bluuebunny/crossref_metadata_2025 huggingface dataset:
A study of the effects of sub‐grid‐scale land use differences on atmospheric stability in prestorm environments
Authors: Doran J. C. , Zhong S.
Published: September 2002

The effects of sub‐grid‐scale land use differences in modifying the stability of prestorm environments has been studied using data from the Department of Energy Cloud and Radiation Testbed (CART) in Kansas and Oklahoma and a mesoscale model. To quantify the atmospheric instability, three indices were used: the lifted index, the modified K index, and convective available potential energy. The spatial variations of these indices were calculated from simulations using spatially varying and spatially uniform surface fluxes as lower boundary conditions. The CART is approximately 105 km2 in area and is characterized by large areas of contrasting vegetation cover and surface sensible and latent heat fluxes. The spatially varying fluxes were calculated with the SiB2 model using data from the CART. Six days, during which isolated thunderstorms developed, were chosen for the study. The results suggest that sub‐grid‐scale variations in land use differences do not modify the spatial distribution of the stability indices in the southern Great Plains to any significant degree. Predictions of areas of preferred development of deep convection, based on changes in the indices, are not improved by accounting for sub‐grid‐scale variations in land use. The indices were found to be potentially useful but imperfect indicators of the occurrence of deep convective precipitation at scales smaller than that of the CART. At the model resolution used (grid spacing of 2.08 km) a close correlation was not found between regions of precipitation and regions with enhanced simulated vertical updrafts related to land‐use differences, but a detailed study of local triggering has not been carried out.
Double‐peaked hydroxyl airglow profiles observed from WINDII/UARS
Authors: Melo Stella M. L. , Lowe R. P. , Russell J. P.
Published: September 2002

The WINDII instrument onboard the UARS satellite made many thousands of measurements of the apparent brightness of the hydroxyl airglow as a function of tangent height during the night. The measurements were mainly at latitudes between 42°S and 42°N. Most of the profiles of volume emission rate indicate a simple layer with a peak between 85 and 90 km. However, from 5 to 25% of the measured profiles have a more complex structure for the layer profile, most frequently with two peaks separated by 4 to 6 km. The horizontal extent varies from 200 to several thousand kilometers, and the duration may be as long as 1½ hours. Here the WINDII data from November 1991 to April 1995 are used to study the frequency of occurrence of the double‐peaked layers as a function of latitude, season, and local time and to explore possible causes of the unusual profiles. Early in the night the double‐peaked layers occur preferentially at low to medium latitudes, migrating toward the equator by midnight where they remain for the rest of the night. There is a pronounced asymmetry between the hemispheres which reverses between the March/April equinox and the September/October equinox. The solstice behavior is similar, though not so pronounced. Through simulation three possible causes to the observed OH* double‐peaked layers were identified. Gravity waves are a likely source of isolated small‐scale events but the large horizontal scale, long‐duration structures are more likely to result from temperature inversion layers or mixing events or a combination of the two.
Global distribution of carbon monoxide
Authors: Holloway Tracey , Levy Hiram , Kasibhatla Prasad
Published: September 2002

This study explores the evolution and distribution of carbon monoxide (CO) using the National Oceanic and Atmospheric Administration (NOAA) Geophysical Fluid Dynamics Laboratory three‐dimensional global chemical transport model (GFDL GCTM). The work aims to gain an improved understanding of the global carbon monoxide budget, specifically focusing on the contribution of each of the four source terms to the seasonal variability of CO. The sum of all CO sources in the model is 2.5 Pg CO/yr (1 Pg = 103Tg), including fossil fuel use (300 Tg CO/yr), biomass burning (748 Tg CO/yr), oxidation of biogenic hydrocarbons (683 Tg CO/yr), and methane oxidation (760 Tg CO/yr). The main sink for CO is destruction by the hydroxyl radical, and we assume a hydroxyl distribution based on three‐dimensional monthly varying fields given bySpivakovsky et al. [1990], but we increase this field by 15% uniformly to agree with a methyl chloroform lifetime of 4.8 years [Prinn et al, 1995]. Our simulation produces a carbon monoxide field that agrees well with available measurements from the NOAA/Climate Monitoring and Diagnostics Laboratory global cooperative flask sampling network and from the Jungfraujoch observing station of the Swiss Federal Laboratories for Materials Testing and Research (EMPA) (93% of seasonal‐average data points agree within ±25%) and flight data from measurement campaigns of the NASA Global Tropospheric Experiment (79% of regional‐average data points agree within ±25%). For all 34 ground‐based measurement sites we have calculated the percentage contribution of each CO source term to the total model‐simulated distribution and examined how these contributions vary seasonally due to transport, changes in OH concentration, and seasonality of emission sources. CO from all four sources contributes to the total magnitude of CO in all regions. Seasonality, however, is usually governed by the transport and destruction by OH of CO emitted by fossil fuel and/or biomass burning. The sensitivity to the hydroxyl field varies spatially, with a 30% increase in OH yielding decreases in CO ranging from 4–23%, with lower sensitivities near emission regions where advection acts as a strong local sink. The lifetime of CO varies from 10 days over summer continental regions to well over a year at the winter poles, where we define lifetime as the turnover time in the troposphere due to reaction with OH.
A modeling study of iodine chemistry in the marine boundary layer
Authors: McFiggans Gordon , Plane John M. C. , Allan Beverley J. , Carpenter Lucy J. , Coe Hugh , O'Dowd Colin
Published: September 2002

