bluuebunny/crossref_metadata_2025 huggingface dataset:
Hemispherical backscattering by biomass burning and sulfate particles derived from sky measurements
Authors: Kaufman Y. J. , Holben B. N.
Published: February 2004

Holben et al.[1995] reported measurements of the aerosol spectral optical thickness and derived volume size distribution from measurements of the sky spectral radiance in the aureole (scattering angle ≤40°). In this paper we use these and other sky data for analysis of the aerosol hemispheric backscattering ratio β for two aerosol types: smoke aerosol, prevailing during the biomass burning season in the Amazon and the Cerrado in 1993, and predominately sulfate aerosol, measured during the Sulfate/Smoke Cloud and Radiation‐Atlantic field experiment in the eastern United States in 1993. The β determines the efficiency (per unit of optical thickness) of the aerosol particles to reflect radiation back to space and therefore to generate a negative radiative forcing of climate. The average value of β, using theWiscombe and Grams[1976] definition, βWG, varied between 0.20 and 0.28 for both aerosol types, as compared to βWG= 0.29 used byCharlson et al.[1992] andPenner et al.[1992] to calculate radiative forcing by sulfate and smoke aerosol, respectively. The variable βWGis an average value of β on all the illumination directions. However, high optical thicknesses occur in the Amazon and eastern United States during the period of July to September, when the solar elevation is high. For these months and the latitude range the actual average value of β is 25% lower than βWGfor the same aerosol type. A combination of these two factors results in values of β, and the corresponding aerosol direct radiative forcing, that are 30–50% lower from these estimates. The climate modeling ofKiehl and Briegleb[1993] uses actual computations of the sulfate aerosol hemispherical backscattering as a function of time and geographic position for a particle radius of 0.2 μm, which corresponds very closely to the present results. The volume size distribution, used to derive β, was obtained from sky radiances for scattering angle ≤40°, assuming spherical homogeneous particles. It can be compared with values of β derived from the whole sky almucantar radiance (scattering angle ≤140°) that reflects the true aerosol scattering phase function. The two values of β did not differ significantly.
Radiative effects of convection in the tropical Pacific
Authors: Collins W. D. , Valero F. P. J. , Flatau P. J. , Lubin D. , Grassl H. , Pilewskie P.
Published: February 2004

The radiative effects of tropical clouds at the tropopause and the ocean surface have been estimated by using in situ measurements from the Central Equatorial Pacific Experiment (CEPEX). The effect of clouds is distinguished from the radiative effects of the surrounding atmosphere by calculating the shortwave and longwave cloud forcing. These terms give the reduction in insolation and the increase in absorption of terrestrial thermal emission associated with clouds. At the tropopause the shortwave and longwave cloud forcing are nearly equal and opposite, even on daily timescales. Therefore the net effect of an ensemble of convective clouds is small compared to other radiative terms in the surface‐tropospheric heat budget. This confirms the statistical cancellation of cloud forcing observed in Earth radiation budget measurements from satellites. At the surface the net effect of clouds is to reduce the radiant energy absorbed by the ocean. Under deep convective clouds the diurnally averaged reduction exceeds 150 W m−2. The divergence of flux in the cloudy atmosphere can be estimated from the difference in cloud forcing at the surface and tropopause. The CEPEX observations show that the atmospheric cloud forcing is nearly equal and opposite to the surface forcing. Based upon the frequency of convection, the atmospheric forcing approaches 100 W m−2 when the surface temperature is 303 K. The cloud forcing is closely related to the frequency of convective cloud systems. This relation is used in conjunction with cloud population statistics derived from satellite to calculate the change in surface cloud forcing with sea surface temperature. The net radiative cooling of the surface by clouds increases at a rate of 20 W m−2K−1 during the CEPEX observing period.
Fragmentation of stratospheric intrusions
Authors: Appenzeller C. , Davies H. C. , Norton W. A.
Published: February 2004

Evidence is presented pointing to the existence of rich and coherent subsynoptic and mesoscale flow features at tropopause levels. These features are related to, and evolve from, the classical V‐shaped intrusions of stratospheric air down to tropospheric elevations. It is shown that intrusions can develop into elongated (∼2000–3000 km) and slender (∼200 km) streamers, and that thereafter such a streamer can roll up to form a train of stalactite‐shaped vortex subentities with an accompanying substantial thinning of the intervening filament. In addition there are indications that the vortices themselves can develop a spirallike interior structure of interleaved stratospheric and tropospheric air. These inferences are based upon two independent but complementary sources: analysis of the potential vorticity distribution on tropopause transcending isentropic surfaces derived from the analysis fields of the European Centre for Medium‐Range Weather Forecasts either directly, or indirectly using a contour advection technique; and imagery from the water vapor channel of the European Space Agency Meteosat 4 satellite. Streamers were observed to occur with a frequency of approximately one per week over central and southern Europe during the winter of 1991–1992. The fragmentation is linked to the instability or self‐development of a filament of enhanced potential vorticity and it can modify or instigate surface weather systems. Moreover, by inducing a substantial and rapid enlargement of the intrusion's surface area it greatly enhances the potential for local irreversible mixing of stratospheric and tropospheric air.
Validation studies using multiwavelength Cryogenic Limb Array Etalon Spectrometer (CLAES) observations of stratospheric aerosol
Authors: Massie Steven T. , Gille John C. , Edwards David P. , Bailey Paul L. , Lyjak Lawrence V. , Craig Cheryl A. , Cavanaugh Charles P. , Mergenthaler John L. , Roche Aidan E. , Kumer John B. , Lambert Alyn , Grainger Roy G. , Rodgers Clive D. , Taylor Frederic W. , Russell James M. , Park Jae H. , Deshler Terry , Hervig Mark E. , Fishbein Evan F. , Waters Joe W. , Lahoz William A.
Published: February 2004

