bluuebunny/crossref_metadata_2025 huggingface dataset:
The ozone hole over Punta Arenas, Chile
Authors: Kirchhoff V. W. J. H. , Casiccia S. C. A. R. , Zamorano B F.
Published: February 2004

Three fundamental questions are addressed for the location of Punta Arenas (53.0°S, 70.9°W), Chile: Does the Antarctic ozone hole extend over the city? If so, at what height is most of the ozone depleted? And by how much does the UVB radiation increase under ozone hole conditions? It is shown that Punta Arenas is affected by the Antarctic ozone hole, even though it is a considerable distance away from the pole. In comparison with the average global trend of −0.25% per year, the downward ozone trend at Punta Arenas is −0.5% per year (twice as large as the global average) and −1.2% per year (almost 5 times larger than the global average) using the monthly October averages. To obtain these figures, an ozone climatology of TOMS (Total Ozone Mapping Spectrometer) data from 1979 to 1992 has been used. Observations of the ozone hole over Punta Arenas are described during the austral spring periods of 1992, 1993, and 1994. Systematic observations with a Brewer spectrophotometer have been made in order to measure the vertical column of ozone and the simultaneous variations in UVB radiation. In addition, the vertical distribution of ozone has been investigated during ozone hole conditions, by using ozone ECC (Electrochemical Concentration Cell) sondes launched on balloons, in order to determine at which height the ozone is reduced most. Using the 220 DU (Dobson Units) threshold as reference, we identified the ozone hole over Punta Arenas on five occasions during the 3 year period of these observations, September 1993, October 1992, 1994; and November 1993 and 1994. The vertical distribution of ozone during ozone hole conditions shows maximum ozone destruction efficiency near 15–17 km, where ozone nearly vanishes completely, on several occasions. The largest amount of ozone depletion, however, occurs above this height, just under and at the main ozone peak, where it may reach about 50%. The UVB radiation in small spectral bands may increase dramatically during ozone hole events; at 295–297 nm the observed factors of increase for Punta Arenas were between 10 and 38. The maximum integrated/weighted intensities in October, during hole events increased over background values between 2 and 3 times, which represent values near the local summer maximum but not yet beyond levels normally observed at low‐latitude stations.
A height‐ and time‐dependent model of polar mesosphere summer echoes
Authors: Klostermeyer J.
Published: February 2004

The generation of VHF radar echoes in the polar summer mesosphere is studied with a height‐ and time‐dependent model including three mechanisms: electron scavenging by microscopic ice particles created by heterogeneous nucleation on meteoric dust, formation of steep gradients in the ice particle and electron concentrations by the Kelvin effect, and reduction of the electron diffusivity by the presence of negatively charged particles. Then turbulent mixing produces a viscous convective subrange in the variance spectrum of the electron concentration, which in turn, can give rise to strong radar echoes. Based on a gravity‐wave‐perturbed model and many sensitivity tests, several conclusions are reached. Taking into account the high variability of relevant parameters, the model can predict the wide range of reflectivities measured on the northern hemisphere and indicates that the virtual absence of detectable radar echoes on the southern hemisphere is probably due to the annual variation of the meteoric mass influx and relatively high mesopause temperatures. The frequently occurring double‐layer structure of the reflectivity is characteristic of almost unperturbed rnesospheric conditions. In the presence of weak harmonic gravity waves, the inherently strong nonlinearity of the model yields steepened and breaking reflectivity structures. In agreement with observational results, the model further predicts a positive (negative) correlation between the vertical velocity variance and the reflectivity of spectrally broad (narrow) echoes. It is suggested that the frequently measured aspect sensitivity of spectrally narrow echoes results from anisotropic turbulence in the electron gas occurring if the inertial subrange is small or absent and the viscous‐convective subrange virtually adjoins the buoyancy subrange. Even under extremely favorable conditions, the calculated reflectivities at radar frequencies near 1 GHz are much weaker than measured ones, confirming an earlier result that in the UHF range, polar mesosphwre summer echoes originate from modified Thomson scattering.
Atmospheric net transport of water vapor and latent heat across 60°S
Authors: Giovinetto Mario B. , Yamazaki Koji , Wendler Gerd , Bromwich David H.
Published: February 2004

The mean annual moisture flux across 60°S is estimated using results of numerical analyses produced by the U. S. National Centers for Environmental Prediction and the European Centre for Medium‐Range Weather Forecasts for the 7‐year period 1985–1991. The atmospheric data indicate a net poleward transport of 17.06 kg m−1 s−1 or 10.74 Tt yr−1. The mean annual moisture transport divergence for the area poleward of 60°S is estimated using a combination of surface and near‐surface data (precipitation and evaporation for the Southern Ocean, net surface accumulation and seaward drifting snow transport for the Antarctic ice sheet). The mass exchange rates at the ice sheet–atmosphere and ocean‐atmosphere interfaces are integrated strictly for the area between 60°S and 70°S and are combined with the results of a preceding surface data estimate of transport divergence for the area poleward of 70°S. The surface data are a combination of diverse sets representative of various multiyear periods distributed through 1941–1990 and indicate a poleward transport of 18.60 kg m−1 s−1 or 11.79 Tt yr−1 across 60°S. The estimates based on atmospheric and surface data show remarkable agreement (the difference is well within the error estimates) and indicate net atmospheric transports of water vapor and latent heat poleward across 60°S of 17.8 kg m−1 s−1 and 50 MJ m−1 s−1, respectively.
The atmospheric column abundance of IO: Implications for stratospheric ozone
Authors: Wennberg P. O. , Brault J. W. , Hanisco T. F. , Salawitch R. J. , Mount George H.
Published: February 2004

