bluuebunny/crossref_metadata_2025 huggingface dataset:
Mean moist circulation for PEM‐Tropics missions
Authors: Hu Yuanlong , Newell Reginald E. , Zhu Yong
Published: February 2004

Features of atmospheric water vapor and water vapor transport fields during the Pacific Exploratory Mission (PEM) in the Tropical Pacific were studied using the data from the National Centers for Environmental Prediction (NCEP) reanalyses, the Global Precipitation Climatology Project, the Climate Analysis Center, and the European Centre for Medium‐Range Weather Forecasts, with a focus on PEM‐Tropics B. Vertically integrated values of the moisture quantities such as the precipitable water and the zonal and meridional water vapor fluxes were examined. Moisture transports across boundaries of major sectors over the tropical Pacific and their budgets were estimated. The differences of the precipitable water between PEM‐Tropics B and PEM‐Tropics A were highlighted owing to its importance for atmospheric chemistry. During PEM‐Tropics B, the NCEP reanalyses showed that there existed clearly a moist Walker circulation over the northern tropical Pacific during the whole period and a moist Hadley circulation over the eastern Pacific during the first half of the mission. The southwest sector where the South Pacific Convergence Zone (SPCZ) was located had the largest areal mean precipitation and the northeast sector had the smallest, whereas the estimated evaporation rates for both regions were comparable. A more vigorous moist circulation during the second half period led to enhanced precipitation for all the regions except the southeast sector. An interesting and potentially very important meteorological phenomenon during PEM‐Tropics B was the double Intertropical Convergence Zone (ITCZ) structure over the eastern tropical Pacific. We suggest that this structure, along with the similar double convective zones over the western tropical Pacific (i.e., ITCZ and SPCZ), may act as double barriers for the interhemispheric exchanges for this particular season with implications for photochemistry in both hemispheres.
Large‐scale latitudinal and vertical distributions of NMHCs and selected halocarbons in the troposphere over the Pacific Ocean during the March‐April 1999 Pacific Exploratory Mission (PEM‐Tropics B)
Authors: Blake Nicola J. , Blake Donald R. , Simpson Isobel J. , Lopez Jimena P. , Johnston Nancy A. C. , Swanson Aaron L. , Katzenstein Aaron S. , Meinardi Simone , Sive Barkley C. , Colman Jonah J. , Atlas Elliot , Flocke Frank , Vay Stephanie A. , Avery Melody A. , Rowland F. Sherwood
Published: February 2004

Nonmethane hydrocarbons (NMHCs) and selected halocarbons were measured in whole air samples collected over the remote Pacific Ocean during NASA's Global Tropospheric Experiment (GTE) Pacific Exploratory Mission‐Tropics B (PEM‐Tropics B) in March and early April 1999. The large‐scale spatial distributions of NMHCs and C2Cl4 reveal a much more pronounced north‐south interhemispheric gradient, with higher concentrations in the north and lower levels in the south, than for the late August to early October 1996 PEM‐Tropics A experiment. Strong continental outflow and winter‐long accumulation of pollutants led to seasonally high Northern Hemisphere trace gas levels during PEM‐Tropics B. Observations of enhanced levels of Halon 1211 (from developing Asian nations such as the PRC) and CH3Cl (from SE Asian biomass burning) support a significant southern Asian influence at altitudes above 1 km and north of 10°N. By contrast, at low altitude over the North Pacific the dominance of urban/industrial tracers, combined with low levels of Halon 1211 and CH3Cl, indicate a greater influence from developed nations such as Japan, Europe, and North America. Penetration of air exhibiting aged northern hemisphere characteristics was frequently observed at low altitudes over the equatorial central and western Pacific south to ∼5°S. The relative lack of southern hemisphere biomass burning sources and the westerly position of the South Pacific convergence zone contributed to significantly lower PEM‐Tropics B mixing ratios of the NMHCs and CH3Cl south of 10°S compared to PEM‐Tropics A. Therefore the trace gas composition of the South Pacific troposphere was considerably more representative of minimally polluted tropospheric conditions during PEM‐Tropics B.
Characteristics of cloud‐nucleating aerosols in the Indian Ocean region
Authors: Twohy Cynthia H. , Hudson James G. , Yum Seong‐Soo , Anderson James R. , Durlak Susan K. , Baumgardner Darrel
Published: February 2004

During the Indian Ocean Experiment (INDOEX), cloud droplets were collected and evaporated using a counterflow virtual impactor (CVI). The nonvolatile residual particles were then analyzed by various instruments. Physical and chemical properties of the cloud droplets and their residual nuclei were compared with properties of the below‐cloud aerosol to evaluate which aerosol particles act as cloud nuclei in different environments, and their effects on cloud microphysics. Four cases, ranging from clean Southern Hemispheric clouds to heavily polluted clouds near India, were analyzed. For the cleaner clouds, droplet concentrations were a much higher fraction of the available particle concentrations than for polluted clouds, but entrainment apparently acted to reduce droplet number concentrations in both regimes. For clean clouds the median critical supersaturation and size of the ambient particles and droplet residual particles were similar. In polluted clouds there were stronger differences between ambient and droplet residual distributions, and particles with lower critical supersaturations were favored as nuclei. Simple model calculations were used to show that polluted clouds are expected to achieve lower water supersaturations than clean clouds; thus only particles with relatively low critical supersaturations are likely to affect clouds in polluted regions. Soluble fractions for the ambient aerosol inferred from the size and cloud condensation nuclei measurements were in general agreement with another study in the region. Droplet residual particles did not necessarily have higher soluble fractions than the ambient aerosol, but did tend to have higher total amounts of soluble material per particle, particularly in the polluted cases.
BME representation of particulate matter distributions in the state of California on the basis of uncertain measurements
Authors: Christakos George , Serre Marc L. , Kovitz Jordan L.
Published: February 2004