Validation studies of multiwavelength Cryogenic Limb Array Etalon Spectrometer (CLAES) observations of stratospheric aerosol are discussed. An error analysis of the CLAES aerosol extinction data is presented. Aerosol extinction precision values are estimated at latitudes and times at which consecutive Upper Atmosphere Research Satellite (UARS) orbits overlap. Comparisons of CLAES aerosol data with theoretical Mie calculations, based upon in situ particle size measurements at Laramie, Wyoming, are presented. CLAES aerosol data are also compared to scaled aerosol extinction measured by the Stratospheric Aerosol and Gas Experiment (SAGE II) and Atmospheric Trace Molecule Spectroscopy (ATMOS) experiments. Observed and calculated extinction spectra, from CLAES, Improved Stratospheric and Mesospheric Sounder (ISAMS), and Halogen Occultation Experiment (HALOE) data, are compared. CLAES extinction data have precisions between 10 and 25%, instrumental biases near 30%, and accuracies between 33 and 43%.
Mesoscale forcing of a boundary layer jet along the California coast
Authors: Holt Teddy R.
Published: February 2004

Mesoscale numerical model simulations were examined to investigate the thermal and topographic forcing of a boundary layer jet along the northern California coast from 0000 UTC May 2 to 1200 UTC May 3, 1990. Routine surface and upper air observations, satellite, and higher‐resolution coastal observations (R/V Point Sur and Doppler wind profiler data) documented the diurnal evolution of a coastal marine atmospheric boundary layer (MABL), coastal cloud structure, and mesoscale features such as the coastal jet and land‐sea breeze circulations that were reasonably well forecast by the numerical model. Sensitivity model simulations to coastal baroclinicity showed that the diurnal development and intensification of the coastal MABL jet located north of the San Francisco Bay area was a result of thermal forcing due to differential land‐sea heating. Simulations in which the coastal baroclinicity was held fixed in time at a minimal strength showed no diurnal development of the MABL jet. The strength of the jet was approximated well by the thermal wind relationship, with the jet core located in the sloping marine inversion layer within one Rossby radius of deformation from the coastline. The steepness of the marine inversion at the coastline was enhanced by regions of strong MABL divergence just offshore and strong convergence onshore resulting from the sea breeze circulation. Modifications to coastal terrain in two separate sensitivity model simulations illustrated that the primary effect of coastal topography was to act as a barrier to the onshore intrusion of higher momentum offshore air. This blocking was due to a topographically forced vertical circulation with a strong descending branch located at the coast. This downslope flow and adiabatic warming steepened the MABL inversion at the coast and enhanced the coastal baroclinicity and the coastal jet. Simulations without coastal terrain showed a jet structure that was spatially diffuse across the coastline and displaced more shoreward. Simulations in which coastal valleys were removed and replaced by a larger coastal mountain range resulted in little modification to the coastal jet intensity or offshore displacement.
TRACE A trajectory intercomparison: 1. Effects of different input analyses
Authors: Pickering Kenneth E. , Thompson Anne M. , McNamara Donna P. , Schoeber Mark R. , Fuelberg Henry E. , Loring Robert O. , Watson Mark V. , Fakhruzzaman Khan , Bachmeier A. Scott
Published: February 2004

We address the problem of air mass trajectory uncertainty through an intercomparison of trajectories computed from operational meteorological analyses from the region and time period of the NASA/GTE/TRACE A experiment. This paper examines the trajectory uncertainty that results from the input meteorological analyses. We first compare the National Meteorological Center (NMC) and European Centre for Medium‐Range Forecasts (ECMWF) meteorological analyses to an independent set of observations, the dropsondes released from the NASA DC‐8 over the South Atlantic during TRACE A. We also compare the gridded wind and temperature fields with selected rawinsonde data that entered the analyses. These comparisons show that the ECMWF fields are marginally better than the ones from NMC, particularly in the tropical regions of the southern hemisphere. The NMC analyses are marginally better in the midlatitude westerlies in some cases. In general, slightly more confidence can be placed in trajectories computed with ECMWF data over the TRACE A region, based on our comparisons of the analyses with observations. Second, we compute 5‐day back trajectories with three different models from a grid of points over the South Atlantic and adjacent portions of South America and Africa as well as on the track of TRACE A flight 15 over the South Atlantic. When using the Goddard Space Flight Center isentropic model, horizontal separations of greater than 1000 km occur for about 50% of the points when trajectories run with the ECMWF and NMC analyses are compared. Greater sensitivity to the input analysis differences is noted when trajectories are computed with the FSU kinematic model (separations exceed 1000 km for 75% of the points). The problem of meteorological uncertainty should be addressed with two approaches. There are large differences between both sets of analyses and the TRACE A soundings; this is also likely to be the case in other remote regions. Therefore we recommend that a test set of trajectories be computed with both sets of input data to quantify the uncertainty due to analysis differences. In addition, clusters of trajectories about the points of interest should be run to assess the uncertainty due to wind shear. These recommendations are applicable to any region of the globe with sparse observations. The companion paper [Fuelberg et al., this issue, part 2] addresses uncertainties due to trajectory technique.
Measurements of high‐resolution ultraviolet‐visible absorption cross sections at stratospheric temperatures: 2. Chlorine dioxide
Authors: Frost Gregory J. , Goss Lisa M. , Vaida Veronica
Published: February 2004