Absorption attributable to atmospheric IO is observed in high‐resolution, high air mass solar spectra taken at the National Solar Observatory, Kitt Peak, Arizona, in March 1995. These observations, together with cross sections measured in the laboratory for the IO {A2Π3/2←X2Π3/2 (2,0)} rotationally resolved electronic transition, are consistent with a total stratospheric iodine mixing ratio of 0.2 (+0.3 −0.2) parts per trillion by volume. This result, combined with recent laboratory measurements of the rate of the reactions of IO with other halogen species, suggests that iodine chemistry is not responsible for the reductions observed in lower stratospheric ozone during the last several decades.
Laboratory investigation of the catalytic reduction technique for measurement of atmospheric NOy
Authors: Kliner Dahv A. V. , Daube Bruce C. , Burley Joel D. , Wofsy Steven C.
Published: February 2004

We report laboratory studies of the detection scheme employed for in situ measurement of NOy in the atmosphere. In this technique, an air stream is passed over a hot metal (usually 24 karat (k) Au) catalyst in the presence of a reducing agent (usually CO), which converts the NOx compounds to NOx Using the NOy species NO, NO2, HNO3, and isopropyl nitrate and the potential interferences HCN, CH3CN, NH3, and N2O, we investigated: (1) conversion efficiencies as a function of pressure and catalyst temperature; (2) conversion efficiencies as a function of reducing‐agent concentration with both H2 and CO; (3) the effect of humidity and O3 on conversion efficiency; (4) loss of NO in the catalyst; and (5) the efficacy and suitability as catalytic converters (or inlets) of several metals (24 k Au, 18k Au, Au with 1% Co, Ag, Pt, stainless steel) and quartz. The most significant results are the discovery of a gas‐phase process that contributes to the conversion of HNO3 to NO and the identification of conditions under which HCN, CH3CN, and NH3 are converted to NO with high efficiency. We discuss the implications of these results for in situ measurement of atmospheric NOy.
Aircraft encounters with strong coherent vortices over the boreal forest
Authors: MacPherson J. Ian , Betts Alan K.
Published: February 2004

A number of intense low‐level vortices were encountered at a height of 30 m over the boreal forest by a Twin Otter atmospheric research aircraft. They imposed strong vertical and lateral accelerations on this aircraft: the most energetic had an updraft speed in excess of 11 m s−1, only 20 m above the forest. All were associated with large sensible heat fluxes, low wind speeds, and deep boundary layers.
Absolute electron impact ionization cross sections for N2O and NO from threshold up to 1000 eV
Authors: Iga I. , Rao M. V. V. S. , Srivastava S. K.
Published: February 2004

Cross sections for the production of singly charged ions by electron impact on N2O and NO have been obtained by utilizing the relative flow technique in which the accurately known values of cross sections for the production of singly charged ions of rare gases have been used for normalization. By summing the cross sections for direct ionization and dissociative ionization, the total ionization cross sections have been obtained and compared with the previously available data. Energies for the appearance of various ions have also been measured and compared with previous determinations from photoelectron spectroscopy.
Low‐temperature space observatories: The effect of realistic surface properties
Authors: Blake R. P. , Jones B. W.
Published: February 2004

Space‐based infrared telescopes will be of great importance in the search for extrasolar planets, and in their subsequent investigation. Such telescopes benefit enormously from radiative cooling. We have investigated this cooling with the aid of a Monte‐Carlo model of a cylindrical telescope tube shielded from the Sun by two flat plates. Our primary concern has been to establish to what extent radiative exchanges have to be modeled more accurately than has been the case up to now. We have shown that the equilibrium temperatures depend significantly on the directionality of the radiative properties of the materials coating the tube and shields. Also significant is the polarization of the radiation exchanged between the tube and shields, the change of radiative properties with wavelength and temperature, and the spacing between the shields and tube. There is a need for further measurements of infrared radiative properties at cryogenic temperatures, and we are planning to make some.
Star clusters: Optimal targets for a photometric planetary search program
Authors: Janes Kenneth
Published: February 2004