Maps of temporal and spatial values of annual averages of daily particulate matter (PM10) concentrations were generated throughout the state of California using uncertain forms of physical data. The PM10estimates were derived in an integrated space/time domain using the Bayesian maximum entropy (BME) mapping approach of modern spatiotemporal geostatistics. The approach possesses some interesting features which allow an insightful analysis of the PM10space/time distribution. A complete stochastic characterization of the pollutant involves the probability density function of the PM10map, which is the result of a rigorous knowledge‐integration process. This process is considerably flexible, it can account for several physical knowledge bases and sources of uncertainty, and it may involve Bayesian or material conditionalization rules. Taking advantage of BME's flexibility, PM10estimates were chosen which offered an appropriate representation of the real distribution in space/time, and a meaningful assessment of the representation accuracy was derived. Depending on the space scales/timescales considered, the PM10distributions depicted considerable levels of variability, which may be associated with topographic features, climatic changes, seasonal patterns, and random fluctuations. The importance of integrating soft information available at surrounding sites as well as at the estimation points themselves was discussed. Comparisons were designed which demonstrated the usefulness of the BME‐based maps to represent PM10distributions in space/time. Areas were identified where the annual PM10geometric mean reached or exceeded the California standard, which is valuable information for regulatory purposes.
Airborne measurements of tropospheric ice‐nucleating aerosol particles in the Arctic spring
Authors: Rogers David C. , DeMott Paul J. , Kreidenweis Sonia M.
Published: February 2004

Instrumented aircraft flights were made during field experiments in the Arctic Ocean, the NASA FIRE Arctic Cloud Experiment and SHEBA (Surface Heat Budget of the Arctic). Airborne measurements of ice nucleating aerosol particles (IN) used a continuous flow diffusion (CFD) chamber, covering −10° to −30°C and humidities from ice saturation to water supersaturation. During selected time periods, ice crystals that grew on ice nuclei in the chamber were sampled onto electron microscope (EM) grids for later examination of the nucleating particles. Samples of total aerosol (IN and non‐IN) were also collected for comparative analyses. Concentrations of IN ranged from zero to rare very high values (hundreds per liter at −25°C), making the frequency distribution of IN highly skewed: when accumulated as 10 s average concentrations (volume ∼0.17 L), 50% were zero. Additional evidence of few IN was seen in thin low‐level stratus clouds at −10° to −20°C that persisted for several days with low concentrations of ice crystals (∼0.1 L−1) and a few tenths g m−3 liquid water. On occasion, small regions of high IN concentrations (hundreds per liter) were detected near the surface. The EM analyses indicated that ice nuclei a few tenths micrometer in size contained crustal materials (primarily Si) and had widely varying morphology. Many IN particles produced weak or no X‐ray signatures, suggesting a dominant low‐molecular‐weight component not detected by the energy dispersive X‐ray (EDX) system, and were probably carbonaceous. In contrast, for the total aerosol, S or S and Si were dominant components, and few particles had no X‐ray signature.
The δ17O and δ18O measurements of atmospheric sulfate from a coastal and high alpine region: A mass‐independent isotopic anomaly
Authors: Lee Charles Chi‐Woo , Thiemens Mark H.
Published: February 2004

Atmospheric sulfate extracted from rainwater and aerosols has been sampled from two environments: La Jolla and White Mountain Research Station (WMRS), California. In general, the multiple stable oxygen isotopic analysis of these sulfate samples shows that they possess mass‐independent oxygen isotopic compositions, unlike laboratory nonatmospheric standards. As suggested by the recent laboratory findings, the source of mass independent oxygen isotopic composition in sulfate is implied to be aqueous phase S(IV) oxidation by atmospheric H2O2 and O3. The relative S(IV) oxidation contribution by H2O2 and O3, which together generally represent the overall aqueous phase S(IV) oxidation, is estimated utilizing the average 17O enrichment value of La Jolla rainwater sulfate samples as an index. The estimate indicates a relative dominance of the aqueous over gas phase S(IV) oxidation. Although inconclusive owing to small data sets, both La Jolla and WMRS aerosol sulfate samples display seasonal 17O enrichment variations with a maximum in winter and a minimum in summer. Such seasonal isotopic variations are believed to derive from climatic effects that favor aqueous phase S(IV) oxidation in winter compared to summer. The radioactivity measurements of the cosmogenic 35S radionuclide, coupled with double oxygen isotope ratio measurements suggests an atmospheric sulfate contribution from high‐altitude air mass (perhaps stratospheric) at WMRS, in particular during late winter/early spring season. The results of this work provide a new way to quantify relative oxidative pathways of S(IV) species in the atmosphere and to improve understanding atmospheric processes associated with sulfate.
Identifying the agricultural imprint on the global N2O budget using stable isotopes
Authors: Pérez T. , Trumbore S. E. , Tyler S. C. , Matson P. A. , Ortiz‐Monasterio I. , Rahn T. , Griffith D. W. T.
Published: February 2004