Measurements of absorption cross sections of chlorine dioxide at 200 ± 20 K in the wavelength region 390–454 nm and at resolutions of 0.5 and 0.1 cm−1 are reported. The measurements were made on an experimental system combining high‐resolution Fourier transform spectroscopy with supersonic jet expansions for sample preparation. This is the second of two papers describing this new technique for obtaining ultraviolet‐visible absorption cross sections at high resolution and under atmospheric conditions.
The spectral relationships between atmospheric electrical conductivity and air pollution in urban conditions
Authors: Guo Y. , Barthakur N. N. , Bhartendu S.
Published: February 2004

The timescale dependent relationships between the atmospheric electrical conductivity and concentrations of aerosol particles, carbon monoxide, sulfur dioxide, and nitrogen dioxide in downtown Toronto were examined by spectral analyses based on a year of hourly averaged observations at a single site during 1973–1974. The highlights of the relationships were found as follows: (1) The variance spectra (or power spectra) of both electrical conductivity and pollutant concentrations obeyed the −5/3 power law of spectral decay, suggesting that the air in the city of Toronto during the experiment was severely polluted. (2) The variation of the electrical conductivity corresponded significantly to those of pollutant concentrations for most timescales, characterized by a common variance‐contributing scale range from a few hours to a few weeks in the variance spectra. (3) The variations of both electrical conductivity and pollutant concentrations were contributed mainly by daily changes of hourly averaged values and weekly changes of daily averaged values, coinciding with the daily and weekly cycles in the city's traffic and industrial activities. (4) The electrical conductivity correlated negatively with pollutant concentrations for most timescales. However, the correlations were contributed mainly by weekly changes of daily averaged values, different from the variation contributions of these variables. This suggests that the electrical conductivity is poorly synchronized with pollutant concentrations for the timescales smaller than diurnal component, which could be expected to introduce noises if the correlations between the electrical conductivity and pollutant concentrations at the scales smaller than diurnal component are pursued.
Three‐dimensional radiative transfer effects of clouds in the microwave spectral range
Authors: Liu Quanhua , Simmer Clemens , Ruprecht Eberhard
Published: February 2004

A three‐dimensional Monte Carlo transfer model for polarized radiation is developed and used to study three‐dimensional (3‐D) effects of raining clouds on the microwave brightness temperature. The backward method is combined with the forward method to treat polarization correctly within the cloud. In comparison with horizontally homogeneous clouds, two effects are observed: First, brightness temperatures from clouds are reduced in the 3‐D case due to net leakage of radiation from the sidewalls of the cloud. Second, radiation which is emitted by the warm cloud and then reflected from the water surface increases the brightness temperatures of the cloud‐free areas in the vicinity of the cloud. Both effects compete with each other, leading to either lower or higher overall brightness temperatures, depending on the geometry of the cloud, the satellite viewing angle, the coverage, and the position of the cloud within the field of view (FOV) of the satellite. At 37 GHz, for example, up to 10 K differences can occur for a cloud of 50% coverage. Finite homogeneous raining clouds matching the size of the FOV of the satellite show a similar relationship between rain rates and brightness temperatures (TB) as horizontally infinite clouds. Namely, an increase of TB with increasing rain rates at low rain rates, due to emission effects, is followed by a decrease due to temperature and scattering effects. For small horizontal cloud diameter, however, the 3‐D brightness temperatures may show a second maximum due to the decrease of the leakage effect with increasing rain rates. At nadir, 3‐D brightness temperatures are always lower than the 1‐D values with differences up to 20 K for a cloud of 5‐km vertical extent and a base of 1 × 1 km. To quantify the 3‐D effects for more realistic cloud structures, we used results of a three‐dimensional dynamic cloud model as input for the radiative transfer codes. The same 3‐D effects are obtained, but the differences between 1‐D and 3‐D modeling are smaller. In general, most of the differences between the 1‐D and 3‐D results for off‐nadir view angles are pure geometry effects, which can be accounted for in part by a modified 1‐D model.
Comparison of sea ice simulations with interactive and monthly averaged forcing models
Authors: Wu Xingren , Simmonds Ian , Budd W. F.
Published: February 2004