By targeting stars in some of the most populous open star clusters, the stringent requirements of a photometric search for planets as small as the Earth, i.e., that large numbers of stars must be monitored frequently and with extremely high precision, can be achieved in a ground‐based observing program. Data for the cluster NGC 188 confirms previous studies that even with a telescope of modest aperture (0.9 m), high‐precision relative photometry of moderately bright stars is possible; even under marginal conditions of thin clouds and variable seeing, high‐frequency atmospheric fluctuations (scintillation) can be largely corrected for. By continuously monitoring the many hundreds of stars in the richest old clusters simultaneously in two colors, it should also be possible to model explicitly the slower variations in atmospheric extinction that could mask the small light variations caused by transit events. Furthermore, the stars selected will automatically be main sequence stars of known age and distance; there are several old open clusters with ages of billions of years that together contain several thousand stars of solar mass or less and which are virtually identical to the Sun in chemical composition. An effective search program will require one or more telescopes of at least 4 m in aperture dedicated to the project for a period of several years. Even very large ground‐based telescopes can be built for a small fraction of the cost of a space mission and would be usable for other projects at the end of the search period. By targeting four of the “best” of the old open clusters, the project could uncover as many as 25 planetary systems in a few years of observing, or at least set meaningful upper limits on the frequency of planetary systems. Furthermore, the same data set will also be useful to investigate stellar surface activity as a function of stellar age, to derive precise parallaxes of the target clusters (which will help to set the cosmic distance scale) and possibly to conduct asteroseismological studies. The data will also likely lead to the discovery of serendipitous events such as cosmologically distant supernova explosions along the line of sight.
Surface deformation and coherence measurements of Kilauea Volcano, Hawaii, from SIR‐C radar interferometry
Authors: Rosen P. A. , Hensley S. , Zebker H. A. , Webb F. H. , Fielding E. J.
Published: February 2004

The shuttle imaging radar C/X synthetic aperture radar (SIR‐C/X‐SAR) radar on board the space shuttle Endeavor imaged Kilauea Volcano, Hawaii, in April and October 1994 for the purpose of measuring active surface deformation by the methods of repeat‐pass differential radar interferometry. Observations at 24 cm (L band) and 5.6 cm (C band) wavelengths were reduced to interferograms showing apparent surface deformation over the 6‐month interval and over a succession of 1‐day intervals in October. A statistically significant local phase signature in the 6‐month interferogram is coincident with the Pu'u O'o lava vent. Interpreted as deformation, the signal implies centimeter‐scale deflation in an area several kilometers wide surrounding the vent. Peak deflation is roughly 14 cm if the deformation is purely vertical, centered southward of the Pu'u O'o caldera. Delays in the radar signal phase induced by atmospheric refractivity anomalies introduce spurious apparent deformation signatures, at the level of 12 cm peak‐to‐peak in the radar line‐of‐sight direction. Though the phase observations are suggestive of the wide‐area deformation measured by Global Positioning System (GPS) methods, the atmospheric effects are large enough to limit the interpretation of the result. It is difficult to characterize centimeter‐scale deformations spatially distributed over tens of kilometers using differential interferometry without supporting simultaneous, spatially distributed measurements of refractivity along the radar line of sight. Studies of the interferometric correlation of images acquired at different times show that L band is far superior to C band in the vegetated areas, even when the observations are separated by only 1 day. These results imply longer wavelength instruments are more appropriate for studying surfaces by repeat‐pass observations.
The Pathfinder Microrover
Authors:   The Rover Team
Published: February 2004

An exciting scientific component of the Pathfinder mission is the rover, which will act as a mini‐field geologist by providing us with access to samples for chemical analyses and close‐up images of the Martian surface, performing active experiments to modify the surface and study the results, and exploring the landing site area. Structures, textures, and fabrics revealed in the rover camera images will provide a bounty of geologic information, which can be put in the larger context of geologic features seen in lander camera images. Rover technology experiments will improve our understanding of “soil” properties (grain size, bulk density, friction angle, cohesion, and compressibility) by contributing information not provided by the other Pathfinder instruments. Unplanned opportunities and activities are also likely to provide valuable science information, such as examination of any slumped or eroded soil that has been piled up by the rover wheels, and examination of overturned rocks. Using its ability to actively explore and experiment, the rover will fill in key gaps in our understanding of the landing site.
A radar study of the Chryse region, Mars
Authors: Harmon J. K.
Published: February 2004

Arecibo 12.6‐cm delay‐Doppler measurements are used to study the topography and radar scattering properties of the Chryse region, including the Viking 1 and Mars Pathfinder lander sites and environs. The Viking 1 lander lies on a gentle (0.13°) east facing slope at a 6.1‐mbar altitude of −1.35 km. The Mars Pathfinder lander site lies slightly lower (−1.7 km) on a flat, low‐relief plain. Quasi‐specular roughness across Chryse is considerably higher than the Martian average and shows some strong spatial variation not apparent from older CW observations. Some of the rougher regions coincide with the distal ends of the Tiu and Ares Vallis channels, while another is located just to the northeast of the Viking 1 lander. Chryse shows asymmetric scattering (skew) similar to that seen in Mars' southern hemisphere and in recent Magellan radar observations of Venus. Comparison of 12.6‐cm and 3.5‐cm data indicates that Chryse's quasi‐specular reflectivity is strongly wavelength dependent (being higher at the longer wavelength), suggesting a rapid density increase within the top decimeter of the regolith. Depolarized reflectivity images of the general Chryse area show modest enhancements (from decimeter‐scale roughness or rocks) associated with Maja and Kasei Valles and the plateau bordering Simud Vallis. These images indicate that the rock abundance at the proposed Mars Pathfinder lander site is similar to that at the Viking lander sites.
Effect of impact angle on vaporization
Authors: Schultz Peter H.
Published: February 2004

