from days to years after the eruption. We cite examples below from Kilauea (Hawaii), Etna (Sicily) and Cerro Negro (Nicaragua) and caution against undue reliance on spectral reflectance to determine the bulk properties of lava flows on Io, where sulfur and/or sulfur compounds are likely to dominate comparable (but probably more widespread) coatings.
Evolution of the solar ionizing flux
Authors: Ayres Thomas R.
Published: February 2004

A young magnetically active Sun, with enhanced ionizing radiations and an elevated solar wind, might have contributed to erosion of the primordial atmosphere of Mars (which is particularly vulnerable to dissociative recombination and sputtering by solar wind pickup ions). Spacecraft and ground‐based observations of solar‐type dwarfs in young galactic clusters have yielded a unified view of the early evolution and subsequent systematic decline of magnetic activity with age. Rotational braking by the coronal wind ultimately quenches the spin‐catalyzed “dynamo” at the heart of stellar magnetism. Decay of the 106–107 K corona is much faster than the 104 K chromosphere, but XUV emissions of both can be predicted reliably, and photoionization of key planetary atmospheric constituents can be modeled. The early Martian atmosphere (age ∼1 Gyr) probably was subjected to photoionization rates ∼5 times contemporary peak values (sunspot maximum), possibly more if the ages of galactic clusters have been underestimated.
Size‐frequency distributions of rocks on Mars and Earth analog sites: Implications for future landed missions
Authors: Golombek M. , Rapp D.
Published: February 2004

The size‐frequency distribution of rocks at the Viking landing sites and a variety of rocky locations on the Earth that formed from a number of geologic processes all have the general shape of simple exponential curves, which have been combined with remote sensing data and models on rock abundance to predict the frequency of boulders potentially hazardous to future Mars landers and rovers. Rock data from the near field of the Viking landers where dimensions can be measured accurately in stereo images and estimates from the far field of Viking 1 have convex up curved shapes on log‐log graphs of cumulative frequency per square meter or cumulative fractional area versus diameter. The rock data show a sharp drop‐off at large diameters and a progressive approach to a plateau with decreasing diameter (approaching the total rock coverage), which can be fit well with simple exponential functions. Similar shaped size‐frequency distributions of rocks are found at a wide variety of rocky surfaces on the Earth and can be fit well with simple exponential functions. This distribution is compatible with fracture and fragmentation theory, which provides a physical basis for its wide application. A combined fit to rock area data at both Viking sites was made with a general exponential function, in which the pre‐exponential is the total area covered by rocks. Simple linear height versus diameter relationships were also derived from height‐diameter ratios at the Viking sites, which suggest that rockier areas on Mars have higher standing rocks than less rocky areas. Height was then substituted into the general exponential function derived for diameter, yielding the cumulative fractional area of rocks versus height for any given total rock coverage on Mars. Results indicate that most of Mars is rather benign with regard to hazards from landing on large rocks. For total rock coverage of 8%, equivalent to modal rock coverage on Mars and the Viking 1 site without the outcrops, about 1% of the surface is covered by 20 cm or higher rocks. A lander designed to accommodate landing on 0.5‐m‐high boulders, such as the Mars Pathfinder airbag system, could land on a surface covered by about 20% rocks, similar to the Viking 2 site (which is rockier than ∼95% of the planet), with ∼1% of the surface covered by rocks of 0.5 m or higher.
Low‐level jets in the NASA Ames Mars general circulation model
Authors: Joshi M. M. , Haberle R. M. , Barnes J. R. , Murphy J. R. , Schaeffer J.
Published: February 2004

Previous simulations of the Martian atmosphere have shown how topography acts to confine the low‐level Hadley cell flow into intense jets on the eastern flanks of Tharsis and Syrtis Major. We now conduct detailed studies of these jets using the NASA Ames Mars general circulation model (MGCM). The structure of the flow is found to be sensitive to local topography as well as large‐scale diabatic heating patterns, consistent with terrestrial studies, and MGCM studies carried out with simplified topography. The summer subtropical zonal winds associated with the Hadley circulation also form spatially confined intense jet cores. Diurnal variations in heating affect jet structure in three distinct ways. Global tides interact with the jets, resulting in effects such as the two reinforcing each other at the summer subtropics near midday, leading to high winds and surface stresses at this time. Slope winds act to change the character of the jets during the course of a day, especially at Syrtis Major and the Hellas basin, where slopes are large. Vertical mixing acts to decrease low‐level winds during the late afternoon. The sensitivity of the results to atmospheric dust loading is examined. We finally show how a decrease in boundary layer height due to dust loading actually augments mid‐afternoon jet strength near the surface. The resulting increase in maximum surface stress indicates that this is a positive feedback to dust lifting.
Variation of the 3‐μm absorption feature on Mars: Observations over eastern Valles Marineris by the Mariner 6 infrared spectrometer
Authors: Calvin Wendy M.
Published: February 2004