A dynamic‐thermodynamic sea ice model is developed and coupled with the “21 wave 9 level” Melbourne University general circulation model to simulate the seasonal cycle of the global sea ice distribution. We have run the coupled system and obtained a creditable seasonal simulation of global sea ice. When monthly averaged atmospheric data (taken from the mean of the coupled run) are used to force the sea ice model, the seasonal cycle of sea ice extent (to the outer ice edge) is quite similar to that simulated in the interactive run. However, the actual sea ice covered area (i.e., excluding leads) differs considerably between the two simulations. Sea ice is more compact in the monthly averaged forced run than in the interactive run throughout the year in both hemispheres. The sea ice thickness distribution also differs between the two runs. In general, the sea ice is more open and thicker in the seasonal ice zone of the two polar regions for the interactive coupled case than for the mean forcing. We have also run the model forced with daily atmospheric data and the simulated sea ice distribution differs significantly from both the interactive model and the monthly averaged forcing results. These differences highlight the dangers of undertaking studies with sea ice models forced with prescribed atmospheric conditions rather than using a fully interactive atmosphere‐sea ice system.
Measurement of O3 and related compounds over southern Nova Scotia: 1. Vertical distributions
Authors: Kleinman Lawrence I. , Daum Peter H. , Lee Yin‐Nan , Springston Stephen R. , Newman Leonard , Leaitch W. Richard , Banic Catharine M. , Isaac George A. , MacPherson J. Ian
Published: February 2004

As part of the North Atlantic Regional Experiment, the National Research Council of Canada Twin Otter aircraft was used to measure the concentration of O3 and related compounds in the atmosphere over southern Nova Scotia. Forty‐eight flights were conducted, primarily over the surface sampling site in Chebogue Point, Nova Scotia, or over the adjoining Atlantic Ocean. A typical flight included one or more vertical profiles from 30 m above the surface to an altitude of 3 or 5 km. We present here O3 measurements and supporting chemical and meteorological data including NOy, CO, accumulation mode aerosol particles, winds, temperature, and dew point. Data are presented in a format which illustrates day‐to‐day variability and vertical structure. We find that Nova Scotia is impacted by a wide variety of air masses with varying chemical content depending on flow conditions relative to the locations of upwind emission regions. As an aid to understanding the chemical composition of the air, we characterize four types of events: (1) moist continental boundary layer air with high concentrations of O3 and other anthropogenic pollutants which is advected to Nova Scotia in relatively thin vertical layers, usually with a base altitude of several hundred meters; (2) “background” air with concentrations of anthropogenic ingredients much lower than experienced in continental pollution episodes but higher than observed in more remote regions of Canada, suggesting a dilute anthropogenic or biomass burning influence; (3) near‐surface air which because of a strong temperature inversion over the Atlantic Ocean, is decoupled from air aloft, with the consequence that near‐surface measurements do not give a representative view of the eastward transport of the North American plume; and (4) dry air masses with high O3 concentration in which we have to distinguish between boundary layer and upper atmosphere source regions.
Photometry and surface structure of the icy Galilean satellites
Authors: Buratti Bonnie J.
Published: February 2004

Observations of the icy Galilean satellites from both spacecraft and ground‐based telescopes have been fit to radiative transfer models describing the surfaces in terms of their roughness, compaction state, and single‐particle phase function and albedo. The macroscopically rough nature of Ganymede and Callisto is similar to the Moon's, while Europa is significantly smoother. The sharply peaked opposition surge on Europa may be due to an extremely uncompacted upper regolith. However, if the peak is attributed to coherent backscatter, its surface may be the most compacted of the Galilean satellites. The compaction state of Callisto is similar to that of the Moon, while Ganymede's surface is more compacted. Callisto and possibly Europa exhibit textural differences in their leading and trailing hemispheres. This dichotomy is due to enhanced erosion by meteorites on the leading side.
Thermal inertias in the upper millimeters of the Martian surface derived using Phobos' shadow
Authors: Betts Bruce H. , Murray Bruce C. , Svítek Tomáš
Published: February 2004

The first thermal images of Phobos' shadow on the surface of Mars, in addition to simultaneous visible images, were obtained by the Phobos '88 Termoskan instrument. The best observed shadow occurrence was on the flanks of Arsia Mons. For this occurrence, we combined the observed decrease in visible illumination of the surface with the observed decrease in brightness temperature to calculate thermal inertias of the Martian surface. The most realistic of our three models of eclipse cooling improves upon our preliminary model by including nonisothermal initial conditions and downward atmospheric flux. Most of our derived inertias fall within the range 38 to 59 J m−2 s−1/2 K−1 (0.9 to 1.4 10−3 cal cm−2 s−1/2 K−1), corresponding to dust‐sized particles (for a homogeneous surface), consistent with previous theories of Tharsis as a current area of dust deposition. Viking infrared thermal mapper (IRTM) inertias are diurnally derived and are sensitive to centimeter depths, whereas the shadow‐derived inertias sample the upper tenths of a millimeter of the surface. The shadow‐derived inertias are lower than those derived from Viking IRTM measurements (84 to 147), however, uncertainties in both sets of derived inertias make conclusions about layering tenuous. Thus, near‐surface millimeter versus centimeter layering may exist in this region, but if it does, it is likely not very significant. Both eclipse and diurnal inertias appear to increase near the eastern end of the shadow occurrence. We also analyzed a shadow occurrence near the crater Herschel that showed no observed cooling. This analysis was limited by cool morning temperatures and instrument sensitivity, but yielded a lower bound of 80 on eclipse inertias in that region. Based upon our results, we strongly recommend future spacecraft thermal observations of Phobos' shadow, and suggest that they will be most useful if they improve upon Termoskan's geographic and temporal coverage and its accuracy.
The magnetic bode fallacy
Authors: Cain Joseph C. , Beaumont Paul , Holter William , Wang Zhigang , Nevanlinna Heikki
Published: February 2004