An observationally constrained photochemical box model has been developed to investigate the atmospheric chemistry of iodine in the marine boundary layer, motivated by recent measurements of the iodine monoxide (IO) radical (Allan et al., this issue). Good agreement with the time series of IO measured at a midlatitude coastal station was achieved by using a reaction scheme that included recycling of iodine through marine aerosol. The strong diurnal variation in IO observed in the subtropical Atlantic was satisfactorily modeled by assuming a constant concentration of iodocarbons that photolyzed to produce roughly 1×104 iodine atoms cm−3 s−1 at midday. The significance of the occurrence of IO at concentrations of up to 4 parts per trillion in the marine boundary layer was then considered from three angles. First, the iodine‐catalyzed destruction of ozone was shown to be of a magnitude similar to that caused by odd‐hydrogen photochemistry, with up to 13% of the available ozone destroyed per day in a marine air mass. Second, the enrichment factor of iodine in marine aerosol compared with surface seawater was predicted to increase to values of several thousand, in sensible accord with observations. Most of the enrichment should be due to the accumulation of iodate, although other iodine species may also be present, depending on the rate of aerosol recycling. Third, the denoxification of the marine boundary layer was found to be significantly enhanced as a result of aerosol uptake of IONO2, formed from the recombination of IO with NO2.
Ozone loss in soot aerosols
Authors: Disselkamp R. S. , Carpenter M. A. , Cowin J. P. , Berkowitz C. M. , Chapman E. G. , Zaveri R. A. , Laulainen N. S.
Published: September 2002

The fractal‐like structure of atmospheric soot (e.g., elemental carbon) provides a large surface area available for heterogeneous chemistry in the upper troposphere and lower stratosphere [Blake and Kato, 1995]. One potentially important reaction is ozone decomposition on soot. Although extensively studied in the laboratory, a wide range of reaction probabilities have been observed (γ∼10−3 to γ∼10−7) which have been attributed to differences in reactivity between fresh (i.e., nonoxidized) versus aged (i.e., oxidized) soot [Schurath and Naumann, 1998]. The importance in understanding soot‐ozone chemistry is particularly important in light of recent nighttime field measurements [Berkowitz et al., 2000] made over Portland, Oregon. The data revealed episodes of an anticorrelation between ozone mixing ratio and aerosol surface area density. During these episodes a single scattering albedo in the range 0.8–0.9 was measured, indicating an increased absorptive component of the aerosol, perhaps due to elemental carbon. In addition, an increase in the concentration of aerosols contained in the small size range of the fine mode (<0.1–0.15 μm) was observed, suggestive of new aerosol formation. In this article we attempt to explain these field observations. One explanation of the field observations is ozone loss occurring on atmospheric soot aerosol. Here we present laboratory results obtained using a static aerosol reactor that indicate that direct ozone loss on soot aerosol is unlikely under ambient conditions in the troposphere. An alternative and more likely explanation of the field data is based on ozone‐mediated organic aerosol production. This could occur by either nighttime nitrate radical oxidation or direct ozone oxidation of hydrocarbons as suggested previously [Starn et al., 1998; Griffin et al., 1999; Kamens et al., 1999; Yu et al., 1999; De Gouw and Lovejoy, 1998].
On the parameterization of activation spectra from cloud condensation nuclei microphysical properties
Authors: Cohard Jean‐Martial , Pinty Jean‐Pierre , Suhre Karsten
Published: September 2002

A simple parametric relationship is established between factors describing the shape of cloud condensation nuclei (CCN) activation spectra and observable properties of the aerosol population they grow on (size distribution and solubility). This is done independently for maritime and continental aerosol types because of their very different characteristics. The data used for the multiple statistical adjustments in the procedure described in this paper are generated by running a numerical model of aerosol growth coupled to a simple cloud droplet activation scheme. Each aerosol population (maritime and continental) is assumed to be of homogeneous chemical composition, lognormally distributed and with variable solubility. The parameterization is then evaluated using a large set of aerosol populations with randomized properties. Finally, the study presents a preliminary analysis of the most important aerosol properties that influence the shape of the CCN spectra. An idealized scenario of a clean maritime boundary layer cloud perturbed by anthropogenic emissions (such as the ship track problem) illustrates the capability of the parameterization to selectively increase the cloud droplet concentration in a partially polluted cloud. The calibration results presented in this paper are not meant to be the definitive activation spectra produced by any lognormally distributed aerosols. These results are indeed a step toward an objective initialization of CCN spectra and hence toward the computation of cloud droplet concentrations based on measurable multimodal aerosol features, as required by three‐dimensional numerical models with a coupled interactive aerosol module.
Comparison of aerosol optical properties and water vapor among ground and airborne lidars and Sun photometers during TARFOX
Authors: Ferrare R. , Ismail S. , Browell E. , Brackett V. , Clayton M. , Kooi S. , Melfi S. H. , Whiteman D. , Schwemmer G. , Evans K. , Russell P. , Livingston J. , Schmid B. , Holben B. , Remer L. , Smirnov A. , Hobbs P. V.
Published: September 2002

