bluuebunny/crossref_metadata_2025 huggingface dataset:
Extrapolating branch enclosure measurements to estimates of regional scale biogenic VOC fluxes in the northwestern Mediterranean basin
Authors: Owen Susan Margaret , Hewitt C. Nicholas
Published: September 2002

Simple summation and averaging methods were used to extrapolate 870 branch‐scale measurements of volatile organic compound (VOC) emissions from 40 native Mediterranean plant species to give estimates of habitat‐scale emission factors (EH) for dunes, macchia, garrigue, forest, and riverside habitats in the northwest Mediterranean area. The summation method derived biomass‐weighted VOC EH using local vegetation cover data and component species emission factors and leaf area indices (LAIs). The averaging method generated mean and median EH and LAI from component species. Summer biomass‐weighted EH (ng m−2 s−1) for monoterpenes and isoprene, respectively, were 1700, 125 (forest, Italy); 460, 8 (macchia, Italy); 280, 40 (dunes, Italy); 160, 1600 (forest, southern France); 34, 550 (garrigue, southern France); 0, 12000 (riverside, Spain); 90, 240 (garrigue, Spain). Autumn biomass‐weighted EH (ng m−2 s−1) for monoterpenes and isoprene, respectively were 700, 31 (dunes, Italy); 2030, 1620 (forest, southern France); 64, 110 (garrigue, Spain). In general, mean and median EH underestimated biomass‐weighted monoterpene EH and overestimated isoprene EH. Habitat isoprene flux estimates followed an annual cycle with a peak in summer. An autumn peak for habitat monoterpene flux estimates may be due to drought relief following autumn rains. When extrapolated to the total area of land covered by natural habitats in the northwest Mediterranean area (71,300 km2), total monoterpene and isoprene fluxes were estimated to be 0.07 and 0.1 Tg yr−1, respectively. Within the limits of uncertainty, the VOC flux estimates presented here for the Mediterranean region agree with previously published work. However, there are indications that previous published studies may underestimate monoterpene fluxes from this area.
An observational approach for determining aerosol surface radiative forcing: Results from the first field phase of INDOEX
Authors: Conant William C.
Published: September 2002

This paper presents one of the few quantitative estimates of surface aerosol forcing made directly from surface irradiance observations. The method described within yields estimates of the forcing accurate to 20%. The study was conducted from February to March 1998 at the Kaashidhoo Climate Observatory (KCO) during the First Field Phase of the Indian Ocean Experiment (INDOEX‐FFP). For the 400–700 nm region studied here, the forcing is −7.6±1.5 W m−2. The data are obtained from two photodiode radiometers measuring global and diffuse irradiance in five channels in the visible and ultraviolet. The instruments were chosen, calibrated, and deployed specifically for a precise measurement of aerosol forcing. The angular, spectral, and absolute response characteristics of the instruments are determined in the laboratory and used to calibrate the data, as described here. The accuracy in the calibrated data is 2.4% for the global irradiance and 1.8% for the diffuse irradiance. Direct aerosol forcing is obtained from the measured aerosol forcing efficiency, which is determined by two methods: hybrid and differential. The hybrid method uses a radiative transfer model to subtract out the contribution from the aerosol‐free atmosphere. The differential method assumes that changes in 400–700 nm solar flux are forced by changes in aerosol optical depth. By using flux changes, the differential method is not sensitive to the small calibration uncertainties, and is independent of model assumptions about the single‐scatter properties of the aerosol. For this soot‐laden marine region south of India, a 0.1 change in aerosol optical depth produces a −4.0±0.8 W m−2; change in the 400–700 nm surface flux; 55% of this forcing is observed in the 400–540 nm region. The global and diffuse data agree to within 5 W m−2 of results calculated by a Monte Carlo radiative transfer model. The model assumes an aerosol consistent with the spectral optical depth, lidar vertical profiles, and surface optical properties measured simultaneously at KCO.
Validation of stratospheric ClO measurements from the Millimeter‐wave Atmospheric Sounder (MAS)
Authors: Feist D. G. , Aellig C. P. , Kämpfer N. , Solomon P. M. , Barrett J. W. , Zoonematkermani S. , Hartogh P. , Jarchow C. , Waters J. W.
Published: September 2002

During three missions in 1992, 1993, and 1994, the Millimeter‐wave Atmospheric Sounder (MAS) measured volume mixing ratio profiles of stratospheric chlorine monoxide (ClO) at 204 GHz from the space shuttle. Owing to the space shuttle orbit, measurements were restricted to tropical and midlatitudes. We compared zonal mean profiles to correlative ClO measurements by an airborne 649 GHz radiometer, a ground‐based 278 GHz instrument on Mauna Kea, Hawaii, and Version 4 ClO profiles by the Microwave Limb Sounder (MLS) on the Upper Atmosphere Research Satellite (UARS). The agreement between MAS and all the other instruments was well within the combined error bars over a pressure range of 0.4–40 hPa. Further comparisons of MAS and MLS day‐night difference profiles produced an agreement of typically better than 0.1 ppbv. A detailed analysis proved that this agreement was independent of the a priori information that was used for the retrieval of the different data sets.
Cloud thermodynamical phase classification from the POLDER spaceborne instrument
Authors: Goloub P. , Herman M. , Chepfer H. , Riedi J. , Brogniez G. , Couvert P. , Séze G.
Published: September 2002

