bluuebunny/crossref_metadata_2025 huggingface dataset:
Descent of long‐lived trace gases in the winter polar vortex
Authors: Bacmeister Julio T. , Schoeberl Mark R. , Summers Michael E. , Rosenfield Joan R. , Zhu Xun
Published: February 2004

Recent observations of CH4 and HF from the UARS Halogen Limb Occultation Experiment (HALOE) suggest that vigorous descent occurs within the polar winter vortex with “mesospheric” values of CH4 evident down to 30 mbar. This study shows that a highly accurate two‐dimensional model advection scheme coupled with a modern radiation scheme, parameterized planetary and gravity wave drag algorithms can produce tracer distributions consistent with HALOE observations. The modeled tracer distribution within the polar vortex is found to be principally dependent on the strength of dynamical drag in the middle atmosphere and the strength of the planetary wave forcing. However, the strong downward transport of tracers at the poles during winter can be disrupted in midwinter by planetary wave mixing. Thus the weaker planetary wave forcing in the southern hemisphere winter allows for a more coherent descent of long‐lived tracers from the mesosphere than during the northern hemisphere winter. Multiple‐year integrations of the model reveal a general circulation of the stratosphere which lofts tracers to mesospheric altitudes. Material removed from the mesosphere returns to the stratosphere principally within the polar regions. Upward vertical transport of material is found to be enhanced by horizontal planetary wave mixing.
Climatology of a midlatitude mesopause region observed by a lidar at Fort Collins, Colorado (40.6°N, 105°W)
Authors: Yu J. R. , She C. Y.
Published: February 2004

On the basis of 147 nights of quality lidar data over a period of 3 years the thermal structure of the mesopause region over Fort Collins, Colorado (40.6°N, 105°W), has been constructed. False color contour plots depicting seasonal temperature variations are given. Monthly mean temperatures between 81 and 105 km are tabulated, from which a cool mean summer mesopause of 178 K at 86 km and a warm mean winter mesopause of 195 K at 101 km are determined. The mean temperature profiles exhibit double temperature minima in the mesopause region in spring and fall, but they are less obvious in summer and winter. Seasonal variations of the associated temperature maximum show a minimum in temperature (196 K) and a maximum in altitude (95 km) a month before summer solstice. These results elucidate the heating mechanisms in the mesopause region. Harmonic analysis of the temperature profiles identifies a node at 99 km in the annual amplitude, below which point an out‐of‐phase evolution between temperature and mean solar flux forcing is observed, suggesting a dynamics‐dominated mesopause.
Latitudinal distribution of black carbon soot in the upper troposphere and lower stratosphere
Authors: Blake David F. , Kato Katharine
Published: February 2004

Black carbon soot from the upper troposphere and lower stratosphere has been systematically collected at latitudes from 90°N to 45°S. The measured latitudinal distribution of this soot at 10‐ to 11‐km altitude is found to covary with commercial air traffic fuel use, suggesting that aircraft fuel combustion at altitude is the principal source. In addition, at latitudes where the commercial air traffic is high, measured black carbon soot values are high even at 20‐km altitude, suggesting that aircraft‐generated soot injected just above the tropopause may be transported to higher altitudes. During the volcanically influenced period in which these samples were collected, the number abundances, total mass, and calculated total surface area of black carbon soot are 2–3 orders of magnitude lower than similar measures of sulfuric acid aerosol. During volcanically quiescent periods, the calculated total surface area of black carbon soot aerosol is of the same order of magnitude as that of the background sulfuric acid aerosol. It appears from this comparison that black carbon soot is only capable of influencing lower stratosphere or upper troposphere chemistry during periods when the aerosol budget is not dominated by volcanic activity. It remains to determine the extent to which black carbon soot particles act as nuclei for sulfuric acid aerosol formation. However, mass balance calculations suggest that aircraft soot injected at altitude does not represent a significant source of condensation nuclei for sulfuric acid aerosols.
Seasonal cycle of the residual mean meridional circulation in the stratosphere
Authors: Rosenlof Karen H.
Published: February 2004

The transformed Eulerian‐mean (TEM) residual circulation is used to study the zonally averaged transport of mass in the stratosphere. The residual circulation is estimated from heating rates computed with a radiative transfer model using data from the Upper Atmosphere Research Satellite (UARS) as inputs. An annual cycle exists in the resulting circulation in the lower stratosphere, with a larger net upward mass flux across a pressure surface in the tropics during northern hemisphere winter than during northern hemisphere summer. The annual cycle in upward tropical mass flux follows the annual cycle in downward mass flux across a pressure surface in the northern hemisphere extratropics. It is argued that the annual cycle in zonal momentum forcing in the northern hemisphere stratosphere is controlling mass flux across a pressure surface in the lower stratosphere both in the tropics and in the northern hemisphere extratropics.
Measurements of alkyl and multifunctional organic nitrates at a rural site in Ontario
Authors: O'Brien J. M. , Shepson P. B. , Muthuramu K. , Hao C. , Niki H. , Hastie D. R. , Taylor R. , Roussel P. B.
Published: February 2004

