bluuebunny/crossref_metadata_2025 huggingface dataset:
In situ single‐particle characterization at Cape Grim
Authors: Murphy D. M. , Thomson D. S. , Middlebrook A. M. , Schein M. E.
Published: February 2004

This paper describes operation of the particle analysis by laser mass spectrometry (PALMS) instrument at Cape Grim. Several changes to the instrument improved the uniformity of ionization, simplified data analysis, and reduced the minimum analyzable particle size to 0.16 μm. Almost all of the particles analyzed by PALMS at Cape Grim during clean marine conditions contained some sea salt. Almost all of the sulfate found during clean marine conditions was mixed with sea salt rather than in separate ammonium sulfate or sulfuric acid aerosols. Metals such as Cr, Fe, and Sn were detected in a few particles even in clean marine air masses. An NO+ peak in the mass spectra was associated with Mg, Ca, and Sr. This association could be caused either by the water retention of various salts or by different reactivities. It provides a reminder that other components of sea salt besides NaCl need to be considered for heterogeneous chemistry.
Absorption of solar radiation by water vapor, oxygen, and related collision pairs in the Earth's atmosphere
Authors: Solomon S. , Portmann R. W. , Sanders R. W. , Daniel J. S.
Published: February 2004

Absorption of solar radiation by collision pairs of oxygen is a small but significant part of the total budget of incoming shortwave radiation. It is shown that the 1.26‐μm band of O4 and O2·N2 is likely to contribute 0.64–1.55 W m−2 to the total atmospheric absorption for overhead Sun, clear sky conditions, bringing the total estimated absorption by all collision pairs of oxygen over all wavelengths from 300‐ to 1300‐nm to 2.2–3.11 W m−2 for overhead Sun under clear sky. The globally averaged all‐sky absorption by collision complexes of oxygen is likely to be about 0.9–1.3 W m−2, roughly twice the value of previous estimates neglecting the 1.26‐μm band and the contribution of O2·N2. Significant uncertainties are shown to remain in the spectroscopy of O4 and O2·N2 in the near‐infrared band at 1.26‐μm (and possibly also at 1.06‐μm), requiring modern laboratory measurements to better quantify these processes. Atmospheric measurements are also presented in this paper to verify the spectral shape of the absorption by O4 and other trace gases (O2, H2O, NO2, and O3) in the 610‐ to 680‐nm and 400‐ to 450‐nm regions. These observations rule out the existence of significant unknown “anomalous” or unexplained absorbers displaying structured features under either clear or cloudy conditions at these wavelengths. However, the possibility of anomalous broad continuum or quasi‐continuum absorption cannot be evaluated using our measurement technique. The observed wavelength dependence of water vapor absorption in the bands near 442‐ and 650‐nm are shown to agree well with their expected spectral structure under both clear and cloudy sky conditions for the instrumental resolution employed. Further, both the shape and magnitude of the measured atmospheric O4 absorption near 630‐nm agree with expectations based on recent laboratory measurements of this band to better than 10% over a broad range of zenith angles.
A technique for determining the spatial and temporal distributions of surface fluxes of heat and moisture over the Southern Great Plains Cloud and Radiation Testbed
Authors: Doran J. C. , Hubbe J. M. , Liljegren J. C. , Shaw W. J. , Collatz G. J. , Cook D. R. , Hart R. L.
Published: February 2004

Land surface parameterization schemes such as the Simple Biosphere Model (SiB2) have found considerable use in climate simulation models, where they provide lower boundary conditions in the form of surface sensible and latent heat fluxes. A methodology is described to apply models of this type at high resolution, using data from the Department of Energy's Cloud and Radiation Testbed in Oklahoma and Kansas, to determine the spatial variations of heat fluxes over the domain and to determine area‐weighted flux averages for use in single‐column model studies. Data from a dense array of meteorological instruments are interpolated to provide the wind, temperature, vapor pressure, radiation, and precipitation values needed by SiB2. The state of the vegetation is characterized through the use of the normalized difference vegetation index determined from satellites. The performance of the SiB2 model is evaluated by comparing its predictions with flux data from seven Bowen ratio stations over a 6‐month period. No tuning of the model parameters for individual sites was allowed during the simulation period. Although there is significant scatter in the results, the performance of the model was generally good, accounting for over 60% of the variance in sensible heat fluxes and over 80% of the variance in latent heat fluxes. The model was therefore used to prepare “flux maps” for the study area. These maps show large contrasts in sensible and latent heat fluxes associated primarily with differences in vegetation cover and soil moisture over the site. The differences in vegetation, in turn, result from the planting of large areas with winter wheat, which leaves some regions nearly devoid of actively growing vegetation in midsummer, while other areas are still covered with thriving crops or naturally occurring vegetation. Implications for determining large‐area averages of fluxes from a limited number of measurement sites are discussed.
Concept of a two‐level mesopause: Support through new lidar observations
Authors: She C. Y. , von Zahn U.
Published: February 2004

