bluuebunny/crossref_metadata_2025 huggingface dataset:
Local meteorological features affecting chemical measurements at a North Atlantic coastal site
Authors: Angevine Wayne M. , Buhr Martin P. , Holloway John S. , Trainer Michael , Parrish David D. , MacPherson J. Ian , Kok Gregory L. , Schillawski Richard D. , Bowlby Don H.
Published: February 2004

One of the foci of the North Atlantic Regional Experiment (NARE) 1993 summer intensive campaign was Chebogue Point, approximately 10 km south of Yarmouth, Nova Scotia. Measurements were taken at this site with a 915‐MHz boundary layer wind profiler, the Canadian Twin Otter and National Center for Atmospheric Research King Air aircraft, and a variety of surface instruments. This paper discusses features observed in the meteorological measurements and the implications of those features for the interpretation of chemical measurements. The meteorology of this coastal site is complex. A strong surface‐based temperature inversion was almost always present, producing strong layering in the lower atmosphere. As a result, surface chemistry measurements were not often representative of the state of the overlying atmosphere. A low‐level jet was also frequently present. A variety of turbulence structures were observed by the profiler, including convective boundary layers and complex layering. Ozone concentrations above 60 parts per billion at the surface occurred on four occasions late in August, in conjunction with strong stability and winds off the Atlantic Ocean (Gulf of Maine). Ozone levels of more than 60 ppbv were observed by the aircraft at altitudes between 400 and 1600 m on five other occasions earlier in the month. Particular (but not necessarily unusual) combinations of transport, mixing, and source conditions appear to be required to produce ozone episodes at Chebogue Point.
Interaction between nitrogen and sulfur cycles in the polluted marine boundary layer
Authors: Yvon S. A. , Plane J. M. C. , Nien C.‐F. , Cooper D. J. , Saltzman E. S.
Published: February 2004

Simultaneous measurements are reported of the nitrate radical (NO3), nitrogen dioxide (NO2), ozone (O3), and dimethylsulfide (DMS) in the nighttime marine boundary layer over Biscayne Bay in South Florida. These field observations are analyzed and used to initialize a boundary layer box model which examines the relative importance of the various sinks for NOx in the marine boundary layer. The results show that the observed lifetime of NO3 (≤6 min.) is probably controlled both by the loss of nitrogen pentoxide (N2O5) to reaction with water vapor and aerosols and by the reaction between NO3 and DMS. The model is then extended to investigate the loss of nitrogen oxides from an air parcel that remains in the boundary layer with a constant sea‐to‐air DMS flux for several days. The principal conclusions are (1) that DMS is a much more important sink for NO3 at lower NO2 levels and (2) that the reaction between NO3 and DMS is an important sink for DMS in the marine boundary layer and could exceed that of the daytime removal by OH.
Comparison of Mount Pinatubo and El Chichon volcanic events: Lidar observations at 10.6 and 0.69 μm
Authors: Post Madison J. , Grund Christian J. , Weickmann Ann M. , Healy Kathleen R. , Willis Ron J.
Published: February 2004

Analyses of a dense set of lidar observations of the volcanic clouds from Mount Pinatubo and El Chichon, primarily over Boulder, Colorado, and at a wavelength of 10.59 μm, reveal notable similarities and differences. The importance of tropopause folding events for purging stratospheric debris into the troposphere is confirmed, as well as the role of gravitational settling to bring debris into the grasp of the folding mechanism. Seasonal influences appear more important than time after eruption in both poleward transport and in purging stratospheric material downward. We discuss the lengthening of the decay process for the Mount Pinatubo clouds by either an equatorial reservoir of debris or by eruptions of Mount Spurr. Simultaneous observations at 0.69 and 10.59 μm help monitor aerosol effective radii versus altitude with good temporal and vertical resolution.
Evolution of the stratospheric aerosol in the northern hemisphere following the June 1991 volcanic eruption of Mount Pinatubo: Role of tropospheric‐stratospheric exchange and transport
Authors: Jónsson Hafliòi H. , Wilson James C. , Brock Charles A. , Dye J. E. , Ferry G. V. , Chan K. R.
Published: February 2004

Since the eruption of Mount Pinatubo in June, 1991, measurements of particle size and concentration have intermittently been carried out from an ER‐2 aircraft at altitudes of up to 21 km at midlatitudes and high latitudes in the northern hemisphere. They show the evolution and purge of the volcanic aerosol to be due to an interaction of aerosol mechanics with tropospheric‐stratospheric exchange processes, transport, and mixing. During the first 5 months after the eruption the volcanic plume spread to higher latitudes in laminae and filaments, producing steep spatial gradients in the properties of the stratospheric aerosol. At the same time the concentration of newly formed particles in the plume rapidly decreased toward background values as a result of coagulation while particle size and aerosol surface area continued to increase. By December 1991, the particle number mixing ratios and aerosol surface area mixing ratios had become spatially uniform over a wide range of latitudes above 18 km. The surface area mixing ratios peaked in this region of the stratosphere at ∼35 times their background values in the winter of 1992. The corresponding condensed mass mixing ratio enhancement was by a factor of ∼200. After the winter of 1992, a gradual removal of the volcanic mass began and initially was dominated by sedimentation above 18 km. The aerosol surface area mixing ratio thus decreased by an order of magnitude over 2.5 years, and the aerosol volume, or condensed mass, mixing ratio decayed by an order of magnitude over approximately 1.7 years. Below 18 km, the purging of the Pinatubo aerosol at mid‐latitudes appeared sporadic and disorderly and was strongly influenced by episodal rapid quasi‐isentropic transport and dilution by tropical air of tropospheric origin having high condensation nuclei mixing ratios but low mixing ratios of aerosol surface area or condensed mass compared to the volcanic aerosol.
A three‐year gravity wave climatology of the mesosphere and lower thermosphere over Kauai
Authors: Connor L. N. , Avery S. K.
Published: February 2004

