bluuebunny/crossref_metadata_2025 huggingface dataset:
Joint characterization of vegetation by satellite observations from visible to microwave wavelengths: A sensitivity analysis
Authors: Prigent Catherine , Aires Filipe , Rossow William , Matthews Elaine
Published: February 2004

This study presents an evaluation and comparison of visible, near‐infrared, passive and active microwave observations for vegetation characterization, on a global basis, for a year, with spatial resolution compatible with climatological studies. Visible and near‐infrared observations along with the Normalized Difference Vegetation Index come from the Advanced Very High Resolution Radiometer. An atlas of monthly mean microwave land surface emissivities from 19 to 85 GHz has been calculated from the Special Sensor Microwave/Imager for a year, suppressing the atmospheric problems encountered with the use of simple channel combinations. The active microwave measurements are provided by the ERS‐1 scatterometer at 5.25 GHz. The capacity to discriminate between vegetation types and to detect the vegetation phenology is assessed in the context of a vegetation classification obtained from in situ observations. A clustering technique derived from the Kohonen topological maps is used to merge the three data sets and interpret their relative variations. NDVI varies with vegetation density but is not very sensitive in semiarid environments and in forested areas. Spurious seasonal cycles and large spatial variability in several areas suggest that atmospheric contamination and/or solar zenith angle drift still affect the NDVI. Passive and active microwave observations are sensitive to overall vegetation structure: they respond to absorption, emission, and scattering by vegetation elements, including woody parts. Backscattering coefficients from ERS‐1 are not sensitive to atmospheric variations and exhibit good potential for vegetation discrimination with ∼10 dB dynamic range between rain forest to arid grassland. Passive microwave measurements also show some ability to characterize vegetation but are less sensitive than active measurements. However, passive observations show sensitivity to the underlying surface wetness that enables detection of wetlands even in densely vegetated areas. Merging the data sets using clustering techniques capitalizes on the complementary strengths of the instruments for vegetation discrimination and shows promising potential for land cover characterization on a global basis.
Multiobjective calibration and sensitivity of a distributed land surface water and energy balance model
Authors: Houser Paul R. , Gupta Hoshin V. , Shuttleworth W. James , Famiglietti James S.
Published: February 2004

The feasibility of using spatially distributed information to improve the predictive ability of a spatially distributed land surface water and energy balance model (LSM) was explored at the U.S. Department of Agriculture Agricultural Research Service (USDA‐ARS) Walnut Gulch Experimental Watershed in southeastern Arizona. The inclusion of spatially variable soil and vegetation information produced unrealistic simulations that were inconsistent with observations, which was likely an artifact of both discretely assigning a single set of parameters to a given area and inadequate knowledge of spatially varying parameter values. Because some of the model parameters were not measured or are abstract quantities a multiobjective least squares strategy was used to find catchment averaged parameter values that minimize the prediction error of latent heat flux, soil heat flux, and surface soil moisture. This resulted in a substantial improvement in the model's spatially distributed performance and yielded valuable insights into the interaction and optimal selection of model parameters.
Sulfur dioxide oxidation in clouds at Whiteface Mountain as a function of drop size
Authors: Rattigan O. V. , Reilly Jill , Judd C. D. , Moore Katharine F. , Das Mita , Sherman D. Eli , Dutkiewicz Vincent A. , Collett Jeffrey L. , Husain Liaquat
Published: February 2004

In situ oxidation of SO2 has been determined in clouds as a function of droplet size using a trace element technique during July 1998 at Whiteface Mountain, New York. The pH of the cloud water ranged from 2.8 to 4.7 with a mean of 3.4, and therefore SO2 oxidation was dominated by hydrogen peroxide. Size‐fractioned cloud samples were collected from six different events at the mountain's summit (1.5 km above mean sea level) using a size‐fractionating California Institute of Technology Active Strand Cloudwater Collector. Bulk samples were collected using both passive and active collectors. During each event, below‐cloud and interstitial aerosols were collected every 2 hours. Cloud water and aerosol samples were analyzed for major ions and selected trace elements. Continuous measurements of gas phase species SO2, H2O2, and O3 were carried out at the summit and below‐cloud sites. Concentrations of cloud water SO42−, NO3−, H2O2, and trace elements, as well as pH, were largely independent of droplet size. The component of cloud water SO42− produced from in situ oxidation (SO2−4in) was also largely independent of droplet size. The results are in agreement with calculated relative production rates in the small and large drop sizes based on known laboratory reaction rates.
Overview of the trace gas measurements on board the Citation aircraft during the intensive field phase of INDOEX
Authors: de Gouw J. A. , Warneke C. , Scheeren H. A. , van der Veen C. , Bolder M. , Scheele M. P. , Williams J. , Wong S. , Lange L. , Fischer H. , Lelieveld J.
Published: February 2004

