bluuebunny/crossref_metadata_2025 huggingface dataset:
Heterogeneous uptake of ClONO2 and N2O5 by sulfuric acid solutions
Authors: Robinson G. N. , Worsnop D. R. , Jayne J. T. , Kolb C. E. , Davidovits P.
Published: February 2004

A droplet train apparatus has been used to measure the heterogeneous reactive uptake of gaseous N2O5 and ClONO2 by concentrated sulfuric acid solutions. H2SO4 concentrations in the range of 39 to 69 wt% were investigated between 229 and 260 K. Uptake rates normalized to the gas‐liquid collision frequency, γ0, for N2O5 ranged from 0.086 to 0.16, decreasing moderately with increasing temperature and decreasing H2SO4 concentration. Uptake rates for ClONO2, measured over a slightly narrower concentration range of 39–59 wt% H2SO4, ranged between 0.0037 and 0.056, decreasing moderately with H2SO4 temperature but significantly with increasing concentration. Results are compared with measurements from other laboratories using different experimental techniques. In general, the data from the different groups agree well. A phenomenological model is presented which addresses the solubility, diffusion, and chemical reactivity of XNO3 (X=Cl, NO2) in sulfuric acid solutions and accounts for the dependence of the observed uptake rates on H2SO4 concentration and temperature. Two XNO3 hydrolysis pathways are proposed, one involving direct reaction with H2O and the other involving participation of H+ ions to promote bond dissociation. Differences between the concentration dependencies of γ0 for ClONO2 and N2O5 can be ascribed largely to different rates of acid‐catalyzed hydrolysis. The implications of these results for the effects of lower stratospheric sulfuric acid aerosols on ozone depletion chemistry are discussed.
Photooxidant dynamics in the Mediterranean basin in summer: Results from European research projects
Authors: Millán M. M. , Salvador R. , Mantilla E. , Kallos G.
Published: February 2004

Most of the Mediterranean Sea is surrounded by mountains 1500 m or higher. Their east and south facing slopes favor the early formation of upslope winds, reinforcing the sea breezes. These slopes also act as orographic chimneys which link the surface winds directly with their return flows aloft, creating recirculations. To characterize the dynamics of pollutants in the Mediterranean basin and to compose a mosaic of the atmospheric circulations involved, the European Commission (EC) supported the following projects: (1) meso‐meteorological cycles of air pollution in the Iberian Peninsula (MECAPIP), 1988–1991, intended to document the atmospheric circulations over the Iberian Peninsula; (2) regional cycles of air pollution in the west central Mediterranean area (RECAPMA), 1990–1991, which extended the characterization from the Atlantic coast of Portugal to Italy; and (3) south European cycles of air pollution (SECAP), 1992–1995, for the whole of the basin. The level of interpretation of the data and the elaboration and validation of working hypotheses across the basin have followed, in turn, with the corresponding lags in space and time. The purpose of this paper is to present a summary (to 1995) of the documented, as well as the postulated, processes involved. The MECAPIP and RECAPMA projects have shown that stacked layer systems form along the Spanish Mediterranean coasts, 2–3 km deep and more than 300 km wide, with the most recent layers at the top and the older ones near the sea. These act as a reservoir for aged pollutants to reenter land the next day, and tracer experiments have shown that turnover times are from 2 to 3 days. During the night, part of this system drifts along the coast. Under strong insolation these circulations become “large natural photochemical reactors,” where most of the NOx emissions and other precursors are transformed into oxidants, acidic compounds, aerosols, and O3 (exceeding some EC directives for several months). Finally, the preliminary analysis of the data obtained in the SECAP project supports the hypothesis that pollutants emitted in the Mediterranean basin could be transported toward the Intertropical Convergence Zone, located along northern Africa in summer, and pumped directly into the upper troposphere. If this is verified, the Mediterranean basin could be one place where all the links from the local to the global scales could be identified and documented.
Diurnal migrating tide as seen by the high‐resolution Doppler imager/UARS: 1. Monthly mean global meridional winds
Authors: Khattatov B. V. , Yubin V. A. , Geller M. A. , Hays P. B. , Vincent R. A.
Published: February 2004

The high‐resolution Doppler imager (HRDI) instrument on board the upper atmosphere research satellite measures global winds in the mesosphere and lower thermosphere on a day‐to‐day basis. The horizontal coverage of the HRDI data is excellent and provides a unique opportunity to study global‐scale dynamic phenomena; however, the local time resolution and coverage are limited because of the nature of the satellite sampling. The lack of local time coverage makes conventional methods of data analysis (e.g., Fourier analysis) both difficult and erroneous. An original method of analysis, based on a numerical model of atmospheric thermal tides, is proposed and applied to the HRDI data. The tidal model is solved for the tidal oscillations in the meridional wind component. The simulated diurnal meridional tidal winds are used as a first guess in the analysis. The results of the model are adjusted to give meridional migrating tidal winds that have maximum consistency with HRDI measurements. This technique is used to derive monthly mean tidal oscillations of the meridional velocity. The derived tidal amplitudes show profound seasonal changes that seem to be consistent with gravity wave breaking theory. The results are compared with MF radar data. It is found that in the upper mesosphere and lower thermosphere, the tidal amplitudes obtained by HRDI can be bigger than those from MF radars by a factor of 2.
Generation and evaluation of test gas mixtures for the Gas‐Phase Sulfur Intercomparison Experiment
Authors: MacTaggart Douglas L. , Farwell Sherry O. , Haakenson Timothy J. , Bamesberger W. Lee , Dorko William D.
Published: February 2004