We compare aerosol optical thickness (AOT) and precipitable water vapor (PWV) measurements derived from ground and airborne lidars and Sun photometers during the Tropospheric Aerosol Radiative Forcing Observational Experiment. Such comparisons are important to verify the consistency between various remote sensing measurements before employing them in any assessment of the impact of aerosols on the global radiation balance. Total scattering ratio and extinction profiles measured by the ground‐based NASA Goddard Space Flight Center scanning Raman lidar system, which operated from Wallops Island, Virginia (37.86°N, 75.51°W), are compared with those measured by the Lidar Atmospheric Sensing Experiment (LASE) airborne lidar system aboard the NASA ER‐2 aircraft. Bias and root‐mean‐square differences indicate that these measurements generally agreed within about 10%. Aerosol extinction profiles and estimates of AOT are derived from both lidar measurements using a value for the aerosol extinction/backscattering ratio Sa = 60 sr for the aerosol extinction/backscattering ratio, which was determined from the Raman lidar measurements. The lidar measurements of AOT are found to be generally within 25% of the AOT measured by the NASA Ames Airborne Tracking Sun Photometer (AATS‐6). However, during certain periods the lidar and Sun photometer measurements of AOT differed significantly, possibly because of variations in the aerosol physical characteristics (e.g., size, composition) which affect Sa. Estimates of PWV, derived from water vapor mixing ratio profiles measured by LASE, are within 5–10% of PWV derived from the airborne Sun photometer. Aerosol extinction profiles measured by both lidars show that aerosols were generally concentrated in the lowest 2–3 km.
Secondary organics and atmospheric cloud condensation nuclei production
Authors: Kerminen Veli‐Matti , Virkkula Aki , Hillamo Risto , Wexler Anthony S. , Kulmala Markku
Published: September 2002

The influence of secondary organics on atmospheric cloud condensation nuclei (CCN) production was investigated using a zero‐dimensional box model that simulates the production of secondary organics in the gas phase, the transportation of these organics from gas to the particulate phase, and the resulting growth of the particles. Model simulations demonstrated that the growth of nanometer‐size nuclei to a CCN size requires the presence of organics of extremely low volatility. These “nonvolatile” organics need to have saturation vapors pressures of the order of 0.01–0.1 parts per trillion or lower and, in order to induce sufficient nuclei growth, must have gas phase production rates of the order of 0.3–1 μg m−3 d−1 under conditions typical for continental background areas. As the nuclei grow in size, they start to uptake volatile organics more efficiently. As a result, organic matter in both the nuclei grown into a CCN size and in the preexisting accumulation mode particles is expected to be dominated by “low‐volatile” organics rather than organics that actually are responsible for the nuclei growth. The modeling results suggest that the monoterpene oxidation products identified so far in field or laboratory experiments, although important contributors to secondary particulate matter, are unlikely to be the ones that grow nuclei to a CCN size. In field experiments, positive identification of organics producing new CCN would require information on the chemical composition of particles smaller than about 0.1 μm in diameter, which is the size range where nonvolatile organics are likely to be enriched compared with other secondary or primary organics. Since the gas phase production rate of nonvolatile organics needs not to be very large in order to induce significant nuclei growth, more attention should also be paid to reaction products that have minor yields in smog chamber experiments.
Chamber measurement of surface‐atmosphere trace gas exchange: Numerical evaluation of dependence on soil, interfacial layer, and source/sink properties
Authors: Hutchinson G. L. , Livingston G. P. , Healy R. W. , Striegl R. G.
Published: September 2002

We employed a three‐dimensional finite difference gas diffusion model to simulate the performance of chambers used to measure surface‐atmosphere trace gas exchange. We found that systematic errors often result from conventional chamber design and deployment protocols, as well as key assumptions behind the estimation of trace gas exchange rates from observed concentration data. Specifically, our simulations showed that (1) when a chamber significantly alters atmospheric mixing processes operating near the soil surface, it also nearly instantaneously enhances or suppresses the postdeployment gas exchange rate, (2) any change resulting in greater soil gas diffusivity, or greater partitioning of the diffusing gas to solid or liquid soil fractions, increases the potential for chamber‐induced measurement error, and (3) all such errors are independent of the magnitude, kinetics, and/or distribution of trace gas sources, but greater for trace gas sinks with the same initial absolute flux. Finally, and most importantly, we found that our results apply to steady state as well as non‐steady‐state chambers, because the slow rate of gas diffusion in soil inhibits recovery of the former from their initial non‐steady‐state condition. Over a range of representative conditions, the error in steady state chamber estimates of the trace gas flux varied from −30 to +32%, while estimates computed by linear regression from non‐steady‐state chamber concentrations were 2 to 31% too small. Although such errors are relatively small in comparison to the temporal and spatial variability characteristic of trace gas exchange, they bias the summary statistics for each experiment as well as larger scale trace gas flux estimates based on them.
High‐resolution photoabsorption cross‐section measurements of SO2 at 295 K between 198 and 220 nm
Authors: Stark G. , Smith Peter L. , Rufus J. , Thorne A. P. , Pickering J. C. , Cox G.
Published: September 2002