Cloud phase recognition is important for cloud studies. Ice crystals correspond to physical process and properties that differ from those of liquid water drops. The angular polarization signature is a good mean to discriminate between spherical and nonspherical particles (liquid and ice phase, respectively). POLDER (Polarization and Directionality of Earth Reflectances) has been launched on the Japanese ADEOS platform in August 1996. Because of its multidirectional, multispectral, and multipolarization capabilities this new radiometer gives useful information on clouds and their influence on radiation in the shortwave range. The POLDER bidirectional observation capability provides the polarization signatures within a large range of scattering angles in three spectral bands centered on 0.443, 0.670, and 0.865 μm with a spatial resolution of 6.2 km×6.2 km. These original features allow to obtain some information both on cloud thermodynamic phase and on cloud microphysics (size/shape). According to POLDER airborne observations, liquid cloud droplets exhibit very specific polarization features of a rainbow for scattering angles near 140°. Conversely, theoretical studies of scattering by various crystalline particles and also airborne measurements show that the rainbow characteristics disappear as soon as the particles depart from the spherical shape. In the paper the POLDER algorithm for cloud phase classification is presented, as well as the physical principle of this algorithm. Results derived from the POLDER spaceborne version are also presented and compared with lidar ground‐based observations and satellite cloud classification. This cloud phase classification method is shown to be reliable. The major limitation appears when thin cirrus clouds overlap the liquid cloud layer. In this case, if the cirrus optical thickness is smaller than 2, the liquid phase may be retrieved. Otherwise, the ice phase is correctly detected as long as cloud detection works.
Observations of iodine monoxide in the remote marine boundary layer
Authors: Allan Beverley J. , McFiggans Gordon , Plane John M. C. , Coe Hugh
Published: September 2002

We report measurements of the iodine monoxide (IO) radical in the marine boundary layer at three remote sites: Mace Head (Ireland), Tenerife (Canary Islands), and Cape Grim (Tasmania). IO was observed by long‐path differential optical absorption spectroscopy using the A2Π3/2 ‐ X2Π3/2 electronic transition between 415 and 450 nm. The daytime IO concentration at these three locations was found to vary from below the detection limit (≤0.2 parts per trillion (ppt)) to a maximum of 4 ppt, with an average of about 1 ppt. Of particular note is that the IO observed off the north coast of Tenerife, which is probably typical of the open ocean sub‐tropical North Atlantic, exhibited a distinct diurnal cycle which correlated strongly with the solar actinic flux in the near UV. IO was also observed at Cape Grim to be present at much lower levels (≈0.3 ppt) in westerly air from the Southern Ocean. As is shown in the companion paper (McFiggans et al., this issue), these measurements of IO are satisfactorily reproduced by a photochemical box model incorporating the recycling of iodine through marine aerosol. This model indicates that the direct iodine‐catalyzed destruction of ozone in the boundary layer may well be similar to the losses caused by odd‐hydrogen photochemistry and dry deposition. The significance of this work is that IO is probably present in much of the open ocean boundary layer, at levels where it may cause significant depletion of ozone.
Thirty year variability of UV irradiance in Moscow
Authors: Chubarova N. Y. , Nezval' Y. I.
Published: September 2002

Continuous measurements of ultraviolet irradiance in the 300–380 nm spectral range (UV380) carried out at the Meteorological Observatory of the Moscow State University are analyzed for the 1968–1997 warm periods. Variability in UV irradiance is examined together with variations of ozone content, aerosol optical thickness, cloud amount, and cloud optical thickness. We estimated the maximum contribution of each parameter to interannual variability of UV380 and erythemally weighted (EW) irradiance. The most important parameters are cloud amount, whose variation contributed to changes of up to 12% in the level of UV irradiance, and total ozone content, which contributed to changes of up to 8% in the level of EW irradiance during this period. According to the developed algorithm the variability in EW irradiance was calculated for the entire 30 year period. The analysis of variability in UV380 and EW irradiance shows a slight increase in UV values since the middle of the 1980s, though no statistically significant UV trend over the 30 year period is observed. Comparison between measurements of UV380 in a cloudless atmosphere and the results of radiative transfer modeling, using a two stream delta‐Eddington approximation, reveals a good agreement. Since the end of the 1980s there is an increase of UV380 and especially EW irradiance under cloudless conditions. Extremely high values of EW irradiance and corresponding atmospheric conditions are of particular concern. It is shown that approximately in 50% of the cases, these extremely high values are detected in the presence of clouds.
Dynamics and energetics of the cloudy boundary layer in simulations of off‐ice flow in the marginal ice zone
Authors: Olsson Peter Q. , Harrington Jerry Y.
Published: September 2002