Regular Doppler temperature lidar measurements at Fort Collins, United States (41°N, 105°W), and Kühlungsborn, Germany (54°N, 12°E), during 1996/1997 have provided two new data sets to further detail the annual variation of the thermal structure of the mesopause region, i.e., the 80‐ to 105‐km altitude range. We present a quantitative analysis of these data sets taken at two stations with considerable north‐south and east‐west differences. In addition, we expand this observational base by including data taken earlier, between 70°S and 70°N. These data permit us to deduce what we consider to be a genuine global picture depicting the thermal structure of the mesopause region, a prominent and globally unified feature is that the mesopause exists at only two altitudes: A winter state at an altitude near 100 km and a summer state near 88 km. In the winter state, the mesopause is at a nearly constant temperature of about 180 K, independent of latitude. The summer state mesopause does not seem to exist in the equatorial region; its temperature decreases drastically as one goes from midlatitudes toward the poles. Relatively sharp transitions take place from the winter state to the summer state and vice versa in spring and autumn, respectively. Since our data show that the winter high mesopause lasts longer at a given geographical latitude and covers more geographical latitudes at a given time, we estimate the globally averaged altitude of the mesopause to be 98 km.
Effect of clouds on direct aerosol radiative forcing of climate
Authors: Liao Hong , Seinfeld John H.
Published: February 2004

The effect of a cloud layer on top‐of‐atmosphere (TOA) aerosol radiative forcing is examined by means of a one‐dimensional vertical column simulation. To span the range between nonabsorbing and strongly absorbing particles, (NH4)2SO4 and soot aerosols are considered individually and in internal and external mixtures. For a cloud layer embedded within an aerosol layer it is shown that direct aerosol radiative forcing still occurs. For a nonabsorbing aerosol a maximum in (negative) forcing actually occurs for a thin cloud layer (100 m thickness for the set of parameters considered). The presence of an embedded cloud layer enhances the heating effect of soot aerosol, producing, for thick clouds, forcing values as much as a factor of 3 over those under cloud‐free conditions. An absorbing aerosol layer can lead to an increase of in‐cloud solar heating rates by up to 3% for the parameter values considered here. A cirrus cloud layer above an aerosol layer leads to only modest changes of TOA aerosol forcing from those in the absence of the cloud layer; thus aerosol forcing in the presence of typical cirrus clouds cannot be neglected.
A global interactive chemistry and climate model: Formulation and testing
Authors: Wang Chien , Prinn Ronald G. , Sokolov Andrei
Published: February 2004

In order to elucidate interactions between climate change and biogeochemical processes and to provide a tool for comprehensive analysis of sensitivity, uncertainty, and proposed climate change mitigation policies, we have developed a zonally averaged two‐dimensional model including coupled biogeochemical and climate submodels, as a part of an integrated global system model. When driven with calculated or estimated trace gas emissions from both anthropogenic and natural sources, it is designed to simulate centennial‐scale evolution of many radiatively and chemically important tracers in the atmosphere. Predicted concentrations of chemical species in the chemistry submodel are used interactively to calculate radiative forcing in the climate submodel, which, in turn, provides winds, temperatures, and other variables to the chemistry submodel. Model predictions of the surface trends of several key species are close to observations over the past 10–20 years. Predicted vertical distributions of climate‐relevant species, as well as seasonal variations, are also in good agreement with observations. Runs of the model imply that if the current increasing trends of anthropogenic emissions of climate‐relevant gases are continued over the next century, the chemical composition of the atmosphere would be quite different in the year 2100 than that currently observed. The differences involve not only higher concentrations of major long‐lived trace gases such as CO2, N2O, and CH4 but also about 20% lower concentrations of the major tropospheric oxidizer (OH free radical), and almost double the current concentrations of the short‐lived air pollutants CO and NOx.
Spectra of gravity wave density and wind perturbations observed during Arctic Noctilucent Cloud (ANLC‐93) campaign over the Canadian Prairies: Synergistic airborne Na lidar and MF radar observations
Authors: Manson A. H. , Meek C. E. , Qian J. , Gardner C. S.
Published: February 2004

The airborne Na lidar and the Saskatoon and Canadian Network for Space Research (CNSR) medium frequency (MF) radars were used to measure the spectra of gravity wave density and wind perturbations during the Arctic Noctilucent Cloud (ANLC‐93) campaign. The National Center for Atmospheric Research Electra flight between Sylvan Lake, Saskatoon, and Great Falls occurred on August 12, 1993. This provided vertical and horizontal wave number spectra for relative atmospheric density fluctuations along the flight legs. Slopes are −2.5±0.1 and −1.3±0.1, respectively, for the vertical and one‐dimensional horizontal wave number spectra. The radars provided temporal frequency and vertical wave number spectra for the flight mission day and duration (July 15 to August 15) of the campaign, so that the fluxes on August 12 could be related to typical summer values. The slopes of frequency spectra are typically −2.1 (1–6 hours) and −1.0 (10–100 min). Gravity wave (GW) parameters from the two observation systems are compared on August 12, including horizontal anisotropies, and dominant horizontal and vertical scale sizes. In particular, several new analyses are developed to illustrate the synergy that is possible between such systems. The radar winds data, using variance ovals and a new correlation‐vector technique, provide evidence for GW propagation into the NE quadrant. This anisotropy of GW propagation is also shown to be evident in the lidar data. An innovative analysis is applied to the Saskatoon MF radar, using Doppler and angle‐of‐arrival data, and used to characterize a unique GW event, which is also observed spatially by the lidar when close to Saskatoon.
Postdepositional change in snowpack nitrate from observation of year‐round near‐surface snow in coastal Antarctica
Authors: Mulvaney R. , Wagenbach D. , Wolff E. W.
Published: February 2004

