bluuebunny/crossref_metadata_2025 huggingface dataset:
Isotopic composition of stratospheric ozone
Authors: Johnson David G. , Jucks Kenneth W. , Traub Wesley A. , Chance Kelly V.
Published: September 2002

Using recently measured rates for isotopic variants of the ozone formation reaction O + O2 + M, we derive expressions for the enrichment of symmetric and asymmetric 50O3 and 49O3 in stratospheric ozone and calculate total enrichments of 123‰ and 107‰ for 50O3 and 49O3, respectively. We compare the calculations with measurements made with the Smithsonian Astrophysical Observatory far‐infrared spectrometer during seven balloon flights which took place between 1989 and 1997. We find that the calculations are in reasonable agreement with the average observed enrichments of 102±43‰ and 73±60‰ for 50O3 and 49O3, respectively, where the errors indicate the estimated accuracy; we also find that our measurements are in agreement with other recent stratospheric and tropospheric measurements. Using our measurements of the enrichment of the asymmetric and symmetric forms of 50O3 and 49O3, we estimate branching ratios of 0.43±0.03 and 0.46±0.06 for production of the symmetric isotopomer by the reaction of 16O with 34O2 and 33O2, respectively.
Arctic ozone destruction and chemical‐radiative interaction
Authors: MacKenzie I. A. , Harwood R. S.
Published: September 2002

Three northern hemisphere winter/springs with contrasting thermal and dynamic characteristics were simulated with a mechanistic model of the middle atmosphere incorporating an interactive chemistry scheme. The three seasonal simulations used different planetary‐wave forcings at the lower boundary, and consequently their respective temperature evolutions represented (1) a relatively mild Arctic winter; (2) a winter comparable to the coldest on record; and (3) a winter more persistently cold than any yet observed. The radiative response to the varying degrees of chemical ozone depletion in these winters was investigated by comparing results from runs that included and excluded the heterogeneous reactions on polar stratospheric clouds (PSCs) which initiate the enhanced destruction of polar ozone. Cooling of the lower stratosphere induced by the enhanced ozone loss ranges from virtually zero in the warmest winter to ∼5 K in the coldest. The extent of any self‐perpetuation of the ozone destruction processes via the induced cooling was examined by using the meteorological fields calculated both with and without PSC‐initiated ozone depletion to drive an off‐line chemical transport model. In winters 1 and 2 the longevity of PSCs and enhanced reactive halogen concentrations is increased only marginally by the cooling arising from the ozone loss, and consequently there is little impact on the net ozone destruction. In winter 3, however, the radiative feedback from the ozone loss leads to a marked prolongation of the lifetime of the enhanced reactive halogens and causes a chemically induced diminution of up to ∼20 Dobson units (DU) in the April ozone column compared with the no‐feedback case. Moreover, in winter 3 the cooling caused by PSC‐initiated ozone depletion delays the springtime weakening of the polar vortex and inhibits the replenishment of Arctic ozone, resulting in a ∼25 DU depression of the ozone column in early May that is due mainly to dynamics. These results suggest that chemical‐radiative interaction via PSC processes has had only a small impact on the ozone distribution in recent Arctic winters, but that it has the potential to become more important should an unprecedentedly cold winter occur while atmospheric halogen concentrations remain artificially high.
Effects of prolonged soil drought on CH4 oxidation in a temperate spruce forest
Authors: Borken W. , Brumme R. , Xu Y.‐J.
Published: September 2002

Our objective was to determine potential impacts of changes in rainfall amount and distribution on soil CH4 oxidation in a temperate forest ecosystem. We constructed a roof below the canopy of a 65‐year‐old Norway spruce forest (Picea abies (L.) Karst.) and simulated two climate change scenarios: (1) an extensively prolonged summer drought of 172 days followed by a rewetting period of 19 days in 1993 and (2) a less intensive summer drought of 108 days followed by a rewetting period of 33 days in 1994. CH4 oxidation, soil matric potential, and soil temperature were measured hourly to daily over a 2‐year period. The results showed that annual CH4 oxidation in the drought experiment increased by 102% for the climate change scenario 1 and by 41% for the climate change scenario 2, compared to those of the ambient plot (1.33 kg CH4 ha−1 in 1993 and 1.65 kg CH4 ha−1 in 1994). We tested the relationships between CH4 oxidation rates, water‐filled pore space (WFPS), soil matric potential, gas diffusivity, and soil temperature. Temporal variability in the CH4 oxidation rates corresponded most closely to soil matric potential. Employing soil matric potential and soil temperature, we developed a nonlinear model for estimating CH4 oxidation rates. Modeled results were in strong agreement with the measured CH4 oxidation for the ambient (r2 = 0.80) and drought plots (r2 = 0.89) over two experimental years, suggesting that soil matric potential is a highly reliable parameter for modeling CH4 oxidation rate.
Characteristics of ground‐level ozone distributions in Korea for the period of 1990–1995
Authors: Ghim Young Sung , Chang Young‐Soo
Published: September 2002

