bluuebunny/crossref_metadata_2025 huggingface dataset:
Comparison of inundation patterns among major South American floodplains
Authors: Hamilton Stephen K. , Sippel Suzanne J. , Melack John M.
Published: October 2002

A comparative view of inundation patterns in the large floodplains of South America was derived by analysis of the 37‐GHz polarization difference observed by the Scanning Multichannel Microwave Radiometer (SMMR; Nimbus‐7 satellite, 1979–1987). The following floodplains were analyzed: (1) mainstem Amazon River floodplain in Brazil; (2) Llanos de Moxos (Beni and Mamoré rivers) in Bolivia; (3) Bananal Island (Araguaía River) in Brazil; (4) Llanos del Orinoco (Apure and Meta rivers) in Venezuela and Colombia; (5) Roraima savannas (Branco and Rupununi rivers) in Brazil and Guyana; and (6) Pantanal wetland (Paraguay River) in Brazil. The maximum areas subject to inundation in each region, which include permanent open waters of river channels and lakes, were as follows (in km2): mainstem Amazon 97,360, Moxos 92,094, Bananal 58,550, Orinoco 107,530, Roraima 16,530, and Pantanal 130,920. The duration of inundation was correlated with the maximum area inundated. Predictive relationships between flooded area and water levels in the nearby rivers allowed extension of the inundation record over nearly a century for the Amazon and Pantanal and several decades for the other floodplains. Interannual variability in the maximum extent of inundation is greatest in the Pantanal, followed by Roraima and Bananal. Based on these extended records, the long‐term mean inundation areas (in km2 and including rivers and lakes) were as follows: mainstem Amazon 46,920, Moxos 29,460, Bananal 13,110, Orinoco 34,700, Roraima 3,480, and Pantanal 34,880. This information is fundamental for hydrological, biogeochemical, and ecological studies of these floodplain‐river systems and will improve estimates of methane and other trace gas emissions to the atmosphere from these vast wetlands.
Deconstructing Atlantic Intertropical Convergence Zone variability: Influence of the local cross‐equatorial sea surface temperature gradient and remote forcing from the eastern equatorial Pacific
Authors: Chiang John C. H. , Kushnir Yochanan , Giannini Alessandra
Published: October 2002

We investigate causes of interannual variability in Atlantic Intertropical Convergence Zone (ITCZ) convection using a monthly mean global precipitation data set spanning 1979–1999. Starting from the hypothesis of two dominant influences on the ITCZ, namely, the cross‐equatorial gradient in tropical Atlantic sea surface temperature (SST) and the anomalous Walker circulation due to the rearrangement of tropical Pacific convection associated with the El Niño–Southern Oscillation, we analyze anomaly composites over the 1979–1999 period that best isolate the effects of each mechanism. Our results suggest that to first order, a strong anomalous Walker circulation suppresses precipitation over the tropical Atlantic, whereas an anomalous warm north/cool south SST gradient shifts the meridional location of maximum ITCZ convection anomalously north. We examined the processes underlying each of the two mechanisms. For the anomalous Walker circulation we find consistency with the idea of suppression of convection through warming of the tropical troposphere brought about by anomalous convective heating in the eastern equatorial Pacific. For the SST gradient mechanism our results confirm previous studies that link convection to cross‐equatorial winds forced by meridional SST gradients. We find that positive surface flux feedback brought about through the cross‐equatorial winds is weak and confined to the deep tropics. On the basis of the results of this and other studies we propose an expanded physical picture that explains key features of Atlantic ITCZ variability, including its seasonal preference, its sensitivity to small anomalous SST gradients, and its role in the context of tropical Atlantic SST gradient variability.
A macroscale hydrological data set of river flow routing parameters for the Amazon Basin
Authors: Costa Marcos Heil , Oliveira Carlos Henrique C. , Andrade Ricardo G. , Bustamante Thiago R. , Silva Fabrício A. , Coe Michael T.
Published: October 2002

Continental‐scale hydrologic routing models, also known as macrohydrological routing models, have evolved considerably in the past few years. As the models have become more sophisticated, they have represented a variety of new processes and expanded their data requirements—either as input data or as validation for the model output. This paper presents a new data set of large‐scale hydrological river flow routing parameters for the Amazon and Tocantins basins. Part of this data set was required by the development of the continental‐scale hydrological routing model HYDRA and its application to the Amazon Basin. HYDRA represents phenomenalike floods, backwater effects, and seasonal hydrograph much more realistically than the previous generation of macrohydrological routing models. The data set contains data on (1) river network at 5‐min (∼9 km) resolution, (2) time series of monthly means of river discharge and river stage for 122 fluviometric stations spread throughout the basin, (3) sinuosity of each of the main rivers measured at 111 river sections in the basin, and (4) depth to the water table and transmissivity of the aquifer derived from measurements taken at 81 points throughout the basin.
Synoptic tracer gradients in the upper troposphere over central Canada during the Stratosphere‐Troposphere Experiments by Aircraft Measurements 1998 summer campaign
Authors: Fischer H. , Brunner D. , Harris G. W. , Hoor P. , Lelieveld J. , McKenna D. S. , Rudolph J. , Scheeren H. A. , Siegmund P. , Wernli H. , Williams J. , Wong S.
Published: October 2002