During the intensive field phase of the Indian Ocean Experiment (INDOEX), measurements of the atmospheric chemical and aerosol composition over the Indian Ocean were performed from a Cessna Citation aircraft. Measurements were performed during February and March 1999 over the northern Indian Ocean from 70°E to 80°E, and from 8°N to 8°S in the 0–13 km altitude range. An overview of the trace gas measurements is presented. In the lowest 3 km the highest levels of pollution were found during February 1999, mostly originating from northeastern India and southeastern Asia. Lower levels of pollution were detected in March 1999, when the sampled air mostly originated from the rabian Sea region. The mixing ratios of a number of trace compounds, indicative of biomass burning, were well correlated. The pollutant emission factors inferred from the measurements are consistent with literature values for fire plumes, confirming that the residential use of biofuels in Asia is a major source of gaseous pollutants to the atmosphere over the Indian Ocean, in accord with emission databases. The removal of reactive trace gases was studied over an extended area without interfering local emissions, and is shown to be governed by photochemical processes rather than by mixing and deposition. At intermediate altitudes of 3–8 km the mixing ratios of all trace gases other than ozone were generally lower, and the measurements suggest that the photochemical processing of these air masses is much more extensive than in the 0–3 km range. In the 8–13 km altitude range some evidence is obtained for the importance of convective cloud systems in the transport of gaseous pollutants to the upper troposphere.
Atmospheric water vapor over Antarctica derived from Special Sensor Microwave/Temperature 2 data
Authors: Miao Jungang , Kunzi Klaus , Heygster Georg , Lachlan‐Cope Tom A. , Turner John
Published: February 2004

In polar regions, satellite microwave radiometry has not been successful in measuring the total water vapor (TWV) in the atmosphere. The difficulties faced in these regions arise from the very low water vapor burden of the atmosphere and the large and highly variable emissivities of ice surfaces in the microwave frequency range. By exploiting the advantages of the Special Sensor Microwave/Temperature 2 (SSM/T2), a method is developed to retrieve TWV over Antarctica from satellite data. This method shows very low sensitivities to the change of surface emissivity and to the presence of water clouds. However, ice clouds may have considerable effects. Results of radiative transfer model simulation show that they may cause one to underestimate TWV using the proposed method and that the amount of underestimation is proportional to the ice water path of the ice cloud. Validations using radiosonde measurements and numerical model analyzes suggest that SSM/T2 retrievals have a high accuracy (maximum error <10%) as long as TWV is <4.0 kg m−2. Above this value, retrievals show a systematic overestimation. Presumably, this is a result of the seasonal difference between the validation and the training radiosonde data sets. TWV retrievals of 1 year's SSM/T2 data show clearly the seasonal variation of water vapor over Antarctica. Throughout the year the mean TWV over West Antarctica is nearly twice as high as that over East Antarctica; the temporal fluctuation of TWV over West Antarctica is also significantly stronger than over East Antarctica. This suggests that precipitation and water vapor transport in West Antarctica are more active than in East Antarctica. Using the same year's TWV data, we estimated the mean residence time of atmospheric water vapor over the Antarctica to be merely 3–4 days. This, however, is much shorter than the global mean of 9–10 days.
Atomic oxygen density profiles from ground‐based nightglow measurements at 23°S
Authors: Melo Stella M. L. , McDade Ian C. , Takahashi Hisao
Published: February 2004

In this paper we demonstrate how ground‐based measurements of selected airglow emission features may be used to determine the vertical distribution of atomic oxygen in the upper mesosphere and lower thermosphere. The technique used is based on a method first suggested by Sharp and McDade [1996] and exploits differences between the kinetics of specific oxygen airglow features to determine key parameters related to the atomic oxygen vertical distribution. The technique is applied to ground‐based measurements of the OI 557.7 nm green line, the O2 Atmospheric (0–1) band, and the OH Meinel (9–4) band emissions made at Cachoeira Paulista, Brazil (23°S, 45°W) from January 1987 to June 1994. In order to compare the results obtained here with observations, the derived atomic oxygen density profiles are used to forward calculate hydroxyl nightglow profiles which are then compared with profiles measured by the Wind Imaging Interferometer (WINDII) experiment on board the Upper Atmosphere Research Satellite (UARS). This comparison shows good agreement for the seasonally averaged nocturnal behavior of both the peak altitude and the peak volume emission rate.
Toxaphene in the United States: 1. Usage gridding
Authors: Li Y. F.
Published: February 2004