Measurements of alkyl and multifunctional organic nitrates were conducted at a rural site in Ontario during periods with varying levels of photochemical activity. On August 6 and August 21–23, 1992, a total of 17 organic nitrates (12 C3‐C6 alkyl nitrates, 4 C2‐C4 hydroxynitrates and 1,2‐dinitrooxybutane) were quantitatively determined in atmospheric samples. The sum of the organic nitrate concentrations was found to be correlated with ozone and ranged from 12 to 140 parts per trillion (volume). The total concentration of organic nitrates measured contributed from 0.5 to 3.0% to the total odd nitrogen species. On average, the alkyl nitrates represented 82%, the hydroxynitrates 16%, and the dinitrate 2% of the total measured organic nitrates. Unidentified organic nitrate peaks determined from an organic nitrate selective detector were found to contribute an additional 0.25% to the total odd nitrogen budget. The distribution of alkyl and hydroxy nitrates measured was found to be reasonably consistent with computed relative production rates, for photochemically active air masses. Although the inclusion of the multifunctional organic nitrates does not significantly change the measured contribution of these species to NOy, it is shown that the isoprene nitrates may contribute as much as the combined contribution of all the measured organic nitrates.
An analytic radiative transfer model for a coupled atmosphere and leaf canopy
Authors: Liang Shunlin , Strahler Alan H.
Published: February 2004

A new analytical radiative transfer model of a leaf canopy is developed that approximates multiple‐scattering radiance by a four‐stream formulation. The canopy model is coupled to a homogeneous atmospheric model as well as a non‐Lambertian lower boundary soil surface. The same four‐stream formulation is also used for the calculation of multiple scattering in the atmosphere. Comparisons of radiance derived from the four‐stream model with those calculated by an iterative numerical solution of the radiative transfer equation show that the analytic model has a very high accuracy, even with a turbid atmosphere and a very dense canopy in which multiple scattering dominates. Because the coupling of canopy and atmospheric models fully accommodates anisotropic surface reflectance and atmospheric scattering and its effect on directional radiance, the model is especially useful for application to directional radiance and measurements obtained by remote sensing. Retrieval of biophysical parameters using this model is under investigation.
IMAGES: A three‐dimensional chemical transport model of the global troposphere
Authors: Müller Jean‐François , Brasseur Guy
Published: February 2004

A new three‐dimensional chemical transport model of the troposphere is presented. This model, named intermediate model of global evolution of species, has been developed to study the global distributions, budgets, and trends of 41 chemical compounds, including the most important species that determine the oxidation capacity of the atmosphere. The chemical mechanism is made of approximately 125 chemical reactions and 26 photodissociations. The model accounts for surface emissions, chemical transformations, dry and wet deposition, and aerosol reactions of trace constituents. The model is formulated in σ coordinates and includes 25 layers in the vertical. Its horizontal resolution is 5° in longitude and 5° in latitude. To keep the requirements in computer time limited, a simplified representation of the transport is adopted: the advection, solved by a semi‐Lagrangian scheme, is driven by monthly mean climatological winds provided by an European Center for Medium‐Range Weather Forecasts analysis. The effect of wind variability at timescales smaller than a month is taken into account by an eddy diffusion parameterization. Convection in cumulonimbus clouds is also represented. All input field, such as the distribution of winds, clouds, eddy diffusion coefficients, and the boundary conditions, are monthly means constrained by observational data. The modeled global distributions of species such as methane, carbon monoxide, nitrogen oxides, and ozone are generally in good agreement with observations. The lifetime of methane, which can be regarded as a measure of the oxidizing capacity of the atmosphere, is found to be equal to 11 years, in agreement with recent estimates. The model also shows that the deposition of ozone at the Earth's surface (1100 Tg/yr) balances the sum of the net photochemical production (550 Tg/yr) and the flux from the stratosphere (550 Tg/yr). In the case of carbon monoxide, surface emissions (1400 Tg/yr) are approximately 50% larger than in situ production by hydrocarbon oxidation (900 Tg/yr).
The effect of riming on the ion concentrations of winter precipitation: 1. A quantitative analysis of field measurements
Authors: Dixon Roy W. , Mosimann Lukas , Oberholzer Beat , Staehelin Johannes , Waldvogel Albert , Collett Jeffrey L.
Published: February 2004

Although riming has been demonstrated to influence precipitation chemistry, its relative importance as a mechanism for the transfer of trace species into snow is not well known. To overcome the qualitative measure of riming in past studies, the mass fraction of snow originating from accreted cloud droplets (rimed mass fraction) is estimated indirectly in this study from replicas of collected snow crystals and flakes. The rimed mass fraction is then compared with the concentrations of various ionic trace species in collected snow and cloud water samples at a prealpine site in Switzerland from 10 case studies. The ratio of the precipitation to the cloud water concentration for a given ion, which indicates the transfer efficiency of the species from clouds to snow, was found to be correlated weakly to the rimed mass fraction for all sample periods investigated. However, much of the variability in the relationship came from sampling periods when the precipitation intensity was low. The relative importance of riming and other scavenging processes was estimated from the sample set, although this estimate is complicated by differences in concentration between cloud droplets collected by snow and those sampled at the site and by case to case variability. Under conditions of higher precipitation intensities, riming appears to be the dominant transfer mechanism for the incorporation of the major ionic species into snow.
Martian plate tectonics
Authors: Sleep Norman H.
Published: February 2004