Postdepositional loss of nitrate in near‐surface snowfall is well known, with mean levels of nitrate in ice cores of around 20 to 80 ng g−1, while nitrate in surface snow may occasionally reach over 300 ng g−1. This has been explained as reemission of nitrate (as nitric acid) during aging, via processes that are not yet clear. However, clear seasonal cycles remain in nitrate profiles from higher accumulation rate ice cores and, across Antarctica, the mean concentration of nitrate is remarkably similar despite widely varying deposition conditions, marking nitrate out as quite different to other major ice core species. This paper examines the year‐round deposition of nitrate at the snow surface at a coastal Antarctic site and discusses the degree and timing of nitrate loss. At Halley Station, Antarctica, the mean concentration of near‐surface snow was 96 ng g−1 over a 2.6 year daily sampling period, while the mean concentration in newly accumulated snow was 79 ng g−1. At the end of this period, a shallow core integrating the sampling period had a mean nitrate concentration of 65 ng g−1 taken over 2 full years of accumulation. Nitrate concentrations in the surface layer were in general highest during the summer period reaching 400 ng g−1 with a mean of about 150 ng g−1 in each of December, January, and February, and lowest during the winter with a mean of around 50 ng g−1 in June. Fresh snow data from Neumayer Station shows a similar seasonal signal, with a mean nitrate concentration of 77 ng g−1, while year‐round aerosol data shows total nitrate (particulate and gas phase) in the air is at a minimum in April to June and reaches a maximum in late November [Wagenbach et al., this issue], slightly out of phase with snowfall nitrate. The observation of a reduction in high nitrate concentrations in new snowfall over a few days does not appear to be general, and at Halley, there is evidence of both uptake and loss of nitrate in the surface snow layer, possibly indicating an equilibrium with changing air concentrations. However, there is attenuation of the nitrate signal over the longer period, with concentrations in the ice core taken at the end of sampling never reaching values seen in the upper surface layer.
Investigation of the effect of surface heterogeneity and topography on the radiation environment of Palmer Station, Antarctica, with a hybrid 3‐D radiative transfer model
Authors: Ricchiazzi Paul , Gautier Catherine
Published: February 2004

We have developed and used a Monte Carlo radiative transfer code to investigate how surface topography and heterogeneous snow/ice distributions affect the downwelling irradiance at Palmer Station, Antarctica (64.76°S, 64.07°W). The Monte Carlo calculations treat a three‐dimensional (3‐D) atmospheric volume which extends from the surface to 100 km altitude and has a 20 km × 20 km footprint on the southwest coast of Anvers Island. The radiative transfer calculations include the effects of molecular absorption, Rayleigh scattering, and clouds. The surface interaction is modeled explicitly. The trajectories of reflected photons are computed from stochastic bidirectional‐directional reflectance functions, and their paths are traced through multiple interactions with complex surface features. Computed results for a range of cloud optical depth, solar zenith angle, and surface albedo are presented. Comparisons of the 3‐D model calculations to plane‐parallel model predictions show that the effective albedo which characterizes a given ice distribution is affected by regions surprisingly far from the point of interest. Under low clouds (Zcloud = 1 km), surface irradiance measurements over a snow surface are significantly affected by the dark ocean surface more than 7 km away. For the opposite case of irradiance observations over ocean, the effect of a distant snow surface is not significant at ranges greater than 2 or 3 km. Since the radius of influence depends on atmospheric transmission and surface albedo, the effective albedo varies spectrally. Neglect of this nonlocal albedo effect may significantly degrade the accuracy of radiation diagnostics that depend on spectral intensity ratios.
Wet deposition in a global size‐dependent aerosol transport model: 1. Comparison of a 1 year210Pb simulation with ground measurements
Authors: Guelle W. , Balkanski Y. J. , Schulz M. , Dulac F. , Monfray P.
Published: February 2004

We present and discuss results from a 1 year (1991) global simulation of the transport and deposition of210Pb with a new size‐resolved aerosol transport model. The model accounts for aerosol size distribution and its evolution during transport. Our wet deposition scheme is size‐dependent and distinguishes between scavenging by deep and shallow convective rains. It treats separately below‐ and in‐cloud scavenging by synoptic rains. Although the model is formulated to treat all aerosol sizes, the validation was done for the210Pb submicronic aerosol for which the main sink is wet deposition. We assess the model transport and deposition of submicron aerosols by a comparison of model results with available surface measurements. Annual mean surface concentrations are compared at 117 stations throughout the globe; seasonal variations are examined for 35 of these sites. The mean bias between simulated and measured yearly averaged surface concentrations is −2.7%, and the correlation coefficient is 0.80. The observed seasonal cycle and the annual mean concentrations are particularly well reproduced, although the model's poor vertical resolution does not capture the strong winter peak at some continental stations, nor the transport to Indian Ocean stations. Using the observed precipitation at or near the sites studied, we were able to explain a large part of the bias in model annual deposition. Deposition at coastal sites deserves also a special treatment since influenced by the land‐ocean partition inherent to the model. When we represent correctly these coastal stations, we reduce the mean bias between observed and predicted annual deposition fluxes from 7.7% to 1.2% at 147 stations, and the correlation coefficient improves from 0.70 to 0.77.
Effects of wind speed and particulate matter source on surface microlayer characteristics and enrichment of organic matter in southern Chesapeake Bay
Authors: Liu Kewen , Dickhut Rebecca M.
Published: February 2004