The case under consideration occurred on March 4, 1993, and was observed as part of the Radiation and Eddy Flux Experiment (REFLEX II) 1993 observational campaign northwest of Spitsbergen. The off‐ice flow on this day brought very cold surface air temperatures (‐35°C) over a relatively warm ocean surface. The resultant latent and sensible surface heat fluxes produced intense convection and a thermal internal boundary layer (TIBL) which deepened with distance from the ice edge. Two‐dimensional cloud‐resolving model (CRM) simulations were performed to determine the impact of various cloud parameterizations on the structure and evolution of the TIBL. The model was able to reproduce the observed thermal structure of the boundary layer to within the acknowledged limitations of the CRM approach. Sensitivity studies of cloud type showed that inclusion of mixed‐phase microphysics had a large impact of BL depth and structure. Radiative heating of the cloud near cloud base and cooling near cloud top along with latent heat release were found to be significant sources of turbulence kinetic energy even in the present case where very strong surface heat fluxes occur. Ice‐phase precipitation processes rapidly depleted the BL of condensate, weakening the radiative thermal forcing. A further consequence of condensate depletion in the mixed‐phase cloud was a less humid boundary layer that was able to maintain a larger surface latent heat flux and continuously extract heat through condensation and deposition. Not surprisingly, the presence of clouds had a profound impact on the radiative budget at the surface, with the cloudy BL reducing surface radiative losses more that 60% over clear‐sky values. Inclusion of the ice phase significantly affected the radiative budget as compared to purely liquid clouds, illustrating the importance of ice‐phase‐radiative couplings for accurate simulations of arctic clouds and boundary layer dynamics.
Spectroscopic characterization of hypervelocity jetting: Comparison with a standard theory
Authors: Sugita Seiji , Schultz Peter H.
Published: September 2002

Symmetric collision between two identical plates has yielded successful theoretical models for the jetting process. Consequently, assessment of impact jetting at planetary scales has been largely based on the theories developed for such specific types of collisions. Little experimental work has been done, however, to measure both temperature and target‐to‐projectile mass ratio of jetting created by spherical projectiles impacting planar targets, which typify planetary impacts. The goal of this study is to examine the validity of applying planar‐impact theories to jetting due to impacts of spherical projectiles into planar targets. Using a newly developed spectroscopic approach, we observe jetting created by copper spheres impacting planar dolomite targets at hypervelocities. In contrast with previous experiments using quartz projectiles, the observed mean temperatures of jets due to copper projectiles does not correlate well with the vertical component of impact velocity. Instead, the observed temperatures of jets show much better correlation with impact velocity than the vertical component of impact velocity and impact angle. The experiments also reveal that the target‐to‐projectile mass ratio within a jet increases with impact angle (measured from the horizontal). In order to understand the significance of these experimental results, they were then compared with a jetting model for asymmetric collisions based on standard theories. Such a comparison indicates qualitative consistencies, such as complete vaporization of the carbonate target (as opposed to mere degassing of carbon dioxide due to incomplete vaporization of carbonate) and higher target‐to‐projectile mass ratio in a jet at higher impact angles. Quantitative comparison, however, also reveals significant inconsistencies between theory and experiments, such as an impact‐angle effect on jet temperature and a correlation in jet temperatures between projectile and target components. In order to resolve these inconsistencies, new factors such as viscous shear heating and the nonsteady state nature of the jetting processes may need to be considered.
On the source regions for mare picrite glasses
Authors: Hess Paul C.
Published: September 2002

Models for the crystallization of the magma ocean, phase equilibria constraints, and geochemical criteria constrain the mare basalt source region to lie deep within the Moon and within olivine‐orthopyroxene cumulate layers of the magma ocean. The Mg* compositions of the mare basalt source region, the ferroan anorthosite crust and the mg‐suite troctolites, require that most if not all the cumulates were produced by a magma ocean undergoing fractional rather than equilibrium crystallization. Estimates of the depth of the elastic lithosphere, the existence of mascons, and the need to sequester some heat‐producing elements in the lunar interior are consistent with the lunar overturn model which transports the refractory cumulates, the more fertile ilmenite‐diopside cumulates and some of the KREEP components into the lunar interior.
Compositional analyses of small lunar pyroclastic deposits using Clementine multispectral data
Authors: Gaddis Lisa R. , Hawke B. Ray , Robinson Mark S. , Coombs Cassandra
Published: September 2002