The northern lowlands of Mars may have been produced by plate tectonics. Preexisting old thick highland crust was subducted, while seafloor spreading produced thin lowland crust during Late Noachian and Early Hesperian time. In the preferred reconstruction, a breakup margin extended north of Cimmeria Terra between Daedalia Planum and Isidis Planitia where the highland‐lowland transition is relatively simple. South dipping subduction occurred beneath Arabia Terra and east dipping subducuon beneath Tharsis Montes and Tempe Terra. Lineations associated with Gordii Dorsum are attributed to ridge‐parallel structures, while Phelegra Montes and Scandia Colles are interpreted as transform‐parallel structures or ridge‐fault‐fault triple junction tracks. Other than for these few features, there is little topographic roughness in the lowlands. Seafloor spreading, if it occurred, must have been relatively rapid. Quantitative estimates of spreading rate are obtained by considering the physics of seafloor spreading in the lower (∼0.4 g) gravity of Mars, the absence of vertical scarps from age differences across fracture zones, and the smooth axial topography. To the first order, the height of vertical scarps across fracture zones does not involve gravity. Crustal thickness at a given potential temperature in the mantle source region scales inversely with gravity. Thus, the velocity of the rough‐smooth transition for axial topography also scales inversely with gravity. Plate reorganizations where young crust becomes difficult to subduct are another constraint on spreading age. Possible plate reorganizations, for example, the end of spreading through Alba Patera, occur when the ridge axis is far from the trench. That is, rapid plate motions are inferred to have placed young oceanic crust far from the ridge axis. The preferred full spreading rate 90° from the plate pole is 80 mm yr−1. Plate tectonics, if it occurred, dominated the thermal and stress history of the planet. A geochemical implication is that the lower gravity of Mars allows deeper hydrothermal circulation through cracks and hence more hydration of oceanic crust so that more water is easily subducted than on the Earth. Age and structural relationships from photogeology as well as median wavelength gravity anomalies across the now dead breakup and subduction margins are the data most likely to test and modify hypotheses about Mars plate tectonics.
Laboratory studies of organic chemistry in planetary atmopheres: From simulation experiments to spectroscopic determinations
Authors: Bruston Paul , Khlifi Mohammed , Bénilan Yves , Raulin François
Published: February 2004

Possible approaches to the study of organic chemistry in planetary atmospheres are threefold: they comprise theoretical modeling, simulation experiments, and observational programs. Because of their respective merits and limitations, these approaches are quite complementary, and their simultaneous improvement is the way to progress further in the field. All three ask for laboratory work, and the lack, or limited accuracy, of laboratory data is the main restriction to future improvement. Together with the development of theoretical modeling (based on chemical kinetics and depending on laboratory studies of reaction pathways and rate constants) laboratory simulation remains a powerful technique. Despite the inaccurate reproduction of all planetary conditions, this experimental approach yields precious information on the nature of middle and higher order molecular weight organics that can be expected in an atmosphere of a given overall composition; and there is, in general, good agreement between the data obtained from simulations and those derived from observations. Indeed, several of the organic species highlighted in such experiments, and their relative abundances, are compatible with those detected in related planetary atmospheres. This is shown in the particular case of Titan. Thus experimental results furnish information on the nature of organics to be searched for in planetary atmospheres, while, in turn, the detection of such candidates and possible indications of their concentration profiles, or the setting of upper limits to their abundancies, constrain the kinetic approach. Given the lists of candidates from simulation experiments, experimental programs for a systematic determination of spectroscopic characteristics, including frequencies and band or line intensities, of the likely organics in planetary atmospheres, have to be developed. As an example, experimental requirements and current results, both in the IR and the UV range, are presented concerning Titan's atmosphere in view of the Cassini‐Huygens mission.
A non‐local thermodynamic equilibrium radiative transfer model for infrared emissions in the atmosphere of Mars: 2. Daytime populations of vibrational levels
Authors: Lopèz‐Valverde M. A. , Lopèz‐Puertas M.
Published: February 2004

A non‐local thermodynamic equilibrium (LTE) radiative transfer model has been applied to study the CO2 and CO emissions in the infrared (1–20 μm) in the atmosphere of Mars during daytime conditions. An extensive set of vibrational‐translational (V‐T) and vibrational‐vibrational (V‐V) collisional exchanges among the vibrational levels responsible for these emissions has been considered. Radiative transfer has been included for most of the transitions and its importance illustrated for some of them. The populations of the most important vibrational levels of GO and of the ν2 and ν3 modes of CO2 are presented. The CO2(0,ν2,0) levels follow LTE up to about 80 km at daytime, some 5 km lower than at nighttime conditions. The absorption of solar radiation at 1.6, 2.0, and 2.7 μm, and subsequent relaxation by V‐V and radiative processes, significantly populates these levels in the lower thermosphere, increasing all their vibrational temperatures with respect to nighttime conditions. Solar excitation and radiative transfer in 4.3 μm constitute the main sources of excitation of the (0,0°,1) level in the thermosphere, where this level shows a very large vibrational temperature. The V‐V transfer from highly excited CO2 levels is even larger than the direct radiative excitation of the (0,0°,1) level in the mesosphere. The model predicts that the known inversion population between this vibrational level and the lower (0,2°,0) and (1,0°,0) levels will occur in the high mesosphere and above. The CO(1) level also shows much larger populations than during nighttime conditions, due to direct solar absorption at 4.7 μm and the role played by radiative transfer. A sensitivity study of the effect of current uncertainties in rate constants on the level populations is also presented. The uncertainties in the rate for ν3 quanta exchange among CO2 levels have significant effects on the deactivation of high energy states, leading to changes of importance in the daytime populations of the 2.7‐μm states in the mesosphere and in the (0,0°,1) level in the lower thermosphere.
Observations, models, and mechanisms of failure of surface rocks surrounding planetary surface loads
Authors: Schultz R. A. , Zuber M. T.
Published: February 2004