Toxaphene, as a general‐purpose insecticide, was widely used in the United States. The use of toxaphene in the United States can be divided into four different periods between 1947 and 1986, with total usage of about 490 kt, and total production around 720 kt. Inventories of gridded usage of toxaphene in the United States with 1/6° by 1/4° latitude/longitude resolution have been created by using different gridded cropland and cattle as surrogate data. The intensive use of toxaphene on croplands was concentrated in the southeastern part of the United States with the highest usage of 2 kt per grid cell. The results show that the state of Alabama was the largest user of toxaphene, reaching as much as 87 kt, followed by Mississippi at 60 kt. The total usage in the top 10 states is 410 kt, 84% of the national usage in the United States. The use in the first six states, Alabama, Mississippi, Georgia, Arkansas, Texas, and South Carolina, is 350 kt, 71% of the national usage in the United States.
Atmospheric particle size and composition measurements to support light extinction calculations over the Indian Ocean
Authors: Chowdhury Zohir , Hughes Lara S. , Salmon Lynn G. , Cass Glen R.
Published: February 2004

The size distribution and chemical composition of the atmospheric aerosol at the Kaashidhoo Climate Observatory (KCO) in the Republic of Maldives was determined during the winter northeast monsoon season to aid in determining the light scattering and light absorption properties of the aerosol particles in that region. These experiments were conducted over 8 two‐day periods during February 11–26, 1999, using filter‐based samplers and cascade impactors operated at ambient relative humidity which was in the range of 80–89% relative humidity over 83% of the period sampled. Fine particle concentrations (Da<1.8 μm) averaged 17.7(±0.22) μg m−3 and varied between 8.4(±0.33)–24.7(±0.21) μg m−3 over the period studied. Sulfate ion and carbonaceous aerosols are the largest contributors to the fine particle mass concentration, accounting for 33–37% and 26–27% of the fine mass, respectively. Calcium carbonate contributes 3% of the mass measured on the impactor stages. Ammonium, nitrate, and chloride ion account for 7–9, 1, and 0–1% of the fine particle mass, respectively. The residual mass of as yet undetermined fine particle material stands at 28–30%. Black elemental carbon particles contribute 6–11% of the fine particle mass concentration and dominate light absorption in the atmosphere at KCO (A. Eldering et al., unpublished manuscript, 2001). These fine particle concentrations are comparable to those found in major cities in the United States and are surprisingly high for a remote location such as the Maldive Islands, which is located downwind of the Indian subcontinent.
Classification of tropical precipitating systems and associated Z‐R relationships
Authors: Rao T. Narayana , Rao D. Narayana , Mohan K. , Raghavan S.
Published: February 2004

Simultaneous observations of Indian Mesosphere‐Stratosphere‐Troposphere (MST) radar, Lower Atmospheric Wind Profiler (LAWP) and disdrometer, located at Gadanki (13.5°N, 79.2°E), over two seasons are used to derive the Z‐R relationships, of the form Z = ARb, for different types of precipitation and for different seasons. Disdrometer data are classified into three types, convection, transition, and stratiform, based on the variation of the median volume diameter D0 with the rain rate R. The disdrometer classification is verified by comparing with the profilers classification, based on reflectivity (in terms of range corrected signal‐to‐noise ratio), Doppler velocity, spectral width, and vertical air velocity. Overall agreement between the two classification schemes is found to be reasonable. The variations in the coefficient A and the exponent b in the Z‐R relation for the case studies presented here are explained with the help of variations in drop size distribution parameters. From the total data the percentage occurrence of precipitation is found to be 55% stratiform, 9% convective, and 36% transition, whereas the total rainfall is 12, 54, and 34%, respectively. Interestingly, the exponent b (coefficient A) is found to be smaller (larger) in the case of stratiform (convective) precipitation than that in convective (stratiform) precipitation, in contrast to the earlier results. The values of A and b are also derived for different monsoon seasons.
Constraints from 210Pb and 7Be on wet deposition and transport in a global three‐dimensional chemical tracer model driven by assimilated meteorological fields
Authors: Liu Hongyu , Jacob Daniel J. , Bey Isabelle , Yantosca Robert M.
Published: February 2004