SO2 is an important constituent of the atmospheres of Io and Venus. Accurate photoabsorption cross‐section data at the temperatures of these planetary atmospheres are required for the interpretation of SO2 observations and for reliable photochemical models. As part of a planned series of measurements over the 160–300 K range, photoabsorption cross sections of SO2 at 295 K, in the wavelength region 198–220 nm, have been determined from very high resolution (resolving power (λ/Δλ) ≈ 450,000) absorption spectra recorded with a vacuum ultraviolet Fourier transform spectrometer. These new measurements are compared with earlier, lower‐resolution (λ/Δλ ≈ 100,000) photoabsorption measurements at 213 K spanning the same wavelength region, with lower‐resolution (λ/Δλ ≈ 100,000) measurements at 295 K in a more limited region, and with a recent Doppler‐limited laser diode measurement at 295 K over a very narrow wavelength region.
Correlations between topography and intraflow width behavior in Martian and terrestrial lava flows
Authors: Peitersen Matthew N. , Crown David A.
Published: September 2002

Local correlations between topography and width behavior within lava flows at Puu Oo, Mount Etna, Glass Mountain, Cerro Bayo, Alba Patera, Tyrrhena Patera, Elysium Mons, and Olympus Mons were investigated. For each flow, width and slope data were both referenced via downflow distance as a sequence of points; the data were then divided into collections of adjacent three‐point features and two‐point segments. Four discrete types of analyses were conducted: (1) Three‐point analysis examined positional correlations between width and slope features, (2) two‐point analysis did the same for flow segments, (3) mean slope analysis included segment slope comparisons, and (4) sudden width behavior analysis measured abruptness of width changes. The distribution of types of correlations compared to random combinations of features and segments does not suggest a significant correlation between flow widths and local underlying slopes and indicates that for these flows at least, other factors have more influence on changes in width than changes in underlying topography. Mean slopes underlying narrowing, widening, and constant flow width segments were calculated. An inverse correlation between slope and width was found only at Mount Etna, where slopes underlying narrowing segments were greater than those underlying widening in 62% of the examined flows. For the majority of flows at Mount Etna, Puu Oo, and Olympus Mons, slopes were actually greatest under constant width segments; this may imply a topographically dependent resistance to width changes. The rate of change of width was also examined. Sudden width changes are relatively common at Puu Oo, Mount Etna, Elysium Mons, and Tyrrhena Patera and relatively rare at Glass Mountain, Cerro Bayo, Olympus Mons, and Alba Patera. After correction for mapping scale, Puu Oo, Mount Etna, Olympus Mons, and Alba Patera appear to fall on the same trend; Glass Mount exhibits unusually small amounts of sudden width behavior, and Tyrrhena Patera exhibits a relatively large number of sudden width behavior occurrences.
Stability of hydroxylated minerals on Mars: A study on the effects of exposure to ultraviolet radiation
Authors: Yen Albert S. , Murray Bruce , Rossman George R. , Grunthaner Frank J.
Published: September 2002

The density and composition of the Martian atmosphere allow solar ultraviolet photons with wavelengths as short as 190 nm to reach the surface. We investigate the hypothesis that this UV radiation is capable of inducing the release of water from iron oxyhydroxide minerals resulting in the formation of oxide phases. These experiments, which utilize a quadrupole mass spectrometer to monitor the water vapor pressure above mineral samples during cyclic exposure to ultraviolet radiation, offer 5 to 6 orders of magnitude greater sensitivity than previous attempts to establish and quantify this process. We find no evidence that UV photons are capable of liberating OH from the crystal lattice of minerals, and we set a minimum ultraviolet radiation‐induced dehydroxylation time of 108 years for removal of this structural OH from mineral particles at the Martian surface. The overturning timescales for surface fines are likely to be shorter than this lower limit for exposure time. Thus we conclude that UV‐stimulated dehydroxylation is not a significant process at the Martian surface and that iron oxyhydroxides, if formed during an earlier water‐rich environment, should still be found on Mars today. The lack of clear evidence for iron oxyhydroxides at the Martian surface further suggests that Mars' surface was never warm and wet for a long enough period of time for Earth‐like weathering to have occurred.
Initial results from radio occultation measurements with Mars Global Surveyor
Authors: Hinson D. P. , Simpson R. A. , Twicken J. D. , Tyler G. L. , Flasar F. M.
Published: September 2002