During the July 1998 Stratosphere‐Troposphere Experiments by Aircraft Measurements (STREAM) intensive campaign, eight measurement flights were conducted from Timmins airport (Ontario, Canada, 48.2°N, 79.3°W). In situ measurements of ozone, carbon monoxide, carbon dioxide, and nonmethane hydrocarbons, as well as three‐dimensional back trajectories based on European Centre for Medium‐Range Weather Forecasts wind field analyses, are used to characterize upper tropospheric air masses with respect to their origin. In the upper troposphere between 8 km and the local tropospause, about 40% of the air masses originated from the subtropics/tropics south of 30°N, about 50% originated from midlatitudes (between 30° and 50°N), and 12% originated from the polar region north of 50°N. In general, highest trace gas levels were observed in air masses originating from midlatitudes (e.g., CO, 107 ± 25 ppbv; ethane, 983 ± 385 pptv; acetylene, 119 ± 43 pptv), with the exception of CO, which had highest mixing ratios (121 ± 11 ppbv) in air masses of polar origin. The lowest concentrations were generally observed in air masses of (sub)tropical origin (e.g., CO, 64 ± 2 ppbv; ethane, 424 ± 86 pptv; acetylene, 39 ± 14 pptv). For CO and most of the alkanes a significant positive latitudinal gradient was observed, while CO2 and CH3Cl exhibited an inverse gradient with lowest mixing ratios at high northern latitudes. This reflects the different source and sink distributions of the investigated species. Ozone mixing ratios significantly increased in the upper troposphere above 8 km altitude, at least partially because of stratosphere‐troposphere exchange. Approximately 10% of the upper tropospheric air was directly influenced by recent cross‐tropopause exchange. A case study indicates that the convergence of air masses of different origin along a cold front creates sharp gradients not only in temperature and humidity but also in trace gas concentrations. It is shown that these mesoscale gradients result from the organized flows of tropical, arctic, and stratospheric air masses.
Eight centuries of volcanic signal and climate change at Talos Dome (East Antarctica)
Authors: Stenni B. , Proposito M. , Gragnani R. , Flora O. , Jouzel J. , Falourd S. , Frezzotti M.
Published: October 2002

During the 1996 Programma Nazionale di Ricerche in Antartide‐International Trans‐Antarctic Scientific Expedition traverse, two firn cores were retrieved from the Talos Dome area (East Antarctica) at elevations of 2316 m (TD, 89 m long) and 2246 m (ST556, 19 m long). Cores were dated by using seasonal variations in non‐sea‐salt (nss) SO42− concentrations coupled with the recognition of tritium marker level (1965–1966) and nss SO42− spikes due to the most important volcanic events in the past (Pinatubo 1991, Agung 1963, Krakatoa 1883, Tambora 1815, Kuwae 1452, Unknown 1259). The number of annual layers recognized in the TD and ST556 cores was 779 and 97, respectively. The δD record obtained from the TD core has been compared with other East Antarctic isotope ice core records (Dome C EPICA, South Pole, Taylor Dome). These records suggest cooler climate conditions between the middle of 16th and the beginning of 19th centuries, which might be related to the Little Ice Age (LIA) cold period. Because of the high degree of geographical variability, the strongest LIA cooling was not temporally synchronous over East Antarctica, and the analyzed records do not provide a coherent picture for East Antarctica. The accumulation rate record presented for the TD core shows a decrease during part of the LIA followed by an increment of about 11% in accumulation during the 20th century. At the ST556 site, the accumulation rate observed during the 20th century was quite stable.
Chasma Boreale, Mars: Topographic characterization from Mars Orbiter Laser Altimeter data and implications for mechanisms of formation
Authors: Fishbaugh Kathryn E. , Head James W.
Published: October 2002

Chasma Boreale, a large reentrant in the north polar cap of Mars, is distinct in scale, detailed topography, and orientation from the spiraling troughs that characterize the majority of the polar cap. We use new, high‐resolution Mars Orbiter Laser Altimeter (MOLA) data and Mars Orbiter Camera (MOC) and Viking images of the chasma region to assess hypotheses of origin of Chasma Boreale. These hypotheses include formation by glacial flow and ablation, by eolian processes (such as katabatic winds) and sublimation, by outflow of meltwater, or by a combination of two or more of these processes. We find that these new data support initial formation by outflow of meltwater with later modification by katabatic winds and sublimation. On the basis of our analysis we suggest a possible scenario in which outflow has occurred in several stages. The outflow appears to have begun near an enclosed depression at the head of the chasma; meltwater migrated within and below the ice down the regional slope and eventually broke out to the surface. Removal of material by melting caused subsidence and erosion of the polar deposits forming the chasma, which has subsequently undergone modification by eolian and sublimation processes. The outflow deposited most of its sediment in the form of a lobe that stratigraphically overlies the polygonal plains mapped at the mouth of the chasma and scoured some of the circumpolar sediment mantle in the surrounding region. The lowest portions of the North Polar Basin received the rest of the sediment as a deposit a few meters thick. MOLA data also reveal evidence for similar events of smaller magnitude elsewhere on the margins of the polar cap.
Flexure of Venusian lithosphere measured from residual topography and gravity
Authors: Barnett David N. , Nimmo Francis , McKenzie Dan
Published: October 2002