A new approach for calibration of the shortest wavelength channel (1.8 to 6.0 μm) of the Mariner 6 infrared spectrometer was derived. This calibration provides a new description of the instrument response function from 1.8 to 3.7 μm and accounts for the thermal contribution to the signal at longer wavelengths. This allows the two segments from 1.8 to 6 μm to be merged into a single spectrum. The broad water of hydration absorption spans these two segments and is examined in these merged spectra using a method of band integration. Unlike previous analyses which rely on ratios at two wavelengths, the integration method can assess the band strength independently from the albedo in the near infrared. Spectra taken over the eastern end of the Valles Marineris are examined for variations of the band‐integrated value, and three distinct clusters are found. Within the estimated uncertainty, two clusters (both low and high albedo) have approximately the same integrated band depth. The third cluster (medium albedo) has an integrated band depth about 10% higher. This difference cannot be systematically attributed to either surface or atmospheric parameters and suggests variation in the amount of water either chemically or physically bound in surface materials. Approximately one‐half of the high integrated band depth cluster is associated with chaotic terrain at the source of outflow channels, the other half occurs over lower inertia plains adjacent to chasmata. This suggests both surface physical properties and mineralogy as well as water in exchange with the atmosphere contribute to the 3‐μm bound water absorption.
Near‐surface thermal gradients and mid‐IR emission spectra: A new model including scattering and application to real data
Authors: Henderson Bradley G. , Jakosky Bruce M.
Published: February 2004

We model the radiative and conductive heat transfer in the top few millimeters of a particulate medium in order to investigate near‐surface thermal gradients and their effects on mid‐IR emission spectra for different planetary environments. The model extends our previous work by including scattering in the radiative heat transfer. Our results show that significant thermal gradients will form in the top few hundred microns of particulate materials on the surfaces of the Moon and Mercury. Their presence alters spectral contrast and creates emission maxima in the transparent regions of the spectra. The results also show that thermal gradients cause the wavelength position of the Christiansen emission peak to shift by as much as 0.5 μm with variations in thermal conductivity and grain size, in agreement with previous laboratory investigations. These wavelength shifts are due to increased emission in the transparent regions of the spectrum which are superimposed upon the emissivity signature. The results are applied to telescopic spectra of the surfaces of the Moon and Mercury and can account for certain features seen in these data. Additional calculations show that thermal gradients will be minor on the surface of Mars and negligible for Earth.
Variations in Mg/Ca and Sr/Ca ratios of planktonic foraminifera caused by postdepositional dissolution: Evidence of shallow Mg‐dependent dissolution
Authors: Brown Sarah J. , Elderfield Henry
Published: February 2004

A comparative study has been made of two species of planktonic foraminifera, G. tumida and G. sacculifer, in a depth transect on the Ontong Java Plateau, western equatorial Pacific. G. tumida tests from core‐top sediments showed decreasing Mg/Ca (2.65–1.25 mmol/mol) and Sr/Ca (1.50–1.16 mmol/mol) ratios with increasing water depth (1600–4400 m), while no such variation was found in G. sacculifer tests in excess of 355 µm (average Mg/Ca, 3.6 mmol/mol, Sr/Ca, 1.4 mmol/mol). Artificial dissolution of G. tumida tests led to a decrease in both Mg/Ca and Sr/Ca, but in G. sacculifer, there was no significant change in the ratios. Analyses by electron microprobe revealed that Mg/Ca of the inner (chamber) calcite in G. tumida tests was much higher than that of the calcite crust (keel), whereas Sr/Ca was only slightly elevated. There were no consistent spatial differences in either Mg/Ca or Sr/Ca for G. sacculifer tests. Dissolution in the oceans gives rise to the removal of the chamber calcite in G. tumida (about 30% of the total calcite), leaving the keel calcite. Although G. tumida is thought of as a dissolution‐resistant form, the majority of Mg/Ca and Sr/Ca depletion (and chamber loss) occurs above the lysocline. It is also possible that much of the Mg‐enriched gametogenic calcite has been lost by dissolution from G. sacculifer above 1600 m. Mg content has a significant effect on the dissolution susceptibility of planktonic foraminifera. Calculations show that the saturation horizon for Mg‐rich parts of tests may be elevated by several hundred meters compared with normal calcite.
A comparison of spatially resolved and global mean reconstructions of continental denudation under ice‐free and present conditions
Authors: Sloan L. Cirbus , Bluth Gregg J. S. , Filippelli G. M.
Published: February 2004