A series of radio occultation experiments conducted with Mars Global Surveyor in early 1998 has yielded 88 vertical profiles of the neutral atmosphere. The measurements cover latitudes of 29°N to 64°S and local times from 0600 through midnight to 1800 during early summer in the southern hemisphere (Ls = 264°–308°). Retrieved profiles of pressure and temperature versus radius and geopotential extend from the surface to the 10‐Pa pressure level. Near‐surface uncertainties in temperature and pressure are about 1 K and 2 Pa, respectively, far smaller than in previous radio occultation measurements at Mars. The profiles resolve the radiative‐convective boundary layer adjacent to the surface and also reveal gravity waves, particularly at northern and equatorial latitudes, which appear to be breaking in some cases. Distinctive meridional gradients of pressure and temperature indicate the presence of a low‐altitude westerly jet at latitudes of 15°–30°S at southern summer solstice. This jet appears in predictions of general circulation models in connection with the strong, seasonal, cross‐equatorial Hadley circulation. The pressure gradient at ∼2 km altitude implies a wind speed of 33 m s−1, stronger than predicted, which may help explain the occurrence of numerous local dust storms within this latitude band in late southern spring. These measurements also characterize the response of the atmosphere to stationary thermal forcing at midsouthern latitudes, where high terrain south of Tharsis and low terrain in Hellas Planitia produce large, zonal temperature variations in the lowest scale height above the surface. Pressure measured at constant geopotential decreases at an average rate of 0.13% per degree Ls, due primarily to condensation of CO2 at the North Pole.
Mars Global Surveyor Thermal Emission Spectrometer (TES) observations of dust opacity during aerobraking and science phasing
Authors: Smith Michael D. , Pearl John C. , Conrath Barney J. , Christensen Philip R.
Published: September 2002

The Mars Global Surveyor (MGS) arrived at Mars in September 1997 near Mars's southern spring equinox and has now provided monitoring of conditions in the Mars atmosphere for more than half a Mars year. The large majority of the spectra taken by the Thermal Emission Spectrometer (TES) are in a nadir geometry (downward looking mode) where Mars is observed through the atmosphere. Most of these contain the distinct spectral signature of atmospheric dust. For these nadirgeometry spectra we retrieve column‐integrated infrared aerosol (dust) opacities. TES observations during the aerobraking and science‐phasing portions of the MGS mission cover the seasonal range Ls = 184°–28°. Excellent spatial coverage was obtained in the southern hemisphere. Northern hemisphere coverage is generally limited to narrow strips taken during the periapsis pass but is still very valuable. At the beginning of the mission the 9‐μm dust opacity at midsouthern latitudes was low (0.15–0.25). As the season advanced through southern spring and into summer, TES observed several regional dust storms (including the Noachis dust storm of November 1997) where peak 9‐μm dust opacities approached or exceeded unity, as well as numerous smaller local storms. Both large and small dust storms exhibited significant changes in both spatial coverage and intensity over a timescale of a day. Throughout southern spring and summer the region at the edge of the retreating southern seasonal polar ice cap was observed to be consistently more dusty than other latitudes.
Determining the modal mineralogy of mafic and ultramafic igneous rocks using thermal emission spectroscopy
Authors: Hamilton Victoria E. , Christensen Philip R.
Published: September 2002

The modal mineralogies of 20 mafic and ultramafic igneous rocks were determined from their thermal infrared emission spectra using a linear deconvolution approach, which uses a library of end‐member mineral spectra to model a bulk rock spectrum. Over 90% of the modes obtained from thermal emission spectra agree with modes obtained by traditional optical analyses to within the stated error of the optical analyses (5–15 vol%). Library spectra of several compositions within a solid solution series (e.g., plagioclase feldspars labradorite and bytownite) were commonly used in each best fit model and are assumed to represent, in combination, a composition in the rock for which an identical mineral spectrum was unavailable in the library. The accuracy of this assumption was evaluated by calculating a weighted average solid solution composition for plagioclase and/or pyroxenes from the library minerals used in the model best fit of 14 rocks and comparing these compositions to the actual chemistries measured by electron microprobe or the optically estimated An#. The derived solid solution compositions are generally within 10–15 An# (or Mg#) of the measured composition. Modal data derived from the spectra via the deconvolution were summed and converted to weight percent (wt%) oxides for comparison to standard bulk chemistry data. SiO2, Al2O3, and Na2O + K2O were generally slightly overestimated, and FeO and MgO were typically slightly underestimated. Modes and wt% oxides were plotted on standard rock type classification diagrams and provide broadly accurate classifications, demonstrating that the linear deconvolution technique is successful at deriving useful mineralogical information from thermal infrared emission spectra of bulk rock samples. Furthermore, convolution of the laboratory data to spectral resolutions comparable to remote sensing instruments (specifically, the Mars Global Surveyor Thermal Emission Spectrometer) demonstrates only a ∼5 vol% increase in modal uncertainty for minerals present in abundances ≥10 vol%. This study thus provides a strong foundation for the application of this technique to mafic igneous sample spectra from both laboratory and remote sensing instruments.
Greenhouse warming by CH4 in the atmosphere of early Earth
Authors: Pavlov Alexander A. , Kasting James F. , Brown Lisa L. , Rages Kathy A. , Freedman Richard
Published: September 2002

Earth appears to have been warm during its early history despite the faintness of the young Sun. Greenhouse warming by gaseous CO2 and H2O by itself is in conflict with constraints on atmospheric CO2 levels derived from paleosols for early Earth. Here we explore whether greenhouse warming by methane could have been important. We find that a CH4 mixing ratio of 10−4 (100 ppmv) or more in Earth's early atmosphere would provide agreement with the paleosol data from 2.8 Ga. Such a CH4 concentration could have been readily maintained by methanogenic bacteria, which are thought to have been an important component of the biota at that time. Elimination of the methane component of the greenhouse by oxidation of the atmosphere at about 2.3–2.4 Ga could have triggered the Earth's first widespread glaciation.
A komatiite analog to potential ultramafic materials on Io
Authors: Williams David A. , Wilson Allan H. , Greeley Ronald
Published: September 2002