Agricultural soils are the most important anthropogenic source of nitrous oxide to the atmosphere. We observed large shifts with time in the emission rate (from 170 to 16 ng N cm−2 h−1) and in δ15N of N2O emitted (from −46‰ to +5‰ relative to atmospheric N2) from a urea‐fertilized and irrigated agricultural field in Mexico. We calculated overall instantaneous enrichment factors for the sampling period, which suggest that the microbial N2O production shifts from nitrification (week 1) to denitrification (week 2). Isotopic signatures of N2O emissions were not always in accord with other proxies (such as NO/N2O emission ratio or water‐filled pore space) used to estimate the relative importance of nitrification and denitrification as N2O sources. These observations strongly suggest that the soil surface emissions integrate processes occurring at different depths in the soil and a decoupling of NO and N2O production in this system. Further clues as to the source of N2O come from the positional dependence of 15N in the emitted N2O, reported here for the first time in soil emissions. Enrichment at the central N position increased relative to the terminal N position by 9.3‰ during the first 4 days after irrigation, implying that nitrification preferentially enriches the central N position compared to denitrification. The overall δ15N signature we measured for N2O emitted from N‐fertilized agricultural systems is more depleted than observed δ15N values for N2O emitted from more N‐limited forest soils. Assuming that one half of the total agricultural N2O emissions associated with the global increase in soil nitrogen fertilizer use have an isotopic composition comparable to those of the agricultural fields reported here, we predict a decline in the isotopic signature of tropospheric N2O during this century of as much as 3‰ for 15N. Although many uncertainties remain, we suggest that measurements of δ15N‐N2O in firn air will provide constraints on how the N2O budget has changed during the past century.
Effect of tropopause height changes on the calculation of ozone trends and their radiative forcing
Authors: de F. Forster Piers M. , Tourpali Kleareti
Published: February 2004

Three separate estimates of ozone trends since 1970 are obtained from ozonesonde analyses in the Northern Hemisphere, at up to 11 point locations. Our analysis shows that there has been an increase in the annually averaged tropopause height of between 330–520 m at all stations, accompanied by an overall upward movement of the ozone profile. This movement of the ozone profile has led to an extra 2–15% decade−1 ozone depletion between roughly 8 and 22 km. We argue that as the part of the ozone trend associated with “dynamical” movement of the ozone profile is not a direct result of anthropogenic chemical ozone depletion, it ought not to be included in anthropogenic radiative forcing calculations. This leads to a stratospheric ozone radiative forcing which is roughly 50% of the original estimate. Combining stratospheric and tropospheric ozone changes to calculate the total ozone radiative forcing in the Northern Hemisphere for the 1970–1997 period gives a radiative forcing of −0.09 W m−2, excluding the effects of dynamical movement of the ozone profile, which were calculated to have led to a further “ozone feedback” of −0.1 W m−2. These results suggest estimating ozone radiative forcing from observational ozone trend data, without accounting for the movement of the ozone profile could be fundamentally flawed, due to the unquantified feedbacks in the climate system, making it difficult to separate out the part of the ozone trend associated directly with the anthropogenic forcing.
Horizontal velocity structure functions in the upper troposphere and lower stratosphere: 1. Observations
Authors: Cho John Y. N. , Lindborg Erik
Published: February 2004

We compute horizontal velocity structure functions using quasiglobal data accumulated by specially equipped commercial aircraft on 7630 flights from August 1994 to December 1997. Using the ozone concentration measurements, we classify the results as tropospheric or stratospheric. We further divide the results into four absolute latitude bands. For separation distance r between 10 and 100 km, the lower stratospheric diagonal third‐order structure functions are proportional to negative r. This implies a downscale energy cascade, and we estimate the mean energy dissipation rate to be 〈ε〉 ≈ 6 × 10−5 m2 s−3. For r between 300 and 1500 km, a positive r3 dependence was visible for the polar stratospheric data. This may be the result of a two‐dimensional (2D) turbulence downscale enstrophy cascade, and we estimate the average enstrophy flux to be Πω ≈ 2 × 10−15 s−3 and the energy spectral constant to be κ ≈ 2. The negative sign of these third‐order functions at mesoscales in both the upper troposphere and lower stratosphere provide no support for an inverse energy cascade 2D turbulence. At scales above ∼100 km, the second‐order structure functions increase with latitude in the troposphere and decrease with latitude in the stratosphere. The off‐diagonal third‐order functions in the stratosphere show a remarkably clean negative r2 dependency from 10 to 1000 km in scale.
Horizontal velocity structure functions in the upper troposphere and lower stratosphere: 2. Theoretical considerations
Authors: Lindborg Erik , Cho John Y. N.
Published: February 2004