The elastic lithosphere thickness Te for various locations on Venus is estimated by modeling lithospheric flexure associated with rifts, coronae, chasmata, and the moats visible around certain large volcanoes. By modeling flexure using the residual topography, a range of elastic thicknesses was found, from around 10 to 40 km or greater. A number of these values are not well‐determined and only constrain Teto be >10 km. The shear stresses predicted from the topography, given these values for Te, reach several hundred MPa, with surface faulting visible in the synthetic aperture radar (SAR) images at many locations. The elastic thickness was also estimated at seven volcano‐like structures by modeling the gravity predicted from the observed topography. This yielded elastic thickness estimates varying between approximately 20 and 60 km, which were generally more tightly constrained. However, an elastic thickness of 25 km fits almost all the observed profiles within uncertainty, and the results from modeling the gravity yield an average global elastic thickness of 29 ± 6 km. There is no evidence that the elastic thickness falls anywhere outside this range. The lack of large‐scale regional variations in Teon Venus, in contrast to the situation observed on the Earth, is consistent with a lack of water and plate tectonics on Venus.
Dust coatings on basaltic rocks and implications for thermal infrared spectroscopy of Mars
Authors: Johnson Jeffrey R. , Christensen Philip R. , Lucey Paul G.
Published: October 2002

Thin coatings of atmospherically deposited dust can mask the spectral characteristics of underlying surfaces on Mars from the visible to thermal infrared wavelengths, making identification of substrate and coating mineralogy difficult from lander and orbiter spectrometer data. To study the spectral effects of dust coatings, we acquired thermal emission and hemispherical reflectance spectra (5–25 μm; 2000–400 cm−1) of basaltic andesite coated with different thicknesses of air fall‐deposited palagonitic soils, fine‐grained ceramic clay powders, and terrestrial loess. The results show that thin coatings (10–20 μm) reduce the spectral contrast of the rock substrate substantially, consistent with previous work. This contrast reduction continues linearly with increasing coating thickness until a “saturation thickness” is reached, after which little further change is observed. The saturation thickness of the spectrally flat palagonite coatings is ∼100–120 μm, whereas that for coatings with higher spectral contrast is only ∼50–75 μm. Spectral differences among coated and uncoated samples correlate with measured coating thicknesses in a quadratic manner, whereas correlations with estimated surface area coverage are better fit by linear functions. Linear mixture modeling of coated samples using the rock substrate and coating materials as end‐members is also consistent with their measured coating thicknesses and areal coverage. A comparison of ratios of Thermal Emission Spectrometer (TES) spectra of dark and bright intracrater and windstreak deposits associated with Radau crater suggests that the dark windstreak material may be coated with as much as 90% areal coverage of palagonitic dust. The data presented here also will help improve interpretations of upcoming mini‐TES and Thermal Emission Imaging System (THEMIS) observations of coated Mars surface materials.
Lunar Prospector epithermal neutrons from impact craters and landing sites: Implications for surface maturity and hydrogen distribution
Authors: Johnson Jeffrey R. , Feldman William C. , Lawrence David J. , Maurice Sylvestre , Swindle Timothy D. , Lucey Paul G.
Published: October 2002

Initial studies of neutron spectrometer data returned by Lunar Prospector concentrated on the discovery of enhanced hydrogen abundances near both lunar poles. However, the nonpolar data exhibit intriguing patterns that appear spatially correlated with surface features such as young impact craters (e.g., Tycho). Such immature crater materials may have low hydrogen contents because of their relative lack of exposure to solar wind‐implanted volatiles. We tested this hypothesis by comparing epithermal* neutron counts (i.e., epithermal −0.057 × thermal neutrons) for Copernican‐age craters classified as relatively young, intermediate, and old (as determined by previous studies of Clementine optical maturity variations). The epithermal* counts of the crater and continuous ejecta regions suggest that the youngest impact materials are relatively devoid of hydrogen in the upper 1 m of regolith. We also show that the mean hydrogen contents measured in Apollo and Luna landing site samples are only moderately well correlated to the epithermal* neutron counts at the landing sites, likely owing to the effects of rare earth elements. These results suggest that further work is required to define better how hydrogen distribution can be revealed by epithermal neutrons in order to understand more fully the nature and sources (e.g., solar wind, meteorite impacts) of volatiles in the lunar regolith.
Plinian eruptions and passive collapse events as mechanisms of formation for Martian pit chain craters
Authors: Scott Evelyn D. , Wilson Lionel
Published: October 2002