General characteristics and trends of ground‐level ozone were investigated by using observations from over 80 stations in Korea for the period of 1990–1995. In general, seasonal ozone variations are characterized by a spring peak with decreased levels in the summer, followed by increases in the fall. In summer, owing to frequent precipitation, monthly average ozone levels notably decreased in most coastal areas, while a weak depression was observed in inland stations, sustained by photochemical ozone buildup under relatively low wind speeds. Coupled with frequent precipitation in summer, with high ozone‐forming potential, low wind speeds in inland areas play a significant role in local variations of ozone concentrations. Accordingly, ozone concentrations vary in a quite different manner among monitoring stations in the Greater Seoul Area, as well as among major cities nationwide. On the whole, monthly average ozone concentrations in major cities, whether located in inland or coastal areas, are about 10–20 ppb lower than background levels of 30–40 ppb typical of the Northern Hemisphere. These lower concentrations are likely due to the titration effect of local NOx emissions, which is a clear indication of urban characteristics for most of the stations. Next, ozone concentration variations at 34–37 stations in the Greater Seoul Area, characterized by typical photochemical air pollution, were closely investigated. Special efforts were made to clarify the effects of transport within the Greater Seoul Area on the ozone concentration variations, a long‐standing question at issue, by examining monthly variations of percentile values and frequency distributions of hourly ozone concentrations. It is believed that complex local variations across the nation are superimposed on underlying effects from the Asian continent and the ocean, located to the west and northeast of the Korean Peninsula, respectively. Further research is needed to quantify the relative importance of these effects in controlling the nationwide distribution of ozone levels.
Statistical characterization of air ion mobility spectra at Tahkuse Observatory: Classification of air ions
Authors: Hõrrak U. , Salm J. , Tammet H.
Published: September 2002

A database of 8615 hourly averaged air ion mobility spectra in the range of 0.00041–3.2 cm2 V−1 s−1 was measured at Tahkuse Observatory, Estonia, during 14 months in 1993–1994. The average mobility spectrum over the whole period shows distinct peaks of small and large ions. Intermediate ions with mobilities of 0.034–0.5 cm2 V−1 s−1 are of low concentration of about 50 cm−3 in the average spectrum. They experience occasional bursts of up to about 900 cm−3 during 6–10 hours at daytime. The number of burst events recorded during 14 months was 101, with maximum frequency in spring and minimum frequency in winter. Physically, large and intermediate ions can be called aerosol ions, and small ions can be called cluster ions. The principal component analysis was applied to detect the structure of an air ion mobility spectrum. As a result, the mobility spectrum in the range of 0.00041–3.2 cm2 V−1 s−1 (diameters of 0.36–79 nm) was divided into five classes: small cluster, big cluster, intermediate, light large, and heavy large ions. The boundaries between the classes are 1.3 cm2 V−1 s−1 (diameter of 0.85 nm), 0.5 cm2 V−1 s−1 (1.6 nm), 0.034 cm2 V−1 s−1 (7.4 nm), and 0.0042 cm2 V−1 s−1 (22 nm). The five principal components that are closely correlated with the respective ion classes explain 92% of total variance. The classification of aerosol ions is in accord with the three‐modal structure of the size spectrum of submicron aerosol particles.
Role of the ambient aerosol in the atmospheric processing of semivolatile contaminants: A parameterized numerical model (Gas‐Particle Partitioning (GASPAR))
Authors: Pirrone Nicola , Hedgecock Ian M. , Forlano Luigi
Published: September 2002

A parameterized description of the ambient aerosol is the basis of a model that treats both gas‐particle partitioning and aqueous phase chemical transformations of semivolatile contaminants. Dividing the aerosol population into source, size, hygroscopic, and compositional classes, it is possible to assess the importance of contaminant‐aerosol interactions under varying meteorological conditions. Using mercury as a test case, the model provides not only the quantity and speciation of mercury associated with particulate matter for use in dry deposition models and in conjunction with dispersion/meteorological models, but shows conclusively that deliquesced aerosol particles are not simply transporters of adsorbed mercury, but play an active and significant role in the transformation of elemental to oxidized mercury. The sensitivity analysis carried out using a version of the Direct Decoupled Method has shown the transfer of Hg(II) to the gas phase from the aqueous phase to be highly dependent on the chloride ion concentration in the initial parameterization array which describes the ambient aerosol. The chloride ion concentration has a notable effect on the oxidized Hg that is associated with the particle when the chemistry model reaches steady state. The reason for this is clarified by the dependencies of the neutral Hg containing species concentrations on the rates of mass transfer and the initial concentrations. The presence of soot in the aerosol particles is shown to be particularly important in the partitioning of Hg(II) between the gas, aqueous and particulate phases. The implications, given the higher solubility of most oxidized mercury species compared to elemental mercury, are fundamental for the understanding of the cycling and fate of mercury in the environment.
Measurement of the settling rate of atmospheric dust on Mars by the MAE instrument on Mars Pathfinder
Authors: Landis Geoffrey A. , Jenkins Phillip P.
Published: September 2002

The Materials Adherence Experiment (MAE) on Pathfinder was designed to measure the rate of dust settling from the Martian atmosphere onto the solar array of the Mars Pathfinder Sojourner Rover. The MAE measurements indicate steady dust accumulation at a rate of about 0.28% per day. This value is consistent with the performance of the lander solar arrays, which decreased in power at an estimated rate of 0.29% per day over the same period.
Magnetic enhancement on the surface of Mars?
Authors: Hargraves R. B. , Knudsen J. M. , Bertelsen P. , Goetz W. , Gunnlaugsson H. P. , Hviid S. F. , Madsen M. B. , Olsen M.
Published: September 2002