Impacts into easily vaporized targets such as dry ice and carbonates generate a rapidly expanding vapor cloud. Laboratory experiments performed in a tenuous atmosphere allow deriving the internal energy of this cloud through well‐established and tested theoretical descriptions. A second set of experiments under near‐vacuum conditions provides a second measure of energy as the internal energy converts to kinetic energy of expansion. The resulting data allow deriving the vaporized mass as a function of impact angle and velocity. Although peak shock pressures decrease with decreasing impact angle (referenced to horizontal), the amount of impact‐generated vapor is found to increase and is derived from the upper surface. Moreover, the temperature of the vapor cloud appears to decrease with decreasing angle. These unexpected results are proposed to reflect the increasing roles of shear heating and downrange hypervelocity ricochet impacts created during oblique impacts. The shallow provenance, low temperature, and trajectory of such vapor have implications for larger‐scale events, including enhancement of atmospheric and biospheric stress by oblique terrestrial impacts and impact recycling of the early atmosphere of Mars.
Catastrophic flood sediments in Chryse Basin, Mars, and Quincy Basin, Washington: Application of sandar facies model
Authors: Rice James W. , Edgett Kenneth S.
Published: February 2004

Viking visible and thermal infrared observations and terrestrial catastrophic flood deposits provide clues to identify the outflow channel sediments that went into Chryse Basin on Mars. On Earth, sandar (outwash plains formed by coalescence of many jökulhlaup floods) are described in terms of three laterally adjacent facies: proximal, midfan, and distal. The Missoula Flood sediments deposited in Quincy Basin, Washington, comprise a miniature analog of Chryse Basin. The terminology and general characteristics of the sandar facies model are applied to Quincy Basin, although the depositional environment and clast sizes are somewhat different (higher‐energy flood, larger clasts, subaqueous rather than subaerial deposition). For example, the Ephrata Fan (a deposit of boulders, cobbles, and pebbles) forms the midfan facies analog; a downfan sandy deposit (reworked into a dune field) comprises the distal facies analog. In Chryse Basin the midfan is defined by a heterogeneous rocky (0–25%), intermediate‐albedo (0.21–0.26), intermediate thermal inertia (260–460 J m−2 s−0.5 K−1) surface, while the distal facies has a low albedo (0.14–0.16) and higher thermal inertia (340–700 J m−2 s−0.5 K−1). The Chryse midfan unit has rocks and windblown dust exposed at the surface. The sand of the distal facies in Chryse/Acidalia is reworked by the wind, as in Quincy Basin. The Viking 1 and Mars Pathfinder landing sites are located on the midfan unit. Observations that can be made at the Mars Pathfinder site might help in reevaluating whether or not Viking 1 landed on outflow channel sediments.
Sedimentary history and mass flow structures of Chryse and Acidalia Planitiae, Mars
Authors: Tanaka Kenneth L.
Published: February 2004

Geologic mapping and crater counting in Chryse and Acidalia Planitiae (CAP) reveal five major sedimentary deposits of Hesperian to Early Amazonian age, including (1) a mass flow deposited during the Early Hesperian near Deuteronilus Mensae (northeast of the map region) that may have resulted from the carving of Kasei Valles, >3000 km southwest of the exposed part of the deposit; (2) knobby plains material consisting of channel (likely from Simud and Tiu Valles and possibly Ares and Shalbatana Valles) and mass‐wasting deposits in central and eastern CAP; (3) material largely from Maja and Ares Valles emplaced in at least western and southern CAP (outcrops in southern Chryse Planitia developed thermokarst); (4) a thin mass flow covering much of southern Chryse Planitia that emanated from Simud and Tiu Valles; and (5) a thick, extensive (perhaps >3500 km across) mass flow deposit in central and northern CAP derived from accumulation and backflow of the preceding thin mass flow or perhaps melting of polar deposits. Other possible deposits may not be recognizable owing to burial by younger materials or a lack of morphologic signature. Various associated landforms appear to be consistent with the mass flow interpretations, including lobate and linear scarps along deposit edges, fractures related to desiccation of thick sediments, troughs, and ridges near the edges of the deposit indicative of secondary mass movement and deformation, pitted domes and fissure‐fed flows possibly formed by sedimentary (mud) eruptions, and longitudinal channel grooves perhaps formed by roller vortices. No convincing evidence for paleoshorelines or stagnant ice sheets is found in CAP. These findings suggest that mass flow and hyperconcentrated flooding may have been the predominant processes of outflow‐channel dissection in CAP. Elsewhere in the northern plains, similar landforms are prevalent. The mass flow interpretation does not require either multiple episodes of extraordinarily high water‐discharge rates achieved by freeing huge volumes of water from the crust, repetitive recycling of immense volumes of water into highland aquifers at the heads of Chryse channels, or profound climate change. Mars Pathfinder will most likely land on and inspect the surface of the thin mass flow that originated from the canyons of Simud and Tiu Valles.
Thermal conductivity measurements of particulate materials 1. A review
Authors: Presley Marsha A. , Christensen Philip R.
Published: February 2004