A 3‐year study of gravity wave variability covering 1991, 1992, and 1993 was conducted using horizontal wind measurements collected by a partial reflection MF radar located on the island of Kauai, Hawaii (22 N, 160 W). High echo return rates between 70 and 98 km provided ample time resolution for the study of gravity wave variability in this height range. Horizontal wind measurements were band pass filtered for periods between 20 min and 6 hours, establishing a gravity wave band time series. Gravity wave variance and polarization calculations were carried out over various time frames and examined with regard to their response to season and background environment. Among the more interesting observed phenomena were the annual and semiannual oscillations in the gravity wave variances, a significant diurnal and semidiurnal variability in the variances, and an extended enhancement of these variances during the latter half of 1992 and the beginning of 1993.
Physical properties of marine boundary layer aerosol particles of the mid‐Pacific in relation to sources and meteorological transport
Authors: Covert D. S. , Kapustin V. N. , Bates T. S. , Quinn P. K.
Published: February 2004

Aerosol measurements were made on three cruises in the mid‐Pacific along longitude 140°W from 55°N to 70°S for a total of about 90 days in 1992 and 1993. The three data sets document the aerosol concentration and general features of its number‐size distribution in the marine boundary layer (MBL) and their variation with latitude and meteorological conditions. Mean concentration varied from 300 cm−3 in the tropics to 500 cm−3 in the midlatitudes outside of continental air masses. Infrequent short‐term spikes in concentration ranged up to 2000 cm−3. Two dominant modes were observed, the Aitken and accumulation, with mean diameters of 25 to 60 nm and 150 to 200 nm, respectively. An intermittent ultrafine mode was noted at diameters less than 25 nm. The concentration and dominance of one mode over another depended on the relative strength of the entrainment of ultrafine and Aitken particles from the free troposphere (FT) into the MBL compared to the rate of growth of Aitken mode into accumulation mode particles and removal rate of the accumulation mode. In general, aging times were shorter in the subtropics, longer in the tropics, and variable in the midlatitudes. The rate of new particle formation within the MBL itself was either low and did not contribute significantly to the observed number concentration or, if the rate was high, it occurred infrequently and was not observed in these experiments.
Sensitivity of buoyant plume heights to ambient atmospheric conditions: Implications for volcanic eruption columns
Authors: Glaze Lori S. , Baloga Stephen M.
Published: February 2004

A theoretical model is developed to investigate the sensitivity of buoyant atmospheric plumes to a wide range of ambient atmospheric conditions, including the temperature gradient, the latitude of the source, and the season. The formulation highlights the compressibility of an ideal gas, internal consistency between the governing equations for the conservation of momentum and energy, and the explicit use of the equation of state. Specific results are presented for water vapor plumes and implications are developed for multicomponent (water vapor, silicate particles, and condensates) volcanic plumes. If plume cooling is due solely to adiabatic expansion and the entrainment and mixing of ambient air, then the atmospheric temperature gradient is shown to be a dominant influence on plume height. Changes in the atmospheric gradient of 10 K/km cause the height of a low‐level plume to diifer by a factor of 2. We estimate the magnitude of this effect on volcanic plumes by considering water vapor erupted with equivalent heat fluxes. The sensitivity of plumes to ambient conditions is a result of the small density difference driving buoyancy. The plume density, in turn, is strongly controlled by the thermal energy of the system. Sensitivities associated with the thermal energy balance in the eruption column are also investigated. A modest thermal loss (1–2%/km) from the column by a process other than entrainment can result in a plume height significantly lower than one that cools by entrainment alone. Additional cooling of this magnitude could be caused by a variety of combinations of phenomena, including radiative heat loss and, possibly, the conversion of heat energy into turbulent rotational energy. For particle‐laden plumes, there is the possibility of additional heat loss through the fallout of solids from the eruption column. To understand the details of the thermal energy balance in a plume, measurements must be made of the bulk plume temperature profile under known atmospheric conditions.
New theoretical framework for studies of vapor growth and sublimation of small ice crystals in the atmosphere
Authors: Nelson J. T. , Baker M. B.
Published: February 2004