Ultramafic volcanism is one model which fits currently available data for some eruptions on Jupiter's moon, Io. Assuming that such activity does occur, it is possible to apply komatiitic analogs and terrestrial ultramafic flow models to Io. We have used such analogs and models along with Galileo data to investigate the nature of ultramafic materials that are assumed to occur at high‐temperature hotspots on Io. The unusual komatiites of the Commondale greenstone belt, South Africa, are consistent with available Galileo data on the temperatures and composition of potential Ionian ultramafic materials. These komatiites had high SiO2 (∼50 wt %) and MgO (∼31 wt %) contents, which are inferred to result in high liquidus temperatures (∼1610°C), low dynamic viscosities (∼0.2 Pa s), and low densities (∼2680 kg/m) and in the crystallization of orthopyroxene phenocrysts (not found in other komatiites). We used the Commondale composition and the model of Williams et al. [1998] to investigate potential ultramafic flow behavior on Io. Compared to their terrestrial counterparts, Ionian ultramafic lava flows would have less potential for turbulent flow, lower maximum emplacement distances, and lower thermal erosion rates because of the lower Ionian gravity and their more silicic composition. Shallow (<10 m deep) thermal erosion channels are predicted to occur more proximally to the lava source than on Earth. Deep (>10 m) thermal erosion channels seem less likely on Io, unless (1) lavas are superheated, (2) lavas are more ultramafic (>31% MgO), (3) eruption rates, eruption durations, or flow volumes are high, or (4) substrates are unconsolidated, partly consolidated, or volatile‐rich and thus more erodable. If volatiles are present in the lava as vesicles, then increasing lava vesicularity results in decreasing maximum emplacement distances, decreasing thermal erosion rates, and decreasing erosion channel depths for given eruption durations.
Crustal history of north central Arabia Terra, Mars
Authors: McGill George E.
Published: September 2002

Detailed geological studies across the highland‐lowland boundary in north central Arabia Terra, Mars provide constraints on the age and nature of processes responsible for a complex array of structures and landforms. The area studied is bounded by 27.5° and 47.5°N, 330° and 335°W. Highland basement was formed in Early Noachian and was extensively resurfaced in Late Noachian. Materials underlying highland plateaus were emplaced in Middle Noachian. Erosion responsible for fretted terrane and fretted channels most likely occurred in Early Hesperian but definitely prior to the middle of the Hesperian and during or after Middle Noachian. There is good evidence for the involvement of liquid water in fretted channel formation. Within the area mapped, the presence of fretted terrane correlates with evidence for extensive development of large grabens. All lowland materials were emplaced after the erosion that formed fretted terrane and channels; the earliest of these materials were emplaced in mid‐Hesperian, and the youngest were emplaced in Late Amazonian. Morphological evidence supports debris flow or rock glacier processes for the formation of the youngest materials. Debris flow materials on the floors of fretted channels are much too young to be related to channel formation. The entire region exhibits a northerly tilt of ∼0.1°–0.15° that formed after deposition of the oldest lowland materials but before emplacement of Upper Hesperian and younger debris flow/rock glacier materials. The nature and ages of structures and landforms in this region are not consistent with the presence of a Late Noachian to Early Hesperian convergent plate boundary in northern Arabia Terra.
Discrimination between maturity and composition of lunar soils from integrated Clementine UV‐visible/near‐infrared data: Application to the Aristarchus Plateau
Authors: Le Mouélic Stéphane , Langevin Yves , Erard Stéphane , Pinet Patrick , Chevrel Serge , Daydou Yves
Published: September 2002

The reflectance spectrum of a lunar soil is mainly dominated by the composition and the degree of exposure to space weathering processes such as micrometeorite bombardment and solar wind implantation. The spectral alteration effects of space weathering should be removed for accurately investigating the composition of the lunar surface using remote sensing data. In this paper we show that the integration of the Clementine UV‐visible (UVVIS) and near‐infrared (NIR) channels provides an improved evaluation of the spectral alteration. The depth of the mafic absorption feature at 0.95 μm is also better defined by combining the UVVIS and NIR data. Laboratory spectra of lunar soil samples indicate that the continuum slope derived from the 1500/750 nm ratio is closely related to the concentration of fine‐grained submicroscopic iron (Is). The continuum slope therefore provides an evaluation of the spectral alteration of the surface, which can be subtracted from the 1 or 2 μm absorption band depths to retrieve compositional information. This method has been applied to the Aristarchus plateau, which exhibits a broad range of mineralogical composition and maturity. A nine‐channel multispectral mosaic of 680 Clementine images of the Aristarchus plateau has been processed. Eight telescopic spectra have been used to check the validity of the reduction process for the near infrared bands. The 1 μm absorption band, once corrected for spectral alteration, provides an evaluation of the initial FeO content in mafic silicates (mafic iron). Lunar soil samples show that it is possible to quantitatively map mafic iron with this technique. Our results are in good agreement with those obtained using the algorithm of Lucey et al. [1995,1998a], which is based on UVVIS bands alone. The mafic iron content and total iron content which can be derived from the combined UVVIS and NIR data sets are less sensitive to local slopes than that derived from Lucey et al.'s method. This new method could therefore be useful for investigating areas at middle to high latitudes. Removing spectral alteration from the 2000/1500 nm ratio also makes possible a better discrimination between olivine and pyroxene within identified mare basalts on the Aristarchus plateau.
Localization of bedding plane slip and backthrust faults above blind thrust faults: Keys to wrinkle ridge structure
Authors: Schultz Richard A.
Published: September 2002