The magnetic moments and angular momenta of Solar System bodies are compared to evaluate the statistical reality of the relation that has come to be called the “magnetic Bode law.” Runcorn has suggested that this “law”, giving a slope of about 3/5 on a log‐log plot, is only a geometrical effect of the angular momenta being proportional to the fifth power of radius and the magnetic moment the cube. The best fit line to the six planets with nonzero magnetic fields is log(m/me) = −0.2 (±0.2) + 0.82 (±0.05) log(L/Le), where the subscripts denote the values for Earth. The value 0.82 is shown to be consistent with the 3/5 slope once estimation bias is accounted for. Monte Carlo analysis was used to construct (2500) synthetic solar systems from the variables representing the range of the planet's radii, densities, coefficients of inertia, periods of rotation, and surface poloidal field intensity. If these variables are considered independent, it was found that the probability of obtaining a slope as different from 0.6 as suggested by Runcorn is 34%, but only a 0.2% chance of obtaining values that are so linear as the actual data on this plot. If instead the covariances of the actual planets are included in the analysis, the mean slope is 0.82 with small deviation, but the odds of obtaining such a tight fit as that observed in the actual Solar System becomes 63%. It is concluded that considering the lack of physical plausibility of the correlations between the physical parameters besides field, the strong correlation with rotation as an important factor in computing the angular momentum is spurious. Comparisons are done using both the angular momenta of the whole planet, and the theorized magnetoactive shells and cores. The conducting volumes are also compared with the poloidal dipole fields external to these magnetoactive regions. The studies show that the volumes of the magnetoactive regions appear to be related to the strength of the poloidal field just outside, and that the tighter trend for the log momentum versus log dipole moment plots is mainly due to the geometric factor suggested by Runcorn. Plots of field just outside these regions versus their volumes show that the Earth and Io have significantly higher fields than those predicted by a line extending from the giant planets to Mercury. Such a line also does not miss Dolginov's estimate for Mars by a half order of magnitude. The upper limit of field of Venus falls 3 orders below this trend. If Mars has a core theorized to be some 1780 km in radius, a weak dynamo resulting in a surface field close to the limiting 10 – 100 nT range suggested by earlier spacecraft observations is not inconsistent with the trend of the other planets. The deviation of Earth and Io from a line joining the other planets is thought to be due to the added vigor of convection from their extra sources of energy. For Earth this would likely be caused by freezing of the inner core whereas for Io the enhancement would be from tidal heating as previously suggested.
A simulated geochemical rover mission to the Taurus‐Littrow valley of the Moon
Authors: Korotev Randy L. , Haskin Larry A. , Jolliff Bradley L.
Published: February 2004

We test the effectiveness of using an alpha backscatter, alpha‐proton, X ray spectrometer on a remotely operated rover to analyze soils and provide geologically useful information about the Moon during a simulated mission to a hypothetical site resembling the Apollo 17 landing site. On the mission, 100 soil samples are “analyzed” for major elements at moderate analytical precision (e.g., typical relative sample standard deviation from counting statistics: Si [11%], Al [18%], Fe [6%], Mg [20%], Ca [5%]). Simulated compositions of soils are generated by combining compositions of components representing the major lithologies occurring at the site in known proportions. Simulated analyses are generated by degrading the simulated compositions according to the expected analytical precision of the analyzer. Compositions obtained from the simulated analyses are modeled by least squares mass balance as mixtures of the components, and the relative proportions of those components as predicted by the model are compared with the actual proportions used to generate the simulated composition. Boundary conditions of the modeling exercise are that all important lithologic components of the regolith are known and are represented by model components, and that the compositions of these components are well known. The effect of having the capability of determining one incompatible element at moderate precision (25%) is compared with the effect of the lack of this capability. We discuss likely limitations and ambiguities that would be encountered, but conclude that much of our knowledge about the Apollo 17 site (based on the return samples) regarding the distribution and relative abundances of lithologies in the regolith could be obtained. This success requires, however, that at least one incompatible element be determined.
Large topographic rises on Venus: Implications for mantle upwelling
Authors: Stofan Ellen R. , Smrekar Suzanne E. , Bindschadler Duane L. , Senske David A.
Published: February 2004