We present a new microscopic model of growth and sublimation (g/s) of ice crystals in the atmosphere. This model is based on the assumption that the flux of vapor to the crystal surface is uniform over each flat crystal face. It thus differs fundamentally from the standard “capacitance” model for crystal growth, in which the mixing ratio is assumed uniform at the surface. In the new model the surface influence on growth is calculated self‐consistently in terms of local environmental conditions, again differing sharply from the standard models in which this influence is either ignored or assigned a uniform, externally prescribed value. The new model leads to predictions of the evolution of ice crystal shape as well as mass. We find that predicted g/s rates are generally smaller than those predicted by the earlier models. The general trends both in g/s rates and in crystal hollowing predicted by the model are consistent with field and laboratory observations. The values of certain surface parameters needed for application of our model must be found from experiment. We review and compare the relevant laboratory experiments on ice crystal g/s rates and show their lack of mutual consistency. Therefore the surface parameters inferred from these experiments are necessarily uncertain. We show that the surface parameter values can be inferred from observations of crystal hollowing, since our model allows the prediction of environmental conditions at which hollowing should occur.
New technique for investigating phase transition processes of optically levitated droplets consisting of water and sulfuric acid
Authors: Anders Klaus , Roth Norbert , Frohn Arnold
Published: February 2004

Experimental techniques for studying supercooled or frozen droplets will be presented. Single particles consisting of water or of water contaminated with sulfuric acid have been levitated optically and stabilized in a vertical laser beam by radiation pressure forces. In the experiments the levitated droplet has been observed in a chamber, in which temperatures down to −60°C can be reached, in order to simulate conditions in contrails. After introduction into the chamber the droplet is supercooled rapidly. Depending on temperature and vapor pressure in the chamber, the droplet will grow or evaporate until it disappears. During this process the droplet is observed by a video camera. Furthermore the scattered laser light is evaluated at scattering angles of approximately 45° to characterize the droplet and to determine the droplet size. Simultaneously with the size, the position of the droplet along the axis of the laser beam is determined. The droplet oscillates for a fixed laser power along the axis of the laser beam, when the droplet radius changes with time. These oscillations are due to oscillations of the radiation pressure forces during monotonic decrease or increase of the radius, i.e., for evaporation or condensation. With this new technique it is possible to associate directly the frequency of these oscillations with the radius change rate. Typical experimental results for size and for the radius change rate are shown.
On the feasibility of using the tropospheric ozone residual for nonclimatological studies on a quasi‐global scale
Authors: Vukovich Fred M. , Brackett Vincent , Fishman Jack , Sickles Joseph E.
Published: February 2004

The tropospheric ozone residual (TOR) was estimated using total ozone mapping spectrometer (TOMS), Stratospheric Aerosol and Gas Experiment (SAGE), and solar backscatter ultraviolet (SBUV) data for nonclimatological periods (i.e., daily values, weekly averages, and monthly averages). The TOR is the difference between the total ozone from TOMS and the stratospheric ozone estimated using either SAGE or SBUV ozone data and the National Meteorological Center tropopause pressure data. Comparison of the TOMS/SAGE TOR with ozonesonde data and with area‐averaged surface ozone data for nonclimatological periods was very poor. The month‐to‐month variations of the TOMS/SBUV TOR, on the other hand, compared very well with the month‐to‐month variations of the area‐averaged surface ozone concentrations, and the day‐to‐day variations of the tropospheric ozone using ozonesonde data from five stations compared reasonably well with the day‐to‐day variations of the TOR. However, the RMS difference between the TOMS/SBUV TOR and the tropospheric ozone from ozonesonde data was about 14.4 Dobson units (DU) and the mean difference about 8 DU, though the mean difference was about 1.5 DU at Boulder, Colorado.
Temporal and spatial characteristics of Saharan dust outbreaks
Authors: Swap Robert , Ulanski Stanley , Cobbett Matthew , Garstang Michael
Published: February 2004

Temporal and spatial characteristics of moderate to heavy northern African aerosol outbreaks over the North Atlantic Ocean for the period of January 1989 through December 1992 are presented using NOAA's advanced very high resolution radiometer (AVHRR) aerosol optical thickness (AOT) data. The episodic nature of the transport and deposition of northern African aeolian material as well as the intra‐annual variability of these aerosol outbreaks is detailed. Saharan dust outbreaks are generally observed within relatively well‐defined zonal corridors that are approximately 10° wide latitude. The latitude of these westward outbreaks progresses from south to north during the first 6–7 months of a year. After reaching its northernmost extent (∼20°−25°N) around August, the area of aerosol outbreaks then moves southward. The first six months of a year are the most active in terms of aerosol outbreak frequency and spatial extent. The annual peak in outbreak activity occurs most frequently during the months of February through April. The annual westward mass flux of northern African aeolian material over the North Atlantic during the years detailed in this study range from an annual minimum of 130 Mt for 1990 to an annual maximum of 460 Mt for 1991. These fluxes are not supplied in a steady fashion. Approximately 50% of the annual westward mass flux of northern African aerosols is transported during 20% of a given year. The annual mass fluxes presented in this study are found to be consistent with other independent estimates of the westward mass flux of northern African aerosols. The relationship of these aerosol loadings to sub‐Saharan rainfall is consistent as well with earlier observations. Aerosol loadings are also presented and compared to regional sedimentation rates. The impact of the transport and deposition of this aeolian material on marine biogeochemical cycles of the North Atlantic is addressed briefly with the calculation of the annual mass flux of various nutritive species. Lastly, the continued development of the National Oceanographic and Atmospheric Administration's AVHRR AOT observations and analyses may lead to a North African aeolian emission system analog useful in the geologic sedimentary record.
Modeling soil moisture: A Project for Intercomparison of Land Surface Parameterization Schemes Phase 2(b)
Authors: Shao Yaping , Henderson‐Sellers Ann
Published: February 2004