Clementine ultraviolet‐visible (UVVIS) data are used to examine the compositions of 18 pyroclastic deposits (15 small, three large) at 13 sites on the Moon. Compositional variations among pyroclastic deposits largely result from differing amounts of new basaltic (or juvenile) material and reworked local material entrained in their ejecta upon eruption. Characterization of pyroclastic deposit compositions allows us to understand the mechanisms of lunar explosive volcanism. Evidence for compositional differences between small pyroclastic deposits at a single site is observed at Atlas crater. At all sites, compositional variation among the small pyroclastic deposits is consistent with earlier classification based on Earth‐based spectra: three compositional groups can be observed, and the trend of increasing mafic absorption band strength from Group 1 to Group 2 to Group 3 is noted. As redefined here, Group 1 deposits include those of Alphonsus West, Alphonsus Southeast, Alphonsus Northeast 2, Atlas South, Crüger, Franklin, Grimaldi, Lavoisier, Oppenheimer, Orientale, and Riccioli. Group 1 deposits resemble lunar highlands, with weak mafic bands and relatively high UV/VIS ratios. Group 2 deposits include those of Alphonsus Northeast 1, Atlas North, Eastern Frigoris East and West, and Aristarchus Plateau; Group 2 deposits are similar to mature lunar maria, with moderate mafic band depths and intermediate UV/VIS ratios. The single Group 3 deposit, J. Herschel, has a relatively strong mafic band and a low UV/VIS ratio, and olivine is a likely juvenile component. Two of the deposits in these groups, Orientale and Aristarchus, are large pyroclastic deposits. The third large pyroclastic deposit, Apollo 17/Taurus Littrow, has a very weak mafic band and a high UV/VIS ratio and it does not belong to any of the compositional groups for small pyroclastic deposits. The observed compositional variations indicate that highland and mare materials are also present in many large and small pyroclastic deposits, and they suggest that volcanic glasses or spheres may not be dominant juvenile components in all large pyroclastic deposits.
Long‐wavelength topographic relaxation for self‐gravitating planets and implications for the time‐dependent compensation of surface topography
Authors: Zhong Shijie , Zuber Maria T.
Published: September 2002

The support of planetary surface topography is controlled by the thickness of the elastic lithosphere and thus has implications for the thermomechanical structure of shallow planetary interiors. Previous analyses of the support of long‐wavelength topography have generally utilized elastic formulations, which preclude consideration of the time evolution of lithospheric stresses and thermal state. Here we formulate a viscoelastic model for the support of topography on a spherical planet. Our stress relaxation model employs a multilayer linear viscoelastic rheology and includes the effect of self‐gravitation. In these models we approximate internal rheology with an olivine flow law, and we estimate the internal thermal structure assuming simple conductive cooling. We analyze how relief at the surface and internal density interfaces evolves with time in response to a surface or internal load. Our analysis demonstrates that crustal compensation is strongly dependent on planetary radius, with a tendency for a large terrestrial planet like Earth to reach complete Airy isostasy at long wavelengths within a time scale controlled by the mantle viscosity. The correlation of degree of compensation with planetary radius is consistent with previous work [Turcotte et al., 1981], which showed that membrane stress can support significant long‐wavelength gravity anomalies. Applied to the Moon, the degree of compensation associated with loading of 100‐Ma lithosphere is less than 0.3, and the load Love numbers are greater than −0.6 for all harmonics including degree 2, even after 109 years of relaxation. This conclusion does not change significantly even with a moderately low viscosity for the crust. The tendency for a small body like the Moon to support long wavelength stresses may be relevant in understanding the anomalously large degree 2 terms in the lunar shape. The results also suggest that major impact basins would not be compensated throughout lunar geological history (>4 b.y.), even if the surfaces on which the basins were initially formed were relatively young (i.e., 100–300 Ma). The fact that the South Pole‐Aitken basin is largely compensated suggests that the lateral variations in thermal and/or mechanical structure induced by impact processes play significant roles in the compensation of this structure.
Tectonic effects of climate change on Venus
Authors: Anderson F. Scott , Smrekar S. E.
Published: September 2002

Venusian plains regions are commonly crossed by small‐strain deformation features such as wrinkle ridges, polygonal terrains, and gridded terrains. Of these, polygonal terrains are observed to have a relatively uniform spacing and are widely distributed on plains that formed during global resurfacing. Models of Venusian climate that assume resurfacing occurred through massive volcanic events suggest that surface temperatures could have dramatically changed over nearly a billion years, propagating thermal stresses into the surface and potentially causing small‐strain features. We investigate this hypothesis by approximating the temperatures predicted by climate models as a step function and employ a fixed plate and strength envelope model to predict the resulting depth of failure and amount of strain. Our calculations indicate that strains due to temperature changes of 50–100 K, which are favored for volcanic resurfacing events of 1–10 km thickness, are consistent with the observed spacing of polygonal and gridded terrains as well as some wrinkle ridges. The global nature of the climate change event is consistent with the global distribution and uniformity of such features and implies that such terrains may be a global stratigraphic marker.
Mars Global Surveyor Thermal Emission Spectrometer (TES) observations: Atmospheric temperatures during aerobraking and science phasing
Authors: Conrath Barney J. , Pearl John C. , Smith Michael D. , Maguire William C. , Christensen Philip R. , Dason Shymala , Kaelberer Monte S.
Published: September 2002