A mechanically based model for wrinkle ridge development is developed that combines wrinkle ridge morphologies, regional topographic offsets suggestive of subsurface thrust faults, and folding of near‐surface layers. This model provides explicit relationships between observed morphologic elements characteristic of wrinkle ridges and plausible mechanisms in the subsurface, a key component that is absent in previous qualitative fault‐based scenarios for these structures. As developed in this paper, wrinkle ridges are the surface expression of anticlines that grow above a blind thrust fault as a result of both flexural slip folding of near‐surface strata and the nucleation and growth of echelon arrays of backthrust faults. Calculations of displacements (both horizontal shortening and vertical uplift) and Coulomb stress change related to slip along blind thrust faults in the model demonstrate physically important spatial inhomogeneities in these quantities, with revealing and useful implications. (1) The ratio of shortening due to folding at the surface to the shortening due to faulting at depth is characteristically small for coupled wrinkle ridge‐blind thrust fault systems and decreases with increasing fault depth; the depth of the blind thrust fault's upper tip thus profoundly influences the surface strains. (2) Folding and uplifted topography, forming the topographic ridge, are produced above the area of the slipping blind thrust fault plane. Horizontal and vertical deformation at the surface extend over several ridge widths, or a total of at least 50 km for a 10‐km‐wide ridge, implying that topographic profiles and geologic studies must extend sufficiently far from the wrinkle ridge to fully characterize the surface deformation. (3) Calculations of Coulomb stress changes suggest that fault slip can localize both bedding plane slip in overlying strata and new backthrust faults that propagate upward to become wrinkles on the trailing side of the ridge. Initiation of bedding plane slip in association with slip along the blind thrust fault likely determines whether the resulting surface structure becomes a wrinkle ridge or a lobate scarp. Wrinkle ridge spacing may also be related to stress changes associated with slip along the underlying blind thrust fault.
The shape of Mars before Global Surveyor: Results from reanalysis of the Viking control point network
Authors: Zeitler Wolfgang , Oberst Jürgen
Published: September 2002

Three‐dimensional coordinates of 3739 globally distributed control points derived from photogrammetric analysis of Viking Orbiter image data are studied with respect to the shape and large‐scale morphology of Mars. Spheres, spheroids, spherical functions, and a Digital Terrain Model (DTM) of 50 km grid spacing are fitted to the data. Prominent topographic features in the terrain model include the Tharsis volcanoes, Olympus Mons, Alba Patera and prominent mare and highland regions as well as the major impact basins. While the global dichotomy is clearly visible in our data, our model does not reveal any distinct signature associated with the presumed dichotomy boundary at this global scale. Rather, it suggests a smooth topographic transition from southern to northern hemisphere. This model is unique among existing topographic data sets for Mars, as it combines global coverage, a spatial resolution sufficient to resolve regional topography, and absolute elevation data more accurate than in previous control point network analyses. Formal errors and a comparison with the first released topographic profiles obtained by Mars Global Surveyor's Laser Altimeter (MOLA) suggest that 70% and 90% of the DTM grid elements represent the topography of the planet to better than 1000 m and 2000 m, respectively.
Calibration of stable isotopic data: An enriched δ18O standard used for source gas mixing detection and correction
Authors: Ostermann D. R. , Curry W. B.
Published: September 2002

We present empirically based calibrations of our measurements made on a Finnigan MAT252 equipped with a Kiel Device to Vienna Pee Dee belemnite, using an enriched δ18O standard. Calibrations include corrections for biases caused by the differences in isotopic composition of carbonate standards measured on the two parallel extraction lines of the Kiel Device and for decreases in the isotopic difference between the reference and sample gas caused by mixing in the source. After correcting for these biases, the precision of 2200 NBS19 analyses (10–300 µg ) is ±0.07 for δ18O and ±0.03 for δ13C. We have shared our standard enriched in δ18O with 18 laboratories engaged in paleoceanographic research, producing the first large‐scale interlaboratory calibrations for this community. Using correction procedures reported here, water mass reconstructions using data produced on multiple mass spectrometers may now be possible with a precision approaching the level necessary to reconstruct temperature‐salinity and density variability in the deep ocean.
Evidence for a Warm Last Glacial Maximum in the Nordic Seas or an example of shortcomings in UK37′ and UK37 to estimate low sea surface temperature?
Authors: Rosell‐Melé Antoni , Comes Pau
Published: September 2002