The automated sulfur gas dilution system (ASGDS) was used during the Gas‐Phase Sulfur Intercomparison Experiment (GASIE) to generate and analyze test gas mixtures containing pptv levels of SO2 in the presence of various suites of potential measurement interferent gases. These test gas mixtures were provided in a 100 standard liters per minute air matrix manifold, allowing simultaneous sampling by all seven intercomparison participants. The test gas mixtures were homogeneous within the 90‐min test periods of the synthetic air phases of GASIE, allowing rigorous intercomparison of any two or more data points obtained during a given test period. A number of delivered SO2 concentrations were successfully replicated throughout the experiment, permitting the collection of a sufficient amount of data for statistical evaluation. The SO2 concentrations measured by the ASGDS quality control instrumentation agreed with values obtained by the intercomparison participants but were different from the set point values.
Mapping of boreal forest biomass from spaceborne synthetic aperture radar
Authors: Ranson K. Jon , Sun Guoqing , Lang Roger H. , Chauhan Narinder S. , Cacciola Randal J. , Kilic Ozlem
Published: February 2004

As part of the Boreal‐Ecosystem Atmosphere Study (BOREAS), an investigation is being made of the use of satellite data including shuttle imaging radar‐C (SIR‐C), X‐band synthetic aperture radar (XSAR), and Landsat‐Thematic Mapper data for estimating total and component aboveground woody biomass in boreal forest study sites in Canada. The goal of this paper is to present progress in mapping above ground woody biomass over portions of the BOREAS southern study area using spaceborne sensor data. Relationships of backscatter to total biomass and total biomass to foliage, branch, and bole biomass are used to estimate biomass across the landscape. The procedure involves image classification with SAR and Landsat data and development of simple mapping techniques using combinations of SAR channels. The analysis uses measurements from forest stands representing a range of biomass and structures. Field measurements included plot level mensuration (species, stem diameter, height, density, and basal area) and tree geometry measurements (leaf, branch, bole size, and angle distributions). The results indicate that aboveground biomass can be estimated to within about 1.6 kg/m2 and up to about 15 kg/m2 across the SIR‐C image evaluated. A general method produced equivalent results with those obtained by treating forest type (pine, spruce, and aspen) separately. The biomass mapping was extended to bole, branch, and foliage components from relationships with total aboveground biomass developed from detailed tree measurements. Average biomass within the imaged area was estimated to be about 7.3 kg/m2 with biomass components of bole, branch, and foliage comprising 83, 12, and 5% of the total. Examination of the scaling of biomass estimates from remote sensing images of varying resolution shows that information at scales useful for ecosystem models can be obtained. In addition, the biomass estimation technique provides similar information at different image resolutions.
Mesospheric Na layer at extreme high latitudes in summer
Authors: Plane John M. C. , Cox Rachel M. , Qian Jun , Pfenninger William M. , Papen George C. , Gardner Chester S. , Espy Patrick J.
Published: February 2004

Summertime observations of the mesospheric Na layer at high latitudes are reported from the 1993 Airborne Noctilucent Cloud (ANLC‐93) campaign in the Canadian Arctic and at the Amundsen‐Scott South Pole Station. Measurements at the South Pole reveal a layer that has a smaller column abundance and is significantly higher and thinner than at midlatitudes. Using a model that was essentially optimized to wintertime conditions at high northern latitudes, the South Pole layer can be modeled satisfactorily if the rate coefficient for the reaction between sodium bicarbonate and atomic hydrogen is set to k(NaHCO3 + H → Na + H2O + CO2) = 1.1 × 10−11 exp (−910/T) cm3 molecule−1 s−1. In particular, the model is able to reproduce the small scale height of about 2 km observed on the underside of the layer. It is then shown that this steep gradient in the atomic Na mixing ratio can be sustained against vertical eddy diffusion because of the sufficiently rapid chemical cycling between Na its major reservoir, NaHCO3. This justifies the assumption in the model that the vertical transport of Na species can be treated in terms of a single continuity equation describing total sodium. The observations from the campaigns in both hemispheres show that the Na abundance has a temperature dependence of about 2 × 108 cm−2 K−1 at temperatures below 170 K, in good accord with the model. About 40% of this dependence appears to be caused by the activation energies of the reactions that partition sodium between atomic Na and NaHCO3, and the remainder by the temperature dependence of the odd‐oxygen/odd‐hydrogen chemistry in the upper mesosphere.
Testing the daytime oxidizing capacity of the troposphere: 1994 OH field campaign at the Izaña observatory, Tenerife
Authors: Armerding W. , Comes F. J. , Crawack H. J. , Forberich O. , Gold G. , Ruger C. , Spiekermann M. , Walter J. , Cuevas E. , Redondas A. , Schmitt R. , Matuska P.
Published: February 2004