Three distinct and intersecting sets of graben exist near the Martian volcano Alba Patera. One set formed as a response to the evolution of the Alba Patera edifice. A second set includes mainly linear structures widely interpreted to have formed in response to very large‐scale regional stresses. The members of the third set contain numerous pit craters, and we propose that these craters formed as a result of volcanic activity above long regional dikes emanating from a source to the south within the Tharsis region. The pit craters are of two morphologies: small (<2 km), regular‐shaped ones, usually completely contained within a graben but occasionally unbounded, and larger (4–10 km), irregular ones which commonly coalesce with one another. We argue that these two types of crater formed by different but related mechanisms. The smaller ones developed as the magma in a stalled dike degassed and volatiles were lost to the atmosphere via cracks and fractures in the overlying crustal rocks, which then collapsed into the potential void space at the top of the dike, producing the pit craters. The larger craters formed subsequent to short‐lived explosive eruptions sourced from the underlying dike when the dike walls imploded and collapsed, destroying evidence of any vent structures. There are no visible products of these eruptions because the ambient Martian atmospheric conditions caused the explosive eruptions to be very vigorous, dispersing the products over very wide areas. Evidence indicates that both formation mechanisms occurred within very short distances over the same dike. Preexisting local tectonic stresses appear to have influenced the locations of the pit craters, and groundwater may have been involved in determining their locations and the style of eruptive activity.
Cold springs in permafrost on Earth and Mars
Authors: Andersen Dale T. , Pollard Wayne H. , McKay Christopher P. , Heldmann Jennifer
Published: October 2002

Perennial springs located on west central Axel Heiberg Island in the Canadian High Arctic occur in a region with a mean annual air temperature of −15°C and flow through continuous permafrost 600 m thick. The spring water is a low‐temperature (up to 6°C) brine that maintains constant discharge temperatures and flow rates throughout the year. Here we report on observations of temperature and discharge rate of these springs and develop a combined flow and thermal model of the subsurface flow using the measured geothermal gradient. We also consider the implications these springs have for the search for similar environments, past or present, on Mars.
Combined remote mineralogical and elemental identification from rovers: Field and laboratory tests using reflectance and laser‐induced breakdown spectroscopy
Authors: Wiens Roger C. , Arvidson Raymond E. , Cremers David A. , Ferris Monty J. , Blacic James D. , Seelos Frank P. , Deal Kim S.
Published: October 2002

Rover mast‐based stereo imaging, reflectance or emission spectra, and laser‐induced breakdown spectroscopy (LIBS) provide powerful synergistic data sets for rapidly determining geologic setting, mineralogy, and elemental composition for rock and soil targets. We report results from hand specimen examination, multiple spectral reflectance (VISIR, 0.4–2.5 μm), and remote LIBS measurements for representative samples (basalt, iron oxides, altered rhyolite, and dolostone) collected during the May 2000 rover trials at Black Rock Summit, Nevada. Major findings are (1) reflectance data indicate the presence of olivine and montmorillonite for the basaltic sample, whereas LIBS data show variations with depth as an alumino‐silicate coating is ablated and a basaltic composition is revealed beneath; (2) iron oxide reflectance data are dominated by goethite and hematite with the additional detection of hydrated cryptocrystalline SiO2 (an interpretation fully supported by the LIBS data); (3) VISIR reflectance data for rhyolite show significant alteration to kaolinite, an inference that would be difficult to make from LIBS data alone; (4) dolostone reflectance data show dominantly dolomite with minor iron oxides, inferences confirmed through analyses of LIBS data; and (5) LIBS data acquired for samples as a function of depth and spatial location significantly aid in understanding the extent of sample coatings and compositional and mineralogical diversity. Given the hand specimen examination and results from joint analysis of the two rapid‐acquisition spectral techniques, a scenario for acquisition and analysis of mast‐based imaging reflectance or emission spectroscopy and LIBS is presented within the context of the scientific objectives for the Mars Exploration Program.
On lunar asymmetries 1. Tilted convection and crustal asymmetry
Authors: Loper D. E. , Werner C. L.
Published: October 2002

It is proposed that lunar crustal asymmetries are the result of convective processes acting early in the Moon's history, during the magma‐ocean phase and after synchronous rotation was established. Buoyant anorthositic crystals were transported to the farside by a large‐scale circulation, called tilted convection, known to occur spontaneously in chaotic convective systems. The circulation was oriented toward the farside of the Moon by a small temperature contrast produced by radiative thermal shielding due to the proximity of Earth. Crustal thickening near the equator resulted from the modification of tilted convection by the Coriolis force. The anorthosite that was transported to the farside and equator may be a major cause of the observed crustal asymmetries and of the associated offset of the center of figure from the center of mass and bimodal hyposgram. The selective transport may have resulted in a nearside excess of incompatible elements, leading to the formation of the Procellarium KREEP terrane.
On lunar asymmetries 2. Origin and distribution of mare basalts and mascons
Authors: Werner C. L. , Loper D. E.
Published: October 2002

A novel mechanism is proposed for the emplacement of mare basalts and the formation of mascons that does not involve melting or remelting of the source region. The source magmas are the remnants of the primordial lunar magma ocean and are rich in incompatible elements, including radioactive isotopes; potassium, rare earth elements, and phosphorus (KREEP); and volatiles. Exsolution of volatiles as the magmas cooled and crystallized (a process called second boiling) produced overpressures which drove the dense basalts upward to the surface, creating the mascons. This process occurred preferentially beneath nearside mare basins, where the crust is thinnest and lithostatic pressure is lowest. The delay between impact and magmatic extrusion reflects the time needed for the volatile‐rich magma to cool to its second boiling point. Radioactive heating buffered the magmas against cooling and solidification, allowing for an extended period of magmatism.
Localization of Mars rovers using descent and surface‐based image data
Authors: Li Rongxing , Ma Fei , Xu Fengliang , Matthies Larry H. , Olson Clark F. , Arvidson Raymond E.
Published: October 2002