Discussion of the thermal conductivity of particulate materials is dispersed over several decades and a wide range of disciplines. In addition, there is some disparity among the reported values. This paper presents a review of the methodology available for the study of thermal conductivity of particulate materials, with an emphasis on low atmospheric pressures, and an assessment of the dependability of the data previously reported. Both steady state and nonsteady state methods of thermal conductivity measurement are reviewed, delineating the advantages, disadvantages, and sources of error for each. Nonsteady state methods generally are simpler and more efficient. The transient hot wire and differentiated line‐heat source are the preferred methods for the laboratory. These methods are better suited for small samples and short measurement times and are therefore the best methods to use for a series of comprehensive studies. Results of previous studies are presented, compared, and evaluated. A good way to assess the relative accuracy is to compare the values of thermal conductivity versus atmospheric pressure obtained from several experimenters. The lowest values of thermal conductivity at vacuum and very low atmospheric pressure, and the steepest slopes on the thermal conductivity versus atmospheric pressure curves, are indicative of the most accurate data. Previous thermal conductivity studies have shown that the thermal conductivity of particulate materials increases with increasing atmospheric pressure, with increasing particle size, and with increasing bulk density of the material. At vacuum, the thermal conductivity of particulate materials is proportional to the cube of the temperature. The temperature dependence of thermal conductivity is much less obvious at higher atmospheric pressures.
Metal‐silicate fractionation in the surface dust layers of accreting planetesimals: Implications for the formation of ordinary chondrites and the nature of asteroid surfaces
Authors: Huang Shaoxiong , Akridge Glen , Sears Derek W. G.
Published: February 2004

Some of the most primitive solar system materials available for study in the laboratory are the ordinary chondrites, the largest meteorite class. The size and distribution of the chondrules (silicate beads) and metal, which leads to the definition of the H, L, and LL classes, suggest sorting before or during aggregation. We suggest that meteorite parent bodies (probably asteroids) had thick dusty surfaces during their early evolution that were easily mobilized by gases evolving from their interiors. Density and size sorting would have occurred in the surface layers as the upward drag forces of the gases (mainly water) acted against the downward force of gravity. The process is analogous to the industrially important process of fluidization and sorting in pyroclastic volcanics. We calculate that gas flow velocities and gas fluxes for the regolith of an asteroid‐sized object heated by the impact of accreting objects or by 26Al would have been sufficient for fluidization. It can also explain, quantitatively in some cases, the observed metal‐silicate sorting of ordinary chondrites, which has long been ascribed to processes occurring in the primordial solar nebula. Formation of the chondrites in the thick dynamic regolith is consistent with the major properties of chondritic meteorites (i.e., redox state, petrologic type, cooling rate, matrix abundance). These ideas have implications for the nature of asteroid surfaces and the virtual lack of asteroids with ordinary chondrite‐like surfaces.
Titan's ion exosphere wake: A natural ion mass spectrometer?
Authors: Luhmann J. G.
Published: February 2004

According to one possible picture of the Titan atmosphere interaction with Saturn's magnetosphere, newborn exospheric pickup ions may exhibit a marked asymmetry with respect to the corotation wake due to their large gyroradii in the local ∼5 nT magnetic field. These finite gyroradius effects should persist until the ions are scattered by wave‐particle interactions (a scale length that is presently unknown). To the extent that simple test‐particle‐like motion is maintained, the combination of the small (on the scale of most ion gyroradii) size of Titan and the small, steady local magnetic field at ∼20 Saturn Radii provides a natural ion mass spectrometer configuration. The resulting spatial dispersion of the pickup ion trajectories can be used for both analyzing the composition of the Titan exosphere and determining the ionization rates of its various constituents. Possible evidence of this ion mass spectrometer effect may be visible as a magnetic field perturbation in the Voyager 1 Titan flyby data.
The contribution by interplanetary dust to noble gases in the atmosphere of Mars
Authors: Flynn G. J.
Published: February 2004

Interplanetary dust particles accumulate significant concentrations of noble gases, implanted by the solar wind and solar energetic particles, while they are in space. When these dust particles enter the atmosphere of a planet they are heated and decelerated, depositing some of the implanted noble gases directly into the atmosphere and, if the particle survives atmospheric entry, emplacing the remainder on the surface of the planet, where it can be released by particle decomposition or by episodic surface heating. Because of the low mass of the atmosphere of Mars, compared to the atmospheres of Venus and Earth, the atmosphere of Mars is more severely influenced by the addition of noble gases by interplanetary dust. The contribution of noble gases to Mars has been modeled using estimates of the interplanetary dust flux at Mars and measurements of the noble gas contents of interplanetary dust collected from the atmosphere of the Earth. Over the past 3.6 b.y., interplanetary dust particles have contributed quantities of 3He, 4He, 20Ne and 22Ne comparable to the current total atmospheric inventories of these isotopes. In the present era, the rates of addition of 3He, 20Ne, 22Ne, 36Ar, and 38Ar to the atmosphere of Mars by interplanetary dust range from about 2% to 20% of the modeled rates of addition by planetary outgassing [Pepin, 1994; Krasnopolsky and Gladstone, 1996], and the isotopic compositions of the He and Ne added by interplanetary dust are distinctly different from those assumed for planetary outgassing: 3He/4He ∼2.8×10−4 in the interplanetary dust [Nier and Schlutter, 1992] versus almost pure 4He for outgassing of U and Th decay products, and 20Ne/22Ne ∼11.8 in the interplanetary dust [Niert 1994] versus 20Ne/22Ne=13.7 assumed for planetary outgassing in Pepin's [1994] model. Since the actual outgassing rate of Mars in the present era is not well constrained by observations, the contributions of Ne and Ar to the atmosphere of Mars by the interplanetary dust must serve, at least, as a lower limit on the current sources of these noble gases.
Trace element geochemistry of Martian iddingsite in the Lafayette meteorite
Authors: Treiman Allan H. , Lindstrom David J.
Published: February 2004