Between September 1997, when the Mars Global Surveyor spacecraft arrived at Mars, and September 1998, when the final aerobraking phase of the mission began, the Thermal Emission Spectrometer has acquired an extensive data set spanning approximately half of a Martian year. Nadir‐viewing spectral measurements from this data set within the 15 μm CO2 absorption band are inverted to obtain atmospheric temperature profiles from the surface up to about the 0.1 mbar level. The computational procedure used to retrieve the temperatures is presented. Mean meridional cross sections of thermal structure are calculated for periods of time near Northern Hemisphere fall equinox, winter solstice, and spring equinox as well as for a time interval immediately following the onset of the Noachis Terra dust storm. Gradient thermal wind cross sections are calculated from the thermal structure. Regions of possible wave activity are identified using cross sections of rms temperature deviations from the mean. Results from both near‐equinox periods show some hemispheric asymmetry with peak eastward thermal winds in the north about twice the magnitude of those in the south. The results near solstice show an intense circumpolar vortex at high northern latitudes and waves associated with the vortex jet core. Warming of the atmosphere aloft at northern midlatitudes suggests the presence of a strong cross‐equatorial Hadley circulation. Although the Noachis dust storm did not become global in scale, strong perturbations to the atmospheric structure are found, including an enhanced temperature maximum aloft at high northern latitudes resulting from intensification of the Hadley circulation. TES results for the various seasonal conditions are compared with published results from Mars general circulation models, and generally, good qualitative agreement is found.
Quantifying groundwater‐sapping landforms with a hypsometric technique
Authors: Luo Wei
Published: September 2002

The groundwater‐sapping process generates peculiar landform features, including amphitheater channel head, U‐shaped cross section with steep wall and flat floor, and short stubby tributaries. However, previous attempts to quantify this landform were primarily limited to the planform geometry of the channel networks and the network topology. The elevation or relief information, which is more revealing of the processes that gave rise to the landform, has not received enough attention in the literature. The hypsometric curve (area‐altitude relation) combines area and relief elegantly. By treating the hypsometric curve as a probability distribution, one can quantitatively distinguish small differences in the shape of the curve (and thus in the characteristics of the landform) on the basis of hypsometric integral, the skewness and kurtosis of the curve, and the skewness and kurtosis of its density function. Preliminary results of applying this technique within an automated Geographic Information System (GIS) environment to typical terrestrial sapping landforms show that they are characterized by high hypsometric integral, low hypsometric skewness, negative density skewness, and high density kurtosis as compared with typical fluvial landforms in humid regions. Statistical analyses indicate that the differences on these hypsometric attributes between typical sapping and typical terrestrial fluvial landforms are statistically significant. Thus the hypsometric analysis technique and hypsomtric attributes can be used to quantify landforms. This technique, along with other lines of evidence, can be applied to help identify the origin of landforms on other planets such as Mars, where groundwater‐sapping is thought to have played an important role in its landform evolution.
Thermal infrared emission spectroscopy of the pyroxene mineral series
Authors: Hamilton Victoria E.
Published: September 2002

The thermal infrared emissivity spectra of coarse particulate samples of compositions in the pyroxene series display reststrahlen features (absorptions) that distinguish not only orthorhombic from monoclinic structures, but also major end‐members within the two structural groups, as well as minerals within solid solution series. The exact number of reststrahlen features observed and their positions are dependent on mineral structure and cation occupancy of the M1 and M2 sites. End‐member quadrilateral pyroxenes (Mg2Si2O6‐Fe2Si2O6‐Ca[Mg, Fe]Si2O6) are easily distinguished from each other and from minerals in the nonquadrilateral series (NaFeSi2O6‐Na[Al, Fe]Si2O6‐LiAlSi2O6). Furthermore, among quadrilateral pyroxenes, variations in Mg/(Mg + Fe) are linearly correlated with several band locations, as are variations in Ca content in high‐Ca clinopyroxenes. In both quadrilateral and nonquadrilateral compositions, Christiansen feature positions are also diagnostic. No correlations with minor constituents (of the order of 0.05 atoms per formula unit) were observed. The detailed spectral characteristics of pyroxenes and their variability as a function of structure and cation occupancy are presented here with determinative curves for the identification of pyroxene composition. These data have important implications for the interpretation of spectral data from both laboratory and remote sensing instruments because they should permit a more detailed determination of pyroxene composition in measured unknown pure mineral and bulk compositions dominated by surface scattering, i.e., all particulates greater than ∼65 μm, and solid samples.
Lunar fast neutron leakage flux calculation and its elemental abundance dependence
Authors: Gasnault O. , d'Uston C. , Feldman W. C. , Maurice S.
Published: September 2002

A numerical code has been developed to simulate fast neutron leakage flux from planetary surfaces. These neutrons are produced in copious number by interactions between galactic cosmic rays and the nuclei of surface material. Their leakage flux spectrum depends on the composition of the upper layers of planetary bodies. Measurement of these neutrons allows the mapping of major compositional units. This technique is successfully used on Lunar Prospector data to obtain global geochemical maps of the lunar surface. With the help of simulations, relations between soil composition and neutron fluxes in the fast energy range [500 keV, 10 MeV] are established. The numerical code reproduces correctly the measurements of fast neutrons over the Moon. Variation in the neutron flux between highland and mare soils is calculated. A simple formula is suggested to estimate quickly the integrated fast neutron leakage flux for a given soil composition.
Methanogenesis as a potential source of chemical energy for primary biomass production by autotrophic organisms in hydrothermal systems on Europa
Authors: McCollom Thomas M.
Published: September 2002