Surface microlayer (SM) samples were collected with a rotating cylinder sampler from the York and Elizabeth River estuaries of lower Chesapeake Bay bimonthly from May 1994 through June 1995. Two intensive samplings were also conducted in the York River during different seasons: one in December 1994 and another during June 1995. Four SM samples were collected during each intensive sampling within 4 days. All the samples were analyzed for total suspended particulates (TSP), particulate nitrogen (PN), particulate organic carbon (POC) and dissolved organic carbon (DOC). The thickness of the SM was observed to decrease linearly with increased wind speed. TSP and POC in the SM were found to be enriched up to 1000‐fold over the corresponding subsurface water, and to be exponentially related with wind speed at sampling. Enrichment of DOC in the SM relative to subsurface bulk water was also observed at both sampling sites. Larger average POC/PN ratios and consistently smaller particle sizes were observed in the Elizabeth River compared with the York River, suggesting that the former is heavily influenced by atmospheric deposition, as well as urban and terrestrial runoff, whereas in situ production of particles dominates in the latter. No seasonal trends were observed in the SM characteristics with the exception of particle size fractions in the York River, which fluctuate seasonally presumably due to changes in the dominant phytoplankton species. TSP and POC concentrations in the SM can be described by a first‐order wind‐driven mixing model. Wind‐driven mixing of SM DOC was less apparent than for TSP and POC.
International Global Atmospheric Chemistry (IGAC) Project's First Aerosol Characterization Experiment (ACE 1): Overview
Authors: Bates Timothy S. , Huebert Barry J. , Gras John L. , Griffiths F. Brian , Durkee Philip A.
Published: February 2004

The southern hemisphere marine Aerosol Characterization Experiment (ACE 1) was the first of a series of experiments that will quantify the chemical and physical processes controlling the evolution and properties of the atmospheric aerosol relevant to radiative forcing and climate. The goals of this series of process studies are to reduce the overall uncertainty in the calculation of climate forcing by aerosols and to understand the multiphase atmospheric chemical system sufficiently to be able to provide a prognostic analysis of future radiative forcing and climate response. ACE 1, which was conducted from November 15 to December 14, 1995, over the southwest Pacific Ocean, south of Australia, quantified the chemical, physical, radiative, and cloud nucleating properties and furthered our understanding of the processes controlling the aerosol properties in this minimally polluted marine atmosphere. The experiment involved the efforts of scientists from 45 research institutes in 11 countries.
Martian drainage densities
Authors: Carr Michael H. , Chuang Frank C.
Published: February 2004

Drainage densities on Mars range from zero over large areas of volcanic plains to 0.3–0.5 km−1 locally on some volcanoes. These values refer to geologic units, not to drainage basins, as is normal for terrestrial drainage densities. The highest values are close to the lowest terrestrial values derived by similar techniques. Drainage densities were determined for every geologic unit portrayed on the 1:15,000,000 geologic map of Mars. Except for volcanoes the geologic unit with the highest drainage density is the dissected Noachian plains with a drainage density of 0.0074 km−1. The average drainage density for Noachian units is 0.0032 km−1, for Hesperian units is 0.00047 km−1, and for Amazonian units is 0.00007 km−1, excluding the volcanoes. These values are 2–3 orders of magnitude lower than typical terrestrial densities as determined by similar techniques from Landsat images. The low drainage densities, despite a cumulative record that spans billions of years, indicate that compared with the Earth, the channel‐forming processes have been very inefficient or have operated only rarely or that the surface is extremely permeable. The high drainage density on volcanoes is attributed to a local cause, such as hydrothermal activity, rather than to a global cause such as climate change.
Mineral layers around coarse‐grained, Ca‐Al‐rich inclusions in CV3 carbonaceous chondrites: Formation by high‐temperature metasomatism
Authors: Ruzicka Alex
Published: February 2004

Coarse‐grained (Type A, B) Ca‐Al‐rich inclusions (CAIs) in carbonaceous chondrites typically are surrounded by thin mineral layers (“rims”) that have puzzled researchers for two decades. Quantitative reaction‐diffusion models can account for the overall mineral zoning structures of rims and the major‐element zoning of the ubiquitous clinopyroxene layer, suggesting that the layers formed by metasomatism. Melilite‐bearing CAIs appear to have reacted with an external medium that primarily contained Mg‐Si‐rich vapor (with atomic Mg/[Mg+Si] ≤0.66) and forsteritic olivine. Different reactant compositions in the external medium appear to have been largely responsible for producing different rim types. Various rims formed either in different local environments or at different times in an evolving system. It is suggested that layer formation occurred in a nebular setting, while silicates were being vaporized and olivine was condensing around CAIs. Steady state layer growth models do not adequately explain the presence of melilite layers or patches in some rims and consistently underestimate the spinel/clinopyroxene ratios of rims, probably because of a failure to attain complete steady state conditions as a result of changing pressure, temperature, or reactant compositions during layer growth. Roughly 3–50% of the spinel in rims can be attributed to metasomatic growth, but the remaining spinel formed by another process, possibly as a residue of partial melting during a brief vaporization event, or by preferential nucleation on the surfaces of molten CAIs. The thermal events accompanying CAI metasomatism can be constrained by modeling Mg isotope exchange that occurred between some CAIs and the external medium. Based on one well‐studied CAI, it is inferred that isotopic exchange and layer formation was initiated either in a high‐temperature (>1450°C) heating event <10 hours in duration, or at lower temperatures (≤1450°C) during cooling at a rate of ≤0.1–2°C/hr.
Planetary vortices and Jupiter's vertical structure
Authors: Williams G. P.
Published: February 2004