The planned 2003 Mars Exploration Rover (MER) Mission and follow‐on surface activities associated with landed missions will focus on long distance roving and sample return, which require detailed knowledge of vehicle locations in both local and global reference systems. In this paper we argue that this rover localization should be known to within 0.1 of the distance traversed for local coordinate systems. To test the ability to meet this goal using only descent and rover‐based data, we conducted experiments with simulated descent images and Field Integrated Design and Operations Rover data collected during field tests at Silver Lake, California, in April 1999. Specifically, an integrated bundle adjustment system incorporating both descent and rover‐based images was developed and used to localize the rover positions. On the basis of surveyed ground control points it is demonstrated that the joint analysis produces RMS errors of 0.24, 0.15, and 0.38 m in x, y, and z% directions in a local coordinate system, respectively, for ground points within 500 m from the landing point and 0.23, 0.21, and 0.46 m within a distance of 1.5 km. Results show that it is possible to meet the 0.1 goal using descent and rover‐based data only.
Ion‐neutral chemistry model of the lower ionosphere of Mars
Authors: Molina‐Cuberos Gregorio J. , Lichtenegger Herbert , Schwingenschuh Konrad , López‐Moreno José J. , Rodrigo Rafael
Published: October 2002

The ion neutral chemistry of the lower Martian atmosphere is more complex than any other ionospheric region and similar, in some aspects, to the terrestrial D region. Two ionization sources operate at these altitude levels: solar radiation, which is the most important one below around 70 km, and galactic cosmic radiation. The interaction of positive ions and electrons produced by these ionization sources with the Martian atmosphere, which is abundant in species able to be charged negatively, produces a high variety of positive and negative ions. We present here a new model of the ion chemistry developed to calculate the concentration of electrons and most abundant ions in the atmosphere of Mars below 100 km. The present knowledge of Mars' atmosphere allows us to include new neutral and ionic species, such as nitrogen derivated, in a more detailed ion‐chemistry calculation than previous lower ionopheric models of Mars. We obtain that the electron density profile has a low‐altitude secondary peak below the main photon‐induced maximum. This peak is placed at around 35 km and is due to the high efficiency of the electron attachment to Ox molecules, which entails that the concentration of negative ions is higher than that of electrons below around 30 km. The most abundant ionic species predicted are positive ions like hydronimum hydrates H3O+(H2O)n and O2+ and negative ions like CO3−(H2O)n and NO3−(H2O)n.
Brines exposed to Europa surface conditions
Authors: McCord Thomas B. , Teeter Glenn , Hansen Gary B. , Sieger Mathew T. , Orlando Thomas M.
Published: October 2002

Evidence for an ocean beneath the icy crust of Europa includes reflectance spectra of disrupted surface regions indicating hydrated materials such as salts. We simulated exposure of salty brine on the cold surface of Europa by flash‐freezing sulfate and carbonate solutions. This produces materials that have near‐infrared reflectance spectra distinct from those for crystalline minerals and more similar to those for Europa's non‐ice regions. These new spectroscopic data, along with geophysical evidence, geochemical models, and meteorite studies, strongly suggest that the non‐ice materials in the disrupted regions on Europa's surface contain large amounts of disordered and heavily hydrated MgSO4 and perhaps Na2SO4 that are endogenic in origin.
Morphology of Europan bands at high resolution: A mid‐ocean ridge‐type rift mechanism
Authors: Prockter Louise M. , Head James W. , Pappalardo Robert T. , Sullivan Robert J. , Clifton Amy E. , Giese Bernd , Wagner Roland , Neukum Gerhard
Published: October 2002

We utilize imaging data from the Galileo spacecraft to investigate band formation on one of Jupiter's moons, Europa. Bands are polygonal features first observed in Voyager data close to Europa's anti‐Jovian point and represent areas where preexisting terrain has been pulled apart, allowing new material to move up into the gap. We examine the detailed morphology of several bands imaged at different resolutions and lighting geometries. We identify several distinct morphological characteristics, including central troughs, hummocky textures, and ridge and trough terrains, some of which are common among the bands studied. In many cases, bands have initiated along segments of one or more preexisting double ridges, ubiquitous within Europa's ridged plains. Distinctive morphological features and high standing topography imply that the bands formed from compositionally or thermally buoyant ice, rather than liquid water. Comparisons between Europan band morphologies and features found on terrestrial mid‐ocean ridges reveal several similarities, including axial troughs, subcircular hummocks, normal faults, and indications of symmetrical spreading. We conclude that terrestrial mid‐ocean ridge rifting is a good analogy for Europan band formation. If a terrestrial seafloor‐spreading model is applicable to Europan bands, we speculate that band morphologies might be related to the relative rate of spreading of each band. Bands may have contributed significantly to the resurfacing of Europa. Europan bands we examine predate (but do not postdate) lenticulae and related features, implying that the style of resurfacing on Europa has changed over recent geological time in these regions.
Midinfrared spectral features of rocks and their powders
Authors: Cooper B. L. , Salisbury J. W. , Killen R. M. , Potter A. E.
Published: October 2002