Geochemical models are used to explore the possibility that lithoautotrophic methanogenesis (the conversion of CO2 plus H2 to methane) could be a source of metabolically useful chemical energy for the production of biomass at putative Europan hydrothermal systems. Two cases are explored: a relatively reduced methane‐rich ocean and a relatively oxidized sulfate‐ and bicarbonate‐rich ocean. In the case of a methane‐rich ocean, a source of CO2 for methanogenesis is provided by conversion of dissolved methane to CO2 during reaction of ocean water with igneous rocks at high temperatures in the subsurface. Fluid‐rock reactions also provide a source of dissolved H2 in the hydrothermal fluid. When this fluid circulates back to the ocean floor and mixes with seawater, conversion of the dissolved CO2 and H2 to methane provides a potential source of chemical energy that can be used to drive metabolic processes. For the case of a sulfate‐ and carbonate‐rich ocean, reaction with reduced igneous rocks at high temperatures will also produce hydrothermal fluids with high H2 concentrations (as occurs in hydrothermal systems on Earth). Mixing of the resulting hydrothermal fluid with seawater in a relatively oxidized ocean could supply energy from either methanogenesis or sulfate reduction. For plausible compositions of a Europan ocean, methanogenesis can supply similar amounts of energy to that which supports the prolific ecosystems surrounding submarine hydrothermal vents on Earth. Even in the most optimistic case, however, the total amount of biomass that could be supported globally by lithoautotrophic microbes on Europa is extremely small compared to the biomass produced photosynthetically on Earth. Nevertheless, sufficient metabolic energy could apparently be available at hydrothermal systems on Europa to support an origin of life and localized ecosystems.
Viking era water‐ice clouds
Authors: Tamppari Leslie K. , Zurek Richard W. , Paige David A.
Published: September 2002

We have spatially and temporally mapped water ice clouds on Mars for 1.25 Martian years using the Viking infrared thermal mapper (IRTM) data. Our technique compares brightness temperatures in the 11 and 20 μm IRTM channels, utilizing the 11 μm water ice absorption feature. A complication arises because of the surface nonunit and wavelength‐dependent emissivities. We developed a technique for removal of this effect. Using a surface thermal model, we calculated brightness temperatures and their differences for the IRTM channels resulting from the surface emissivity effect alone. These were then subtracted from the measured brightness temperatures, yielding brightness temperature differences dominated by atmospheric effects. The ability to identify water ice clouds in the infrared provides potential new information about particle size and physical processes by comparing these clouds to those seen in visible wavelengths. We found that water ice clouds were more widespread and frequent during the Viking period than had been previously recognized, with the northern spring and summer being the cloudiest periods on Mars. We interpret some of the identified cloudy zones as the southern and northern solstice season upwelling branches of the Hadley cell, although these were shifted 15°–20° southward from model predictions. Additionally, the transition between the two branches occurred later in time than in the model predictions. We observed the extension of the north polar hood below 60°N in longitudes 120°–200°. We did not find evidence for a south polar hood north of 60°S nor any evidence for interannual variability within our limited data set.
Mariner 10 stereo image coverage of Mercury
Authors: Cook A. C. , Robinson M. S.
Published: September 2002

A computer search was performed on all useable Mariner 10 images of Mercury for which refined camera positions and orientations are available. Over 2000 valid stereo pairs have been found. The results of this search allowed us to construct a map illustrating the available stereo coverage and the corresponding stereo height accuracy across the surface of Mercury. Examples are given to illustrate the quality of the available stereo pairs and the resulting digital elevation models.
Distributions of hot oxygen for Venus and Mars
Authors: Hodges R. Richard
Published: September 2002

Hot atomic oxygen velocity distributions and corresponding profiles of density and temperature have been calculated for the Venus and Mars exospheres by Monte Carlo simulation. The Venus results are realistically based on well‐established models of the Venus atmosphere and ionosphere that were derived from Pioneer Venus orbiter measurements near solar maximum. The confidence level is lower for Mars because the only viable data come from Viking entry measurements in daytime at solar minimum, and the global morphologies of the atmosphere and ionosphere are poorly understood. What is clear is that planetary differences arise because the exothermal velocities of hot oxygen created by dissociative recombination of O2+ are below satellite speeds on Venus and greater than escape on Mars. As a result, the distribution of velocities on Venus tends to be nearly isotropic, whereas it is grossly anisotropic on Mars, favoring the vertical with increasing distance from the planet. The calculated lower bound for the Martian oxygen escape rate indicates that the lifetime of CO2 in the Mars atmosphere is probably less than 10 Myr.
Geologic evolution of the Akna Montes‐Atropos Tessera region, Venus
Authors: Marinangeli Lucia , Gilmore Martha S.
Published: September 2002