The Lafayette meteorite contains abundant iddingsite, a fine‐grained intergrowth of smectite clay, ferrihydrite, and ionic salt minerals. Both the meteorite and iddingsite formed on Mars. Samples of iddingsite, olivine, and augite pyroxene were extracted from Lafayette and analyzed for trace elements by instrumental neutron activation. Our results are comparable to independent analyses by electron and ion microbeam methods. Abundances of most elements in the iddingsite do not covary significantly. The iddingsite is extremely rich in Hg, which is probably terrestrial contamination. For the elements Si, Al, Fe, Mn, Ni, Co, and Zn, the composition of the iddingsite is close to a mixture of ∼50% Lafayette olivine + ∼40% Lafayette siliceous glass + ∼10% water. Concordant behavior among these elements is not compatible with element fractionations between smectite and water, but the hydrous nature and petrographic setting of the iddingsite clearly suggest an aqueous origin. These inferences are both consistent, however, with deposition of the iddingsite originally as a silicate gel, which then crystallized (neoformed) nearly isochemically. The iddingsite contains significantly more magnesium than inched by the model, which may suggest that the altering solutions were rich in Mg2+.
Mars surface mineralogy from Hubble Space Telescope imaging during 1994–1995: Observations, calibration, and initial results
Authors: Bell James F. , Wolff Michael J. , James Philip B. , Clancy R. Todd , Lee Steven W. , Martin Leonard J.
Published: February 2004

Visible to near‐infrared observations of Mars were made with the Hubble Space Telescope (HST) during 1994–1995 with the goals of monitoring seasonal variability of the surface and atmosphere and mapping specific spectral units to constrain the planet's surface mineralogy. This paper presents the details of the collection and calibration of the data, concentrating specifically on the near‐IR data that were obtained exclusively for the surface mineralogy aspect of our HST Mars observing program. We also present some initial results from the calibrated data set. Our calibration procedures included the standard “pipeline” processing steps, supplemented by special procedures required for use with the linear ramp filters on the Wide Field/Planetary Camera 2 instrument, and an additional point spread function deconvolution procedure applied in order to realize the full potential spatial resolution of the images (23 to 64 km/pixel between August 1994 and August 1995). The calibration results in a set of images projected onto a standard map grid and presented in radiance factor (I/F) units, having an estimated ≈5% photometric accuracy based on the performance of HST and comparisons with previous ground‐based and spacecraft Mars spectra. Initial scientific analyses of these data reveal (1) distinct red/blue color units within the classical bright regions, similar to those seen in Viking Orbiter images and possibly related to variations in nanophase and/or crystalline ferric mineral abundance; (2) near‐IR spectral slope variations correlated with albedo on a large scale (darker is “bluer” near‐IR slope) but exhibiting wider variations among many of the small‐scale features visible in the data; (3) an absorption at 860 nm that occurs in all regions but which is 3 to 5% stronger in many of the classical dark regions than in the bright regions, possibly because of a greater abundance of a well‐crystalline ferric phase like hematite or a very low Ca pyroxene or opx/cpx mixture; and (4) an absorption from pyroxene at 953 nm with a band depth that is inversely correlated with albedo (bright regions 0 to 5% deep; dark regions 7 to 15% deep) and which shows the highest band depth values in individual craters, calderas, and other small geologic units that are resolved in the images.
Absorption and scattering properties of the Martian dust in the solar wavelengths
Authors: Ockert‐Bell Maureen E. , Bell James F. , Pollack James B. , McKay Christopher P. , Forget François
Published: February 2004

A new wavelength‐dependent model of the single‐scattering properties of the Martian dust is presented. The model encompasses the solar wavelengths (0.3 to 4.3 μm at 0.02 μm resolution) and does not assume a particular mineralogical composition of the particles. We use the particle size distribution, shape, and single‐scattering properties at Viking Lander wavelengths presented by Pollack et al. [1995]. We expand the wavelength range of the aerosol model by assuming that the atmospheric dust complex index of refraction is the same as that of dust particles in the bright surface geologic units. The new wavelength‐dependent model is compared to observations taken by the Viking Orbiter Infrared Thermal Mapper solar channel instrument during two dust storms. The model accurately matches afternoon observations and some morning observations. Some of the early morning observations are much brighter than the model results. The increased reflectance can be ascribed to the formation of a water ice shell around the dust particles, thus creating the water ice clouds which Colburn et al. [1989], among others, have predicted.
Late Paleocene Arctic Ocean shallow‐marine temperatures from mollusc stable isotopes
Authors: Bice Karen L. , Arthur Michael A. , Marincovich Louie
Published: February 2004