We assess the impact of continental‐scale processes on global denudation through the use of spatially resolved information for both an ice‐free paleoclimate and the present climate. Runoff from general circulation modeling cases representing the early Eocene is superimposed upon an Eocene paleogeologic reconstruction, and this information is combined with chemical denudation rates for silica (dissolved moles Si) and bicarbonate (dissolved moles HCO3−). Global fluxes of silica and bicarbonate to the global ocean are then calculated. A parallel procedure is carried out with present‐day distributions of lithology and model‐derived runoff. This work demonstrates that fluxes produced by a simple model such as the one used here are significantly different when calculated with spatially uniform runoff values versus those calculated with a spatially varying runoff distribution having the same global mean value. Use of a uniform runoff distribution produces denudation rates that are significantly higher than the global results derived from a spatially varying runoff distribution. We show that present‐day fluxes of silica calculated by our model containing spatially varying runoff and lithology are similar to observations of current fluxes, suggesting that our model captures the first‐order relationship accurately; however, the bicarbonate value compares less well to observations. Comparison of Eocene and present‐day flux results shows that present‐day fluxes of Si are greater than Eocene values, while calculated present‐day HCO3− fluxes are greater than or equal to Eocene values. This result occurs despite the existence of greater global mean annual runoff for the Eocene cases and despite the existence of ice‐covered areas (by definition, not contributing to chemical weathering in our model) in the present case. The increase in Si global denudation fluxes from the Eocene to the present are caused primarily by the large increase in exposed granitic, basaltic, and shale lithologies, and a decrease in exposed sandstone areas, between the Eocene and present.
The polar wind
Authors: Ganguli Supriya B.
Published: February 2004

The polar wind is an ambipolar outflow of thermal plasma from the terrestrial ionosphere at high latitudes to the magnetosphere along geomagnetic field lines. The polar wind plasma consists mainly of H+, He+, and O+ ions and electrons. Although it was initially believed that O+ ions play a major role only at low altitudes, it is now clear from observations that relatively large amounts of suprathermal and energetic O+ ions are present in the polar magnetosphere. Recently, thermal O+ outflow has been observed at altitudes of 5000–10,000 km together with H+ and He+ ions. The polar wind undergoes four major transitions as it flows from the ionosphere to the magnetosphere: (1) from chemical to diffusion dominance, (2) from subsonic to supersonic flow, (3) from collision‐dominated to collisionless regimes, and (4) from heavy to light ion composition. The collisions are important up to about 2500 km, after which the ions and electrons exhibit temperature anisotropies. The direction of the anisotropy varies with geophysical conditions. The polar wind outflow varies with season, solar cycle, and geomagnetic activity. The O+ flux exhibits a summer maximum, while the H+ flux reaches a maximum in the spring. The He+ flux increases by a factor of 10 from summer to winter. At both magnetically quiet and active times the integrated H+ ion flux is largest in the noon sector and smallest in the midnight sector. The integrated upward H+ ion flux exhibits a positive correlation with the interplanetary magnetic field. In the sunlit polar cap the photoelectrons can increase the ambipolar electric field, which in turn increases the polar wind ion outflow velocities. The outflowing polar wind plasma flux tubes also convect across the polar cap. When the flux tubes cross the cusp and nocturnal auroral regions, the plasma can be heated and become unstable. Similar mixing of hot magnetospheric plasma with cold polar wind may result in instabilities. A number of free energy sources in the polar wind, including temperature anisotropy, relative drift between species, and spatial inhomogeneities, feed various fluid and kinetic instabilites. The instabilities can produce plasma energization and cross‐field transport, which modify the large‐scale polar wind outflow.
Earth's magnetospheric cusps
Authors: Smith M. F. , Lockwood M.
Published: February 2004