The investigated area comprises an arcuate mountain belt, Akna Montes, in Western Ishtar Terra, associated with an outboard plateau, Atropos Tessera, to the west and a volcanic plateau, Lakshmi Planum, to the east. Eight geologic units have been recognized on the basis of their geomorphic and structural characteristics as they appear on Magellan radar images. Our stratigraphic analysis shows that the geological evolution of the study area can be explained by four main steps: (1) formation of the older substrata of Atropos Tessera and Lakshmi, (2) extensive plains emplacement, (3) an orogenic phase including the formation of Akna Montes, and (4) local emplacement of younger plains. The tectonic evolution shows a deformational sequence characterized by contraction, shear, and topographic relaxation. This sequence is interpreted to be a consequence of the variation of crustal stresses and crustal thickening during orogenic events as observed for terrestrial high plateaus associated with a mountain belt (i.e., Himalaya and Tibet, Andes and Altiplano). In order to estimate the amount of crustal shortening associated with the Akna Montes, we considered two end‐members for structural style of the mountain belt: a symmetric fold model and a fault‐bend fold model. The models are theoretical because terrestrial orogenic belts are often formed by a combination of different compressional structures. However, symmetric and fault‐bend faults represent the minimum and maximum crustal shortening, respectively, and thus they do place bounds on the amount of strain recorded by Akna Montes. The first model yields a shortening value less than 1%, whereas a range of 17–34% is derived for the second model. The large difference between these values underscores the importance of fold geometries for estimating strain and to place constraints on geodynamic models for mountain belt formation. On the basis of our study we think that a combination of mantle downwelling and horizontal convergence may provide a good explanation of the geology and tectonics we observed in the Akna Montes‐Atropos Tessera region.
Phobos: Regolith and ejecta blocks investigated with Mars Orbiter Camera images
Authors: Thomas P. C. , Veverka J. , Sullivan R. , Simonelli D. P. , Malin M. C. , Caplinger M. , Hartmann W. K. , James P. B.
Published: September 2002

Four flybys of Phobos by the Mars Global Surveyor spacecraft in 1998 allowed imaging at resolutions (∼2–7 m) better than obtained for any other satellite or asteroid except Earth's Moon. The images show the interior and vicinity of the large crater Stickney. There are ∼2000 identifiable ejecta blocks, the largest about 85 m across. The great majority of these blocks come from Stickney as part of low‐velocity ejecta spread eastward by the influence of Phobos's rapid rotation. They appear to have experienced only a few meters, at most, of burial or modification since emplacement. The images also show materials of different albedos within Stickney that mark downslope motion of regolith. The number of craters on the steep slopes of Stickney that are subject to the downslope motion suggests that <10 m depth of material has moved since Stickney formed. Depths of regolith east of Stickney inferred from crater morphologies and from lengths of groove slopes are greater than can be attributed to ejecta from Stickney or other visible craters. This finding suggests many large craters (>4 km) have been degraded to the point that they are no longer recognizable.
Downflow width behavior of Martian and terrestrial lava flows
Authors: Peitersen Matthew N. , Crown David A.
Published: September 2002

Examination of the downflow width behavior of 59 terrestrial lava flows at Puu Oo (Hawaii) and Glass Mountain (California) and 86 Martian flows at Alba Patera, Tyrrhena Patera, Elysium, and Olympus Mons was completed using aerial photographs, topographic maps, previously published flow maps, and Viking Orbiter images. The examined lava flows exhibit diverse width behavior, from which information about flow processes and conditions was assessed. For Puu Oo flows, no significant correlation was found between the average width of a flow and flow length or average underlying slope. A significant, but weak relationship was found between average width and average flow thickness. In analyses of the downflow width behavior of individual flows, no consistent correlations were observed between width and thickness or underlying slope. When width was analyzed as a function of distance from the source for all flows, a variety of flow width behavioral trends were recognized and quantitatively classified. The most common behavior observed on Earth and Mars involved variations of width (sometimes significant) about a mean without a significant downflow narrowing or widening trend. The distributions of width behavior trends for the Alba Patera and Puu Oo flows examined were similar, with this type of “constant” behavior dominating. In contrast, Tyrrhena Patera flows showed a tendency to widen with distance downflow, and silicic flows at Glass Mountain were more likely to narrow. Flows were also subdivided by distance from the vent, and the width behavior of each division classified. Subdivision of flows resulted in significant changes in the classification of width behavior. While width behavior in the medial regions of flows was similar to that over entire flow lengths, proximal regions show more variability (possibly due to greater fluidity of lavas near the vent) and distal regions tend to uniformly narrow (possibly due to limited supply). In certain cases, classification and subdivision analysis can be used to link flow emplacement processes to the resulting morphology. In particular, width behavior can be correlated to the presence or absence of lateral levees.
Raman spectroscopic characterization of a Martian SNC meteorite: Zagami
Authors: Wang Alian , Jolliff Bradley L. , Haskin Larry A.
Published: September 2002

To demonstrate the ability of Raman spectroscopy to determine the mineralogical character of a rock that originated on Mars, we analyzed a small slab of “normal Zagami” by point analyses and multipoint scans using laboratory spectrometers. Spectra of clinopyroxenes were dominant; their compositions were estimated from a calibration of Raman peak positions with Mg/(Mg+Fe) based on lunar pyroxenes of known composition, and these agree with compositions obtained by electron microprobe. A few spectra of orthopyroxene were observed. The broad spectrum of maskelynite was observed, but not that of plagioclase feldspar. Spectra of minor phosphates, magnetite, and pyrrhotite were obtained, as were spectra of an organic contaminant and of hematite, both apparently introduced during sample handling prior to Raman analysis. The modal analysis based on the multipoint scans agrees well with published values. If the spectra had been obtained on the surface of Mars by Raman spectroscopic analysis as a stand‐alone method and no other information about the sample was available (and by ignoring the spurious hematite and organic material), we could rule out sedimentary and plutonic rock types and conclude that the sample was a pyroxene‐phyric basalt.
Chemistry of the Jovian auroral ionosphere
Authors: Perry J. J. , Kim Y. H. , Fox J. L. , Porter H. S.
Published: September 2002