A field campaign was carried out during May 1994 at the Izaña station, Tenerife. This campaign was part of the program Environment and Climate sponsored by the European Commission to study the influence of European emissions on the oxidizing capacity of a clean tropospheric environment. Daytime and also nighttime measurements were made, covering the OH as well as the NO3 chemistry. This paper presents the OH measurements taken with a multipass optical absorption spectrometer (MOAS) and discusses the daytime chemistry in a statistical and therefore more preliminary way. All relevant parameters influencing the OH concentration were monitored. From the data the two main contributions to the OH production can clearly be discerned and are given by the primary production following the ozone photolysis and the O(1D)‐H2O reaction and by the catalytic reactions of NOx in the recycling process. The latter processes prove to contribute a dominant part to the OH concentration. The measurements of the nonmethane hydrocarbons (NMHC) especially of the biogenics, indicate a considerable influence of the NMHC on the absolute values of the OH concentration at Tenerife.
Cloud simulations with the Max Planck Institute for Meteorology general circulation model ECHAM4 and comparison with observations
Authors: Chen C.‐T. , Roeckner Erich
Published: February 2004

In this study, cloud parameters as simulated by the latest version of the Max Planck Institute for Meteorology general circulation model are documented and compared with observations. The model simulations generally agree with the observed spatial distribution and temporal variation of the total cloud amount. There are, however, biases in the details. Underestimation in the total cloud amount found over the midlatitude oceans in summer leads to significant biases in the simulated radiation budget. Considerable uncertainties of the observed total cloud amount in the polar region call for improved measurement techniques for further model validation. With a similar mean state of total cloud amount in the tropics between model and observation, fewer daily and interannual variabilities are found in the model. Despite large uncertainties in the current cloud liquid water path retrievals, the main pattern and magnitude of the space‐time distribution of cloud liquid water path is reseasonably well reproduced by the model. Lack of contrast between simulated cloud liquid water path in the subtropics and midlatitudes and failure to capture the observed summer local maxima of cloud liquid water path in the subtropical eastern ocean basins are the major discrepancies found in the model in comparison with the observations. The systematic differences in the magnitude of cloud liquid water path retrievals as shown in the comparison reveal a need for careful calibration of satellite retrieval algorithms. With a simple approach to prescribing land‐sea contrast in cloud droplet number concentrations the simulated distribution of effective radii of cloud water droplets in the tropics is in good agreement with observations. However, some physical processes (e.g., precipitation efficiency and air mass advection) in regulating cloud droplet number concentrations cannot be resolved by the simple prescription. The model simulation of cloud effective drop radii over the midlatitude oceans reveals errors larger than those in other regions of the globe.
In situ observations of air traffic emission signatures in the North Atlantic flight corridor
Authors: Schlager H. , Konopka P. , Schulte P. , Schumann U. , Ziereis H. , Arnold F. , Klemm M. , Hagen D. E. , Whitefield P. D. , Ovarlez J.
Published: February 2004

Focussed aircraft measurements have been carried out over the eastern North Atlantic to search for signals of air traffic emissions in the flight corridor region. Observations include NO, NO2, HNO3, SO2, O3, H2O, total condensation nuclei (CN), and meteorological parameters. A flight pattern with constant‐altitude north‐south legs across the major North Atlantic air traffic tracks was flown. Signatures of air traffic emissions were clearly detected for NOx, SO2, and CN with peak concentrations of 2 ppbv, 0.25 ppbv, and 500 cm−3, respectively, exceeding background values by factors of 30 (NOx), 5 (SO2), and 3 (CN). The observed NOx, SO2, and CN peaks were attributed to aircraft plumes based on radar observations of the source air traffic and wind measurements. Major aircraft exhaust signatures are due to relatively fresh emissions, i.e., superpositions of 2 to 5 plumes with ages of about 15 min to 3 hs. The observed plume peak concentrations of NOx compare fairly well with concentrations computed with a Gaussian plume model using horizontal and vertical diffusivities as obtained by recent large‐eddy simulations, measured vertical wind shear, and the corridor air traffic information. For the major emission signatures a mean CN/NOx abundance ratio of 300 cm−3ppbv−1 was measured corresponding to an emission index (EI) of about 1016 particles per 1 kg fuel burnt. This is higher than the expected soot particle EI of modern wide‐bodied aircraft. For the most prominent plumes no increase of HNO3 concentrations exceeding variations of background values was observed. This indicates that only a small fraction of the emitted NOx is oxidized in the plumes within a timescale of about 3 hs for the conditions of the measurements.
Stratospheric effects of Mount Pinatubo aerosol studied with a coupled two‐dimensional model
Authors: Rosenfield Joan E. , Considine David B. , Meade Paul E. , Bacmeister Julio T. , Jackman Charles H. , Schoeberl Mark R.
Published: February 2004