The magnetic properties experiments on the Viking missions and the Pathfinder mission indicate that the Martian soil and airborne dust are somewhat magnetic (average saturation magnetization, σS ∼ 4 A m2kg−1). While hematite, superparamagnetic or macrocrystalline, is not sufficiently magnetic to yield the results obtained, pyrogenetic titaniferous magnetite (TiMt) might conceivably be the cause. However, the σS of the dust is considerably higher than that in any of the known Martian meteorites, some of which may be representative of the bedrock from which the Mars soil formed. Furthermore if the reported TiO2 content of Mars soil (∼1% by weight) was entirely present as TiMt of composition Usp 60 (that typical of terrestrial ocean floor basalts), the calculated abundance (<4%) would yield σS of only 1.2 A m2kg−1. As the Pathfinder magnetic properties experiment results pertain only to the airborne dust particles on Mars, the likelihood of aeolian concentration of such TiMt grains is minimal. Ferrous iron in the bedrock silicates must have been converted to maghemite (γ‐Fe2O3) by some unknown oxidative mechanism; this “magnetic enhancement” should be incorporated in any process envisioned for the origin of Martian soil.
Discussion of the 1.04‐μm water ice absorption band in the Europa NIMS spectra and a new NIMS calibration
Authors: McCord Thomas B. , Hansen Gary B. , Shirley James H. , Carlson Robert W.
Published: September 2002

Measurements made by the Galileo Near Infrared Mapping Spectrometer (NIMS) have been used to calculate reflectance spectra for regions of Europa. Many of these spectra show features indicative of water ice. Others show asymmetric H2O absorption features suggesting hydrated non‐ice constituents such as hydrated sulfates and carbonates. Often, both types of features are present. However, some previously published NIMS spectra with strongly developed ice features did not show the expected 1.04‐μm ice absorption band. This discrepancy has led to questions regarding earlier interpretations of Europa's spectral features. Recent work reveals that unexpected changes have occurred in the spectral range covered by each NIMS detector and that an additional calibration is required. Preliminary calibration values are given. When this new calibration is applied, the 1.04‐μm feature emerges clearly in those spectra where it was expected but absent previously, thereby resolving the 1.04‐μm band discrepancy.
Evidence for a sill emplacement event on the upper flanks of the Ascraeus Mons shield volcano, Mars
Authors: Scott Evelyn D. , Wilson Lionel
Published: September 2002

Channels within the SSW rift zone of Ascraeus Mons, a large shield volcano forming part of the Tharsis volcanic province on Mars, have morphological features extremely similar to those of channels identified elsewhere on Mars as being due to fluvial action. The channel sources are too far from the caldera rim and have the wrong morphology for it to be plausible that the channels were eroded during pyroclastic flow eruptions, and the sinuosities are not consistent with scenarios involving lava flows. We propose that these channels were formed by meltwater generated and released, mainly by relatively slow seepage, when a sill was emplaced within or beneath an ice‐rich permafrost layer. Smaller, fan‐like channels coalescing into the amphitheater headwall of the wider ones are secondary features produced by further sapping of the undermined flank. The minimum dimensions required for the sill (∼13 by 13 km in lateral extent and ∼120 m thick) are calculated using the thermal exchanges between the sill and the permafrost implied by the volumes of the channel head craters. The corresponding minimum sill volume, ∼19 km3, is similar to that of many surface lava flows associated with the Tharsis shield volcanoes and is of the order expected for an intrusion resulting from an elastic inflation‐deflation event in the summit magma reservoir.
Obliquity‐oblateness feedback on Mars
Authors: Bills Bruce G.
Published: September 2002

A simple model is presented for the coupled dynamics of the orbit‐rotation‐climate system of Mars. Changes in the orientation of the spin pole, relative to the orbit pole, influence the spatiotemporal pattern of incident radiation and thus drive climatic mass transport into and out of the polar regions on a variety of timescales. Changes in the mass distribution occur from direct climatic forcing and compensating viscous flow in the interior. The net change in mass distribution influences the rate of spin axis precession and thereby influences obliquity. The rate of secular obliquity drift depends on several poorly known parameters, including the magnitudes and response times of volatile inventories and viscosity structure within Mars. Even relatively modest secular obliquity drift can lead to trapping in nearby resonances. The dissipative nature of the coupled dynamical system makes reconstruction of past evolution much more difficult than for a purely inertial system. The long‐term obliquity history of Mars is dominated by climate.
Brine mobilization during lithospheric heating on Europa: Implications for formation of chaos terrain, lenticula texture, and color variations
Authors: Head James W. , Pappalardo Robert T.
Published: September 2002