Earth's cusps are magnetic field features in the magnetosphere associated with regions through which plasma from the Sun can have direct access to the upper atmosphere. Recently, new ground‐based observations, combined with in situ satellite measurements, have led the way in reinterpreting cusp signatures. These observations, combined with theoretical advances, have stimulated new interest in the solar wind‐magnetosphere‐ionosphere coupling chain. This coupling process is important because it causes both momentum and energy from the solar wind to enter into the near‐Earth region. Here we describe the current ideas concerning the cusps and the supporting observational evidence which have evolved over the past 30 years. We include discussion on the plasma entry process, particle motion between the magnetopause and ionosphere, ground optical and radar measurements, and transient events. We also review the important questions that remain to be answered.
Radioisotopic studies of submarine hydrothermal vents
Authors: Kadko David
Published: February 2004

The importance of mid‐ocean ridge hydrothermal systems has been recognized for their role in the regulation of ocean and sediment chemistry, as well as for providing a chemosynthetic source of carbon which drives a unique population of animals found at hydrothermal vents. Despite the importance of these systems the rates, length, and depth scale of submarine hydrothermal processes are not precisely known because they are, for the most part, inaccessible to observational tools. We must therefore rely on indirect methods to quantify these processes. One way of investigating the rates, or timescales, of processes in a hydrothermal (or any natural) system is through the study and modeling of naturally occurring radioisotopes. Disequilibria among the naturally occurring radioactive decay series in vent fluids, associated mineral deposits, and overlying effluent plume have provided geochemical tools to investigate the rates of various processes occurring in submarine hydrothermal systems. Because the half‐lives of the radioisotopes vary from days to many years, processes which encompass a wide range of spatial and temporal scales can be studied. This paper presents a review of methods that estimate the residence time of hydrothermal fluids in the ocean crust, establish the geochronology of seafloor sulfide deposits, investigate the rates of chemical reactions within hydrothermal effluent plumes, and derive the heat and mass flux from seafloor hydrothermal areas.
Precipitation recycling
Authors: Eltahir Elfatih A. B. , Bras Rafael L.
Published: February 2004

The water cycle regulates and reflects natural variability in climate at the regional and global scales. Large‐scale human activities that involve changes in land cover, such as tropical deforestation, are likely to modify climate through changes in the water cycle. In order to understand, and hopefully be able to predict, the extent of these potential global and regional changes, we need first to understand how the water cycle works. In the past, most of the research in hydrology focused on the land branch of the water cycle, with little attention given to the atmospheric branch. The study of precipitation recycling, which is defined as the contribution of local evaporation to local precipitation, aims at understanding hydrologic processes in the atmospheric branch of the water cycle. Simply stated, any study on precipitation recycling is about how the atmospheric branch of the water cycle works, namely, what happens to water vapor molecules after they evaporate from the surface, and where will they precipitate? Estimation of precipitation recycling over any large basin, such as the Mississippi or the Amazon, is a necessary step before developing a quantitative description of the regional water cycle. This paper reviews the research on the concept of precipitation recycling and emphasizes the basic role of this process in defining the different components of the atmospheric branch in any regional water cycle. To illustrate the assumptions and limitations involved in estimation of precipitation recycling, we present and discuss a general formula for estimation of precipitation recycling. The recent estimates of annual precipitation recycling ratio from different regions are reviewed and compared. Finally, the dependence of precipitation recycling over any region on the spatial scale is discussed and illustrated by the example of the Amazon basin.
Mesoscale shallow convection in the atmosphere
Authors: Atkinson B. W. , Wu Zhang J.
Published: February 2004