A new interactive radiative‐dynamical‐chemical zonally averaged two‐dimensional model has been developed at Goddard Space Flight Center. The model includes a linear planetary wave parameterization featuring wave‐mean flow interaction and the direct calculation of eddy mixing from planetary wave dissipation. It utilizes family gas phase chemistry approximations and includes heterogeneous chemistry on the surfaces of both stratospheric sulfate aerosols and polar stratospheric clouds. This model has been used to study the effects of the sulfate aerosol cloud formed by the eruption of Mount Pinatubo in June 1991 on stratospheric temperatures, dynamics, and chemistry. Aerosol extinctions and surface area densities were constrained by satellite observations and were used to compute the aerosol effects on radiative heating rates, photolysis rates, and heterogeneous chemistry. The net predicted perturbations to the column ozone amount were low‐latitude depletions of 2–3% and northern and southern high‐latitude depletions of 10–12%, in good agreement with observations. In the low latitudes a depletion of roughly 1–2% was due to the altered circulation (increased upwelling) resulting from the perturbation of the heating rates, with the heterogeneous chemistry and photolysis rate perturbations contributing roughly 0.5% each. In the high latitudes the computed ozone column depletions were mainly a result of heterogeneous chemistry occurring on the surfaces of the volcanic aerosol. Temperature anomalies predicted were a low‐latitude warming peaking at 2.5 K in mid‐1992 and high‐latitude coolings of 1–2 K which were associated with the high‐latitude ozone reductions. The sensitivity of the predicted perturbations to changes in the specification of the planetary wave forcings was examined. The maximum globally averaged column ozone depletions ranged from 2 to 4% for the cases studied.
Hierarchical characterization of canopy architecture for boreal forest
Authors: Fournier R. A. , Rich P. M. , Landry R.
Published: February 2004

Canopy architecture, the geometric organization of aboveground vegetation, provides an essential link between patterns observable by remote sensing and fundamental ecological processes. As part of the international Boreal Ecosystem‐Atmosphere Study (BOREAS); (1) we developed a hierarchical approach for sampling canopy architecture, and (2) we acquired a comprehensive data set for characterizing canopy architecture for the major BOREAS study sites. The approach involves a series of four sets of measurements at different spatial scales, ranging from the ecosystem to the leaf level: (1) regional characterization, involving measurements of the climate, ecosystem, and landscape features; (2) stand characterization, involving measurements of crown geometry (diameter at breast height (DBH), height, and crown extent), individual tree location, and understory cover; (3) tree vectorization, involving detailed sampling of the three‐dimensional distribution of canopy elements and crown form; and (4) characterization of canopy geometry as seen from beneath, involving acquisition of a multitemporal catalog of hemispherical photographs. The last three sets of measurements were then used to reconstruct the three‐dimensional geometry of the canopy. By comparing simulated hemispherical views upward from beneath this reconstructed canopy with in situ hemispherical photographs, the methodological approach was validated. Simulated photographs faithfully reproduced patterns observed for in situ hemispherical photographs, in particular, for gap fraction distributions of the middle ranges of zenith angles (20°–70°). Moreover, simulation of the gap fraction for this middle portion of the zenith angle was insensitive to exact mapping of the stand. The hierarchical data acquisition approach, involving mapping of tree locations and tree reconstruction, permits realistic representation of canopy material distribution. Our approach and our comprehensive data set provide a solid basis from which to integrate data gathered at the stand and tree scales, and a powerful tool for the simulation of the light regime anywhere in the canopy.
Selected ion chemical ionization mass spectrometric measurement of OH
Authors: Tanner D. J. , Jefferson A. , Eisele F. L.
Published: February 2004

Ion‐assisted OH measurements are presented from two different standpoints. First, instrument operation and calibration are discussed along with recent improvements in measurement techniques. A chemical model of inlet titration chemistry in both clean and polluted air is also included in this section. The results of OH measurements using the present technique are presented for the entire tropospheric OH photochemistry campaign. Hydroxyl radical concentrations of 2–4×106 molecules cm−3 were typical at noon for clean, clear days. In polluted air masses, OH concentrations up to 8×106 molecules cm−3 were observed. Variation of OH with changing and differing air masses is clearly shown and discussed.
In situ CO concentrations at the sites Zugspitze (47°N, 11°E) and Cape Point (34°S, 18°E) in April and October 1994
Authors: Scheel H. E. , Brunke E.‐G. , Sladkovic R. , Seiler W.
Published: February 2004

Continuous measurements of CO have been performed at the coastal site Cape Point (34°S, 18°E) and at the continental mountain site Zugspitze (47°N, 11°E) since 1978 and 1990, respectively. The CO concentrations observed at both stations in 1994 during the periods April 9–18, and September 30 to October 10 were studied in detail with the aim to supplement CO observations from space made by NASA during two shuttle flights (measurement of air pollution from satellites (MAPS) project). All CO values reported were adjusted to the NOAA Climate Monitoring and Diagnostics Laboratory (CMDL) calibration scale. CO mixing ratios at Cape Point comprised episodes of baseline CO levels in maritime air (average: 48.8 ppb (April); 69.4 ppb (September/October)) and of elevated levels related to continental air and regional pollution (average: 89.5 ppb (April); 102.1 ppb (September/‐October)). During the same periods the mean CO mixing ratio at the Zugspitze amounted to 220.1 ppb (range 150 ‐ 358 ppb) and 124.8 ppb (range 78–175 ppb), respectively. Due to the prevailing airflow, CO levels at the Zugspitze were unusually high compared to the long‐term average during the April mission (time of seasonal maximum). In contrast, they were close to the average during the October mission. Baseline concentrations at Cape Point were about 3–4 ppb above the long‐term average for both April (increasing side of seasonal cycle) and October (time of seasonal maximum), which is within the range of normal interannual variability. A preliminary comparison with the MAPS data for the respective geographic regions showed good agreement of the CO mixing ratios for clean air at Cape Point in April and at the Zugspitze in October. In addition, the CO levels observed in continental air at Cape Point in October agreed well with the MAPS data. Obvious differences in CO concentrations for other parts of the records mainly reflected uncomparable atmospheric conditions. In particular, they seemed to be related to either vertical gradients of CO or small‐scale differences in air mass composition as indicated by meteorological analyses.
Mass dependence of energy loss in collisions of icy spheres: An experimental study
Authors: Dilley James , Crawford Darren
Published: February 2004