The background lineated plains on Europa are locally highly modified and destroyed in regions known as chaos and lenticulae. Produced there are (1) isolated fragments and polygons of background material which rotate and translate, (2) matrix, which fills in the areas between the fragments and polygons, and (3) surface discolorations. Using observations and constraints from high‐resolution Galileo images, we find that a model for the formation of these terrains which involves mobilization and migration of brines, and a possible percolation phase transition as the Europan lithosphere is warmed, can readily explain the vast majority of their characteristics. In addition, the presence of melt fractions of a few percent in the adjacent ice framework may enhance the creep rate and the accompanying deformation rates. The characteristics and distribution of lenticulae suggest that among the strong candidates for heat sources for brine migration and ice mobilization processes is diapirism linked to solid‐state convection in a layer underlying a brittle lid and possibly overlying a liquid water layer.
Effects of sterilizing doses of gamma radiation on Mars analog rocks and minerals
Authors: Allen Carlton C. , Albert Fred G. , Combie Joan , Banin Amos , Yablekovitch Yehuda , Kan Ido , Bodnar Robert J. , Hamilton Victoria E. , Jolliff Bradley L. , Kuebler Karla , Wang Alian , Lindstrom David J. , Morris Penny A. , Morris Richard V. , Murray Richard W. , Nyquist Laurence E. , Simpson Paul D. , Steele Andrew , Symes Steven J.
Published: September 2002

Rock and soil samples from the planet Mars are due to be returned to Earth within a decade. Martian samples initially will be tested for evidence of life and biological hazard under strict biological containment. Wider distribution of samples for organic and inorganic analysis may occur only if neither evidence of life nor hazard is detected, or if the samples are first sterilized. We subjected a range of Mars analog rocks and minerals to high doses of gamma radiation in order to determine the effects of gamma sterilization on the samples' isotopic, chemical, and physical properties. Gamma photons from 60Co (1.17 and 1.33 MeV) in doses as high as 3×107 rads did not induce radioactivity in the samples and produced no measurable changes in their isotopic and chemical compositions. This level of irradiation also produced no measurable changes in the crystallographic structure of any sample, the surface areas of soil analogs, or the fluid inclusion homogenization temperature of quartz. The only detectable effects of irradiation were dose‐dependent changes in the visible and near‐infrared spectral region (e.g., discoloration and darkening of quartz and halite and an increase in albedo of carbonates) and increases in the thermoluminescence of quartz and plagioclase. If samples returned from Mars require biological sterilization, gamma irradiation provides a feasible option.
Mars Pathfinder landing site: Evidence for a change in wind regime from lander and orbiter data
Authors: Greeley Ronald , Kraft Michael D. , Kuzmin Ruslan O. , Bridges Nathan T.
Published: September 2002

Surface features related to the wind are observed in the vicinity of the Mars Pathfinder (MPF) landing site data from the lander and in data from orbit by the Viking Orbiter and Mars Global Surveyor missions. Features seen from the surface include wind tails associated with small rocks, barchanoid duneforms, ripplelike patterns, and ventifact flutes cut into some rocks. Features seen from orbit include wind tails associated with impact craters, ridges inferred to be duneforms, and modified crater rims interpreted to have been eroded and mantled by windblown material. The orientations of these features show two prevailing directions. One is inferred to represent winds from the northeast, which is consistent with strongest winds predicted by a general circulation model to occur during the Martian northern winter under current conditions. A second wind blowing from the ESE was responsible for modifying the crater rims and cutting some of the ventifacts. The two wind regimes could reflect a change in climate related to Mars' obliquity or some other, unknown factor. Regardless of the cause, the MPF area has been subjected to a complex pattern of winds and supply of small particles, and the original surface formed by sedimentary processes from Tiu and Ares Valus flooding events has been modified by repeated burial and exhumation.
Modal mineralogic distribution in the regolith at Apollo landing sites
Authors: Basu Abhijit , Riegsecker Sue
Published: September 2002

We have reduced published petrographic data on 90 surface soils from Apollo sample return sites in terms of olivine, pyroxene, plagioclase, and opaques (mostly ilmenite) for a “best estimate”. To do so, we have recast the best estimates of modal mineralogy of mare and highland lithic fragments to add to the monomineralic population of soils. We have assumed that agglutinates and regolith breccias mimic those of local soils. We have thus arrived at a working summary of mineral distribution in areas traversed by Apollo astronauts. In addition, we have proportioned these averages to geologic units in the premission maps to estimate the average mineral composition of areas up to 30 km × 30 km around the landing sites. The calibration and testing of remote sensing signals for planetary exploration depend largely on the ground truth provided by lunar samples. Chemical compositions and the modal distribution of petrologic constituents of most lunar soils are known. These can be grouped spatially to represent areas that coincide with pixels of remote sensing observations and measurements. Signals from certain petrographic constituents such as lithic fragments, breccias, and agglutinates, however, are understandably ambiguous. Petrologic inference from remote sensing signals would be facilitated if petrographic data from actual samples were to be reduced and expressed as sums of single phases occurring in each pixel of observation.
The thermal inertia of Mars from the Mars Global Surveyor Thermal Emission Spectrometer
Authors: Jakosky Bruce M. , Mellon Michael T. , Kieffer Hugh H. , Christensen Philip R. , Varnes E. Stacy , Lee Steven W.
Published: September 2002