We examined the thermal IR spectra of 35 rock samples and their fine‐powder equivalents to better understand the relationship between spectral features, particle size, and composition. The Christiansen features of solid crystalline rocks differ from those of their powdered samples, and the magnitude of this difference changes systematically with rock type. Both the Christiansen feature (CF) and the transparency feature (TF) in spectra of particulate igneous rocks are useful in rock type determination. They correlate better with the SCFM chemical index of rock type than with other numerical rock type indices that we considered. Major rock type divisions (acidic, intermediate, basic, and ultrabasic) could be distinguished using both CF and TF, but some overlap occurred if one or the other spectral feature was used alone. Spectral contrast decreases as rocks become more basic, probably because of increasing opaque mineral content. Likewise, metallic iron, formed as a result of space weathering in an airless environment, appears to be the most likely cause of reduced spectral contrast in mature lunar soils. Thus reststrahlen features should be difficult to detect in remote‐sensing measurements of such bodies as the Moon and Mercury. Moreover, the vacuum environment of an airless planetary surface appears to enhance the CF, but it may eliminate the TF, depending critically on particle size. The Martian pressure and radiative environment, on the other hand, appears to enhance the TF, which should be sought in spectra of finely particulate surface areas on Mars. Additional environmental simulation spectral experiments are needed to more accurately define the roles of pressure, particle size, opacity, and insolation angle on apparent emissivity.
FIDO prototype Mars rover field trials, Black Rock Summit, Nevada, as test of the ability of robotic mobility systems to conduct field science
Authors: Arvidson R. E. , Squyres S. W. , Baumgartner E. T. , Schenker P. S. , Niebur C. S. , Larsen K. W. , SeelosIV F. P. , Snider N. O. , Jolliff B. L.
Published: October 2002

The Field Integration Design and Operations (FIDO) prototype Mars rover was deployed and operated remotely for 2 weeks in May 2000 in the Black Rock Summit area of Nevada. The blind science operation trials were designed to evaluate the extent to which FIDO‐class rovers can be used to conduct traverse science and collect samples. FIDO‐based instruments included stereo cameras for navigation and imaging, an infrared point spectrometer, a color microscopic imager for characterization of rocks and soils, and a rock drill for core acquisition. Body‐mounted “belly” cameras aided drill deployment, and front and rear hazard cameras enabled terrain hazard avoidance. Airborne Visible and Infrared Imaging Spectrometer (AVIRIS) data, a high spatial resolution IKONOS orbital image, and a suite of descent images were used to provide regional‐ and local‐scale terrain and rock type information, from which hypotheses were developed for testing during operations. The rover visited three sites, traversed 30 m, and acquired 1.3 gigabytes of data. The relatively small traverse distance resulted from a geologically rich site in which materials identified on a regional scale from remote‐sensing data could be identified on a local scale using rover‐based data. Results demonstrate the synergy of mapping terrain from orbit and during descent using imaging and spectroscopy, followed by a rover mission to test inferences and to make discoveries that can be accomplished only with surface mobility systems.
Surface runoff features on Mars: Testing the carbon dioxide formation hypothesis
Authors: Stewart Sarah T. , Nimmo Francis
Published: October 2002

Fresh gully‐like features on Mars strongly suggest that fluid flowed on the surface in the recent past. Here we consider the possibility that CO2 vapor‐supported flows formed the gullies. We find that neither condensed CO2 nor CO2 clathrate hydrate are likely to accumulate in significant quantities in the Martian crust. In addition, if condensed CO2 were present under lithostatic pressures, exposure to the atmosphere would produce features analogous to terrestrial pyroclastic flows, not surface runoff. Finally, the source volume of CO2 required to support a flow excludes clustered or episodic gully formation. Therefore, we conclude that CO2 cannot have formed the gullies. In light of these results, liquid water flow remains the preferred formation mechanism for the recent surface runoff features.
Spectral reflectance properties of zeolites and remote sensing implications
Authors: Cloutis Edward A. , Asher Pranoti M. , Mertzman Stanley A.
Published: October 2002

The 0.3‐ to 26‐μm reflectance spectra of a suite of 28 zeolites were measured and analyzed to derive spectral‐compositional‐structural relationships. Below ∼7 μm, the spectra are largely dominated by absorption features associated with zeolitic water. At longer wavelengths, the spectra are dominated by absorption features associated with the aluminosilicate framework. The spectra exhibit a number of systematic variations which can be used for both structural and compositional determinations. These include: (1) distinguishing different structural groups on the basis of wavelength position variations associated with absorption features in the 8.5‐ to 26‐μm region that are related to differences in the structure of the aluminosilicate framework; (2) determining the major cation which is present (Ca, Na, K) and the associated electronic environment of the zeolitic water on the basis of how these cations hydrogen bond to the water molecules in the void spaces and consequently affect water‐related absorption band positions, particularly in the 1.4, 1.9, and 2.0‐ to 2.5‐μm regions; (3) determining the Al:(Al + Si) ratio and SCFM chemical index on the basis of absorption features in the 7‐ to 26‐μm region which are most sensitive to these compositional variations; and (4) identifying iron‐bearing zeolites on the basis of absorption features in the 0.35‐ to 0.9‐μm region. The wavelength position and number of H2O‐associated absorption bands are sensitive to factors such as the type of major cation, degree of hydrogen bonding, and size of the void space, all of which are somewhat interrelated.
On rates and styles of late volcanism and rifting on Venus
Authors: Basilevsky Alexander T. , Head James W.
Published: October 2002