Geophysical models of flexural stresses in an elastic lithosphere due to an axisymmetric surface load typically predict a transition with increased distance from the center of the load of radial thrust faults to strike‐slip faults to concentric normal faults. These model predictions are in conflict with the absence of annular zones of strike‐slip faults around prominent loads such as lunar maria, Martian volcanoes, and the Martian Tharsis rise. We suggest that this paradox arises from difficulties in relating failure criteria for brittle rocks to the stress models. Indications that model stresses are inappropriate for use in fault‐type prediction include (1) tensile principal stresses larger than realistic values of rock tensile strength, and/or (2) stress differences significantly larger than those allowed by rock‐strength criteria. Predictions of surface faulting that are consistent with observations can be obtained instead by using tensile and shear failure criteria, along with calculated stress differences and trajectories, with model stress states not greatly in excess of the maximum allowed by rock fracture criteria.
Formation of a hot proto‐atmosphere on the accreting giant icy satellite: Implications for the origin and evolution of Titan, Ganymede, and Callisto
Authors: Kuramoto Kiyoshi , Matsui Takafumi
Published: February 2004

Judging from accretion energy and accretion time for the giant icy satellites it is suggested that a proto‐atmosphere is formed by the evaporation of icy materials during accretion of these bodies around the proto‐gaseous giant planets. We study the blanketing effect of proto‐atmosphere during accretion of these satellites in the gas‐free environment. We use a gray atmosphere model in which the condensation of H2O in a convective atmosphere is taken into account. The numerical results strongly suggest that the accretion energy flux is large enough to increase the surface temperature higher than ∼500 K during accretion due to the blanketing effect of proto‐atmosphere as long as the accretion time is shorter than 105 years. Such a high surface temperature causes the formation of a deep water‐rich ocean due to the melting of icy materials. Also a rocky core should be eventually formed by sinking of rocky materials through the water‐rich ocean during accretion. Therefore, the apparent difference in the surface geologic features between Ganymede and Callisto can hardly be explained by whether or not these bodies have experienced the formation of rocky core. Stability of hydrostatic structure of the proto‐atmosphere is also studied. Vigorous escape of the proto‐atmosphere is likely to occur under high surface temperature. A large portion of accretional energy is possibly consumed by the vigorous escape during accretion. Thus, the giant icy satellites may lose a significant amount of icy materials during their accretions. This can explain the ice‐depleted composition of Titan inferred from the observed mean densities of Saturnian satellites, if the accretion occurs within 104–105 years. Such a significant loss of icy materials is also expected for Ganymede and Callisto. The escape and catalytic reaction in the hot proto‐atmosphere may play an important role in formation of the present N2‐abundant and CO‐depleted atmosphere of Titan.
Spatial variability in the seasonal south polar cap of Mars
Authors: Calvin Wendy M. , Martin Terry Z.
Published: February 2004

We present the first comprehensive discussion of the south seasonal polar cap spectra obtained by the Mariner 7 infrared spectrometer in the short‐wavelength region (2–4 μm). In addition, we correlate the infrared spectra with images acquired by the wide‐angle camera. Significant spectral variation is noted in the cap interior and regions of varying water frost abundance, CO2 ice/frost cover, and CO2‐ice path length can be distinguished. Many of these spectral variations correlate with heterogeneity noted in the camera images, but certain significant infrared spectral variations are not discernible in the visible. We use simple reflectance models to classify the observed spectral variations into four regions. Region I is at the cap edge, where there is enhanced absorption beyond 3 μm inferred to be caused by an increased abundance of water frost. The increase in water abundance over that in the interior is on the level of a few parts per thousand or less. Region II is the typical cap interior characterized by spectral features of CO2 ice at grain sizes of several millimeters to centimeters. These spectra also indicate the presence of water frost at the parts per thousand level. A third, unusual region (III), is defined by three spectra in which weak CO2 absorption features are as much as twice as strong as in the average cap spectra and are assumed to be caused by an increased path length in the CO2. Such large paths are inconsistent with the high reflectance in the visible and at 2.2 μm and suggest layered structures or deposition conditions that are not accounted for in current reflectance models. The final region (IV) is an area of thinning frost coverage or transparent ice well in the interior of the seasonal cap. These spectra are a combination of CO2 and ground signatures.
S ≠ NC: Multiple source areas for Martian meteorites
Authors: Treiman Allan H.
Published: February 2004