By varying the mass of icy spheres which collide with large ice blocks, we have determined the mass dependence of energy loss in such collisions. We find, at commercial deep freeze temperatures, that collisions at 1 cm/s become quite inelastic for small iceballs and that the rate at which elasticity decreases with decreasing mass roughly coincide with that predicted by a viscous dissipation model. If such a mass dependence persists at much lower temperatures, it could lead to very large effects in planetary ring systems (rings and dust, ice, surfaces, origin, and evolution).
Phase transitions in the Martian mantle: Implications for the planet's volcanic history
Authors: Breuer D. , Zhou H. , Yuen D. A. , Spohn T.
Published: February 2004

The influences of phase transitions on Martian mantle convection and melting in the Martian mantle have been studied with an axisymmetric spherical‐shell model. An extended Boussinesq model in which viscous and adiabatic heatings are included has been used. There are depth dependences in the thermal expansivity and gravity, which, taken together, decrease by a factor of between 2 and 3 across the Martian mantle. The two destabilizing exothermic phase transitions, olivine to β‐spinel and β‐ and γ‐spinel transitions, above the Martian core‐mantle boundary (CMB) accelerate the mantle flow and result in an amplification and superheating of plumes in Mars. The additional consideration of the endothermic phase transition, spinel to perovskite transition, which was only likely present in the early evolution of the planet, has little influence on Martian mantle convection in this model. Strong localized viscous heating is generated near the CMB and underneath the lithosphere because here the flow bends over sharply. A possible volcanic evolution of Mars can be derived from a comparison of the temperature fields with the solidus of anhydrous peridotite if the cooling of the Martian mantle is taken into account. In the early evolution the mantle was molten to a high degree along the plume axis, which possibly resulted in a strong differentiation of the mantle. As the planet cooled down, the region of melt generation receded to where the maximum of viscous heating occurred: at the CMB and immediately underneath the lithosphere. Upon further cooling, the deep‐mantle melt source region became subsolidus. The most recent volcanism on Mars was most likely generated at a shallow depth below the lithosphere.
Extrasolar planet detection at infrared wavelengths from the Earth
Authors: Yu Ka Chun , Bally John
Published: February 2004

We show that extrasolar planets in orbit around nearby stars can be detected from the ground or from a stratospheric telescope in the infrared region of the spectrum. We present calculations on the detectability of extrasolar planets, using Erich Grossman's Atmospheric Transmission (AT) code to compute atmospheric transmission at bands centered at 11 μm, 20 μm, 27 μm, 200 μm, 225 μm, 350 μm, 450 μm, 640 μm, 750 μm, and 870 μm. Detection limits for terrestrial and Jovian planets orbiting Sun‐like stars are presented by assuming Planck emission. We consider several potential sites including the south pole, a 5000‐m elevation site in the Atacama Desert in Chile, Mauna Kea in Hawaii, and an aerostat‐borne telescope flown near the poles. We consider extrasolar planet detection with a 10‐m class mid‐IR telescope and a dilute aperture stratospheric telescope consisting of 4‐m mirrors optimized for this task. Detection in the submillimeter, even with a 104 m2 collecting area array is extremely difficult because of low atmospheric transparency, and the decrease (∝ λ−2) in planet emission with increasing wavelength in the Rayleigh‐Jeans limit. We discuss critical technologies needed for this undertaking, including tethered aerostats and balloon‐borne telescopes, the development of mid‐IR nulling interferometry, actively cooled optics operating in the atmosphere, and optimized filters that are matched to the atmospheric transmission windows.
Transport timescales in the Martian atmosphere: General circulation model simulations
Authors: Barnes Jeffrey R. , Walsh Thomas D. , Murphy James R.
Published: February 2004

Simulations with a Mars general circulation model (GCM) are used to perform idealized tracer transport experiments, which are analyzed to yield estimates of eddy mixing coefficients as well as “stratospheric” ventilation timescales for the zonal‐mean circulation. The results indicate that relatively moderate values of the vertical eddy mixing coefficient, Kz ∼ 20–100 m2/s, may be most appropriate for the 10 to 45‐km altitude region of the Martian atmosphere. Under dusty solstice conditions, somewhat stronger eddy mixing is present, but the transport is dominated by advection by the mean meridional circulation which acts to ventilate the atmosphere above 1 mbar (∼20 km) in only about 7 days. Such a mean circulation has transport effects which are roughly comparable to those produced by eddy mixing with a much larger Kz value, ∼1500 m2/s. In contrast, the computed mean ventilation timescale for a nondusty equinox circulation is approximately 180 days. In this case, vertical eddy mixing is the dominant transport process. In an intermediate nondusty solstice case, the effects of mean advection and eddy mixing are of comparable importance. An estimate of a mean ventilation timescale from the GCM for a late northern winter seasonal date (∼45 days) is in very good agreement with the value of ∼38 days recently inferred from Mariner 9 infrared imaging spectrometer (IRIS) data [Santee and Crisp, 1995].
Geomorphic processes and remote sensing signatures of alluvial fans in the Kun Lun Mountains, China
Authors: Farr Tom G. , Chadwick Oliver A.
Published: February 2004