This paper is a review of the observational, experimental, theoretical, and numerical studies of mesoscale shallow convection (MSC) in the atmosphere. Typically, MSC is 1 to 2 km deep, has a horizontal length scale of a few to a few tens of kilometers, and takes distinctive planforms: linear and hexagonal. The former is called a cloud street, roll, or band, while the latter is called mesoscale cellular convection (MCC), comprising three‐dimensional cells. MSC is characterized by its shape, horizontal extent, convective depth, and aspect ratio. The latter is the ratio of the horizontal extent to that in the vertical. For cells the horizontal extent is their diameter, whereas for rolls it is their spacing. Rolls usually align along or at angles of up to 10° from the mean horizontal wind of the convective layer, with lengths from 20 to 200 km, widths from 2 to 10 km, and convective depths from 2 to 3 km. The typical value of aspect ratio ranges from 2 to 20. Rolls may occur over both water surface and land surfaces. Mesoscale convective cells may be divided into two types: open and closed. Open‐cell circulation has downward motion and clear sky in the cell center, surrounded by cloud associated with upward motion. Closed cells have the opposite circulation. Both types of cell have diameters ranging from 10 to 40 km and aspect ratios of 5 to 50, and both occur in a convective layer with a depth of about 1 to 3 km. Both the magnitude and direction of horizontal wind in the convective layer change little with height. MSC results from a complex and incompletely understood mix of processes. These processes are outlined, and their interplay is examined through a review of theoretical and laboratory analyses and numerical modeling of MSC.
Atmospheric loading and the oceanic “inverted barometer” effect
Authors: Wunsch Carl , Stammer Detlef
Published: February 2004

The response of the ocean to fluctuating atmospheric pressure loads is reviewed in theory and in observations. Major theoretical issues lie primarily with oceanic boundary reflectivity and with rates of dissipation, generally. Analytical solutions for a stratified ocean show that a static (“inverted barometer”) response is anticipated at all frequencies and wavenumbers not coincident with certain dispersion curves. Nonstatic behavior of two types is predicted: zero motion of the sea surface and a resonant response. Two types of resonance emerge. The first type corresponds to barotropic modes which are long gravity waves or Rossby waves at high and low frequencies, respectively. The second type excites internal modes, either gravity waves or Rossby waves depending on frequency, but modified from a conventional resonant response by the immediate juxtaposition in frequency/wavenumber space of the rigid‐lid modes. The extent to which these actual resonances are generated is obscure owing to the same uncertainties about oceanic dissipation and scattering which affect all other forced oceanic motions, especially including the tides and other barotropic motions. Zero sea surface motion is predicted at frequencies and wavenumbers corresponding to “rigid‐lid” modes. Observations support the wide applicability of the static response except in the tropics and in the western boundary current extension regions; there, the signal‐to‐noise ratios may be inadequate. The only other known clear nonstatic response occurs near 5 days in the Pacific and South Atlantic Oceans, indicative of a low‐Q resonance in the former area and a forced nonresonant response in the latter, but there are remaining problems with these interpretations.
High‐altitude energy source(s) for stable auroral red arcs
Authors: Kozyra J. U. , Nagy A. F. , Slater D. W.
Published: February 2004

Stable auroral red (SAR) arcs have been viewed across the midlatitude night sky with interest since their discovery in 1956. This relatively late discovery (compared to the poleward aurora) is a direct consequence of the subvisual levels of the delicate and diffuse light that makes up the SAR arc. Except in rare instances when the emissions actually crossover the threshold to visible levels, optical instruments are required to register their presence and document their morphology and occurrence frequency. SAR arcs are seen as relatively featureless, slowly changing bands of red light that can extend across the entire night sky. Early observations from the ground and from satellites established the relationship between SAR arcs and magnetic disturbances in near‐Earth space involving the ring current (a population of high‐energy ions trapped in the Earth's magnetic field). The long‐lived, soft, red glow of SAR arcs reflects the slow energy loss from the ring current ions as they bounce back and forth in the Earth's confining field geometry, but the exact sequence of physical processes that feed a portion of the ring current energy to the SAR arc region is a matter of continuing debate. The midlatitude location of the SAR arc bands mirrors the high‐altitude location of the outer portion of the ring current torus, but exactly how this positioning is related to gradients or enhancements in the high‐altitude extension of the cold ionospheric plasma remained an open question. Our knowledge of SAR arcs and associated signatures now spans more than two full solar cycles. Detailed observations of the high‐altitude space environment overlying the SAR arc region have been made, indicating that heavy ions from the lower atmosphere, energized by the solar wind interaction with the magnetosphere, play an important role in the SAR arc process. Sophisticated models of the coupled high‐ and low‐altitude regions, involved in SAR arc formation, paint an interesting picture of the flow of energy between the atmosphere and near‐Earth space. This energy flow culminates in the SAR arc's glowing red bands, which act, in turn, as sensitive markers of the interaction regions and processes. This emerging view of the SAR arc process and the new observational information that provided the underpinnings of its development from a rich framework of past theoretical and observational work is the subject of the present review.