In an intensive investigation of soil moisture simulation in land surface schemes, a number of numerical experiments was conducted with 14 representative schemes and the results compared with Hydrological and Atmospheric Pilot Experiment ‐ Modelization du Bilan Hydrique (HAPEX‐MOBILHY) data. The results show that soil moisture simulation in current land surface schemes varies considerably. After adjustment of land surface parameters, the disagreement in soil moisture for a 1.6‐m soil layer remains around 100 mm. Correspondingly, the range of variation in predicted annual cumulative evaporation as well as total runoff plus drainage is around 250 mm (annual precipitation being 856 mm for HAPEX‐MOBILHY). The partitioning of surface available energy into sensible and latent heat fluxes is closely coupled to the partition of precipitation into evaporation and runoff plus drainage. Although, on average, the range of variation in net radiation is about 8 W m−2, that of both the latent and sensible heat fluxes is twice as large. These disagreements are related to different causes but attempts to establish the link between the outcome and the responsible mechanism has had only limited success to date because of the complex interactions embedded in the schemes. This study implies that different schemes achieve different equilibrium states when forced with prescribed atmospheric conditions and that the time period to reach these states differs among schemes; and even when soil moisture is fairly well simulated, the processes (particularly evaporation and runoff plus drainage) controlling the simulation differ among schemes and at different times of the year. These results suggest that prescription of land surface scheme physics may have to be a function of the type of predictions (short‐term weather forecasting, mesoscale modeling or climate ensembles) required as well as the underlying scheme formulation and that scheme simulations must be validated for all components of the prediction.
Measurements of high‐resolution ultraviolet‐visible absorption cross sections at stratospheric temperatures: 1. Nitrogen dioxide
Authors: Frost Gregory J. , Goss Lisa M. , Vaida Veronica
Published: February 2004

A method for obtaining high‐resolution, temperature dependent absorption cross sections in the ultraviolet and visible regions is described. The technique combines a Fourier transform spectrometer with a supersonic jet expansion for sample preparation. Infrared spectroscopy is used to determine the temperature of the sample and to reveal any interferences in the ultraviolet‐visible measurement. This method is applied to the measurement of absorption cross sections of 200 ± 20 K nitrogen dioxide in the 370‐ to 497‐nm region, at resolutions of 2 and 0.5 cm−1.
Transport of ozone and sulfur to the North Atlantic atmosphere during the North Atlantic Regional Experiment
Authors: Banic Catharine M. , Leaitch W. Richard , Isaac George A. , Couture Mark D. , Kleinman Lawrence I. , Springston Stephen R. , MacPherson J. Ian
Published: February 2004

The horizontal transport of O3, SO2, and non‐sea‐salt particulate SO4= over the ocean near Nova Scotia, Canada, is determined from in situ measurements made during the 1993 North Atlantic Regional Experiment (NARE) summer intensive. The average mass of O3 transported through an area 1 m in horizontal extent and 5 km in the vertical 2.8 g s−1, moving from west to east. Anthropogenic O3 accounts for 50% of the transport below 1 km, 35–50% from 1 to 3 km, 25–50% from 3 to 4 km, and 10% from 4 to 5 km. The average mass of SO2 and SO4= transported through the same area is 50 mg S s−1, moving from west to east. Eighty percent of the SO4= and 55% of the SO2 are transported above 1 km, with little transport of these species seen above 3 km. The anthropogenic input of O3 and S to the North Atlantic atmosphere from the North American continent is estimated.
Validation of nitrogen dioxide measurements from the Improved Stratospheric and Mesospheric Sounder
Authors: Reburn W. J. , Remedios J. J. , Morris P. E. , Rodgers C. D. , Taylor F. W. , Kerridge B. J. , Knight R. J. , Ballard J. , Kumer J. B. , Massie S. T.
Published: February 2004