Topographic rises on Venus have been identified that are interpreted to be the surface manifestation of mantle upwellings. These features are classified into groups based on their dominant morphology. Atla and Beta Regiones are classified as rift‐dominated, Dione, western Eistla, Bell, and Imdr Regiones as volcano‐dominated, and Themis, eastern Eistla, and central Eistla Regiones as corona‐dominated. At several topographic rises, geologic indicators were identified that may provide evidence of uplifted topography (e.g., volcanic flow features trending upslope). We assessed the minimum contribution of volcanic construction to the topography of each rise, which in general represents less than 5% of the volume of the rise, similar to the volumes of edifices at terrestrial hotspot swells. The total melt volume at each rise is approximated to be 104‐106 km3. The variations in morphology, topography, and gravity signatures at topographic rises are not interpreted to indicate variations in stage of evolution of a mantle upwelling. Instead, the morphologic variations between the three classes of topographic rises are interpreted to indicate the varying influences of lithospheric structure, plume characteristics, and regional tectonic environment. Within each class, variations in topography, gravity, and amount of volcanism may be indicative of differing stages of evolution. The similarity between swell and volcanic volumes for terrestrial and Venusian hotspots implies comparable time‐integrated plume strengths for individual upwellings on the two planets.
Alteration of volatile inventories by polar clathrate formation on Mars
Authors: Musselwhite Donald , Lunine Jonathan I.
Published: February 2004

Recent models of chaotic variation in the Martian obliquity suggest that CO2 could be released during times of high obliquity and then recaptured in the polar caps as ice or clathrate during times of lower obliquity (Jakosky, et al., 1995). A natural implication of clathrate trapping is that other species in the Martian atmosphere, including noble gases, must incorporate in the water ice structure as well, in varying amounts according to the size and polarizability of the molecules as well as their atmospheric abundances. For nominal estimates of cap volume and amount of incorporated CO2, we find that the current atmospheric inventory of noble gases is not representative of the bulk inventory in the Martian surface‐atmosphere system. In particular, xenon and krypton are underrepresented in the present atmosphere. Models of source regions for Martian volatiles, which are constrained by noble gas abundances, must be modified to take these fractionation effects into account if indeed evidence for large amounts of polar clathrates is found.
Constraints on density models from radial moments: Applications to Earth, Moon, and Mars
Authors: Bills Bruce G. , Rubincam David P.
Published: February 2004

Few objective constraints exist on radial density variations in planetary interiors. Even if the mean density and mean moment of inertia were known with perfect accuracy, they would only provide integral constraints on the density models. However, among the family of monotonic radial density models with a given mean density and mean moment of inertia, the simple two‐layer piecewise constant models have useful extremal properties. The model has only three parameters, an inner density, an outer density, and a transition radius. Once the radial moment constraints are applied, there is only one remaining degree of freedom. If the outer region density is somehow specified, then the inner region density of the two‐layer model provides a firm lower bound on central density of monotonic models. Likewise, if an upper bound on central density can be provided, then the two‐layer model provides a lower bound on outer region density. An envelope of acceptable density models can be generated by scanning the transition depth from the center to the surface. Any monotonic model with specified mean density and mean inertial moment must lie within that envelope. Resulting extremal density envelopes for the Earth, Moon, and Mars are compared to published radial density profiles. For the Moon, the moment constraints are quite restrictive. For the Earth, which is more centrally condensed, the allowed envelope of density profiles is rather broad. Mars is an intermediate case. Present geodetic and astrometric observations only constrain the Martian mean moment of inertia to lie somewhere in the range 0.325 ≤ I/MR2 ≤ 0.365. However, our analysis shows that any value within that range can be accommodated without invoking geochemically implausible density minima or maxima.
Explosive volcanism on Venus: Transient volcanic explosions as a mechanism for localized pyroclast dispersal
Authors: Fagents Sarah A. , Wilson Lionel
Published: February 2004

It is proposed that transient volcanic explosions of the vulcanian type may provide a mechanism for the generation and dispersal of pyroclastic material on Venus. The influence of the Venusian high atmospheric pressure environment implies that continuous discharge plinian eruptive activity is relatively uncommon: the tendency for suppression of exsolution and expansion of magmatic gases favors effusive eruptions. However, it may be possible for explosive activity to occur, in a fashion analogous to vulcanian eruptions on Earth, as a result of the accumulation of hot, pressurized gas under a coherent rock “lid”. The explosion may be initiated by the failure of this retaining caprock, causing the catastrophic release of the high‐pressure gas, which expands out of the vent driving the fragmented caprock material ahead of it and displacing the surrounding atmosphere. On Earth the driving gas may originate either from vaporization of groundwater or from degassing of a stalled magma body in the near‐surface crust, whereas on Venus, where the presence of crustal stores of volatile compounds is uncertain, the latter option only is favored: prolonged degassing may lead to an accumulation of gas sufficient to initiate an explosion. This paper presents the results of a numerical model describing the explosion process under boundary conditions representing the Venusian physical environment. This involves treatments of the acceleration of the driving gas, caprock and displaced atmospheric gas out of the vent and the subsequent motions and aerodynamic interactions between the atmosphere and the ejected blocks of fragmented caprock. In this way, predictions of the eruption velocities and of the resulting distribution of (large) solid ejecta can be obtained for likely conditions on Venus. Deposits of large blocky debris are predicted to range up to a maximum distance of the order of 1 km from the vent on Venus, compared with distances of several kilometers commonly attained by ejecta from transient explosions on Earth. More typical blocky deposits may extend for only a few hundred meters, which implies that they would not be detected in the Magellan radar data. However, the possible presence of associated pyroclastic flow and fine‐grained ashfall deposits may constitute aids to the identification of sites of vulcanian eruptions on Venus.
Ground‐based near‐infrared observations of the Venus nightside: The thermal structure and water abundance near the surface
Authors: Meadows V. S. , Crisp D.
Published: February 2004