Measurements of the vertical structure of Jupiter's circulation have recently been made near the equator by the Galileo spacecraft probe. In other regions, planetary vortices exist selectively for a limited range of generic (exponential) vertical forms and can be used to probe the atmospheric structure theoretically. A study of vortex genesis with a three‐dimensional numerical model produces reasonably realistic simulations of the Great Red Spot for both the generic and galilean forms, provided that Jupiter's winds do not extend much beyond a 500 km depth. However, the actual depth of the winds remains uncertain.
The NEAR magnetic field investigation: Science objectives at asteroid Eros 433 and experimental approach
Authors: Acuña M. H. , Russell C. T. , Zanetti L. J. , Anderson B. J.
Published: February 2004

The NEAR magnetic field investigation is an integrated technical, scientific, and management undertaking that was selected by NASA to achieve the following goals: to coordinate and support the development, calibration, and integration aboard the NEAR spacecraft of the magnetometer facility instrument (MAG), to provide technical assistance to the NEAR project in the area of magnetic cleanliness, and to ensure maximum science data quality. The prime objective of the MAG investigation is to establish the nature of the magnetic field of 433 Eros and any detectable effects of its interaction with the solar wind. The detection of an intrinsic magnetic field would place a strong constraint on the composition and structure of the asteroid's interior and thermal evolution. NEAR magnetometer data will be primarily available through the Johns Hopkins University Applied Physics Laboratory (APL) Science Data Center, http://sd‐www.jhuapl.edu/NEAR. The basic building block of the MAG investigation is the facility‐class magnetometer instrument flown aboard the NEAR spacecraft [Lohr et al., 1997]. The NEAR MAG was built as a cooperative undertaking between the Laboratory for Extraterrestrial Physics of the Goddard Space Flight Center (GSFC) and APL. GSFC provided the sensors and analog systems, and APL provided the digital processing unit and power converter as well as oversight for test and integration activities on the spacecraft. No booms or appendages were used to place the MAG sensor away from the spacecraft; instead and in analogous fashion to ESA's Giotto spacecraft, the sensor assembly was mounted on the high‐gain antenna feed structure. This arrangement results in a minimum detectable magnetic signature of 2–5 nT.
Gradation of the Roter Kamm impact crater, Namibia
Authors: Grant John A. , Koeberl Christian , Reimold Wolf Uwe , Schultz Peter H.
Published: February 2004

Interpretation of ground penetrating radar data from the 3.7 Ma old Roter Kamm impact crater, together with results of petrographic and sedimentologic analysis of sediment samples, indicates that ∼40 m or more of slope sensitive degradation of the rim was accompanied by reduction of wall slopes well below the angle of repose and nearly complete erosion of ejecta from around the crater. Only one patch of in situ ejecta deposit was identified. Degradation of both the interior and exterior of the crater was dominated by fluvial/alluvial activity, likely during the first 1.0–2.7 Ma history of the crater. Eolian modification has dominated gradation since the last half of the Pleistocene. Results from Roter Kamm can help in evaluating evolution of degraded Martian craters that may have experienced fluvial/alluvial gradation followed by eolian modification. On Mars, extended fluvial/alluvial activity should produce a low sloped, incised rim, thereby eroding most of the continuous ejecta well before removal of easily recognizable rim relief. Subsequent eolian deposition can bury all but the raised rim of craters, thereby creating a focal point for efforts geared towards constraint of gradational history.
Geologic context of the Mars radar “Stealth” region in southwestern Tharsis
Authors: Edgett Kenneth S. , Butler Bryan J. , Zimbelman James R. , Hamilton Victoria E.
Published: February 2004

“Stealth” is a region on Mars that has no distinguishable radar return. Stealth was discovered in 1988 by Muhleman et al. [1991] in 3.5 cm Earth‐based radar images, and it was found to extend ∼2000 km along the Martian equator west of Arsia and Pavonis Montes. Stealth was proposed to be a deposit of unconsolidated, granular material with a minimum thickness of several meters, with the most likely explanation being that Stealth is a volcanic tephra deposit [Muhleman et al., 1991, 1995]. Stealth overlies some of the youngest geologic units on Mars, including lava flows and a giant, curvilinear‐ridged lobate unit on the west flank of Arsia Mons. Stealth also overlies part of a region of smooth and wind‐scoured materials called the Medusae Fossae Formation. Examination of high‐resolution Viking orbiter images suggests that Stealth is the youngest of several discontinuous, wind‐worked mantles of fine‐grained (≪1 cm) sediment. Older deposits of fine sediment appear to be interbedded with lava flows and show evidence of aeolian deposition (dunes and deposits in the lee of topographic obstacles) and erosion (yardangs and pedestal craters). The geologic context supports a volcanic ash origin for Stealth. If Stealth is composed of tephra, then its extent and the nature of landforms in the region indicate that it resulted from multiple eruptions that occurred relatively late in Martian history. Possible source vents include linear depressions (flanked by constructive volcanic landforms) that are located on the lower western slopes of Arsia and Pavonis Montes.
Formation conditions of igneous regions in ordinary chondrites: Chico, Rose City, and other heavily shocked H and L chondrites
Authors: Yolcubal Irfan , Sack Richard O. , Wang Ming‐Sheng , Lipschutz Michael E.
Published: February 2004