Measurements of nitrogen dioxide (NO2) from the Improved Stratospheric and Mesospheric Sounder (ISAMS) on the Upper Atmosphere Research Satellite (UARS) are assessed. Channel 5 of the instrument was dedicated to observations of nitrogen dioxide and employed pressure‐modulation and wideband radiometry to make measurements at 6.2 μm. This dual technique allows simultaneous determination of nitrogen dioxide mixing ratio and the aerosol extinction coefficient at this wavelength and therefore provides nitrogen dioxide data even in the presence of heavy aerosol loading. Approximately 180 days of data, in the period from September 1991 to July 1992, were obtained with, typically, over 2600 profiles per day for each retrieved species, covering an altitude range of 100–0.01 mbar. In this paper the version 10 data are assessed and a full error analysis is described. Comparisons with the Cryogenic Limb Array Etalon Spectrometer (CLAES) on UARS and the Limb Infrared Monitor of the Stratosphere (LIMS) on Nimbus 7 are also presented. It is concluded that the morphology of the retrieved ISAMS fields is robust and consistent with concurrent as well as previous infrared satellite measurements. Random errors are estimated to be of the order of 10% for nighttime and 15% for daytime NO2 near the maxima of the distributions, and systematic errors are estimated to be of a similar size. However, there remains an unresolved systematic difference of about a factor of 2 between ISAMS and CLAES. Both random and systematic errors are likely to be reduced in future versions of the processing.
Electric field measurements above mesoscale convective systems
Authors: Marshall Thomas C. , Stolzenburg Maribeth , Rust W. David
Published: February 2004

We show that electric field discontinuities occur above the stratiform clouds associated with mesoscale convective systems. Above cloud top, 12 discontinuities were observed at altitudes between 10 and 16 km. The field changes of the discontinuities ranged from −1.1 to −4.0 kV m−1. The data suggest that the electric field discontinuities were caused by coincident, positive, cloud‐to‐ground lightning flashes. The coincident ground flashes included both single and multiple return stroke flashes, with first‐stroke peak currents between 20 and 154 kA. We modeled the electric field change that would occur if lightning discharged a horizontally extensive positive charge layer within the stratiform cloud. In the model, disks with charge densities of 1 and 3 nC m−3, a thickness of 400 m, and diameters ranging from 20 to 200 km were discharged and produced field changes similar to the observed above‐cloud field discontinuities. Our results support the idea that sprites may be initiated by above‐cloud field changes caused by positive cloud‐to‐ground lightning flashes that discharge a horizontally extensive charge region in the stratiform cloud of a mesoscale convective system. During the time between the electric field discontinuities the electric field above the stratiform clouds was −0.5 to −1.0 kV m−1; this field may be important in the global electrical circuit because the stratiform clouds have large horizontal extents (∼104 km2).
The Martian drainage system and the origin of valley networks and fretted channels
Authors: Carr Michael H.
Published: February 2004

Outflow channels provide strong evidence for abundant water near the Martian surface and an extensive groundwater system. Collapse of the surface into some channels suggests massive subsurface erosion and/or solution in addition to erosion by flow across the surface. Flat floors, steep walls, longitudinal striae and ridges, downstream deflection of striae from channel walls, and lack of river channels suggest that fretted channels form dominantly by mass wasting. A two‐stage process is proposed. In the first stage, extension of valleys at valley heads is favored by seepage of groundwater into debris shed from slopes. The debris moves downstream, aided by interstitial groundwater at the base of the debris, possibly with high pore pressures. In the second stage, because of climate change or a lower heat flow, groundwater can no longer seep into the debris flows in the valleys, their movement almost stops, and more viscous ice‐lubricated debris aprons form. Almost all uplands at elevations greater than +1 km are dissected by valley networks, although the drainage densities are orders of magnitude less than is typical for the Earth. The valley networks resemble terrestrial river systems in planimetric shape, but U‐shaped and rectangular‐shaped cross sections, levéelike peripheral ridges, median ridges, patterns of branching and rejoining, and flat floors without river channels suggest that the networks may not be true analogs to terrestrial river valleys. It is proposed that they, like the fretted channels, formed mainly by mass wasting, aided by groundwater seepage into the mass‐wasted debris. Movements of only millimeters to centimeters per year are needed to explain the channel lengths. Most valley formation ceased early at low latitudes because of progressive dehydration of the near surface, the result of sublimation of water and/or drainage of groundwater to regions of lower elevations. Valley formation persisted to later dates where aided by steep slopes, as on crater and canyon walls, and/or by high heat flows and the presence of water, as on some volcanoes.
The effects of a deep mantle endothermic phase change on the structure of thermal convection in silicate planets
Authors: Weinstein Stuart A.
Published: February 2004

Previous studies of the effects of silicate phase transitions on mantle convection have tended to use numerical models in which the phase transitions have the same depths as found in the Earth's mantle, or in which a single transition is placed at a depth equal to half the layer thickness. This study presents a systematic investigation designed to examine the effects of an endothermic phase transition near the core‐mantle boundary of terrestrial type planets. The results indicate that a deep mantle endothermic phase change may trap the lower thermal boundary layer, allowing only one upwelling to penetrate the phase transition. In these situations the deep endothermic phase change excites a dipolar convection pattern. This mechanism may have a role in the degree one power observed in the distribution of the Earth's hotspots and the development of the Martian crustal dichotomy.
Spectral mixing models of S‐type asteroids
Authors: Clark Beth E.
Published: February 2004