We investigated the possibility of significant variations in the rates and styles of volcanism and rifting during the time postdating the formation of regional plains on Venus. We analyzed the age relations of all known impact craters ≥30 km in diameter (183 craters) with the neighboring geologic units. Of these we selected 164 craters which were superposed on regional plains and determined if post‐regional‐plains (PRP) volcanics and/or PRP rift structures were present in the crater vicinity and if these craters postdated the volcanics and rift structures or predated them. In 53 cases it was possible to determine these relations. On the basis of these relationships, it was found that the general rates of volcanism and rifting during PRP time were close to constant or at least had no drastic changes. This implies a significant change in the rates of volcanism and rifting in the vicinity of the boundary between PRP time and the preceding time, which is marked by the formation of the global wrinkle‐ridge network. It was also found that (1) the role of rift‐associated volcanism during PRP time was close to constant or slightly decreasing, (2) the role of corona‐related volcanism was noticeably decreasing, and (3) the role of noncorona, hot spot volcanism was proportionally increasing. The latter changes may be due to thickening of the lithosphere during PRP time. The results imply also that at most if not all places where PRP rifting occurred, the rift‐associated fracturing, when started, continued for a significant part of PRP time. Our conclusions are valid both (1) for the case of a globally synchronous transition from emplacement of regional plains to the PRP regime and (2) for the case of such a transition occurring at different times in different areas of Venus.
Role of molecular species in pickup ion sputtering of the Martian atmosphere
Authors: Leblanc F. , Johnson R. E.
Published: October 2002

This paper presents a new approach for calculating the sputtering of an atmosphere by impacting particles when there are molecular species at the exobase. We couple a test particle Monte Carlo approach with a molecular dynamic model to describe the collisions between hot particles and cold molecules in the Martian atmosphere. It is shown that both the heating efficiency and sputtering by pickup ions are reduced from that in an atomic thermosphere. The yields are given for three different ratios of atoms to molecules at the exobase and are used to obtain a better estimate of the total loss of atmosphere due to the pickup ion sputtering. Using the pickup ion fluxes of Zhang et al. [1993], ∼120 mbar of O and ∼60 mbar of CO2 are lost. The loss of O if associated with H2O would be equivalent to ∼4 m averaged over the Mars surface.
Velocity distributions of energetic atoms in planetary exospheres from dissociative recombination
Authors: Kabin K. , Shizgal B. D.
Published: October 2002

A kinetic theory description of translational energetic atoms in the upper planetary atmosphere is presented. A new analytical result for the velocity distribution of the products of reactive collisions is described. Our calculation takes into account different temperatures of the reactants and arbitrary dependence of the cross section on the relative velocity of the colliding particles. The final result is applied to the production of hot oxygen and carbon by dissociative recombination of O2+ and CO+, respectively. The nascent distribution of hot atoms generated in this way is compared with the earlier Monte Carlo calculations. We use the Boltzmann equation to study the thermalization of the hot oxygen via collisions with the thermal oxygen population. The results of this calculation demonstrate quasi‐steady state velocity distribution of high‐energy oxygen atoms near the exobase of Venus for daytime conditions.
Terrestrial analogs to wind‐related features at the Viking and Pathfinder landing sites on Mars
Authors: Greeley Ronald , Bridges Nathan T. , Kuzmin Ruslan O. , Laity Julie E.
Published: October 2002

Features in the Mojave Desert and Iceland provide insight into the characteristics and origin of Martian wind‐related landforms seen by the Viking and Pathfinder landers. The terrestrial sites were chosen because they exhibit diverse wind features that are generally well understood. These features have morphologies comparable to those on Mars and include origins by deposition and erosion, with erosional processes modifying both soils and rocks. Duneforms and drifts are the most common depositional features seen at the Martian landing sites and indicate supplies of sand‐sized particles blown by generally unidirectional winds. Erosional features include lag deposits, moat‐like depressions around some rocks, and exhumed soil horizons. They indicate that wind can deflate at least some sediments and that this process is particularly effective where the wind interacts with rocks. The formation of ripples and wind tails involves a combination of depositional and erosional processes. Rock erosional features, or ventifacts, are recognized by their overall shapes, erosional flutes, and characteristic surface textures resulting from abrasion by windblown particles. The physics of saltation requires that particles in ripples and duneforms are predominantly sand‐sized (60–2000 μm). The orientations of duneforms, wind tails, moats, and ventifacts are correlated with surface winds above particle threshold. Such winds are influenced by local topography and are correlated with winds at higher altitudes predicted by atmospheric models.
A mesoscale model for the Martian atmosphere
Authors: Toigo Anthony D. , Richardson Mark I.
Published: October 2002