We have used Mars Global Surveyor (MGS) Thermal Emission Spectrometer thermal emission measurements to derive the thermal inertia of the Martian surface at the ∼100‐km spatial scale. We have validated the use of nighttime‐only measurements to derive thermal inertia as well as the use of a single wavelength band versus bolometric thermal emission measurements. We have also reanalyzed the Viking Infrared Thermal Mapper data set in a similar manner in order to allow a direct comparison between the two. Within the uncertainties of the fit of the data to the model, and the uncertainties inherent in the model, the thermal inertia has not changed substantially in the 21 years between the Viking and the MGS measurements. Although some differences are seen, they are most likely due to changes in albedo during the intervening years or to residual effects of airborne dust that are not fully accounted for in the thermal models. The thermal inertia values that we derive, between about 24 and 800 J m−2 s−1/2 K−1, are thought to better represent the actual thermal inertia of the Martian surface than previous estimates.
An intercomparison of ground‐based millimeter, MGS TES, and Viking atmospheric temperature measurements: Seasonal and interannual variability of temperatures and dust loading in the global Mars atmosphere
Authors: Clancy R. T. , Sandor B. J. , Wolff M. J. , Christensen P. R. , Smith M. D. , Pearl J. C. , Conrath B. J. , Wilson R. J.
Published: September 2002

During the period October 1997 to September 1999 we obtained and analyzed over 100 millimeter‐wave observations of Mars atmospheric CO line absorption for atmospheric temperature profiles. These measurements extend through one full Mars year (solar longitudes LS of 190° in 1997 to 180° in 1999) and coincide with atmospheric temperature profile and dust column measurements from the Thermal Emission Spectrometer (TES) experiment on board the Mars Global Surveyor (MGS) spacecraft. A comparison of Mars atmospheric temperatures retrieved by these distinct methods provides the first opportunity to place the long‐term (1982–1999) millimeter retrievals of Mars atmospheric temperatures within the context of contemporaneous, spatially mapped spacecraft observations. Profile comparisons of 0–30 km altitude atmospheric temperatures retrieved with the two techniques agree typically to within the 5 K calibration accuracy of the millimeter observations. At the 0.5 mbar pressure level (∼25 km altitude) the 30°N/30°S average for TES infrared temperatures and the disk‐averaged millimeter temperatures are also well correlated in their seasonal and dust‐storm‐related variations over the 1997–1999 period. This period includes the Noachis Terra regional dust storm, which led to very abrupt heating (∼15 K at 0.5 mbar) of the global Mars atmosphere at LS = 224° in 1997 [Christensen et al., 1998; Conrath et al., this issue; Smith et al., this issue]. Much colder (10–20 K) global atmospheric temperatures were observed during the 1997 versus 1977 perihelion periods (LS = 200°–330°), consistent with the much (2 to 8 times) lower global dust loading of the atmosphere during the 1997 perihelion dust storm season versus the Viking period of the 1977a,b storms. The 1998–1999 Mars atmosphere revealed by both the millimeter and TES observations is also 10–15 K colder than presented by the Viking climatology during the aphelion season (LS = 0°–180°, northern spring/summer) of Mars. We reassess the observational basis of the Viking dusty‐warm climatology for this season to conclude that the global aphelion atmosphere of Mars is colder, less dusty, and cloudier than indicated by the established Viking climatology even for the Viking period. We also conclude that Mars atmospheric temperatures exhibit their most significant interannual variations during the perihelion dust storm season (10–20 K for LS = 200°–340°) and during the post‐aphelion northern summer season (5–10 K for LS = 100°–200°).
Spectral data set factor analysis and end‐member recovery: Application to analysis of Martian atmospheric particulates
Authors: Bandfield Joshua L. , Christensen Philip R. , Smith Michael D.
Published: September 2002

A method is described that uses target transformation factor analysis techniques to determine the number of independently variable components and recover the spectral end‐members present in a set of mixed spectra. These techniques were tested on two sets of synthetic spectral mixtures and several subsets of Thermal Emission Spectrometer (TES) data. In both synthetic mixture sets, the correct numbers of components were determined, and the original spectral end‐members were accurately recovered. An initial application of this method was used with several subsets of TES apparent emissivity spectra that contain only minor surface spectral components. This method has demonstrated that the spectra can be modeled using linear combinations of three spectral end‐members: atmospheric dust (with atmospheric gas absorptions that vary in unison with the dust shape), water ice cloud, and blackbody. The atmospheric dust and water ice cloud spectral shapes were recovered from several orbits with a wide variety of atmospheric dust opacities during the southern spring and summer seasons. The atmospheric dust spectral shape is nearly constant except for the relative contribution of atmospheric gasses that vary in unison with the dust shape. The water ice spectral shape is also constant with the exception of a small shift in the position of the ∼800 cm−1 absorption that was observed during a period of high opacity.
Formation of large metal nodules in ordinary chondrites
Authors: Rubin Alan E.
Published: September 2002