The Kolmogorov equation for the third‐order velocity structure function is derived for atmospheric mesoscale motions on an f plane. A possible solution is a negative third‐order structure function, varying linearly with separation distance and mean dissipation, just as in three‐dimensional turbulence, but with another scaling constant. On the basis of the analysis and the observed stratospheric third‐order structure function, it is argued that there is a forward energy cascade in the mesoscale range of atmospheric motions. The off‐diagonal part of the general tensor equation is also studied. In this equation there is an explicit Coriolis term that may be crucial for the understanding of the kinetic energy spectrum at scales larger than 100 km.
On‐line simulations of passive chemical tracers in the University of California, Los Angeles, atmospheric general circulation model: 1. CFC‐11 and CFC‐12
Authors: Gupta Mohan L. , Turco Richard P. , Mechoso Carlos R. , Spahr Joseph A.
Published: February 2004

Long‐term simulations of the response of atmospheric CFC‐11 and CFC‐12 to standard emission scenarios have been carried out using the University of California, Los Angeles (UCLA) atmospheric general circulation model (AGCM) coupled on‐line with the UCLA atmospheric chemistry model. For both compounds, photochemical loss rates are computed interactively over the entire model domain at each time step of the integration. Using industrial‐based emission estimates, the simulations for CFC‐12 closely track the long‐term trends recorded in both hemispheres by the Atmospheric Lifetime Experiment/Global Atmospheric Gases Experiment/Advanced Global Atmospheric Gases Experiment (AGA) and Climate Monitoring and Diagnostics Laboratory monitoring networks. The agreement between simulations and observations is best when AGA‐deduced emissions are employed. The predicted surface mixing ratios of CFC‐11, on the other hand, are somewhat overestimated by the model. Because the transport and loss processes, as well as source distributions, are roughly similar for these halocarbons, the divergence in surface concentrations points to the possibility that emissions of CFC‐11 may be overestimated for the period extending from the late 1980s through the early 1990s, and perhaps even at earlier times. As for CFC‐12, the best agreement is achieved using AGA emissions. The simulated interhemispheric exchange time constant for these CFCs is about 0.6 year. In the annual cycle, maximum transport occurs from the Northern to Southern Hemisphere within the lowest atmospheric layers during northern winter. Our best estimates of the annually averaged mean global lifetimes of CFC‐11 and CFC‐12 are about 55 and 100 years, respectively. The simulations indicate that both the mean residence time and interhemispheric exchange rate depend on the assumed model vertical domain. For the mass balance analysis, when the upper boundary of the AGCM is artificially fixed below ∼35 km for CFC‐11, or ∼43 km for CFC‐12, there is a tendency for the timescales (lifetimes and interhemispheric exchange times) to be overestimated. Comparisons between CFC distributions and trends calculated using low and high spatial resolution show relatively small differences in the present case. These results, especially regarding CFC persistence and interhemispheric exchange, suggest that the present model accurately represents the global dispersion of long‐lived chemical tracers.
Isotopic enrichment of nitrous oxide (15N14NO, 14N15NO, 14N14N18O) in the stratosphere and in the laboratory
Authors: Röckmann Thomas , Kaiser Jan , Brenninkmeijer Carl A. M. , Crowley John N. , Borchers Reinhard , Brand Willi A. , Crutzen Paul J.
Published: February 2004

Nitrous oxide (N2O) extracted from stratospheric whole air samples has been analyzed for its 15N and 18O isotopic composition, and strong enrichments in the heavy isotopes are observed concomitant with decreasing N2O mixing ratio. Notably, the 15N enrichment is strongly different at the two nonequivalent positions in the molecule. Laboratory broadband photolysis experiments at wavelengths representative for the stratosphere confirm that photolysis is the prime cause for the observed fractionation in the stratosphere. However, the in situ stratospheric fractionation constants are significantly reduced compared to the laboratory data, reflecting the importance of dynamic processes. In addition, small but significant variations in the ratio of the two 15N fractionation constants indicate the influence of additional chemical processes like the oxidation of N2O by O(1D).
Toxaphene in the United States: 2. Emissions and residues
Authors: Li Y. F. , Bidleman T. F. , Barrie L. A.
Published: February 2004

Emission factors of toxaphene for spraying and tilling events are distributed for the United States on a 1°×1° latitude and longitude grid system. By using the gridded usage and emission factors, inventories of gridded toxaphene emissions and residues in agricultural soil in the United States with 1/6°×1/4° latitude and longitude resolution have been created. Total toxaphene emissions were around 190 kt between 1947 and 1999. At the beginning of 2000, almost 20 years after banning the use of toxaphene, there were still around 29 kt of toxaphene left in the agricultural soil, of which 360 t will emit to the air in 2000. The calculated toxaphene emissions and residues are in general consistent with published monitoring data. The trends of toxaphene emissions due to current use and residues in agricultural soil in the United States match both the historical atmospheric input function for toxaphene extending over the past 40 years derived from the composition of toxaphene in peat core from eastern Minnesota to Nova Scotia, and the trends of air concentration of toxaphene in the Arctic. This indicates that toxaphene residues in the United States agricultural soil could be a major source of toxaphene in the Canadian Arctic and the Great Lakes region.
Formaldehyde measurements in the marine boundary layer of the Indian Ocean during the 1999 INDOEX cruise of the R/V Ronald H. Brown
Authors: Wagner V. , Schiller C. , Fischer H.
Published: February 2004