We have investigated the chemistry of the Jovian auroral thermosphere‐/ionosphere by modeling the precipitation of high‐energy electrons into the auroral zones using a multistream electron‐transport code and three model thermospheres: a standard model based on pressure and temperature data from Galileo, and two additional models that are characterized by enhanced eddy diffusion coefficients. We have predicted the effects of precipitation of monoenergetic electrons with energies between 20 and 100 keV with energy fluxes of about 11 ergs cm−2 s−1. We have derived the column densities of H2, H, CH4, and C2H2 above the altitudes of peak energy deposition. For methane column densities similar to those determined from IUE and Hubble Space Telescope H2 spectral data, we find that for our standard model, the most likely electron energies are in the 45–55 keV range. For the enhanced eddy diffusion models the energies are lower. We present ion density profiles, H density profiles, and production profiles for the most important H2 and H emissions. The predicted H column densities are in the range (1–6)×1018 cm−2 for the standard model and are smaller for the enhanced eddy diffusion models. We find that the temperatures near the altitude of peak energy deposition vary from 160 to 200 K and are significantly lower than those derived from rotational analyses of auroral H2 emissions, which average 400–500 K. This indicates that the auroral thermosphere is considerably warmer than those at lower latitudes that were measured by the Voyager spacecraft or the Galileo probe.
Correction to “Initial results from radio occultation measurements with Mars Global Surveyor” by D. P. Hinson et al.
Authors: Hinson D. P. , Simpson R. A. , Twicken J. D. , Tyler G. L. , Flasar F. M.
Published: September 2002

A general modeling scheme for assessing the suitability for life of extrasolar planets is presented. The scheme focuses on the identification of the “habitable zone” in main sequence star planetary systems accommodating Earth‐like components. Our definition of habitability is based on the long‐term possibility of photosynthetic biomass production under geodynamic conditions. Therefore all the pertinent astrophysical, climatological, biogeochemical, and geodynamic processes involved in the generation of photosynthesis‐driven life conditions are taken into account. Implicitly, a cogenetic origin of the central star and the orbiting planet is assumed. A geostatic model version is developed and investigated in parallel for demonstration purposes. The numerical solution of the advanced geodynamic model yields realistic lookup diagrams for convenient habitability determination. As an illustration, the MACHO‐98‐BLG‐35 event is scrutinized. It is shown that this event is definitely not tantamount to the discovery of one of Gaia's sisters.
Southern ocean contributions to glacial‐interglacial changes of atmospheric pCO2: An assessment of carbon isotope records in diatoms
Authors: Rosenthal Yair , Dahan Maimon , Shemesh Aldo
Published: September 2002

Eleven new Antarctic records of diatom‐bound δ13Corg show significantly lighter values during the Last Glacial Maximum than during the Holocene with a general trend of increasing depletions southward of the modern Antarctic Polar Front (MAPF). In contrast, glacial diatoms from cores farther north of the MAPF show δ13Corg values which are ∼1‰ heavier than during the Holocene. Likely causes for the low glacial δ13Corg in Antarctic diatoms are higher [CO2]aq, lower growth rates, and changes in diatom abundance or size. Taken at face value, the glacial depletions of δ13Corg suggest that Antarctic surface waters south of the MAPF might have been a source of atmospheric pCO2 during the glacial period. Alternatively, the glacial depletions might have been caused by either a relatively small decrease in growth rates or changes in the abundance of the two dominant diatom species or their average cell size. Interpretations of the Subantarctic records are complicated by similar uncertainties.
Intermediate water ventilation on the Northeastern Pacific Margin during the Late Pleistocene inferred from benthic foraminiferal δ13C
Authors: Stott Lowell D. , Neumann M. , Hammond D.
Published: September 2002

Late Pleistocene intermediate water ventilation history in the northeastern Pacific has been inferred from benthic foraminiferal carbon isotopic data from seven California margin basins. Secular variations in oceanic δ13C recorded at North Pacific ODP Site 849 were subtracted from each basin record leaving a residual δ13C history that reflects variations in ventilation. During the previous interglacial intermediate waters above 2000m contained less oxygen than they do today or Pacific deep water at Site 849 was better ventilated. Intermediate water ventilation began to improve during oxygen isotope stage 4 and continued to improve throughout stages 3 and 2. This enhanced ventilation was not contemporaneous at all depths and appears to have progressed upwards through the water column. The diachronous nature of these changes suggest that there was not simply an “on”/”off” mechanism which resulted in higher or lower ventilation in the North Pacific during the last glacial cycle.