We used ground‐based near‐infrared (NIR) observations of thermal emission from the Venus nightside to determine the temperature structure and water vapor distribution between the surface and the 6‐km level. We show that emission from spectral windows near 1.0, 1.1, and 1.18 μm originates primarily from the surface and lowest scale height (∼16 km). These windows include absorption by weak H2O and CO2 lines and by the far wings of lines in strong nearby CO2 bands. Rayleigh scattering by the 90‐bar CO2 atmosphere and Mie scattering by the H2SO4 clouds attenuate this emission, but add little to its spectral dependence. Surface topography also modulates this NIR thermal emission because high‐elevation regions are substantially cooler and emit less thermal radiation than the surrounding plains. These contributions to the emission are clearly resolved in moderate‐resolution (λ/Δλ ∼ 400) spectral image cubes of the Venus nightside acquired with the infrared imaging spectrometer (IRIS) on the Anglo‐Australian Telescope (AAT) in 1991. To analyze these observations, we used a radiative transfer model that includes all of the radiative processes listed above. Synthetic spectra for several topographic elevations were combined with Pioneer Venus altimetry data to generate spatially resolved maps of the NIR thermal emission. Comparisons between these synthetic radiance maps and the IRIS observations indicate no near‐infrared signature of the surface emissivity differences seen at microwave wavelengths by the Magellan orbiter. Assuming constant surface emissivity in the near‐infrared, we derive nightside averaged temperature lapse rates of −7 to −7.5 K/km in the lowest 6 km. These lapse rates are smaller and indicate much greater static stability than those inferred from earlier measurements and greenhouse models (−8 to −8.5 K/km) [Seiff, 1983]. An acceptable fit to the data was obtained with an H2O mixing ratio profile which increases from 20 ppmv at the cloud base to 45 ppmv at 30 km, and then remains constant between that altitude and the surface. There is no evidence for H2O mixing ratios that decrease with altitude, like those inferred from the Pioneer Venus large probe mass spectrometer [Donahue and Hodges, 1992a] or the Venera 11 and 12 Lander spectrophotometers [Moroz, 1983].
Magma generation on Mars constrained from an 40Ar degassing model
Authors: Tajika Eiichi , Sasaki Sho
Published: February 2004

The degassing history of 40Ar on Mars is reconstructed from a model based on the Martian volcanic record. Accumulation history of 40Ar is influenced by the volcanic eruption rate at each Martian epoch in addition to the production rate of 40Ar due to potassium decay in the mantle. In order to degas the present amount of 40Ar to the Martian atmosphere, the volume fraction of erupted magma to the original mantle materials averaged over history should be about 0.006–0.018. The integrated volume of mantle materials that produced the observed volcanic materials and atmospheric 40Ar is about (4–11) × 1018 m3, hence the extent of mantle differentiation after the formation of Mars is only 0.03–0.08. A significant amount of 40Ar (about 73% of the present atmospheric content) has apparently been degassed during the formation of the major volcanic regions on Mars, such as Tharsis and Elysium. This means that a significant portion of 40Ar in the Martian atmosphere would have been degassed by relatively recent volcanic activity, whereas large fractions of other volatile components, such as H2O and CO2, might have been degassed by an early catastrophic degassing. The possibility of ancient plate tectonic activity on Mars, which was proposed recently to explain the origin of the Martian crustal dichotomy, is also examined by using the 40Ar degassing model. Although the possibility of Martian plate tectonics may not be ruled out from the model, we can constrain an upper limit for the duration of plate tectonics. The duration should be much shorter than 6–350 Ma (probably ≪100 Ma), which is rather short compared with the ranges of the northern lowlands ages (600–750 Ma).
Seasonal‐ to decadal‐scale climatic variability in southwest Florida during the Middle Pliocene: Inferences from a coralline stable isotope record
Authors: Roulier Leanne M. , Quinn Terrence M.
Published: February 2004