As part of the Indian Ocean Experiment (INDOEX), measurements of CH2O were carried out with a tunable diode laser absorption spectrometer on board the National Oceanic and Atmospheric Administration research vessel Ronald H. Brown during spring 1999. The average CH2O concentration measured in pristine southern hemispheric air over the Indian Ocean (IO) was 210 parts per trillion by volume (pptv). CH2O levels measured in the Northern Hemisphere show a strong dependence upon the air mass origin. North of the Intertropical Convergence Zone a mean concentration of 430 pptv was found in surface air over the IO. When the origin of the air masses did not change during the day and the air was not influenced by anthropogenic emissions, a clear diurnal cycle was observed with a minimum in the late morning and a maximum in the late afternoon. The amplitude of the mean diurnal pattern is around 150 pptv in the Southern Hemisphere and around 200 pptv in the Northern Hemisphere. In polluted air masses of the Northern Hemisphere, CH2O is positively correlated with CO. The ratio CH2O/CO is 3.2 × 10−3 indicating well aged air with only a reduced amount of CH2O precursors left. To date, the reason for the observed correlation remains open to speculation.
Evaluation of the land surface water budget in NCEP/NCAR and NCEP/DOE reanalyses using an off‐line hydrologic model
Authors: Maurer Edwin P. , O'Donnell Greg M. , Lettenmaier Dennis P. , Roads John O.
Published: February 2004

The ability of the National Centers for Environmental Prediction (NCEP)/National Center for Atmospheric Research (NCAR) reanalysis (NRA1) and the follow‐up NCEP/Department of Energy (DOE) reanalysis (NRA2), to reproduce the hydrologic budgets over the Mississippi River basin is evaluated using a macroscale hydrology model. This diagnosis is aided by a relatively unconstrained global climate simulation using the NCEP global spectral model, and a more highly constrained regional climate simulation using the NCEP regional spectral model, both employing the same land surface parameterization (LSP) as the reanalyses. The hydrology model is the variable infiltration capacity (VIC) model, which is forced by gridded observed precipitation and temperature. It reproduces observed streamflow, and by closure is constrained to balance other terms in the surface water and energy budgets. The VIC‐simulated surface fluxes therefore provide a benchmark for evaluating the predictions from the reanalyses and the climate models. The comparisons, conducted for the 10‐year period 1988–1997, show the well‐known overestimation of summer precipitation in the southeastern Mississippi River basin, a consistent overestimation of evapotranspiration, and an underprediction of snow in NRA1. These biases are generally lower in NRA2, though a large overprediction of snow water equivalent exists. NRA1 is subject to errors in the surface water budget due to nudging of modeled soil moisture to an assumed climatology. The nudging and precipitation bias alone do not explain the consistent overprediction of evapotranspiration throughout the basin. Another source of error is the gravitational drainage term in the NCEP LSP, which produces the majority of the model's reported runoff. This may contribute to an overprediction of persistence of surface water anomalies in much of the basin. Residual evapotranspiration inferred from an atmospheric balance of NRA1, which is more directly related to observed atmospheric variables, matches the VIC prediction much more closely than the coupled models. However, the persistence of the residual evapotranspiration is much less than is predicted by the hydrological model or the climate models.
Characteristics of snowdrift over a bare ice surface in Antarctica
Authors: Bintanja Richard
Published: February 2004

This paper presents various snowdrift quantities evaluated from meteorological and snowdrift data observed over an Antarctic blue ice surface during austral summer. It is found that snowdrift transport rates over ice are much lower than over snow surfaces. The main cause of this is the limited availability of loose snow on the blue ice surface. Observations indicate that a significant part of snowdrift transport over blue ice occurs at relatively weak wind speeds, signifying the importance of erosion of loose snow at the ice surface. No clear relationship between wind speed and transport rate is found because other factors such as snow availability, the age of the snow cover, and the upwind conditions are important parameters as well. The increase in aerodynamic roughness length with friction velocity, which is usually found in snowdrift over snow, is absent over blue ice. This is because the surface friction associated with the movement of saltating particles is negligible over the smooth blue ice surface.
Wavelet analysis of stratospheric gravity wave packets over Macquarie Island: 2. Intermittency and mean‐flow accelerations
Authors: Zink Florian , Vincent Robert A.
Published: February 2004

We calculate the mean‐flow accelerations due to gravity wave packets observed in the lower stratosphere over Macquarie Island (55°S, 159°E) between February 1994 and April 1995. The parameters of the wave packets were extracted from twice‐daily radiosonde soundings using a wavelet‐based analysis method introduced by Zink and Vincent [this issue]. The deduced wave parameters are used to directly compute momentum flux profiles in the lower stratosphere, and the shortcomings of this approach to assess mean‐flow accelerations are discussed. We then use the observed wave packets as an input spectrum in a linear ray‐tracing model. The vertical extent of the detected wave packets allows us to define an expression for wave intermittency, which enables us to compute zonal accelerations in the stratosphere and mesosphere. In the stratosphere the waves produce an acceleration of the mean flow, in accordance with predictions. In the mesosphere the inferred wave drag is 2–3 times larger than previous observational and theoretical estimates.
Early evolution of Martian volatiles: Nitrogen and noble gas components in ALH84001 and Chassigny
Authors: Mathew K. J. , Marti K.
Published: February 2004