Late Paleocene high‐latitude (80°N) Arctic Ocean shallow‐marine temperatures are estimated from molluscan δ18O time series. Sampling of individual growth increments of two specimens of the bivalve Camptochlamys alaskensis provides a high‐resolution record of shell stable isotope composition. The heavy carbon isotopic values of the specimens support a late Paleocene age for the youngest marine beds of the Prince Creek Formation exposed near Ocean Point, Alaska. The oxygen isotopic composition of regional freshwater runoff is estimated from the mean δ18O value of two freshwater bivalves collected from approximately coeval fluviatile beds. Over a 30 – 34‰ range of salinity, values assumed to represent the tolerance of C. alaskensis, the mean annual shallow‐marine temperature recorded by these individuals is between 11° and 22°C. These values could represent maximum estimates of the mean annual temperature because of a possible warm‐month bias imposed on the average δ18O value by slowing or cessation of growth in winter months. The amplitude of the molluscan δ18O time series probably records most of the seasonality in shallow‐marine temperature. The annual temperature range indicated is approximately 6°C, suggesting very moderate high‐latitude marine temperature seasonality during the late Paleocene. On the basis of analogy with modern Chlamys species, C. alaskensis probably inhabited water depths of 30–50 m. The seasonal temperature range derived from δ18O is therefore likely to be damped relative to the full range of annual sea surface temperatures. High‐resolution sampling of molluscan shell material across inferred growth bands represents an important proxy record of seasonality of marine and freshwater conditions applicable at any latitude. If applied to other regions and time periods, the approach used here would contribute substantially to the paleoclimate record of seasonality.
Deficiencies in the diatom evidence for a Pliocene reduction of the East Antarctic Ice Sheet
Authors: Burckle Lloyd H. , Stroeven Arjen P. , Bronge Christian , Miller Urve , Wasell Anders
Published: February 2004

Evidence for Pliocene reduction of the East Antarctic Ice Sheet (EAIS) has been inferred from the presence of Pliocene planktic marine diatoms in tills of the Sirius Group now exposed along the slopes of the Transantarctic Mountains. We highlight deficiencies in this diatom evidence for a Pliocene reduction of the EAIS. First, we examine what diatom habitats are left behind by a retreating ice sheet, in this case the Fennoscandian Ice Sheet (FIS), and find that diatoms should occur in planktic and benthic marine, brackish water, freshwater, and terrestrial habitats. We expect that during former reexpansions, these diverse diatom assemblages were picked up by the advancing FIS and deposited in tills. This is shown to be the case, and diatoms found in tills from Sweden and Finland reflect the many habitats over which the FIS passed. If there was a reduction of the EAIS during the Pliocene followed by renewed glaciation, diatoms from many different habitats should be found in the resulting tills of the Sirius Group. This is not the case. Only marine planktic diatoms and a few freshwater forms are reported. Similarly, vertical (downsection) distribution of diatoms in Sirius Group and Fennoscandian tills differ. While in the Sirius Group, diatoms appear restricted to surface and near‐surface sediments, in Fennoscandia they are found throughout the tills. Such data do not support a Pliocene reduction model for the EAIS.
Sea surface temperature changes in the southern California borderlands during the last glacial‐Interglacial cycle
Authors: Mortyn P. Graham , Thunell Robert C. , Anderson David M. , Stott Lowell D. , Le Jianning
Published: February 2004

A variety of evidence suggests that average sea surface temperatures (SSTs) during the last glacial maximum in the California Borderlands region were significantly colder than during the Holocene. Planktonic foraminiferal δ18O evidence and average SST estimates derived by the modern analog technique indicate that temperatures were 6°–10°C cooler during the last glacial relative to the present. The glacial plankton assemblage is dominated by the planktonic foraminifer Neogloboquadrina pachyderma (sinistral coiling) and the coccolith Coccolithus pelagicus, both of which are currently restricted to subpolar regions of the North Pacific. The glacial‐interglacial average SST change determined in this study is considerably larger than the 2°C change estimated by Climate: Long‐Range Investigation, Mapping, and Prediction (CLIMAP) [1981]. We propose that a strengthened California Current flow was associated with the advance of subpolar surface waters into the Borderlands region during the last glacial.
Application of artificial neural networks to chemostratigraphy
Authors: Malmgren Björn A. , Nordlund Ulf
Published: February 2004

Artificial neural networks, a branch of artificial intelligence, are computer systems formed by a number of simple, highly interconnected processing units that have the ability to learn a set of target vectors from a set of associated input signals. Neural networks learn by self‐adjusting a set of parameters, using some pertinent algorithm to minimize the error between the desired output and network output. We explore the potential of this approach in solving a problem involving classification of geochemical data. The data, taken from the literature, are derived from four late Quaternary zones of volcanic ash of basaltic and rhyolithic origin from the Norwegian Sea. These ash layers span the oxygen isotope zones 1, 5, 7, and 11, respectively (last 420,000 years). The data consist of nine geochemical variables (oxides) determined in each of 183 samples. We employed a three‐layer back propagation neural network to assess its efficiency to optimally differentiate samples from the four ash zones on the basis of their geochemical composition. For comparison, three statistical pattern recognition techniques, linear discriminant analysis, the k‐nearest neighbor (k‐NN) technique, and SIMCA (soft independent modeling of class analogy), were applied to the same data. All of these showed considerably higher error rates than the artificial neural network, indicating that the back propagation network was indeed more powerful in correctly classifying the ash particles to the appropriate zone on the basis of their geochemical composition.
Synchronous, high‐frequency oscillations in tropical sea surface temperatures and North Atlantic Deep Water production during the Last Glacial Cycle
Authors: Curry W. B. , Oppo D. W.
Published: February 2004