We present the results of mixing model determinations of S‐type asteroid mineralogies using the Hapke spectral theory. The goal is to quantify uncertainties resulting from poorly constrained model parameters and to produce sets of model compositions that we can analyze for trends that can be related to what is already known about S asteroids and the meteorites proposed to be derived from them. We present two different approaches to the selection of the albedo input parameter and show the resulting differences in model compositions. Similar grain sizes and composition patterns in terms of metal/silicate and olivine/pyroxene ratios were determined regardless of albedo approach. Encouraged by this agreement, we suggest that the two resulting sets of model compositions can be used to set limits on end‐member abundances. Residual errors from model fits are comparable to those found by other compositional modeling attempts and, furthermore, tend to be located in the same spectral regions. These regions are (1) 0.4–0.7 μm, where the silicate and metal component UV drop‐off in the asteroid spectra is too steep to be modeled with the current end‐members, and (2) 1.5–2.5 μm, where the IR continuum turnover of the asteroid spectra is too flat to be fit with metallic spectrally red components included to match the IR continuum from 0.7 to 1.5 μm. We believe these systematic problems to be compositional information, as yet uninvestigated. More information is needed regarding the grain size conditions and the nature of opaque components present at asteroid surfaces before highly accurate compositional modeling is possible. The technique presented here is thus most useful in conjunction with other spectral deconvolution methods.
The distribution and behavior of Martian ground ice during past and present epochs
Authors: Mellon Michael T. , Jakosky Bruce M.
Published: February 2004

Mars undergoes significant oscillations in its orbit, which will have a pronounced effect on its climate and, in particular, on the behavior of subsurface water ice. We explore and map the behavior of ice in the Martian near‐surface regolith over the past 1 m.y. using a diffusion and condensation model presented in an earlier paper, with two primary modifications to include orbitally induced variations in insolation and atmospheric water abundance. We find that the past behavior of ground ice differs significantly from that at the present epoch, primarily the result of high‐amplitude oscillations in obliquity (presently 25°). In midlatitude and equatorial regions, ground ice will condense from atmospheric water during times of higher obliquity, filling the top few meters of the regolith with significant amounts of ice. At an obliquity of 32°, ground ice becomes stable globally. During times of lower obliquity, ground ice will sublime and diffuse back into the atmosphere, dessicating the regolith to a depth of about 1 to 2 m equatorward of 60° to 70° latitude. In the high‐latitude regions these oscillations are considerably subdued. Below this depth of cyclic saturation and dessication a long‐term stability of ice exists in some geographic regions. We present a map of the distribution of ice expected at the present epoch. Cyclic exchange of water between the global regolith and polar regions will have significant implications for surface geology and the polar layered deposits.
Determining heights and slopes of fault scarps and other surfaces on Venus using Magellan stereo radar
Authors: Connors Chris
Published: February 2004

This report presents relationships and techniques for determining the heights and slopes of discretely dipping surfaces, such as normal fault scarps, on the surface of Venus from measurements of their widths in Magellan stereo synthetic aperture radar (SAR) images. These surfaces are clearly recognizable as distinct bands of increased or decreased radar backscatter relative to flat‐lying areas in the 120 to 280‐m resolution Magellan SAR images, but are not generally imaged in the >10‐km resolution Magellan altimetry. Our methods take into account radar distortion effects, and allow one to determine whether a slope is foreshortened, laid over, elongated, or in radar shadow. The techniques make use of graphs constructed for the Magellan incidence angle profiles so that investigators can determine the local height and slope of an individual surface in a straightforward manner, but the techniques are applicable to any stereo radar data set if the radar incidence angles are known. Additionally, the techniques can be used to improve digital elevation models constructed using radar stereoscopy in areas that have steep local terrain where stereoscopic fusion may be impossible due to the effects of layover or radar shadow. Using these techniques we show that there are normal fault scarps on Venus with heights in the 700‐m range that have remarkable topographic slopes of close to 60°, something unheard of on Earth and which suggests a high effective cohesive strength of the Venusian crust.
Charge‐coupled device spectra of the Galilean satellites: Molecular oxygen on Ganymede
Authors: Spencer John R. , Calvin Wendy M. , Person Michael J.
Published: February 2004

We have obtained 3200–7800 Å CCD spectra of the Galilean satellites at a variety of orbital longitudes, with spectral resolution between 3 and 18 Å and signal‐to‐noise ratios of up to 2000 at 6000 Å. Despite the higher resolution and signal‐to‐noise ratio than previous published spectra, no new features are seen on Io, Europa, and Callisto. However, Ganymede shows an unusual and previously unreported shallow absorption feature at 5773 Å, which is much stronger on the trailing side, and a weaker band at 6275 Å. The features are apparently due to diatomic oxygen, and require simultaneous electronic transitions in two adjacent molecules: they are the strongest visible‐wavelengt h absorption bands in solid or liquid oxygen. Because condensed pure oxygen is not stable at Ganymede surface temperatures and pressures, the oxygen must be trapped in other surface materials, perhaps in water ice, with the constraint that O2 molecules must be close enough together for simultaneous electronic transitions. Magnetospheric bombardment of water ice is a plausible production mechanism for the O2. Continuum shapes on all four satellites are more clearly seen than in previous spectra. The shape of the continuum is very similar on Ganymede and Callisto but is strikingly different on Europa, indicating a different origin for the nonice component on Europa. The shape of the Europa continuum suggests that allotropes or compounds of sulfur may be the dominant spectrally active materials in the visible spectrum. No convincing changes since 1978 are visible in Io's spectrum, despite the high resurfacing rates, suggesting that volcanic resurfacing tends to overpaint areas with more material of the same composition.
Field recovery of layered tektites in northeast Thailand
Authors: Wasson John T. , Pitakpaivan Kaset , Putthapiban Prinya , Salyapongse Sirot , Thapthimthong Boonlom , McHone John F.
Published: February 2004