Studies on SNC meteorites have permitted the characterization of modern Martian atmospheric components as well as indigenous Martian nitrogen and solar‐type xenon. New isotopic and elemental abundances of noble gases and nitrogen in ALH84001 and Chassigny provide important constraints on the early evolution of the planet. A primitive solar Xe component (Chass‐S) and an evolved Xe component (Chass‐E), augmented with fission Xe are identified in Chassigny. Both components represent interior reservoirs of Mars and are characterized by low 129Xe/132Xe (<1.07) and by distinct elemental ratios 36Ar/132Xe <5 and >130, respectively. Light nitrogen (δ15N = −30‰) is associated with the Chass‐S component and is enriched in melt inclusions in olivine. An ancient (presumably incorporated ∼4 Gyr ago) evolved Martian atmospheric component is identified in ALH84001 and has the following signatures: 129Xe/132Xe = 2.16, 36Ar/38Ar ≥ 5.0, 36Ar/132Xe = ∼50, 84Kr/132Xe = ∼6, and δ15N = 7‰. The trapped Xe component in ALH84001 is not isotopically fractionated. We observe major shifts in nitrogen signatures due to cosmogenic N component in both Chassigny and ALH84001. A heavy nitrogen component of comparable magnitude (δ15N>150‰) has previously been interpreted as (heavy) Martian atmospheric N. In situ produced fission Xe components, due to 244Pu in ALH84001 and due to 238U in Chassigny, are identified. The ALH84001 data strongly constrain exchanges of Martian atmospheric and interior reservoirs. Mars retained abundant fission Xe components, and this may account for the low observed fission Xe component in the modern Martian atmosphere. Chronometric information regarding the evolution of the early Martian atmosphere can be secured from the relative abundances of radiogenic and fission Xe, as ∼80% of the Martian 129Xer is observed in the atmospheric 129Xe/132Xe ratio ∼4 Gyr ago.
Geology of Lofn Crater, Callisto
Authors: Greeley Ronald , Heiner Sarah , Klemaszewski James E.
Published: February 2004

Lofn crater is a 180‐km‐diameter impact structure in the southern cratered plains of Callisto and is among the youngest features seen on the surface. The Lofn area was imaged by the Galileo spacecraft at regional‐scale resolutions (875 m/pixel), which enable the general geology to be investigated. The morphology of Lofn crater suggests that (1) it is a class of impact structure intermediate between complex craters and palimpsests or (2) it formed by the impact of a projectile which fragmented before reaching the surface, resulting in a shallow crater (even for Callisto). The asymmetric pattern of the rim and ejecta deposits suggests that the impactor entered at a low angle from the northwest. The albedo and other characteristics of the ejecta deposits from Lofn also provide insight into the properties of the icy lithosphere and subsurface configuration at the time of impact. The “target” for the Lofn impact is inferred to have included layered materials associated with the Adlinda multiring structure northwest of Lofn and ejecta deposits from the Heimdall crater area to the southeast. The Lofn impact might have penetrated through these materials into a viscous substrate of ductile ice or possibly liquid water. This interpretation is consistent with models of the current interior of Callisto based on geophysical information obtained from the Galileo spacecraft.
Is there a Martian atmospheric electric circuit?
Authors: Farrell W. M. , Desch M. D.
Published: February 2004

Laboratory, analytical, and simulation studies suggest that Martian dust clouds can become significantly charged via dust/dust contact electrification. If moderate‐size dust clouds on Mars are like their terrestrial counterparts, they are expected to have the capacity to create electric fields in excess of 1 kV/m. As on Earth, there is an expectation that these storm‐generated electric fields terminate into both the ground and the ionosphere, coupling directly to the capacitor created by these bounding surfaces. Such charged dust storms are expected to drive ionospheric and ground currents, which complete their current path through the “fairweather” atmosphere remote from the storm. This system forms a Martian atmospheric electric circuit analogous (but not identical) to that created in the terrestrial atmosphere via thunderstorms. Here we define the requirements on Martian dust storms and the surrounding environment to create such a global electric circuit and then model the system. We find that the variation in global electric field is a strong function of Martian season, with possible values ranging in excess of hundreds of volts per meter in the active storm season (southern summer) to less than a volt per meter during the nonstorm season.
Strategies for autonomous rovers at Mars
Authors: Gilmore Martha S. , Castaño Rebecca , Mann Tobias , Anderson Robert C. , Mjolsness Eric D. , Manduchi Roberto , Saunders R. Stephen
Published: February 2004