We report microprobe and radiochemical neutron activation analysis (RNAA) data for hosts and igneous inclusions in the heavily shocked ordinary chondrites Rose City (H5), Yanzhuang (H6), Farmington (L5), Malakal (L5), Chantonnay (L6), Chico (L6), and Tuan Tuc (L6). Based on these analytical results, equilibrium crystallization calculations [Ghiorso and Sack, 1995], phase equilibrium analysis [e.g., Sack and Ghiorso, 1994a,b; Sack et al., 1994], and experimental cooling rate studies [e.g., Walker et al., 1976], we have assessed the metamorphic and magmatic histories of these seven heavily shocked chondrites. We infer that (1) unfractionated chondritic liquids were intruded to depths >0.1 km in the parent asteroid of Rose City; (2) early chondritic liquids experiencing 5–10% olivine fractionation were erupted onto the surface of the parent asteroid(s) of Yanzhuang and Chantonnay; and (3) near‐surface crystallization is also indicated for the liquids in Tuan Tue and Farmington, with Chico and Malakal crystallizing at slightly greater depths. In all but Chantonnay, liquids appear to have derived from melting of chondrite types corresponding to their hosts. In the L chondrite Chantonnay, the intrusive liquids derive from melting an H chondrite source region in which chondritic melts were stored for a sufficient time to produce pigeonite in zoned pyroxene xenocrysts. Heating effects are also reflected in the trace element contents of the chondrites. Our RNAA data for Rose City seem to reflect only the siderophile‐lithophile fractionation evident in the metal distribution. The Yanzhuang RNAA data are generally similar to those of other H chondrites: severe shock not involving phase transport seems to leave H chondrites unaffected compositionally. Contents of the four most mobile elements (Cd, Bi, T1, In) and Cs in Chico indicate loss, so that this assemblage experienced extended, low‐temperature cooling after injection of dike material into the Chico host. RNAA data for the other four L chondrites examined indicate more rapid cooling.
In situ compositions of Martian volcanics: Implications for the mantle
Authors: Mustard John F. , Murchie Scott , Erard Stéphane , Sunshine Jessica
Published: February 2004

The primary objective of this analysis is to determine the mineralogic composition of relatively pristine terrains exposed on the surface of Mars. This analysis is conducted using imaging spectrometer data acquired in 1989 by the Imaging Spectrometer for Mars (ISM) instrument that was onboard the Phobos‐II spacecraft. The ISM instrument acquired some of the highest spectral (64 channels between 0.77 and 3.14 μm) and spatial resolution (≈22 km/pixel) observations to date of Mars, focused on the equatorial region. Relatively unaltered surfaces were identified in these data on the basis of their spectral properties (low albedo, weak spectral slope, absence of ferric absorptions), and were found in volcanic terrains of Syrtis Major, Valles Marineris, Ophir Planum, and Sinus Meridiani/Oxia Palus. Spectra representative of the diversity among terrains were extracted for detailed analysis using the Modified Gaussian Model to determine the primary mafic mineralogy, It was determined that the mafic mineralogy of the regions investigated is dominated by low‐calcium and high‐calcium pyroxene (LCP, HCP), and that there was homogeneity within a given region but some heterogeneity between regions in the relative abundance of LCP to HCP. The two‐pyroxene mineralogy is consistent with the mineralogies of the basaltic SNC meteorites believed to have originated on Mars. Two‐pyroxene volcanic mineralogies are indicative of large degrees of partial melting (e.g., komatiites) or a mantle source depleted in aluminum. Integration of these findings with previous studies of Mars and the SNC meteorites indicates that (1) the SNC mineralogies are representative of large volcanic regions of Mars with surface ages as old as 2–3 Ga, (2) the martian mantle was depleted in aluminum at least as long ago as the oldest terrain analyzed (Ophir Planum), and (3) there has been little evolution in the composition of mantle source regions as the 180 Ma SNC mineralogies are comparable to those of the oldest terrains analyzed.
Manganese‐rich olivines: Identification from spectral reflectance properties
Authors: Cloutis Edward A.
Published: February 2004