Literature data show that many large (2–30 mm) metallic Fe‐Ni nodules in ordinary chondrites are depleted in refractory siderophiles (e.g., Re, Os, Ir, Ru, Pt, and Rh) and Cu relative to fine‐grained metal but are not depleted in (normally refractory) W and Mo, common siderophiles (e.g., Ni and Co) and volatile siderophiles (e.g., Au, As, and Sb). It was recently proposed that metamorphic diffusion could form the metal nodules and that partitioning of siderophile elements between kamacite and taenite could account for the nodules' unusual compositions. However, the nodules have the same compositional trends as metal nodules and veins in shocked meteorites that clearly formed by impact‐related processes. If large metal nodules formed during metamorphism, type 6 chondrites should contain abundant large nodules. Instead, the mean metal grain size in H6 chondrites is 0.12±0.15 mm, far smaller than any of the nodules. It seems likely that the metal nodules formed by impact vaporization, oxidation of W and Mo to form volatile oxides, concomitant partial oxidation of Re and Os, rapid fractional condensation of refractory siderophiles, transport of the residual vapor, condensation of Cu, S, and Se into vugs and fractures to form sulfide, and condensation of the remaining siderophiles to form metal nodules and veins. During vapor cooling and condensation (or during subsequent parent body annealing) W, Mo, Re, and Os oxides were reduced.
Identification of a basaltic component on the Martian surface from Thermal Emission Spectrometer data
Authors: Christensen Philip R. , Bandfield Joshua L. , Smith Michael D. , Hamilton Victoria E. , Clark Roger N.
Published: September 2002

The Mars Global Surveyor Thermal Emission Spectrometer (TES) instrument collected 4.8×106 spectra of Mars during the initial aerobraking and science‐phasing periods of the mission (September 14, 1997, through April 29, 1998). Two previously developed atmosphere‐removal models were applied to data from Cimmeria Terra (25°S, 213°W). The surface spectra derived for these two models agree well, indicating that the surface and atmosphere emission can be separated and that the exact atmosphere‐removal model used has little effect on the derived surface composition. The Cimmeria spectra do not match terrestrial high‐silica igneous rocks (granite and rhyolite), ultramafic igneous rocks, limestone, or quartz‐ and clay‐rich sandstone and siltstone. A particulate (sand‐sized) sample of terrestrial flood basalt does provide an excellent match in both spectral shape and band depth to the Cimmeria spectrum over the entire TES spectral range. No unusual particle size effects are required to account for the observed spectral shape and depth. The implied grain size is consistent with the thermal inertia and albedo of this region, which indicate a sand‐sized surface with little dust. The identification of basalt is consistent with previous indications of pyroxene and basalt‐like compositions from visible/near‐infrared and thermal‐infrared spectral measurements. A linear spectral deconvolution model was applied to both surface‐only Cimmeria spectra using a library of 60 minerals to determine the composition and abundance of the component minerals. Plagioclase feldspar (45%; 53%) and clinopyroxene (26%; 19%) were positively identified above an estimated detection threshold of 10–15% for these minerals. The TES observations provide the first identification of feldspars on Mars. The best fit to the Mars data includes only clinopyroxene compositions; no orthopyroxene compositions are required to match the Cimmeria spectra. Olivine (12%; 12%) and sheet silicate (15%; 11%) were identified with lower confidence. Carbonates, quartz, and sulfates were not identified in Cimmeria at detection limits of ∼5, 5, and 10%, respectively. Their presence elsewhere, however, remains open. The Cimmeria spectra are not well matched by any one SNC meteorite spectrum, indicating that this region is not characterized by a single SNC lithology. The occurrence of unweathered feldspar and pyroxene in Cimmeria, together with the inferred presence of pyroxene and unweathered basalts in other dark regions and at the Viking and Pathfinder landing sites, provides evidence that extensive global chemical weathering of materials currently exposed on the Martian surface has not occurred.
Seismic reflection study of Upheaval Dome, Canyonlands National Park, Utah
Authors: Kanbur Z. , Louie J. N. , Chávez‐Pérez S. , Plank G. , Morey D.
Published: September 2002

The origin of Upheaval Dome, in Canyonlands National Park of southeastern Utah, has been a topic of controversy among geologists and planetary scientists. The structure has long been thought to have been created by salt diapirism from the underlying Paradox Formation. Recent studies have suggested that impact could have formed the dome. To test the various hypotheses, we acquired, processed, and interpreted seismic reflection data within and adjacent to the structure. Both conventionally stacked and prestack‐migrated images show <100 m relief in the Paradox Formation, contrary to salt diapirism hypotheses. Further, we have identified features within the images typical of impact structures, such as listric normal faults having displacements toward the center of the dome. Deformation occurs in two depth ranges, with the faulting that created the central uplift appearing only above the Hermosa Formation, in the upper 800 m of the structure. The images also suggest limited fracturing of the Hermosa and salt flow in the Paradox Formation, perhaps due to gravitational relaxation of the crater form. Our image of a nearly flat top of the Paradox salt strongly favors an impact origin for Upheaval Dome.
New views of Mars eolian activity, materials, and surface properties: Three vignettes from the Mars Global Surveyor Mars Orbiter Camera
Authors: Edgett Kenneth S. , Malin Michael C.
Published: September 2002