The science return from future robotic exploration of the Martian surface can be enhanced by performing routine processing using onboard computers. This can be accomplished by using software that recognizes scientifically relevant surface features from imaging and other data and prioritizes the data for return transmission to Earth. Two algorithms have been designed and evaluated with field data to identify the properties of the environment that can be reliably detected with onboard imaging and multispectral observation. One algorithm identifies variations in surface textures in images and successfully distinguishes between rocks and soil and between differences in grain size in a rock of a single composition. A second algorithm utilizes a neural net to recognize selected carbonate minerals from spectral reflectance data and successfully identifies carbonates from a set of spectra collected in the field. These types of algorithms will contribute to the efficiency of a landed instrument suite given the limited resources of time, data storage, and available communications opportunities.
The solar wind as a possible source of lunar polar hydrogen deposits
Authors: Crider Dana Hurley , Vondrak Richard R.
Published: February 2004

We investigate the solar wind as a source for the deposits of hydrogen at the lunar poles. We create a Monte Carlo model that simulates the migration of atmospheric particles through the lunar exosphere. Making the model general enough to incorporate any physical process that might affect the particles, we develop a tool that estimates the number and form of particles that reach and stick to lunar cold traps. Each particle is allowed to follow a series of ballistic trajectories as it hops around the surface of the Moon. We trace the path of the particle until it is removed from the system by photo‐processes such as ionization or dissociation, by thermal escape, or by reaching a cold trap. Accumulating statistics on the outcomes for various input particles, we determine the amount and form of hydrogen able to migrate to the lunar cold traps over time for a typical solar wind input flux. We find that although the fraction of hydrogen delivered to the Moon that ultimately reaches the poles is small, a slow steady source like the solar wind has provided enough hydrogen over 83 Myr to account for the observed deposits. Also, we find that an enrichment in the [D/H] ratio occurs by the migration process. The amount of fractionation is dependent on the molecular form of the migrating hydrogen. Atomic deuterium/hydrogen is enriched by a factor of 4 over the delivered fraction by the migration process.
Thermal and radiation stability of the hydrated salt minerals epsomite, mirabilite, and natron under Europa environmental conditions
Authors: McCord Thomas B. , Orlando Thomas M. , Teeter Glenn , Hansen Gary B. , Sieger Matthew T. , Petrik Nikolay G. , Van Keulen Lisa
Published: February 2004

We report studies on the thermal and radiolytic stability of the hydrated salt minerals epsomite (MgSO4·7H2O), mirabilite (Na2SO4·10H2O), and natron (Na2CO3·10H2O) under the low‐temperature and ultrahigh vacuum conditions characteristic of the surface of the Galilean satellite Europa. We prepared samples, ran temperature‐programmed dehydration (TPD) profiles and irradiated the samples with electrons. The TPD profiles are fit using Arrhenius‐type first‐order desorption kinetics. This analysis yields activation energies of 0.90±0.10, 0.70±0.07, and 0.45±0.05 eV for removal of the hydration water for epsomite, natron, and mirabilite, respectively. A simple extrapolation indicates that at Europa surface temperatures (<130 K), epsomite should remain hydrated over geologic timescales (∼1011–1014 years), whereas natron and mirabilite may dehydrate appreciably in approximately 108 and 103 years, respectively. A small amount of SO2 was detected during and after 100 eV electron‐beam irradiation of dehydrated epsomite and mirabilite samples, whereas products such as O2 remained below detection limits. The upper limit for the 100 eV electron‐induced damage cross section of mirabilite and epsomite is ∼10−19 cm2. The overall radiolytic stability of these minerals is partially due to (1) the multiply charged nature of the sulfate anion, (2) the low probability of reversing the attractive Madelung (mostly the attractive electrostatic) potential via Auger decay, and (3) solid‐state caging effects. Our laboratory results on the thermal and radiolytic stabilities of these salt minerals indicate that hydrated magnesium sulfate and perhaps other salts could exist for geologic timescales on the surface of Europa.
Color and polarization as indicators of comet dust properties and evolution in the near‐nucleus coma
Authors: Kolokolova Ludmilla , Jockers Klaus , Gustafson Bo Å. S. , Lichtenberg Günther
Published: February 2004

We present CCD colorimetric and polarimetric observations of comets Hale‐Bopp, Hyakutake, and Tabur and an analysis of their change in color and polarization with the distance from the nucleus that may indicate dust evolution in the coma. We concentrate on trends along the solar‐antisolar direction in the innermost coma, where all three observed comets demonstrate a correlation between color and polarization. Comparisons to laboratory studies of scattering by complex dust structures reveal that the observed correlation is most likely a result of evaporation/destruction of a dark (organic) material. We study the evolution and properties of the dust in the central coma of these comets using a tentative model of cometary dust as an ensemble of core‐mantle particles with sublimating mantles. Calculations for a variety of optical properties, core‐mantle mass ratios, and dust size distributions showed that this model could describe the observed change in color and polarization in cometary comae when realistic ranges of input parameters are used. The best fit particles have silicate cores of radius 0.05–0.2 μm with a porous organic‐carbon mantle of mantle/core mass ratio 0.5–1. Within the frames of the model the peculiarity of the color and polarization for comet Hale‐Bopp (both increase with the distance from the nucleus unlike the decrease observed in comets Hyakutake and Tabur) can be understood in terms of a compact organic mantle on the Hale‐Bopp dust grains, whereas the Hyakutake and Tabur dust have more porous mantles. We show how polarimetric color restricts the range of satisfactory refractive indices of both the core and the mantle materials.
Mapping and modeling of magnetic anomalies in the northern polar region of Mars
Authors: Hood L. L. , Zakharian A.
Published: February 2004