Reflectance spectra of manganese‐rich olivines were examined to determine which spectral features allow these minerals to be distinguished from forsteritic‐fayalitic olivines. The results indicate that manganese‐rich olivines can be distinguished on the following bases: Fe2+ M2 absorption band depths are reduced relative to Fe2+ M1 depths, the wavelength position of the Fe2+ M2 absorption band is shifted to longer wavelengths (>1.08 μm) for olivines containing between ∼10 and 70 mol% Mn2SiO4, and the presence of an isolated Mn2+ transition band near 0.4 μm. The absence of Fe2+ spin forbidden features in the visible wavelength region is not a reliable method for identifying manganese‐rich olivines.
Yarkovsky thermal drag on small asteroids and Mars‐Earth delivery
Authors: Rubincam David Parry
Published: February 2004

There is an optimal size for the delivery of small asteroids from Mars to the Earth by Yarkovsky thermal drag. Basaltic asteroids with radii of about 6 m take on the average 185 million years (Myr) for their semimajor axes to shrink by 0.52 AU, assuming circular orbits and ignoring planetary perturbations and collisions. All other sizes take longer. Bigger objects are slower because they are more massive, and smaller objects are slower because they are more isothermal. These results are based on treating the asteroids as spheres and solving the heat conduction equation using spherical Bessel functions. The small near‐Earth asteroids show a concentration of sizes in the thermal drag range; thus some of them may come from Mars as survivors of gravitational mechanisms which eliminate them on the 10 Myr timescale. The possible role of thermal drag in Mars‐Earth delivery will remain speculative until it is included in numerical integrations of the orbits of small asteroids.
The Imbrium impact event and the thorium distribution at the lunar highlands surface
Authors: Haskin Larry A.
Published: February 2004

Late in the Moon's heavy bombardment period, the impact that formed the Imbrium Basin excavated and melted a tremendous volume of material that was subsequently distributed over most of the Moon's surface. The Apollo orbiting γ ray experiments [Metzger et al., 1977] show that the Imbrium projectile struck in a Th‐rich area, called the High‐Th Oval Region here and regarded as a unique lunar geochemical province. Imbrium primary ejecta were thus probably rich in Th. It is shown here using ejecta deposit modeling that the distribution of Th in the highlands surface along the ground tracks of the γ ray spectrometers is consistent with the distribution expected for Imbrium ejecta deposits. Deep basins other than Imbrium appear to have excavated regions of lower crust that were mafic and Fe‐bearing but not Th‐rich. The possibility that the surface Th distribution arises mainly from Imbrium ejecta has implications for the nature of the Moon's igneous differentiation and the nature of the proposed “cataclysm” or late bombardment with large meteoroids.
FeO and TiO2 concentrations in the South Pole‐Aitken basin: Implications for mantle composition and basin formation
Authors: Lucey Paul G. , Taylor G. Jeffrey , Hawke B. Ray , Spudis Paul D.
Published: February 2004

We use newly developed spectroscopic methods for determining FeO and TiO2 abundances to study the huge (2500 km diameter) South Pole‐Aitken basin. The floor of this basin shows iron and titanium values which are weakly correlated and largely range from 7 to 14 wt % FeO and from 0.5 to 1.5 wt % TiO2. This composition is consistent with an approximate 1:1 mixture of lower crustal material like that found elsewhere on the Moon and mantle rock containing 10–16 wt % FeO and <0.1 wt % TiO2. Although mantle rock appears to compose about 50% of the floor deposits, models of the formation of large impact basins predict that virtually all the floor materials ought to be derived from the mantle.
The Late Cretaceous: Simulation with a coupled atmosphere‐ocean general circulation model
Authors: Bush Andrew B. G. , Philander S. George H.
Published: February 2004

Results are presented for the climate of the late Cretaceous period (∼75–65 Ma) as simulated by a global climate model that is interactively coupled to a primitive equation global ocean model. Increased values of atmospheric CO2 and altered land surface albedos are invoked to produce the warm Cretaceous temperatures that have been proposed from biogeographic reconstructions. For comparison, a control simulation of the present climate is performed. The globally averaged atmospheric temperature in the Cretaceous simulation stabilizes after 20 years of integration at a value that is 4°C greater than that of the present day. The lower troposphere in high latitudes contributes a majority of the globally averaged warming as a result of the elimination of the Antarctic and Greenland ice sheets. Nevertheless, equatorial surface temperatures are raised by ∼5°C above those of the control simulation and offset somewhat the reduction in near‐surface baroclinicity caused by the absence of the high‐latitude ice sheets. In the Cretaceous simulation, global precipitation is approximately 10% greater than in the present day, with the only region of reduced precipitation occurring beneath the south Eurasian monsoon. Additionally, the amplitude of the seasonal cycle in near‐surface temperatures is smaller in the Cretaceous and, in conjunction with increased mean annual temperatures, precludes the presence of any year‐round snow or ice in the simulation. In high latitudes, however, there are regions that seasonally drop below freezing. The temperatures in these regions are warmer than have been previously observed in atmosphere‐only simulations as a result of poleward heat transport by the ocean's surface currents.
Millennial‐scale deep water oscillations: Reflections of the North Atlantic in the deep Pacific from 10 to 60 ka
Authors: Lund David C. , Mix Alan C.
Published: February 2004