The timing of alluvial deposition in arid and semiarid areas is tied to land‐surface instability caused by regional climate changes. The distribution pattern of dated deposits provides maps of regional land‐surface response to past climate change. Sensitivity to differences in surface roughness and composition makes remote sensing techniques useful for regional mapping of alluvial deposits. Radar images from the Spaceborne Radar Laboratory and visible wavelength images from the French SPOT satellite were used to determine remote sensing signatures of alluvial fan units for an area in the Kun Lun Mountains of northwestern China. These data were combined with field observations to compare surface processes and their effects on remote sensing signatures in northwestern China and the southwestern United States. Geomorphic processes affecting alluvial fans in the two areas include aeolian deposition, desert varnish, and fluvial dissection. However, salt weathering is a much more important process in the Kun Lun than in the southwestern United States. This slows the formation of desert varnish and prevents desert pavement from forming. Thus the Kun Lun signatures are indicative of the dominance of salt weathering, while signatures from the southwestern United States are characteristic of the dominance of desert varnish and pavement processes. Remote sensing signatures are consistent enough in these two regions to be used for mapping fan units over large areas.
Global imaging of Mars by Hubble space telescope during the 1995 opposition
Authors: James Philip B. , Bell James F. , Clancy R. Todd , Lee Steven W. , Martin Leonard J. , Wolff Michael J.
Published: February 2004

Hubble space telescope (HST) imaging of Mars near the 1995 opposition resulted in excellent synoptic‐scale images of the planet during the spring season in the northern hemisphere. Because this season coincides with the aphelion position of Mars in its orbit, it is therefore the most difficult for ground based observation because of the relatively small angular size of Mars. This is the first sequence of images fully utilizing the capability of the new Planetary Camera to produce global synoptic images of the planet. The images reveal bright, discrete clouds associated with topographic features superimposed on a zonal band of condensate clouds between latitudes −10° and 30°; the maximum violet optical depth of the cloud band is about 0.3. In a few instances, the appearance of clouds beyond the morning terminator can be used to infer cloud heights of roughly 8 km. A large, dark albedo feature in the Cerberus region, observed for many years by ground‐based observers, has almost disappeared in the 1995 HST images. Other aspects of Mars, such as the north polar cap, appear much as they did during previous oppositions. Although cloudy regions were observed by spacecraft during this season, the HST images uniquely reveal the global extent of significant optical depth clouds.
Hydrodynamic escape of hydrogen from a hot water‐rich atmosphere: The case of Venus
Authors: Chassefière E.
Published: February 2004

A one‐dimensional model of hydrodynamic escape is used to study the loss of hydrogen from a hot, water‐rich atmosphere of the Venus type. A range of EUV heating rates corresponding to the present solar cycle fluctuations of the EUV flux and different possible heating efficiencies are considered. The present model takes into account the transition to the collisionless state at the exobase through a modified Jeans' approach. For the fluid inner planetary corona, the conservation equations are solved from the base of the expanding flow (z ≈ 200 km) up to the exobase, generally located at an altitude of ≈1 planetary radius. Solutions are found, for which the flux in the collisional region is equal to the Jeans escape flux at the exobase. The ionization state of the planetary wind is calculated, and the position of the obstacle is determined by equilibrating the solar wind ram pressure and the pressure of the expanding plasma. It is shown that ≈2/3 of the escape energy is supplied by energetic neutrals formed by charge exchange between escaping H atoms and solar protons in the heliosphere, a fraction of which intercepts the exobase and heats by collision the upper layers of the fluid planetary corona. Any planetary magnetic field pushing away the obstacle up to an altitude larger than ≈3 planetary radii would inhibit this effect. The combination of (1) the geometric amplification of solar EUV absorption at z > ≈2000 km due to the increase with altitude of the cross‐sectional area of the planetary corona and (2) the substantial inward flux of energy at the exobase through heating by energetic neutrals results in the formation of a stable hot region below the exobase, allowing substantial escape. A detailed energetic budget of the escape process is established. An upper limit of ≈3 × 1011 cm−2 s−1 on the escape flux is found. The ratio of the EUV flux to the solar wind strength is shown to be of prime importance, since the solar wind regulates the escape flux from outside, through the action of energetic neutrals, and the EUV flux acts from inside, by supplying atoms with the energy required to lift them up to the exobase.
An analytical model of the atmospheric entry of large meteors and its application to the Tunguska Event
Authors: Lyne J. E. , Tauber Michael , Fought Richard
Published: February 2004