Stable isotopic measurements of G. sacculifer and C. wuellerstorfi in a core from the western equatorial Atlantic imply that there are parallel, suborbital oscillations in surface water hydrography and deep water circulation occurring during oxygen isotope stages 2 and 3. Low values of G. sacculifer δ18O accompany high values of C. wuellerstorfi δ13C, linking warmer sea surface temperatures (SSTs) in the tropics with increased production of lower North Atlantic Deep Water (NADW). The amplitude of the δ18O oscillations is 0.6‰ (or 2°–3°C), which is superimposed on a glacial/interglacial amplitude of about 2.1‰. Using the G. sacculifer δ18O data, we calculate that surface waters were colder during stage 2 than calculated by CLIMAP [1976, 1981]. The longer‐period (>2 kyr) oscillations in air temperature recorded in the Greenland and Antarctic ice cores appear to correlate with oscillations in sea surface temperature in the equatorial Atlantic. The magnitude of these oscillations in tropical SST is too large to have resulted from changes in meridional heat transport caused by the global conveyor alone. The apparent synchroneity of equatorial SST and polar air temperature changes, as well as the amplitude of the SST changes at the equator, are consistent with the climate effects expected from changes in the atmosphere's greenhouse gas content (H2Ovapor, CO2, and CH4).
Neogloboquadrina pachyderma (sinistral coiling) as paleoceanographic tracers in polar oceans: Evidence from northeast water polynya plankton tows, sediment traps, and surface sediments
Authors: Kohfeld Karen E. , Fairbanks Richard G. , Smith Sharon L. , Walsh Ian D.
Published: February 2004

The abundance and chemistry of the planktonic foraminifera Neogloboquadrina pachyderma (sinistral coiling) have long been used as tools for monitoring polar surface ocean changes and for correlating these changes to atmospheric and thermohaline circulation fluctuations. However, due to its remote habitat, very little is known about how modern N. pachyderma (s.) respond to changing environmental conditions in the polar seas. Modern samples of N. pachyderma (s.) from the Northeast Water Polynya provide a means for studying how environmental conditions affect the vertical distribution and chemistry of this species. Highest abundances of N. pachyderma (s.) were associated with the chlorophyll maximum in the surface 20–80 m, where they are exploiting their primary food source. Evidence suggests that the addition of a calcite crust modifies the calcite tests of some N. pachyderma (s.) between 50 and 200 m, increasing shell density and modifying shell chemistry. The shell mass of encrusted forms is 3–4 times greater than the nonencrusted forms between 50 and 200 m. The oxygen isotope composition of N. pachyderma (s.) shells increase by 1.5‰ in response to local water column gradients. The δ13C values of N. pachyderma (s.) are basically invariant with depth in this region, are consistently 1.0‰ depleted in comparison with the δ13C for equilibrium calcite, and remain basically constant during the shell‐thickening process. Mass balance calculations suggest that encrustation occurs at all depths, but abundance counts suggest that the process occurs mostly at the depth of the main pycnocline. Sediment fluxes of N. pachyderma (s.) occur during a 2‐week bloom event and decrease to almost zero below complete ice cover. The decoupling of the processes controlling abundances and shell chemistry explain the discrepancies between transfer function and isotopically derived paleotemperature estimates of surface conditions, in some oceanic settings. The ability of δ18O to record surface ocean conditions will depend on vertical water column gradients, as evidenced by the differences in core‐top calibrations between the North and South Atlantic.
The Younger Dryas Termination and North Atlantic Deep Water Formation: Insights from climate model simulations and Greenland Ice Cores
Authors: Fawcett Peter J. , Ágústsdóttir Anna Maria , Alley Richard B. , Shuman Christopher A.
Published: February 2004

Results from the GISP2 and GRIP ice cores show that the termination of the Younger Dryas (YD) climate event in Greenland was a large and extremely fast climate change. A reinitiation of North Atlantic Deep Water formation following a shutdown, and its associated winter release of heat to the atmosphere, has been suggested as the most likely cause of this climate transition. To test this idea, two general circulation model experiments using GENESIS have been completed for YD time (12,000 calendar years ago): one with low heat flux in the Nordic Seas (10 W/m², deep water shutdown) and one with high Nordic Sea heat flux (300 W/m², active deep water formation). Comparison of Greenland climate differences between these experiments with the ice core records shows that when deep water is turned on, much of the YD termination warming is achieved. The increase in precipitation is underestimated because of a model tendency to overestimate summertime precipitation, which obscures the dominantly wintertime response to the specified forcing. The winter storm track shift toward Greenland contributes much of the climate change at the YD termination.