The occurrence of ice‐free conditions in the Nordic seas during the Last Glacial Maximum has been demonstrated using microfossil and biomarker approaches. An accurate picture of the prevailing oceanographic conditions may be more difficult to ascertain. Thus, uncertainties for most proxies increase at the cold extreme of temperature calibrations and glacial sediments contain small amounts of microfossils and biomarkers. Here we discuss sea surface temperature estimates derived from alkenone indices for the Last Glacial Maximum in the Nordic seas that provide a surprising scenario, where surface temperatures were warmer than at present. We conclude that the UK37′ and UK37 estimates may not be interpreted in terms of sea surface temperature as several factors may have conspired to bias the molecular signal. However, using an alternative index based on the abundance of the C37∶4 alkenone, we estimate a maximum temperature of 6°C.
Environmental control on Mediterranean salinity and δ18O
Authors: Rohling Eelco J.
Published: September 2002

A box model is presented to simulate changes in Mediterranean long‐term average salinity and δ18O over the past 20,000 years. Simulations are validated by comparison with observations. Sensitivity tests illustrate robustness with respect to the main assumptions and uncertainties. The results show that relative humidity over the Mediterranean remained relatively constant around 70%, apparently narrowly constrained to the lower end of the range observed globally over sea surfaces by the basin's land‐locked character. Isotopic depletion in run off, relative to the present, is identified as the main potential cause of depletions in the Mediterranean δ18O record. Also, slight increases in relative humidity (of the order of 5%) might have caused very pronounced isotopic depletions, such as that in sapropel S5 of the penultimate interglacial maximum. The model shows distinctly non proportional responses of δ18O and salinity to environmental change, which argues against the use of isotope residuals in Mediterranean paleosalinity reconstructions.
Glacial‐interglacial changes in nutrient utilization and paleoproductivity in the Indonesian Throughflow sensitive Timor Trough, easternmost Indian Ocean
Authors: Müller Anne , Opdyke Bradley N.
Published: September 2002

Paleoproductivity in the Timor Trough appears to be inversely proportional to the strength of the Indonesian Throughflow. Today, productivity in the area is inhibited by the narrow band of low‐salinity surface water that moves through the Indonesian Archipelago and spreads out over the equatorial portions of the eastern Indian Ocean. During the Last Glacial Maximum, however, the reduction or absence of this low‐salinity “cap” would have enhanced the possibility of upwelling and higher productivity in the region. Our results indicate that at this time, productivity was enhanced, the surface waters were being depleted of CO2 and relative nitrate utilization was low. This suggests that the thermocline was shallow and that upwelled, nutrient‐rich water was present.
Aspects of bulk atmospheric boundary layer similarity under free‐convective conditions
Authors: Brutsaert Wilfried
Published: September 2002

For many hydrologic and atmospheric dynamic purposes the turbulent surface fluxes of sensible and latent (or evaporative) energy and of momentum must be formulated over areas covering a wide range of spatial scales; these are characteristically the scales of river basins and of the grid sizes for integration in current atmospheric circulation models. The bulk atmospheric boundary layer (ABL) similarity (BAS) approach continues to be one of the very few available formulations to describe turbulent surface fluxes over the range of scales from roughly 1 to 10 km, in terms of average characteristics aloft in the upper reaches of the ABL. The approach is reexamined as it relates to atmospheric stability and surface characteristics under conditions of free convection. Since the largest gradients occur near the surface, first a formulation is proposed for the Monin‐Obukhov profile functions for convective conditions that is consistent with the theoretical advances by Kader and Yaglom [1990] and with recent experimental data. This is then combined with a slab representation of the mixed layer to derive the most plausible form of the BAS functions, on the basis of presently available information. Finally, it is illustrated with available experimental data that any alternative formulation of bulk ABL transport, regardless of its appearance, is of necessity equivalent with BAS and that the same original variables are at work.
Inversions of radio occultation amplitude data
Authors: Sokolovskiy Sergey V.
Published: September 2002

Radio occultation remote sensing of the Earth's atmosphere consists of satellite‐to‐satellite observations of phase and amplitude of radio waves that propagate through the atmosphere. The observed excess phase along with the positions and velocities of the satellites are inverted into bending angle as a function of impact parameter and then into vertical profiles of refractivity, pressure, and temperature in the neutral atmosphere, or into electron density in the ionosphere. The retrieved profiles are assigned to the perigee points of the sounding rays. Amplitude data are normally not used, except when solving diffraction back propagation problems. In this paper a simple method to utilize amplitude radio occultation data is discussed. Equations based on geometric optics are considered for the inversion of an amplitude into bending angle. These inversions do not require high coherence of radio waves or precise orbit determination, as with phase inversions, but they do require precise calibration of the amplitude. Even though amplitude inversions are not so precise as phase inversions, they may still be useful for a number of applications. When compared to phase inversions they allow the optimization of the filter bandwidth for phase inversions, the detection of multipath propagation, and the localization of electron density irregularities in the ionosphere. These applications are demonstrated by processing of the Global Positioning System/Meteorology (GPS/MET) radio‐occultation data collected onboard the satellite Microlab‐1.