Prior to the Mars Global Surveyor (MGS) mission, a very general view had emerged in which Martian surface materials were seen as consisting of a mixture of bright dust, dark sand, and rocks. The configuration of these materials and the winds that transport and deposit sand and dust have been thought to be directly linked to the albedo patterns that have been observed on Mars for centuries. High spatial resolution images (1.4–20 m/pixel) obtained by the MGS Mars Orbiter Camera (MOC) between September 15, 1997, and July 4, 1999, provide new information about the physical nature of the windblown material on the Martian surface and show that the pre‐MGS view was much too simple. In addition to bright dust and dark sand, MOC images show evidence of bright sediment that can be transported by saltation (e.g., sand) and dark material that can be transported in suspension (e.g., silt). New views of eolian wind streaks in Daedalia Planum show that part of this region, thought to be mantled by bright dust based upon Viking and Mariner observations, is instead covered by a thin veneer of bright, windblown sand. MOC images of Sinus Sabaeus and parts of Syrtis Major, two regions thought to be sandy based upon Viking era observations, exhibit thick mantles that are inferred to consist of fine‐grained sediment deposited from eolian suspension. Low albedo wind streaks in western Arabia Terra are also dark mantles, and their association with eroded crater floors and megaripples/dunes on these floors suggest that these particular wind streaks are deposits of silt‐sized material that was only briefly suspended before settling to the surface. MOC images also show evidence that some eolian dunes are active on Mars today (i.e., in 1998 and 1999); the evidence for activity is largely based upon wind‐and avalanche‐induced streaks superposed on or eroded into seasonal frost on high‐latitude dune fields. MOC images show that some other dunes are inactive, but the albedo of dunes relative to surrounding terrain is not a good indicator of dune activity because some inactive dunes are not mantled by dust. Inactive dunes are best identified by superposed features such as impact craters, landslide deposits, and yardangs.
The effects of external material on the chemistry and structure of Saturn's ionosphere
Authors: Moses Julianne I. , Bass Stephen F.
Published: September 2002

We have developed a one‐dimensional coupled ion‐neutral photochemical model for Saturn's upper atmosphere to better understand the structure and chemistry of Saturn's ionosphere. In addition to modeling the chemistry of hydrogen and hydrocarbon ions, we investigate the effects of an oxygen and metal influx from ring or meteoric sources. The Infrared Space Observatory observations of H2O and CO2 in Saturn's stratosphere are used to constrain the influx of extraplanetary material. As expected, the topside ionosphere of Saturn is dominated by H+, with H3+ prevailing just below the electron density peak. When micrometeoroid ablation is considered, we find that metal ions, represented here by Mg+, can take the place of hydrocarbon ions as the major ionic species in the lower ionosphere. The models then exhibit a characteristic double peak, with H+ creating the high‐altitude peak and Mg+ the low‐altitude peak. A pronounced gap forms between the two peaks, especially at night, when H3+ ions rapidly recombine. Neutral winds and electric fields in the presence of magnetic fields can cause vertical plasma motion that can shift the location of both electron density peaks. In addition, multiple sharp layers in the electron density profile can form in the lower ionosphere when oscillatory vertical drifts are introduced into the model to simulate the effects of atmospheric gravity waves. The location and magnitude of the “main peak” as well as the sharper lower‐ionospheric layers observed with the Voyager and Pioneer radio occultation experiments (and eventually with similar Cassini observations) can help constrain the atmospheric structure, wind profiles, or electric field properties in Saturn's upper atmosphere.
Terrestrial analogs and thermal models for Martian flood lavas
Authors: Keszthelyi L. , McEwen A. S. , Thordarson T.
Published: September 2002

The recent flood lavas on Mars appear to have a characteristic “platy‐ridged” surface morphology different from that inferred for most terrestrial continental flood basalt flows. The closest analog we have found is a portion of the 1783–1784 Laki lava flow in Iceland that has a surface that was broken up and transported on top of moving lava during major surges in the eruption rate. We suggest that a similar process formed the Martian flood lava surfaces and attempt to place constraints on the eruption parameters using thermal modeling. Our conclusions from this modeling are (1) in order to produce flows >1000 km long with flow thicknesses of a few tens of meters, the thermophysical properties of the lava should be similar to fluid basalt, and (2) the average eruption rates were probably of the order of 104 m3/s, with the flood‐like surges having flow rates of the order of 105–106 m3/s. We also suggest that these high eruption rates should have formed huge volumes of pyroclastic deposits which may be preserved in the Medusae Fossae Formation, the radar “stealth” region, or even the polar layered terrains.
Strike‐slip duplexing on Jupiter's icy moon Europa
Authors: Prockter Louise M. , Pappalardo Robert T. , Head James W.
Published: September 2002

Agenor Linea is a ∼1500 km long, ∼20–30 km wide geologically young zone of deformation on Jupiter's icy moon, Europa. On the basis of recent Galileo high‐resolution images, we interpret Agenor Linea as a strike‐slip zone formed in three stages by a combination of lithospheric separation, extension, and dextral horizontal shear. Agenor Linea exhibits excellent examples of strike‐slip duplexes in an icy lithosphere, unobscured by vegetation and unaltered by erosion.
Ganymede's ozone‐like absorber: Observations by the Galileo ultraviolet spectrometer
Authors: Hendrix A. R. , Barth C. A. , Hord C. W.
Published: September 2002