The Shergottite, Nakhlite, and Chassigny (SNC) meteorites are igneous rocks from Mars, inferred previously to have been propelled into space (and to Earth) by a single impact event. However, the sum of petrologic, chemical, and chronological data suggests two distinct sites of origin: S from one, NC from another. Differences between the S and NC include distributions of terrestrial residence ages, cosmic ray exposure ages, impact shock effects, preterrestrial aqueous alteration products, bulk magma compositions, trace element magma chemistries, and mantle source compositions. If the S and NC did not come from the same site on Mars, selection criteria for determining source craters are relaxed, and a number of craters become candidate sources. Formation of two meteorite source craters on young Martian surfaces (≤1300 Ma) within the last 20 m.y. (spanning the cosmic ray exposure ages) seems to require either smaller source craters or greater cratering rates than allowed in current theories. Both of these options suggest that meteorites from nonvolcanic and ancient Martian surfaces ought to be more common than the known types of Martian meteorites. The absence of such meteorites may reflect the physical properties of nonvolcanic and ancient surfaces, or a failure to recognize these rock types as meteorites. The ALH84001 meteorite, long classified as a diogenite, may be such a sample of the ancient Martian crust.
Recent deformation rates on Venus
Authors: Grimm Robert E.
Published: February 2004

Constraints on the recent geological evolution of Venus may be provided by quantitative estimates of the rates of the principal resurfacing processes, volcanism and tectonism. This paper focuses on the latter, using impact craters as strain indicators. The total postimpact tectonic strain lies in the range 0.5–6.5%, which defines a recent mean strain rate of 10−18–10−17 s−1 when divided by the mean surface age. Interpretation of the cratering record as one of pure production requires a decline in resurfacing rates at about 500 Ma (catastrophic resurfacing model). If distributed tectonic resurfacing contributed strongly before that time, as suggested by the widespread occurrence of tessera as inliers, the mean global strain rate must have been at least ∼10−155 S−1, which is also typical of terrestrial active margins. Numerical calculations of the response of the lithosphere to inferred mantle convective forces were performed to test the hypothesis that a decrease in surface strain rate by at least two orders of magnitude could be caused by a steady decline in heat flow over the last billion years. Parameterized convection models predict that the mean global thermal gradient decreases by only about 5 K/km over this time; even with the exponential dependence of viscosity upon temperature, the surface strain rate drops by little more than one order of magnitude. Strongly unsteady cooling and very low thermal gradients today are necessary to satisfy the catastrophic model. An alternative, uniformitarian resurfacing hypothesis holds that Venus is resurfaced in quasi‐random “patches” several hundred kilometers in size that occur in response to changing mantle convection patterns. Under such a model, the observed crater strain distribution indicates that about 1% of the planet's surface is tectonically active at any time. However, this model requires a very weak crustal rheology to achieve surface velocities ∼100 mm/yr appropriate to the required “patch” size. Without well‐developed lateral weak zones, Venus is essentially a one‐plate planet, but one in which the lithosphere is able to respond to topography produced by mantle convection through faulting and limited horizontal movement. The net rate of tectonic activity is logarithmically intermediate between Earth and Mars: about 100 times slower than plate tectonics, but up to 100 times faster than planets where tectonic stress arises largely from lithospheric cooling and contraction.
Atmospheric effects on the mapping of Martian thermal inertia and thermally derived albedo
Authors: Hayashi Joan N. , Jakosky Bruce M. , Haberle Robert M.
Published: February 2004

We examine the effects of a dusty CO2 atmosphere on the thermal inertia and thermally derived albedo of Mars and we present a new map of thermal inertias. This new map was produced using a coupled surface atmosphere (CSA) model, dust opacities from Viking infrared thermal mapper (IRTM) data, and CO2 columns based on topography. The CSA model thermal inertias are smaller than the 2% model thermal inertias, with the difference largest at large thermal inertia. Although the difference between the thermal inertias obtained with the two models is moderate for much of the region studied, it is largest in regions of either high dust opacity or of topographic lows, including the Viking Lander 1 site and some geologically interesting regions. The CSA model thermally derived albedos do not accurately predict the IRTM measured albedos and are very similar to the thermally derived albedos obtained with models making the 2% assumption.
Detection of a meteoroid entry into the Earth's atmosphere on February 1, 1994
Authors: McCord Thomas B. , Morris John , Persing David , Tagliaferri Edward , Jacobs Cliff , Spalding Richard , Grady LouAnn , Schmidt Ronald
Published: February 2004

Infrared and visible wavelength sensors on board platforms operated by the U.S. Department of Defense detected an energy release over the central Pacific Ocean on February 1, 1994, estimated to be of the order of at least tens of kilotons of TNT. The event has been assessed by the Defense and Intelligence Community to be a meteoroid entry. The object broke up into several fragments and created debris clouds which were tracked for over an hour. The meteoroid entered at about 24 km/s and an angle of approximately 45° on a heading of approximately 300°. From this, the object's heliocentric orbit just prior to entry was calculated to have a semimajor axis of about 1.6 AU, an eccentricity of about 0.65, and inclination of 2.1°. The radiant energy released is modeled to be between 1.4×1013 J and 2.6×1014 J or equivalent to 3.4 to 63 kilotons of TNT, and the total kinetic energy of the meteoroid is estimated to be in the range 1.4×1014 J to 2.6×1015 J or equivalent to 34 to 630 kilotons of TNT. From the kinetic energy and if we model the object as composed of silicates with a density of 3.5 g/cm3, we derive a mass range of 5×105 to 9×106 kg and a diameter range of 6 to 17 m.
Localization of Martian ridges by impact craters: Mechanical and chronological implications
Authors: Allemand P. , Thomas P. G.
Published: February 2004