We recovered Australasian tektites in place throughout a 40×130 km region in north‐east Thailand extending from the Laotian border westward to a line connecting Na Pho Klang in the northeast through Det Udom to Nam Yun in the south. With two exceptions, in sites near the western edge of this region, all fragments are layered (muong‐Nong‐type) tektites. It appears that large layered tektites are mainly found by rice farmers in fields that were forested until the recent past. The presence of layered tektites in this 40×130 km area implies that impact melt that fell in these areas was hot enough to flow if it was deposited on a sloping surface. The absence of splash‐form tektites from the region indicates that the layer was still molten when masses having shapes (teardrops, dumbbells, etc.) produced by spinning reached the ground. To account for this and to allow time for the melt to flow a few tens of centimeters requires that the atmosphere remained hot (>2300 K) for a few minutes. Tektites that are in place are almost always associated with a widespread 10‐cm to meter‐thick layer of laterite. In two cases (one involving layered, one involving splash‐form tektites), where accurate stratigraphic control demonstrated that the tektites were still in section, they were sited on top of the laterite layer just below a layer of aeolian sand.
Viscoelastic relaxation of topographic highs on Venus to produce coronae
Authors: Janes Daniel M. , Squyres Steven W.
Published: February 2004

Coronae on Venus are believed to result from the gravitationally driven relaxation of topography that was originally raised by mantle diapirs. We examine this relaxation using a viscoelastic finite element code, and show that an initially plateau shaped load will evolve to the characteristic corona topography of central raised bowl, annular rim, and surrounding moat. Stresses induced by the relaxation are consistent with the development of concentric extensional fracturing common on the outer margins of corona moats. However, relaxation is not expected to produce the concentric faulting often observed on the annular rim. The relaxation timescale is shorter than the diapir cooling timescale, so loss of thermal support controls the rate at which topography is reduced. The final corona shape is supported by buoyancy and flexural stresses and will persist through geologic time. Development of lower, flatter central bowls and narrower and more pronounced annular rims and moats enhanced by thicker crusts, higher thermal gradients, and crustal thinning over the diapir.
Volcanic constructs on Ganymede and Enceladus: Topographic evidence from stereo images and photoclinometry
Authors: Schenk Paul M. , Moore Jeffrey M.
Published: February 2004

The morphology of volcanic features on Ganymede differs significantly from that on the terrestrial planets. Few if any major volcanic landforms, such as thick flows or shield volcanoes, have been identified to date. Using new stereo Voyager images, we have searched Ganymede for relief‐generating volcanic constructs. We observed seven major types of volcanic structures, including several not previously recognized. The oldest are broad flat‐topped domes partially filling many older craters in dark terrain. Similar domes occur on Enceladus. Together with smooth dark deposits, these domes indicate that the volcanic history of the dark terrain is complex. Bright terrain covers vast areas, although the style of emplacement remains unclear. Smooth bright materials embay and flood older terrains, and may have been emplaced as low‐viscosity fluids. Associated with smooth bright material are a number of scalloped‐shaped, semi‐enclosed scarps that cut into preexisting terrain. In planform these structures resemble terrestrial calderas. The youngest volcanic materials identified are a series of small flows that may have flooded the floor of the multiring impact structure Gilgamesh, forming a broad dome. The identification of volcanic constructs up to 1 km thick is the first evidence for extrusion of moderate‐to‐high viscosity material on Ganymede. Viscosity and yield strength estimates for these materials span several orders of magnitude, indicating that volcanic materials on Ganymede have a range of compositions and/or were extruded under a wide range of conditions and/or eruptive styles.
Some effects of a dry crustal flow law on numerical simulations of coupled crustal deformation and mantle convection on Venus
Authors: Lenardic A. , Kaula W. M. , Bindschadler D. L.
Published: February 2004