Vector crustal magnetic field maps of the northern polar zone (60°N to 90°N) are constructed from selected Mars Global Surveyor magnetometer data obtained during the period from May 28 to September 13, 1998. Two medium anomalies (amplitudes >50 nT at 170 km altitude) are mapped in locations consistent with earlier studies. No visible surface features correlate with the anomalies, suggesting that the sources lie beneath the visible veneer of polar deposits and volcanic lava flows. If so, then they formed prior to the immediate end of the heavy bombardment (upper Hesperian) period. Modeling of anomaly vector field components combined with independent constraints on the depth to the Curie isotherm yields lower limits on bulk magnetization intensities (0.4–0.9 A/m) that are significantly greater than those measured for Martian (SNC) meteorite samples. Rocks that contain substantially more titanomagnetite than SNC meteorites, or that contain magnetic phases in addition to titanomagnetite, possibly resulting from hydrothermal alteration, are therefore suggested. Alternatively, remanence acquisition in a field of Earthlike intensity (∼50 μT), rather than in the relatively weak inferred paleointensities for SNC meteorites (∼1–10 μT), would also help to explain the relatively strong inferred remanent magnetizations. The approximate south paleomagnetic pole positions corresponding to these two anomaly sources are located in a region between Olympus Mons and the present north rotational pole. This region is adjacent to the approximate location predicted by Melosh [1980] for the paleopole prior to the formation of the Tharsis gravity anomaly.
Energetic processing of laboratory ice analogs: UV photolysis versus ion bombardment
Authors: Gerakines P. A. , Moore M. H. , Hudson R. L.
Published: February 2004

We have the ability to perform both ultraviolet (UV) photolysis (primarily Lyman‐α photons, average E ≈ 10.2 eV per photon) and ion irradiation (protons, E = 0.8 MeV) in the same experimental setup, with ices created under identical conditions. Here we present recent results on the UV and ion processing of ice mixtures at 18 K of the composition H2O + CO2 + CH3OH (1:1:1) and H2O + CO2 + CH4 (1:1:1). H2O, CH3OH, CH4, and CO2 are all major components of ices in most astrophysical environments (whether interstellar, cometary, or planetary). Identifications and formation rates of products were measured. Results for photolyzed and irradiated ices are contrasted. We find that similar chemical products are observed in both cases and that rates of formation are equivalent for most of the major products.
Thermal Emission Spectrometer results: Mars atmospheric thermal structure and aerosol distribution
Authors: Smith Michael D. , Pearl John C. , Conrath Barney J. , Christensen Philip R.
Published: February 2004

Infrared spectra returned by the Thermal Emission Spectrometer (TES) are well suited for retrieval of the thermal structure and the distribution of aerosols in the Martian atmosphere. Combined nadir‐ and limb‐viewing spectra allow global monitoring of the atmosphere up to 0.01 mbar (65 km). We report here on the atmospheric thermal structure and the distribution of aerosols as observed thus far during the mapping phase of the Mars Global Surveyor mission. Zonal and temporal mean cross sections are used to examine the seasonal evolution of atmospheric temperatures and zonal winds during a period extending from northern hemisphere midsummer through vernal equinox (Ls = 104°–360°). Temperature maps at selected pressure levels provide a characterization of planetary‐scale waves. Retrieved atmospheric infrared dust opacity maps show the formation and evolution of regional dust storms during southern hemisphere summer. Response of the atmospheric thermal structure to the changing dust loading is observed. Maps of water‐ice clouds as viewed in the thermal infrared are presented along with seasonal trends of infrared water‐ice opacity. Uses of these observations for diagnostic studies of the dynamics of the atmosphere are discussed.
Mars' “White Rock” feature lacks evidence of an aqueous origin: Results from Mars Global Surveyor
Authors: Ruff Steven W. , Christensen Philip R. , Clark Roger N. , Kieffer Hugh H. , Malin Michael C. , Bandfield Joshua L. , Jakosky Bruce M. , Lane Melissa D. , Mellon Michael T. , Presley Marsha A.
Published: February 2004

The “White Rock” feature on Mars has long been viewed as a type example for a Martian playa largely because of its apparent high albedo along with its location in a topographic basin (a crater). Data from the Mars Global Surveyor Thermal Emission Spectrometer (TES) demonstrate that White Rock is not anomalously bright relative to other Martian bright regions, reducing the significance of its albedo and weakening the analogy to terrestrial playas. Its thermal inertia value indicates that it is not mantled by a layer of loose dust, nor is it bedrock. The thermal infrared spectrum of White Rock shows no obvious features of carbonates or sulfates and is, in fact, spectrally flat. Images from the Mars Orbiter Camera show that the White Rock massifs are consolidated enough to retain slopes and allow the passage of saltating grains over their surfaces. Material appears to be shed from the massifs and is concentrated at the crests of nearby bedforms. One explanation for these observations is that White Rock is an eroded accumulation of compacted or weakly cemented aeolian sediment.