The eastern flank of the Tharsis dome (Mars) is affected by compressional ridges. Two conceptual models exist for these structures: (1) the ridges are the expression of a superficial shortening and are rooted on a rheological boundary located in the megaregolith or (2) the ridges result from a global shortening on a lithospheric scale and are rooted on the deep thermal brittle‐ductile transition inside the lithosphere. It is shown that the position of the ridges is strongly influenced by the existence of impact craters. The distribution of crater‐ridge distances has been measured in the Coprates area. This distribution shows that the intersection of ridges and impact craters with a diameter larger than 4 km is four times as frequent as a homogeneous and random distribution of craters relative to what ridges would produce. Very detailed geometric studies of relationships between intersecting ridges and craters show that these craters are older than the ridges. This localization implies that (1) ridges are superficial structures rooted at shallow depths in the megaregolith, and (2) the ridging phase occurred some time after the deposit of the ridged plain unit material, probably during the upper Hesperian. Detailed geometrical studies of ridge edges and graben/ridge width relationships suggest that ridges are bounded by two reverse faults with different offsets, and that the rheological boundary on which ridges and graben are rooted is around 1.5 km deep. Three ways are possible for an impact crater to localize strain in a regional uniaxial compressional stress field: (1) the strength of impacted material is lower than the strength of the surrounding nonaffected rocks, (2) the lithostatic pressure underneath the crater is lower than the lithostatic pressure at the same depth in the surroundings due to the existence of the crater cavity, and (3) the stress is concentrated on the crater borders if craters can be assimilated to holes in a shortened elastic plate. In all three cases, the initiation of the ridges occurs in the area where a low‐strength and/or stress concentration envelope around the crater is nearest to the low‐strength decollement level.
Low‐temperature and low atmospheric pressure infrared reflectance spectroscopy of Mars soil analog materials
Authors: Bishop Janice L. , Pieters Carlé M.
Published: February 2004

Infrared reflectance spectra of carefully selected Mars soil analog materials have been measured under low atmospheric pressures and temperatures. Chemically altered montmorillonites containing ferrihydrite and hydrated ferric sulfate complexes are examined, as well as synthetic ferrihydrite and a palagonitic soil from Haleakala, Maui. Reflectance spectra of these analog materials exhibit subtle visible to near‐infrared features, which are indicative of nanophase ferric oxides or oxyhydroxides and are similar to features observed in the spectra of the bright regions of Mars. Infrared reflectance spectra of these analogs include hydration features due to structural OH, bound H2O, and adsorbed H2O. The spectral character of these hydration features is highly dependent on the sample environment and on the nature of the H2O/OH in the analogs. The behavior of the hydration features near 1.9 μm, 2.2 μm, 2.7 μm, 3 μm, and 6 μm are reported here in spectra measured under a Marslike atmospheric environment. In spectra of these analogs measured under dry Earth atmospheric conditions the 1.9‐μm band depth is 8–17%; this band is much stronger under moist conditions. Under Marslike atmospheric conditions the 1.9‐μm feature is broad and barely discernible (1–3% band depth) in spectra of the ferrihydrite and palagonitic soil samples. In comparable spectra of the ferric sulfate‐bearing montmorillonite the 1.9‐μm feature is also broad, but stronger (6% band depth). In the low atmospheric pressure and temperature spectra of the ferrihydrite‐bearing montmorillonite this feature is sharper than the other analogs and relatively stronger (6% band depth). Although the intensity of the 3‐μm band is weaker in spectra of each of the analogs when measured under Marslike conditions, the 3‐μm band remains a dominant feature and is especially broad in spectra of the ferrihydrite and palagonitic soil. The structural OH features observed in these materials at 2.2–2.3 μm and 2.75 μm remain largely unaffected by the environmental conditions. A shift in the Christiansen feature towards shorter wavelengths has also been observed with decreasing atmospheric pressure and temperature in the midinfrared spectra of these samples.
Spectra of the icy Galilean satellites from 0.2 to 5μm: A compilation, new observations, and a recent summary
Authors: Calvin Wendy M. , Clark Roger N. , Brown Robert H. , Spencer John R.
Published: February 2004

From a variety of sources in the literature and from recent, unpublished, high‐spectral‐resolution data, we can now piece together spectra of both leading and trailing hemispheres for all three icy Galilean satellites from 0.24 to 4 or, in some cases, to 5 μm. The presentation of composite spectra for both hemispheres of all three satellites lends itself to a brief summary of recent results and observations, discussion of the variation in surface composition and surface processing among the three satellites, evolution of our understanding since the comprehensive reviews of the early to mid‐1980's, and suggestions for future observations. The data compilation will assist those planning telescopic observations or spectral modeling studies in support of the Galileo mission to Jupiter, those making observations of other icy solar system bodies, or those designing observation plans for the Cassini mission to Saturn.
Chemical studies of H chondrites: 4. New data and comparison of Antarctic suites
Authors: Wolf Stephen F. , Lipschutz Michael E.
Published: February 2004