Northeast Pacific benthic foraminiferal δ18O and δ13 reveal repeated millennial‐scale events of strong deep‐sea ventilation (associated with nutrient depletion and/or high gas exchange) during stadial (cool, high ice volume) episodes from 10 to 60 ka, opposite the pattern in the deep North Atlantic. Two climate mechanisms may explain this pattern. North Pacific surface waters, chilled by atmospheric transmission from a cold North Atlantic and made saltier by reduced freshwater vapor transports, could have ventilated the deep Pacific from above. Alternatively, faster turnover of Pacific bottom and mid‐depth waters, driven by Southern Ocean winds, may have compensated for suppressed North Atlantic Deep Water production during stadial intervals. During the Younger Dryas event (∼11.6–13.0 cal ka), ventilation of the deep NE Pacific (∼2700 m) lagged that in the Santa Barbara Basin (∼450 m) by >500 years, suggesting that the NE Pacific was first ventilated at intermediate depth from above and then at greater depth from below. This apparent lag may reflect the adjustment time of global thermohaline circulation.
The role of downhole measurements in marine geology and geophysics
Authors: Goldberg David
Published: February 2004

During the last 25 years, downhole measurements have been increasingly used for scientific applications in marine geology and geophysics, particularly in deep‐sea drilling operations. Used mostly by the oil industry to map promising formations for exploration and production of hydrocarbons, a variety of instruments have been developed that can be lowered down drill holes to extract information about the subsurface geology. In the last decade, advances in computers, software, and data transmission have greatly increased the amount and quality of data that such instruments can provide. Relatively new instruments that image the borehole wall with high resolution can reveal layers and faults that previously could be seen only in core sections. Downhole measurements play a crucial role in linking core data with regional geophysical surveys and in providing data where core sections could not be obtained. Examples of recent scientific applications and approaches are presented that address previous problems with data quality and changes in properties over time after a hole is drilled. The role of downhole measurements is discussed for two broad areas of research: the structure and composition of the Earth's crust, most of which is formed at mid‐ocean ridges, and past changes in Earth's environment recorded in the deep‐sea sediments overlying the crust. Finally, new emerging technologies and experiments that promise significant advantages over current methods for downhole measurements in marine geology and geophysics are discussed.
Some basic concepts of wave‐particle interactions in collisionless plasmas
Authors: Tsurutani Bruce T. , Lakhina Gurbax S.
Published: February 2004

The physical concepts of wave‐particle interactions in a collisionless plasma are developed from first principles. Using the Lorentz force, starting with the concepts of particle gyromotion, particle mirroring and the loss cone, normal and anomalous cyclotron resonant interactions, pitch angle scattering, and cross‐field diffusion are developed. To aid the reader, graphic illustrations are provided.
Turbulence and gravity waves in the vicinity of a midtropospheric warm front: A case study using VHF echo‐intensity measurements and radiosonde data
Authors: Muschinski Andreas
Published: February 2004

It is well known that VHF echo‐intensity data obtained at nonvertical beam directions can be used to retrieve the refractive‐index structure parameter C2n and that vertical profiles of C2n can be used to identify and monitor frontal structures in the free troposphere. In a case study, VHF radar observations of C2n are compared with vertical profiles of M2g (the square of the vertical gradient of the generalized potential refractive index) obtained from the data of two radiosondes launched from 4 km east of the radar site. The experiment was carried out in the Harz Mountains, north Germany, on October 18, 1993, during a high‐pressure period under clear‐sky conditions. Well‐defined maxima of C2n and M2g are found at the surface of a warm front at altitudes slightly below 5 km. At altitudes in the immediate vicinity of the front, echo‐intensity oscillations are observed, which have an amplitude of up to 9 dB and a period of about 40 min and which are attributed to gravity wave‐induced vertical displacements of the frontal surface. Vertical profiles of the outer scale of turbulence, L0, are obtained from the profiles of C2n and M2g. In the stably stratified region below the front (at altitudes between 3 and 5 km), 5–20 m is typical, while in the less stably stratified altitude region between 5 and 7 km, 20–50 m is more representative. It is found that large values of L0 are correlated with low static stability; a relationship between L0 and the Richardson number (dynamic stability), however, was not observed. A continuous temporal increase of C2n at altitudes between 5 and 7 km was observed. It is attributed to a temporal increase of the vertical variability of the humidity above the front.
SPIDR on the Web: Space Physics Interactive Data Resource on‐line analysis tool
Authors: O'Loughlin Karen Fay
Published: February 2004

The National Geophysical Data Center's innovative Space Physics Interactive Data Resource (SPIDR) is a multidisciplinary on‐line system to search, browse, and access space weather and environmental data sets over the Internet. SPIDR is a tool for the online user to select data or imagery by date and geographical location and deliver an image to the user over the World Wide Web. Currently, Defense Meteorological Satellite Program (DMSP) satellite imagery, geomagnetic variations, ionospheric vertical incidence databases, and NOAA Goestationary Operational Environmental Satellites (GOES) space environment monitor data can be accessed by SPIDR. DMSP visible, infrared, and microwave imagery displays aurora, city lights, fires, lightning strikes, and cloud coverage. Ionospheric and geomagnetic data, as well as GOES X ray, energetic particle, and the magnetic field at the spacecraft, can be plotted from interactive menus. Solar and geomagnetic indices can be generated as plots for comparison. The Web user selects the month and year from pull‐down menus and clicks the worldwide map in the region of interest. Worldwide contour maps of maximum electron density can also be generated from a global model of the ionosphere.