We have generated a 49‐year‐long, high‐resolution, stable isotope record from a fossil coral, Solenastrea bournoni, to evaluate regional sea surface conditions during the middle Pliocene (3.0 ± 0.5 Ma) in southwest Florida. Continuous routing of the coral slab at an interval of 0.76 mm produced ∼5 to 6 samples per year. The annual cycle is well defined in δ18O and δ13C records confirming the average yearly growth rate estimated from density banding of 4 to 5 mm/yr. High‐density bands are synchronous with the highest δ18O and δ13C values for 79% and 45% of the annual cycles recorded, respectively. The annual range in δ18O is 0.70 to 2.27‰ and in δ13C it is 0.31 to 2.21‰. Partitioning of the coral δ18O signal into sea surface temperature (SST) changes and δ18Owater changes is difficult to estimate in the ancient. However, isotopic mass‐balance calculations, using modern climate data, suggest a partitioning of the δ18O signal into 8 to 12% salinity and 92 to 88% temperature at the sea surface. If 100% of the δ18O variation is attributed to temperature, the observed mean annual range in δ18O (mean = 1.54 ± 0.37‰) corresponds to a seasonal SST range of ∼7.0 ± 1.7°C (0.22‰/1°C), which is a mean of 3.5°C less than that of the present day. Isotope data are consistent with the hypothesis that δ13C values covary with the number of sunshine hours. Annually, peak δ18O values are often observed to precede peak δ13C values by one sample (i.e., about 2 months). We infer from the isotope pattern that periods of reduced SST were followed about 2 months later by periods of increased number of sunshine hours. This seasonal pattern is similar to that of southwest Florida today. Cross‐spectral analysis of fossil coral δ18O and δ13C documents coherency peaks, above the 95% confidence interval, at annual and subdecadal frequencies (1.0, 1.6, 2.9 and 5 year). The spectral phase angle between δ18O and δ13C at the annual frequency is −60 ± 10°, at the 95% confidence interval. The temporal equivalent of this angle is approximately 2 months, which confirms visual observations of seasonality from the isotope pattern.
Constraints from paleotracer data on the North Atlantic circulation during the Last Glacial Maximum
Authors: LeGrand Pascal , Wunsch Carl
Published: February 2004

We investigate the extent to which the paleotracers, δ13C and δ18O, constrain the North Atlantic Ocean general circulation during the last glacial maximum (LGM). The distinction is made in this work between the depiction of water mass distributions and the inference of the oceanic circulation. The δ13C data indicate that water masses formed in the North Atlantic and in the Southern Ocean were distributed differently during the LGM, but the extent of the differences cannot be quantified because of uncertainties in the end‐member composition. Reasonable choices of values for the end‐members produce volumes of Lower North Atlantic Deep Water (NADW) significantly smaller than in the modern Atlantic, in agreement with previous studies. To explore the range of circulations consistent with this water mass distribution, a coarse resolution inverse model of the North Atlantic is constrained with δ13C and δ18O paleodata. The model shows that the glacial period tracer distribution is fully consistent with the circulation rates estimated for the modern ocean. However, the paleodata are also consistent with a circulation rate of one‐half the modern flux of NADW, and indeed there are an infinite number of rates of deep water movement that are possible. To remove this ambiguity, the central need is for a data type capable of setting the overall rates of water movement, i.e., a “clock”, such as that provided by geostrophy or a well‐sampled radioactive tracer.
Carbonate preservation history in the Peru Basin: Paleoceanographic implications
Authors: Weber Michael E. , Wiedicke Michael , Riech Volkher , Erlenkeuser Helmut
Published: February 2004

We studied preservation/dissolution cycles and paleoproductivity in eight sediment cores from the Peru Basin south of the highly productive surface waters of the eastern equatorial Pacific. Stratigraphy is based on stable oxygen isotopes and on combined magnetostratigraphy and biostratigraphy. Sediment cores which span the last 8 m.y., were retrieved during cruise 79 with RV SONNE close to the carbonate compensation depth (CCD). In general, sediments show Pacific‐type carbonate cycles. We interpret a pronounced carbonate peak between 6 and 7 Ma as the result of a western and northern extension of the highly productive Peru Current. Decreased carbonate contents from the late Miocene to the late Pliocene might be associated with a slow contraction of the latitudinal extent of the high‐productivity belt north of the study areas. During the Pliocene, carbonate variations showed 400 kyr cycles indicating the growth and decay of ice sheets, which should have been associated with pulsations of the Antarctic ice cap. An abrupt collapse of the carbonate system occurred at 2.4 Ma. Higher frequency variations of the carbonate record indicate the major increase of the northern hemisphere glaciation. During the Quaternary, carbonate fluxes are high during glacials and low during interglacials. Large amplitude variations with long broad minima and maxima, associated with small migrations of the lysocline and the CCD (< 200 m), are indicative of the preservation/dissolution history in the Peru Basin. During the early Pleistocene, climatic forcing by the 41 kyr obliquity cycle is not observed in the carbonate record. During the last 800 kyr, variability in the carbonate record was dominated by the 100 kyr eccentricity cycle. Fluxes of biogenic material (calcium carbonate, organic carbon, opal, and barium) were greatest during glacials, which imply higher productivity and export production of the Peru Current during cold climatic periods. Dissolution was greatest during interglacials as inferred from the relatively poor preservation of planktonic foraminifera and from the low accumulation rate of carbonate. After the Mid‐Brunhes Event (400 ka), we observe a plateaulike shift to enhanced dissolution and to intensified productivity.