The Pennsylvania State University/National Center for Atmosphere Research Mesoscale Model Version 5 (MM5) has been converted for use on Mars. Modifications are based on schemes implemented in the Geophysical Fluid Dynamics Laboratory Mars General Circulation Model (GCM). Validation of the Mars MM5 is conducted by comparison to the Mars GCM, examining the large‐scale dynamics in the two models. Agreement between the two models on similar scales (a few hundred kilometers) is good. Validation is also performed against both Viking Landers and Mars Pathfinder meteorological observations with the model run at higher vertical (lowest level at 1.6 m) and horizontal resolution (a few kilometers). We find reasonable agreement with near‐surface air temperature, pressure, and wind direction observations, with caveats. The results demonstrate that the model accurately simulates surface heat balance and the propagation of global thermal tides. However, wind speeds are underpredicted. The model generates the correct phasing of wind speeds with local time at the Viking Lander 2 site during winter but does not generate the correct phasing at the other sites or seasons. We examined the importance of slopes and global tides in generating the diurnal cycle of winds at the lander sites. We find that tides are at least as important as slopes, in contrast to previous studies. This study suggests that when used in combination with a GCM, the Mars MM5 promises to be a powerful tool for the investigation of processes central to the Martian climate on scales from hundreds of kilometers to tens of meters.
Ice in the lunar polar regions revisited
Authors: Hodges R. Richard
Published: October 2002

The common wisdom that water ice may exist in lunar polar cold traps has become a significant factor in the selection of space research objectives. The purpose of this paper is to address two topics that are missing from the discourse on lunar water: the effect that the pristine cleanliness of the regolith has on water transport on the moon, and the limits on water exposure implied by the extremely high adsorption potentials of the surfaces of soil grains. Water transport is characterized by chemisorption on soil grains and the mixing of “wet” grains into the regolith by meteoritic gardening. Ballistic lateral flow, which is generally thought to be an efficient conduit for moving water to the poles, is actually a secondary phenomenon that is facilitated by solar wind and micrometeor erosion but not by thermal desorption, as is the case for the dominant lunar exospheric gases, He and 40Ar. Simulation results show that even under the most optimistic conditions, less than 7% of the water accumulated in the regolith resides in the polar cold traps, where water concentrations cannot be greater than 350 ppm. More important, when realistic transport parameters are used in the simulator, the polar water concentration is reduced by almost 2 orders of magnitude. In a word, the concept of water ice at the lunar poles is insupportable.
Influence of the seasonal winds and the CO2 mass exchange between atmosphere and polar caps on Mars' rotation
Authors: Van den Acker Ellen , Van Hoolst Tim , de Viron Olivier , Defraigne Pascale , Forget François , Hourdin Fréderic , Dehant Véronique
Published: October 2002

The Martian atmosphere and the CO2 polar ice caps exchange mass. This exchange, together with the atmospheric response to solar heating, induces variations of the rotation of Mars. Using the angular momentum budget equation of the system solid‐Mars–atmosphere–polar ice caps, the variations of Mars' rotation can be deduced from the variations of the angular momentum of the superficial layer; this later is associated with the winds, that is, the motion term, and with the mass redistribution, that is, the matter term. For the “mean” Martian atmosphere, without global dust storms, total amplitudes of 10 cm on the surface are obtained for both the annual and semiannual polar motion excited by the atmosphere and ice caps. The atmospheric pressure variations are the dominant contribution to these amplitudes. Length‐of‐day (lod) variations have amplitudes of 0.253 ms for the annual signal and of 0.246 ms for the semiannual signal. The lod variations are mainly associated with changes in the atmospheric contribution to the mass term, partly compensated by the polar ice cap contribution. We computed lod variations and polar motion for three scenarios having different atmospheric dust contents. The differences between the three sets of results for lod variations are about one order of magnitude larger than the expected accuracy of the NEtlander Ionosphere and Geodesy Experiment (NEIGE) for lod. It will thus be possible to constrain the global atmospheric circulation models from the NEIGE measurements.
Patterns of CaCO3 deposition in the eastern tropical Pacific Ocean for the last 150 kyr: Evidence for a southeast Pacific depositional spike during marine isotope stage (MIS) 2
Authors: Lyle Mitchell , Mix Alan , Pisias Nicklas
Published: October 2002

We constructed biogenic mass accumulation rate (MAR) time series for eastern Pacific core transects across the equator at ∼105° and ∼85°W and along the equator from 80° to 140°W. We used empirical orthogonal function (EOF) analysis to extract spatially coherent patterns of CaCO3 deposition for the last 150 kyr. EOF mode 1 (51% variance) is a CaCO3 MAR spike centered in marine oxygen isotope stage 2 (MIS 2) found under the South Equatorial Current. EOF mode 2 (19% of variance) is high north of the equator. EOF mode 3 (9% of variance) is an east‐west mode centered along the North Equatorial Counter Current. The MIS 2 CaCO3 spike is the largest event in the eastern Pacific for the last 150 kyr: CaCO3 MARs are 2–3 times higher at 18 ka than elsewhere in the record, including MIS 6. It is caused by high CaCO3 production rather than minimal dissolution. EOF 2, while it resembles deep water flow patterns, nevertheless, shows coherence to Corg deposition and is probably also driven by CaCO3 production.