We report data for the trace elements Au, Co, Sb, Ga, Rb, Ag, Se, Cs, Te, Zn, Cd, Bi, Tl, and In (ordered by putative volatility during nebular condensation and accretion) determined by neutron activation analysis in 13 H5 chondriles from Victoria Land and 20 H4–6 chondrites from Queen Maud Land, Antarctica. These and earlier results provide Antarctic sample suites of 34 chondrites from Victoria Land and 25 from Queen Maud Land. Treatment of data for the most volatile 10 elements (Rb → In) in these suites by multivariate statistical techniques more robust, as well as more conservative, than conventional linear discriminant analysis and logistic regression demonstrates that compositions differ at marginally significant levels. This difference cannot be explained by trivial (terrestrial) causes and becomes more significant, despite the smaller size of the database, when comparisons are limited to data from a single analyst and when all upper limits arc eliminated from consideration. The Victoria Land and Queen Maud Land suites have different mean terrestrial ages (∼300 kyr and ∼100 kyr, respectively) and age distributions, suggesting that a time‐dependent variation of chondritic sources with different thermal histories is responsible. As a result, these two Antarctic suites are, on average, chemically distinguishable from each other. Since H chondrites serve as a paradigm for other meteorite classes, these results indicate that the near‐Earth populations of planetary materials varied with time on the 105‐year timescale.
Chemical studies of H chondrites: 5. Temporal variations of sources
Authors: Michlovich Edward S. , Wolf Stephen F. , Wang Ming‐Sheng , Vogt Stephan , Elmore David , Lipschutz Michael E.
Published: February 2004

We report 36Cl (301‐kyr half‐life) data obtained by accelerator mass spectrometry allowing nominal terrestrial ages to be determined for 39 Antarctic H4–6 chondrites for which contents of volatile trace elements are known. The compositional difference between these Antarctic meteorites and 58 non‐Antarctic falls increases with terrestrial age and, using multivariate statistical techniques, becomes highly significant for Antarctic samples with ages >50 kyr. The compositional difference is inconsistent with trivial causes such as weathering and seems to reflect differences in thermal histories of parent sources. Temporal source variations for the H chondrite flux on Earth thus exist not only on a short‐term, 40 years, basis (Dodd et al., 1993) but also on a long‐term, >50 kyr, basis.
Oligocene‐Miocene strontium isotopes: Stratigraphic revisions and correlations to an inferred glacioeustatic record
Authors: Oslick Jeffrey S. , Miller Kenneth G. , Feigenson Mark D. , Wright James D.
Published: February 2004

This study tests and improves on previously published early and middle Miocene 87Sr/86Sr marine correlations, presents Sr isotopic age correlations for this interval using the new timescale of Cande and Kent [1992], and evaluates Sr isotopic changes against an inferred glacioeustatic proxy. We generated a latest Oligocene to early late Miocene 87Sr/86Sr isotope record from Ocean Drilling Program (ODP) Hole 747A; this site provides an excellent magnetostratigraphic record during most of this interval for independent age estimates, very good foraminiferal preservation, and excellent core recovery. Comparisons of new 87Sr/86Sr data from Hole 747A with previously published data from Deep Sea Drilling Project (DSDP) Sites 608 [Miller et al., 1991a] and 588 [Hodell et al., 1991] yield the following results: (1) confirmation and refinement of the early Miocene Sr isotope changes, (2) improved definition of the timing of the changes in slope of 87Sr/86Sr near 15.4 Ma and 22.8 Ma, (3) improved Sr isotopic age resolution for the middle Miocene with resolution as good as ±0.7 m.y., and (4) identification of an inflection in the Sr isotope record at 28.0 Ma based on the combined records from DSDP Site 522 [Miller et al., 1988] and ODP Hole 747A. We have been unable to determine the cause of middle Miocene offset between Site 588 and Hole 747A data, although we believe it may be attributed to problems in the age assignments for Hole 588A for the interval ∼14‐11 Ma and Site 747 for the interval 11–8 Ma. Because Hole 747A results provide a better chronology than Site 588 for most of the Miocene and a better middle Miocene Sr isotope record than Site 608, we propose that Hole 747A serves as the best reference section for Miocene 87Sr/86Sr variations from ca. 23 to 11 Ma. Using 87Sr/86Sr data from Sites 522, 608, and 747A, we relate late Eocene to early Miocene inflections in the 87Sr/86Sr isotope record to oxygen isotope increases and decreases inferred to represent glacioeustatic events. The decreases (deglaciations) observed in the δ18O record apparently lead the 87Sr/86Sr inflections by 1 to 1.5 m.y.
Global chemical erosion during the Last Glacial Maximum and the present: Sensitivity to changes in lithology and hydrology
Authors: Gibbs Mark T. , Kump Lee R.
Published: February 2004