The atmospheric distributions of the aerosol tracers 210Pb and 7Be are simulated with a global three‐dimensional model driven by assimilated meteorological observations for 1991–1996 from the NASA Goddard Earth Observing System (GEOS1). The combination of terrigenic 210Pb and cosmogenic 7Be provides a sensitive test of wet deposition and vertical transport in the model. Our simulation of moist transport and removal includes scavenging in wet convective updrafts (40% scavenging efficiency per kilometer of updraft), midlevel entrainment and detrainment, first‐order rainout and washout from both convective anvils and large‐scale precipitation, and cirrus precipitation. Observations from surface sites in specific years are compared to model results for the corresponding meteorological years, and observations from aircraft missions over the Pacific are compared to model results for the days of the flights. Initial simulation of 7Be showed that cross‐tropopause transport in the GEOS1 meteorological fields is too fast by a factor of 3–4. We adjusted the stratospheric 7Be source to correct the tropospheric simulation. Including this correction, we find that the model gives a good simulation of observed 210Pb and 7Be concentrations and deposition fluxes at surface sites worldwide, with no significant global bias and with significant success in reproducing the observed latitudinal and seasonal distributions. We achieve several improvements over previous models; in particular, we reproduce the observed 7Be minimum in the tropics and show that its simulation is sensitive to rainout from convective anvils. Comparisons with aircraft observations up to 12‐km altitude suggest that cirrus precipitation could be important for explaining the low concentrations in the middle and upper troposphere.
Boreal forest CO2 exchange and evapotranspiration predicted by nine ecosystem process models: Intermodel comparisons and relationships to field measurements
Authors: Amthor J. S. , Chen J. M. , Clein J. S. , Frolking S. E. , Goulden M. L. , Grant R. F. , Kimball J. S. , King A. W. , McGuire A. D. , Nikolov N. T. , Potter C. S. , Wang S. , Wofsy S. C.
Published: February 2004

Nine ecosystem process models were used to predict CO2 and water vapor exchanges by a 150‐year‐old black spruce forest in central Canada during 1994–1996 to evaluate and improve the models. Three models had hourly time steps, five had daily time steps, and one had monthly time steps. Model input included site ecosystem characteristics and meteorology. Model predictions were compared to eddy covariance (EC) measurements of whole‐ecosystem CO2 exchange and evapotranspiration, to chamber measurements of nighttime moss‐surface CO2 release, and to ground‐based estimates of annual gross primary production, net primary production, net ecosystem production (NEP), plant respiration, and decomposition. Model‐model differences were apparent for all variables. Model‐measurement agreement was good in some cases but poor in others. Modeled annual NEP ranged from −11 g C m−2 (weak CO2 source) to 85 g C m−2 (moderate CO2 sink). The models generally predicted greater annual CO2 sink activity than measured by EC, a discrepancy consistent with the fact that model parameterizations represented the more productive fraction of the EC tower “footprint.” At hourly to monthly timescales, predictions bracketed EC measurements so median predictions were similar to measurements, but there were quantitatively important model‐measurement discrepancies found for all models at subannual timescales. For these models and input data, hourly time steps (and greater complexity) compared to daily time steps tended to improve model‐measurement agreement for daily scale CO2 exchange and evapotranspiration (as judged by root‐mean‐squared error). Model time step and complexity played only small roles in monthly to annual predictions.
Ages of mare basalts on the lunar nearside
Authors: Hiesinger Harald , Jaumann Ralf , Neukum Gerhard , Head James W.
Published: February 2004