The atmospheric entry of a meteor is quite complex, with the body losing kinetic energy both from atmospheric drag and from mass loss due to aerodynamic heating. Moreover, high pressures on the windward side of the body result in enormous compressive stresses which may exceed the yield strength of the material and cause rapid fragmentation of the meteor. While ablative mass loss is not important for extremely large objects, it must be accurately estimated to correctly predict the trajectories of objects that are several tens of meters in diameter. The current paper describes a computer model which performs calculations of shock layer conditions, accounting for the time varying temperature distribution, radiative cooling of the shocked gases, and blockage of surface heating by ablation products. Application of the model to the well‐known Tunguska Event indicates that the responsible bolide was probably a carbonaceous chondrite, although a stony asteroid or a cometary body cannot be conclusively ruled out.
Absolute partial cross sections for electron impact ionization of SO2 from threshold to 1000 eV
Authors: Lindsay B. G. , Straub H. C. , Smith K. A. , Stebbings R. F.
Published: February 2004

Absolute partial cross sections for the production of SO2+, SO+, (S+ + O2+ + SO22+), O+, and SO2+ from electron impact ionization of SO2 are reported for electron energies from threshold to 1000 eV. The product ions are analyzed using a time‐of‐flight mass spectrometer and detected with a position‐sensitive detector, making it possible to demonstrate complete collection of all product ions. The overall uncertainty in the absolute cross section values is ±3.5% for SO2+ ions and is slightly higher for fragment and multiply charged ions. While much of the previously published data are in reasonable agreement with the data presented here, the present O+ cross section is higher than earlier measurements by approximately a factor of 3. The cross section for production of SO2+ has not been previously reported. The sum of the partial cross sections is compared to previously measured total cross sections.
Paleohydrology and flood geomorphology of Ares Vallis
Authors: Komatsu G. , Baker V. R.
Published: February 2004

Ares Vallis is a Martian outflow channel which is about 1500 km long and locally exceeds 100 km in width and 1000 m in depth. We estimate the channel's paleoflow capacity using paleohydraulic techniques. At a constricted and unusually deep reach, not affected by tributaries or secondary inflows, the estimated possible maximum peak discharges are of the order of 108–109m3/s. These values are 1 to 2 orders of magnitude larger than any known terrestrial floods. These high‐discharge flows were theoretically capable of transporting boulders larger than 10 m in diameter. The downstream depositional plain, where Ares debauches into the Chryse Planitia, displays geomorphological features indicative of high erosional capacity by the flooding. In this area, site of the proposed Pathfinder landing, we expect to find a complex sequence of sediments created by varying paleoflood flow hydraulics, sediment transport characteristics, and probable secondary modification of the primary flood landforms.
Extension across Tempe Terra, Mars, from measurements of fault scarp widths and deformed craters
Authors: Golombek M. P. , Tanaka K. L. , Franklin B. J.
Published: February 2004

Two independent methods, with no common assumptions, have been used to estimate the extension across the heavily deformed Tempe Terra province of the Tharsis region of Mars. One method uses measurements of normal fault scarp width with average scarp slope data for simple grabens and rifts on Mars to estimate the fault throw, which, combined with sparse fault dip data, can be used to estimate extension. Formal uncertainties in this method are only slightly greater than those in other methods, given that the total uncertainty is dominated by the likely uncertainty in the fault dip (assumed to be 60° ± 15°). Measurement of normal fault scarp widths along two N25°–50°W directed traverses across Tempe Terra both yield about 22 ± 16 km of extension (or ∼2% strain across the northern traverse and nearly 3% across the southern one). About three quarters of the extension has occurred during the two main phases of Tharsis‐related deformation from Middle/Late Noachian to Early Hesperian and from Late Hesperian to Early Amazonian, with more extension closer to the center of Tharsis during the first phase. Extension across the region was also determined by measuring the elongation and elongation direction of all ancient Noachian impact craters without ejecta blankets, which predate most of the deformation. Results have been corrected for initial non circularity of craters, established from similar measurements of young (post deformation) impact craters, yielding a statistically significant mean strain of 1.96 ± 0.35% in a N38° ± 10°W direction across Tempe Terra (extension of ∼20 ± 4, comparable in magnitude and direction to the average result from the scarp measurement method). Both methods indicate an average extension for single normal fault scarps (and shortening across wrinkle ridges for the crater method) of ∼100 m. The agreement between the results of the two independent methods in overall extension and average single normal fault extension argues that the average scarp slope and fault dip data in the fault scarp width method accurately represent the actual extension across the observed structures. This conclusion supports existing geometric and kinematic models for structural features on Mars. A preliminary estimate of the total circumferential extension around Tharsis (at a radius of ∼2500 km) is roughly 60 ± 42 km; total hoop strain is about 0.4% distributed heterogeneously (Tempe Terra is the most highly strained region on Mars).
Hydrothermal hydration of Martian crust: Illustration via geochemical model calculations
Authors: Griffith Laura L. , Shock Everett L.
Published: February 2004