Geographically based calculations for 18,000 years ago (18 ka) and today were made to examine the potential effects on terrestrial chemical erosion of changes in lithology and hydrology on glacial‐interglacial timescales. Runoff fields were derived from general circulation model predictions of precipitation minus evaporation. Then global lithologic maps were prepared, so that empirical relationships for runoff versus bicarbonate flux for different rock types could be used to calculate global chemical erosion rates. Assuming that significant chemical erosion does not occur beneath ice sheets, we find that chemical erosion in ice‐free areas at 18 ka was only slightly greater than today. This result arises because the amount of land covered by ice sheets is roughly compensated for by exposed shelf areas and because there is little difference in global runoff. The small (∼20%) increase in the global chemical erosion rate during glacial conditions is due to exposure on the shelves of a relatively high proportion of carbonates (which weather faster than average). Data on glacial/interglacial movements of the calcite compensation depth (CCD) are inconclusive but seem to indicate little change, whereas even a 20% increase in the riverine bicarbonate flux should cause an observable deepening. If the CCD did not indeed change, then our predicted increase in the bicarbonate flux from land during glaciations would have to be accommodated by other means, such as increased carbonate productivity. About half of the observed decrease in atmospheric pCO2 at 18 ka could be explained if increased silicate chemical erosion accompanied increased total chemical erosion in ice‐free areas. Consideration of the maximum possible effect of meltwater at ice margins leads to an 80% increase in the global chemical erosion rate at 18 ka. Such an increase would lead to a larger and faster drop in atmospheric pCO2 but also to an excessive deepening of the global CCD; this scenario seems unrealistic. Our results are contrary to interpretations of a much higher silicate chemical erosion rate during glaciations based on recently published records of Quaternary marine Sr isotopic ratios. Therefore, if these records are indeed globally representative and entirely due to increased silicate chemical erosion (as opposed to changes in source areas or hydrothermal inputs), then other factors, such as a shift in the global weathering regime which we do not explicitly consider here, may be involved. Sensitivity tests show that considering spatial heterogeneities in lithology and runoff leads to lower predictions of global chemical erosion rates than what one would obtain by considering only global averages in a non‐spatially‐resolved calculation.
Modeling sediment drifts: A coupled oceanic circulation‐sedimentation model of the northern North Atlantic
Authors: Haupt Bernd J. , Schäfer‐Neth Christian , Stattegger Karl
Published: February 2004

Modeling of sediment drifts in the northern North Atlantic is achieved by coupling the ocean general circulation model sensitivity and circulation in the northern North Atlantic (SCINNA) to the sedimentation models sedimentation in the northern North Atlantic (SENNA) and particle tracing in the northern North Atlantic (PATRINNA). SCINNA is based on the primitive equations with conservation of mass, momentum, heat, and salt. SENNA and PATRINNA are driven by temperature, salinity, and velocity fields derived from SCINNA. Sediments are supplied from the surface and from the continental margins. The modeling includes three‐dimensional sediment transport in the water column and two‐dimensional processes in a thin bottom layer. Sediments are allowed to resuspend repeatedly, thus offering the possibility of stepwise transport. SENNA calculates erosion, transport, and deposition of sediments, resulting in sedimentation patterns for specific time intervals. PATRINNA models the transport paths of single sediment grains corresponding to the ocean circulation. Sensitivity experiments for the modern state and for the last glacial maximum show differences in the large sediment drifts between the Charlie Gibbs Fracture Zone and the Greenland‐Scotland Ridge for these two time slices. The sediment supply changes with the differing circulation modes and is strongly constrained by topography.
Holocene climate and deep ocean circulation changes: Evidence from accelerator mass spectrometer radiocarbon dated Argentine Basin (SW Atlantic) mudwaves
Authors: Jones Glenn A.
Published: February 2004

The first radiocarbon chronology for sediments of the Argentine basin has been determined using accelerator mass spectrometer (AMS) analyses of 54 total organic carbon samples from four box and two piston cores collected from the downstream and upstream sides of two central Argentine Basin mudwaves. Throughout the Holocene, sediment from the geomorphically defined upstream side of each wave accumulated at rates of 30 to 105 cm/1000 years. Sediments from the downstream side of each wave accumulated at rates of 2 to 10 cm/1000 years in the late and early Holocene, while the mid Holocene is characterized by sedimentation rates less than 1.0 cm/1000 years. During the mid‐Holocene, increased aridity reduced chemical weathering and the flow of the rivers draining to the continental shelf, causing a concomitant decrease in fine‐grained terrigenous input to the basin as evidenced by decreased sedimentation rates, lower N/C ratios, and depleted δ13Corg values. It is estimated that all of the organic carbon deposited in the central basin during the mid‐Holocene was of a marine origin. During the late and early Holocene, however, approximately 35% of the organic carbon deposited was of terrestrial origin. Bottom water flow speeds in the late Holocene were estimated using a lee‐wave model and found to average 14 cm s−1. This estimate is comparable to 10 cm s−1 mean and 15–20 cm s−1 maximum flow speeds measured by current meters deployed within the basin. Flow speeds in the Argentine Basin were 10% higher than today from 8000 to 2000 B.P., and are consistent with a general invigoration of thermohaline circulation that began between 9000 and 8000 B.P. It is proposed that the introduction of warm, salty Indian Ocean water into the northern North Atlantic at 9000 B.P. was the mechanism that provided the excess salt needed to stabilize the North Atlantic Deep Water thermohaline circulation system in its present mode.