We report on Ultraviolet Spectrometer observations of Ganymede from four orbits of the Galileo primary mission. During 12 observation sequences, regions on Ganymede's leading, trailing, and anti‐Jovian hemispheres were observed at several latitudes and varying observational geometries. The measurements show an ozone‐like absorber concentrated in the polar regions and near sunrise and sunset at low latitudes, with a clear correlation between ozone abundance and solar zenith angle. The absorption maximum is shifted to the red of the gaseous ozone absorption maximum (near 2600 Å), confirming disk‐integrated observations by other researchers. Ganymede's absorber may be ozone trapped in the ice lattice, which we propose is due to charged particle bombardment of the surface. At high Sun (small solar zenith angles), the ozone‐like absorber is destroyed. The ozone in the polar regions is likely to be long‐lived, on account of the large solar zenith angles.
Influence of neutral components on relative band contrasts in reflectance spectra of intimate mixtures: Implications for remote sensing: 1. Nonlinear mixing modeling
Authors: Moroz Lyuba , Arnold Gabriele
Published: September 2002

The application of nonlinear mixing to model the band contrasts in the spectra of intimate mixtures shows that not only the band contrasts but also their ratios are sensitive to the presence of a featureless end‐member. The ratio of the contrast of the weaker band to that of the deeper one decreases with increasing content of neutral phase. The degree and the rate of decline depend on a number of factors. When a neutral phase is present, the use of relative band contrasts to estimate the relative abundance of end‐members results in systematic errors. In multicomponent mixtures the presence of a spectrally neutral phase leads to the underestimation of the component with the weaker band. The larger the difference in contrasts between the bands, the larger is the systematic error. Additional factors increasing the error are an opacity of the neutral phase, a large difference in brightness between the neutral phase and the components with absorption features, a decreased grain size of the neutral component relative to those of the other constituents, and decreased grain sizes of all end‐members. The use of calibrations for binary mineral mixtures to analyze the spectra of multicomponent surfaces should be subject to errors when opaque constituents are present. Some variations in relative band contrast of asteroid spectra due to variable contents and/or particle size of opaques may be misinterpreted as variations in silicate modal abundance.
Io's sodium directional feature: Evidence for ionospheric escape
Authors: Wilson Jody K. , Schneider Nicholas M.
Published: September 2002

We present evidence that Io's sodium directional feature is produced by the neutralization of pickup ions over the anti‐Jovian hemisphere of Io. The localized production region requires ion pickup to occur at or below Io's exobase and is consistent with an ionospheric escape mechanism. Numerical modeling reproduces the feature in at least four distinct data sets spanning 15 years. The directional feature is sensitive to local magnetic fields and seems to indicate little or no perturbation to the local Jovian magnetic field beyond that associated with the Io/torus interaction. The sensitivity of the directional feature to electric and magnetic fields near Io makes it a convenient ground‐based observable phenomenon for quantifying the complex interaction of Jupiter's plasma torus with Io's atmosphere.
Unchanged nitrogen isotopic composition of organic matter in the South China Sea during the last climatic cycle: Global implications
Authors: Kienast Markus
Published: September 2002

The δ15N of surface and down‐core sediments spanning the last 20–200 kyr from the entire South China Sea (SCS) ranges only from ∼3.0 to ∼6.5‰, with no correlation with discernible paleoclimatic/oceanographic changes. Detailed profiles of the uppermost sediment column, including fluff samples, indicate a minor diagenetic overprint of 0.3–1.2‰ at the sediment‐water interface. The absence of any correlation with reconstructed (glacial‐interglacial) changes in primary production, terrigenous input, and/or sea level related basin configuration is attributed to a complete consumption of nitrate during primary production in this marginal basin during at least the last 140,000 years. This, in turn, implies that the δ15N of the nitrate used during primary production remained approximately constant during the last climatic cycle. The proposed scenario infers an unchanged nitrogen isotopic composition of the western Pacific subsurface nitrate between glacial and interglacial stages as well as during terminations and thus constrains proposed changes in the oceanic N inventory.
Opening Pandora's Box: The impact of open system modeling on interpretations of anoxia
Authors: Hotinski Roberta M. , Kump Lee R. , Najjar Raymond G.
Published: September 2002

The geologic record preserves evidence that vast regions of ancient oceans were once anoxic, with oxygen levels too low to sustain animal life. Because anoxic conditions have been postulated to foster deposition of petroleum source rocks and have been implicated as a kill mechanism in extinction events, the genesis of such anoxia has been an area of intense study. Most previous models of ocean oxygen cycling proposed, however, have either been qualitative or used closed‐system approaches. We reexamine the question of anoxia in open‐system box models in order to test the applicability of closed‐system results over long timescales and find that open and closed‐system modeling results may differ significantly on both short and long timescales. We also compare a scenario with basinwide diffuse upwelling (a three‐box model) to a model with upwelling concentrated in the Southern Ocean (a four‐box model). While a three‐box modeling approach shows that only changes in high‐latitude convective mixing rate and character of deepwater sources are likely to cause anoxia, four‐box model experiments indicate that slowing of thermohaline circulation, a reduction in wind‐driven upwelling, and changes in high‐latitude export production may also cause dysoxia or anoxia in part of the deep ocean on long timescales. These results suggest that box models must capture the open‐system and vertically stratified nature of the ocean to allow meaningful interpretations of long‐lived episodes of anoxia.