If hydrothermal systems existed on Mars, hydration of crustal rocks may have had the potential to affect the water budget of the planet. We have conducted geochemical model calculations to investigate the relative roles of host rock composition, temperature, water‐to‐rock ratio, and initial fluid oxygen fugacity on the mineralogy of hydrothermal alteration assemblages, as well as the effectiveness of alteration to store water in the crust as hydrous minerals. In order to place calculations for Mars in perspective, models of hydrothermal alteration of three genetically related Icelandic volcanics (a basalt, andesite, and rhyolite) are presented, together with results for compositions based on SNC meteorite samples (Shergotty and Chassigny). Temperatures from 150°C to 250°C, water‐to‐rock ratios from 0.1 to 1000, and two initial fluid oxygen fugacities are considered in the models. Model results for water‐to‐rock ratios less than 10 are emphasized because they are likely to be more applicable to Mars. In accord with studies of low‐grade alteration of terrestrial rocks, we find that the major controls on hydrous mineral production are host rock composition and temperature. Over the range of conditions considered, the alteration of Shergotty shows the greatest potential for storing water as hydrous minerals, and the alteration of Icelandic rhyolite has the lowest potential.
Topography of the Moon from the Clementine lidar
Authors: Smith David E. , Zuber Maria T. , Neumann Gregory A. , Lemoine Frank G.
Published: February 2004

Range measurements from the lidar instrument carried aboard the Clementine spacecraft have been used to produce an accurate global topographic model of the Moon. This paper discusses the function of the lidar; the acquisition, processing, and filtering of observations to produce a global topographic model; and the determination of parameters that define the fundamental shape of the Moon. Our topographic model; a 72nd degree and order spherical harmonic expansion of lunar radii, is designated Goddard Lunar Topography Model 2 (GLTM 2). This topographic field has an absolute vertical accuracy of approximately 100 m and a spatial resolution of 2.5°. The field shows that the Moon can be described as a sphere with maximum positive and negative deviations of ∼8 km, both occurring on the farside, in the areas of the Korolev and South Pole‐Aitken (S.P.‐Aitken) basins. The amplitude spectrum of the topography shows more power at longer wavelengths as compared to previous models, owing to more complete sampling of the surface, particularly the farside. A comparison of elevations derived from the Clementine lidar to control point elevations from the Apollo laser altimeters indicates that measured relative topographic heights generally agree to within ∼200 m over the maria. While the major axis of the lunar gravity field is aligned in the Earth‐Moon direction, the major axis of topography is displaced from this line by approximately 10° to the east and intersects the farside 24° north of the equator. The magnitude of impact basin topography is greater than the lunar flattening (∼2 km) and equatorial ellipticity (∼800 m), which imposes a significant challenge to interpreting the lunar figure. The floors of mare basins are shown to lie close to an equipotential surface, while the floors of unflooded large basins, except for S.P.‐Aitken, lie above this equipotential. The radii of basin floors are thus consistent with a hydrostatic mechanism for the absence of significant farside maria except for S.P.‐Aitken, whose depth and lack of mare require significant internal compositional and/or thermal heterogeneity. A macroscale surface roughness map shows that roughness at length scales of 101–102 km correlates with elevation and surface age.
The xenon isotopic composition of the primordial Martian atmosphere: Contributions from solar and fission components
Authors: Swindle T. D. , Jones J. H.
Published: February 2004

Previous models of the evolution of the isotopic composition of Xe in the Martian atmosphere have little room for any Xe produced by fission of 244Pu, although there is a contribution from the decay of shorter‐lived 129I. We find that the previously calculated ratios of I‐derived to Pu‐derived Xe can only be matched if Mars accreted very early and outgassing of I‐derived Xe after atmospheric loss was much stronger than outgassing of Pu‐derived Xe. Alternatively, we find that if we assume that primordial Xe on Mars had the isotopic composition of the solar wind, rather than the previously assumed chondritic Xe, a larger contribution from 244Pu is allowed, and it is much easier to generate models of outgassing and atmospheric loss that match the present atmosphere. Furthermore, the isotopic composition of solar wind Xe matches that of the Chassigny meteorite (widely considered to represent a sample of Martian interior Xe), so it is not necessary to postulate separate sources of Xe for the interior and atmosphere of Mars.
Comparison of IUE and HST diagnostics of the Jovian Aurorae
Authors: Prangé R. , Maurice S. , Harris W. M. , Rego D. , Livengood T.
Published: February 2004

Analysis of international ultraviolet explorer (IUE) and Voyager ultraviolet spectrometer (UVS) spectra of the Jovian auroral emission indicates that the Jovian auroral brightness is modulated in longitude (brighter near 180° in the north and 20° in the south) and that there is a color ratio asymmetry associated with this brightening. The purpose of this study is to investigate the origin of this apparent asymmetry. To that end, we use a series of six typical images of the north auroral region taken in the H2 Lyman bands with the faint object camera (FOC) aboard the Hubble space telescope (HST) and which cover a full Jovian rotation. Although the images do not display any strong brightening near 180°, once we have simulated the signal IUE would see through its aperture, we find the characteristic longitudinal modulation. We attribute most of this modulation to a combination of viewing geometry effects near the east and west ansae of the auroral oval (already taken into account in previous studies) and of the spatial degradation of the source by the IUE instrument function (never considered so far), and we suggest qualitatively that these effects may also affect the color ratio asymmetry. Nevertheless, part of the asymmetry seems to be due to an intrinsic modulation associated with a bright feature crossing the polar cap along the 160° meridian (transpolar emission) and present in most of the images. We then use a series of FOC images taken during an atypically strong auroral event, and we show that the same effects can again account for the anomalous brightness variations observed simultaneously with IUE.