Models of coupled crustal deformation and mantle convection on Venus are reevaluated in light of recent experimental evidence suggesting that the strength of dry basalt is comparable to that of olivine. A previous model which assumed a relatively weak basalt flow law, more appropriate for hydrated crust, is compared to a similar model that assumes a strong basalt flow law, more appropriate for dry crust. A principal difference is the timescale for significant crustal thickening, of the order of 108 years for the weak basalt model and 109 years for the strong basalt model. These results can be understood in the context of theoretical scaling relations for convecting temperature dependent media which imply that in the absence of concentrated zones of near‐surface weakness, of the type associated with plate margins on Earth, the lithospheric overturn time, which sets the timescale required for convective thickening of crust, scales dominantIy with the Rayleigh number defined by the average viscosity of the lithosphere. Theoretical scalings, as well as numerical models, suggest crustal thickening timescales of the order of 108 years remain possible for high bulk mantle Rayleigh numbers and effective viscosity contrasts from lithosphere to interior mantle of the order of 103. Such timescales imply a relatively thin lithosphere and heat loss comparable to that of the present‐day Earth. The large values inferred for the thickness of the mechanical lithosphere on Venus, if correct, would thus tend to favor timescales of the order of 109 years, if one assumes relatively low convective mantle heat loss over an equivalent time. This, in turn, would argue against the likelihood of highland plateaus being manifestations of crustal thickening above mantle downflows. If, instead, one assumes that convective heat removal and implied lithospheric overturn rates were significantly higher over the last several hundred million years, then highland formation due to crustal thickening cannot be ruled out based solely on the strength of dry crust.
Diagenetic layers in the upper walls of Valles Marineris, Mars: Evidence for drastic climate change since the mid‐Hesperian
Authors: Treiman Allan H. , Fuks Kelly H. , Murchie Scott
Published: February 2004

A packet of relatively resistant layers, totaling ∼400 m thickness, is present at the tops of the chasma walls throughout Valles Marineris. The packet consists of an upper dark layer (∼50 m thick), a central bright layer (∼250 m thick), and a lower dark layer (∼100 m thick). The packet appears continuous and of nearly constant thickness and depth below ground surface over the whole Valles system (4000 km E‐W, 800 km N‐S), independent of elevation (3–10 km) and age of plateau surface (Noachian through upper Hesperian). The packet continues undisturbed beneath the boundary between surface units of Noachian and Hesperian ages, and continues undisturbed beneath impact craters transected by chasma walls. These attributes are not consistent with layer formation by volcanic or sedimentary deposition, and are consistent with layer formation in situ, i.e., by diagenesis, during or after upper Hesperian time. Diagenesis seems to require the action of aqueous solutions in the near subsurface, which are not now stable in the Valles Marineris area. To permit the stability of aqueous solutions, Mars must have had a fairly dense atmosphere, ≥1 bar CO2, when the layers formed. Obliquity variations appear to be incapable of producing such a massive atmosphere so late in Mars' history.
Carbon 14 measurements of the Martian atmosphere as an indicator of atmosphere‐regolith exchange of CO2
Authors: Jakosky Bruce M. , Reedy Robert C. , Masarik Josef
Published: February 2004

A determination of the mixing ratio of 14CO2 in the Martian atmosphere would provide a direct measurement of the degree of exchange of CO2 in the Martian atmosphere with CO2 in the regolith and polar caps. This is the case because production of 14C occurs in the atmosphere and in the very‐near‐surface regolith and decays away in a timescale of 104 years. The resulting steady state abundance in the atmosphere can be diluted by mixing with nonatmospheric reservoirs. The degree of mixing expected is such that an order‐of‐magnitude measurement would provide definitive information on the state of the regolith. Although in situ measurements probably are not possible, determination could be made easily on a sample of pristine Martian atmosphere returned to the Earth.
Comment on “Oxygen ionization rates at Mars and Venus: Relative contributions of impact ionization and charge exchange” by M. H. Zhang, J. G. Luhmann, A. F. Nagy, J. R. Spreiter, and S. S. Stahara
Authors: Krymskii A. M. , Breus T. K.
Published: February 2004

The accuracy of estimates of electron impact ionization which are based on the gasdynamic model of the solar wind interaction with Venus is discussed. Employing the hybrid simulations and in situ data on electron temperature or electron fluxes the electron impact ionization is reevaluated. The electron impact ionization rate estimated in this paper is typically 4–8 times less than the value derived from the gasdynamic model Zhang et al. [1993]. During solar maximum the photoionization rate is typically greater than the electron impact ionization rate.
The European Southern Observatory planetary search program: Preliminary results
Authors: Hatzes A. P. , Kürster M. , Cochran W. D. , Dennerl K. , Döbereiner S.
Published: February 2004

We report on the progress of a program to search for extra‐solar planets using high precision radial velocity (RV) measurements taken at the European Southern Observatory. Instrumental errors in the radial velocity measure are minimized by placing a temperature stabilized molecular iodine absorption cell in front of the entrance slit to the spectrograph. This imposes an iodine absorption spectrum on top of the stellar spectrum. All stellar radial velocity shifts are measured with respect to the iodine absorption spectrum. The current program consists of a sample of 26 F, G, and K dwarfs and 1 dMe star. Radial velocity measurements taken since November 1992 indicate that a typical long‐term radial velocity precision for this program is about 18 m s−1. One of our program stars, τ Cet, shows constant radial velocity variability with a scatter of 6 m s−1. Representative radial velocity measurements are given for three stars showing no significant radial velocity variations, three stars showing variations indicative of low mass companions, and for the triple system α Cen. The radial velocity measurements of α Cen A and B are consistent with the astrometric orbital solution, although the data points have a slightly steeper slope than the calculated orbit. Continued radial velocity monitoring of this star should eventually result in a better orbital solution.