The chronology of lunar volcanism is based on radiometric ages determined from Apollo and Luna landing site samples, regional stratigraphic relationships, and crater degradation and size‐frequency distribution data for units largely defined prior to the end of the Apollo program. Here we report on new crater size‐frequency distribution data for 139 spectrally and morphologically defined basalt units which are exposed in six nearside impact basins (Australe, Tranquillitatis, Humboldtianum, Humorum, Serenitatis, and Imbrium). Crater size‐frequency distribution measurements are a statistically robust and accurate method to derive absolute model ages of unsampled regions of the Moon. Compared to crater degradation ages, crater size‐frequency ages, performed on spectrally defined units, offer significant improvements in accuracy. Our investigation showed that (1) in the investigated basins, lunar volcanism was active for at least 1.5–2.0 b.y., starting at about 3.9–4.0 b.y. and ceasing at ∼2.0 b.y., (2) most basalts erupted during the late Imbrian Period at about 3.6–3.8 b.y., (3) significantly fewer basalts were emplaced during the Eratosthenian Period, (4) basalts of Copernican age were not found in any of the investigated basins, (5) lunar basin‐filling volcanism probably started within ∼100 m.y. after the formation of the individual basins. We also assessed the relationship between impact basin age and the history of mare basalt emplacement in these basins. We found that (1) in all pre‐Nectarian basins (Australe and Tranquillitatis) as well as in the Humboldtianum basin, which is of Nectarian age, the distribution of surface ages is clearly dominated by only a single peak in the number of erupted units at 3.6–3.8 b.y., (2) in the younger basins (Humorum, Serenitatis, and Imbrium) a second peak at 3.3–3.5 b.y. is observed, (3) basalt eruptions younger than 2.6 b.y. occur only intermittently, and (4) in the youngest basins, Serenitatis and Imbrium, we see an extended period of active basin‐filling volcanism (1.5–1.6 b.y.) which is 500 m.y. longer than in the Australe and Humorum and even ∼1.0 b.y. longer than in Tranquillitatis and Humboldtianum.
Correspondence and least squares analyses of soil and rock compositions for the Viking Lander 1 and Pathfinder landing sites
Authors: Larsen Kristopher W. , Arvidson Raymond E. , Jolliff Brad L. , Clark Benton C.
Published: February 2004

Correspondence analysis is a dual Q and R mode factor analysis that allows simultaneous visualization of relationships between normalized variables and samples. The usefulness of the technique is first illustrated with an analysis of a simulated feldspar compositional data set and then applied to analysis of patterns among variables and samples for compositions obtained for Viking Lander 1 soils and Pathfinder rocks and soils. For the Martian data the first two factors capture 90% of the variance. Data projected onto the plane defined by these two factors fall within a mixing triangle defined by three geologically plausible end‐members: basaltic shergottite, andesite, and a halide‐kieserite evaporite. However, examination of data projected onto the second and third factors shows that the compositions are systematically displaced from the mixing triangle plane in the same general direction as the Fe vector. Inclusion of a Fe oxide (e.g., hematite or maghemite) as a reasonable fourth end‐member forms a mixing tetrahedron that encapsulates the data. Least squares mixing analysis of the data with these four end‐members and the elemental suite measured at both landing sites (Si, Al, Ti, Fe, Mg, Ca, S, Cl) implies that aeolian dust on rocks is a physical mixture of basalt and andesite‐derived materials in approximately equal proportions, soil components at the two landing sites are similar to each other and less basalt‐like than the aeolian dust, and Viking 1 soils are more andesite‐rich than the Pathfinder soils.
Carbon dioxide cycling and implications for climate on ancient Earth
Authors: Sleep Norman H. , Zahnle Kevin
Published: February 2004

The crustal Urey cycle of CO2 involving silicate weathering and metamorphism acts as a dynamic climate buffer. In this cycle, warmer temperatures speed silicate weathering and carbonate formation, reducing atmospheric CO2 and thereby inducing global cooling. Over long periods of time, cycling of CO2 into and out of the mantle also dynamically buffers CO2. In the mantle cycle, CO2 is outgassed at ridge axes and island arcs, while subduction of carbonatized oceanic basalt and pelagic sediments returns CO2 to the mantle. Negative feedback is provided because the amount of basalt carbonatization depends on CO2 in seawater and therefore on CO2 in the air. On the early Earth, processes involving tectonics were more vigorous than at present, and the dynamic mantle buffer dominated over the crustal one. The mantle cycle would have maintained atmospheric and oceanic CO2 reservoirs at levels where the climate was cold in the Archean unless another greenhouse gas was important. Reaction of CO2 with impact ejecta and its eventual subduction produce even lower levels of atmospheric CO2 and small crustal carbonate reservoirs in the Hadean. Despite its name, the Hadean climate would have been freezing unless tempered by other greenhouse gases.
Large mass movements on Callisto
Authors: Chuang Frank C. , Greeley Ronald
Published: February 2004

Galileo images reveal the presence of mass movement deposits within impact craters on Callisto. Eleven such deposits were found in 830 candidate craters imaged at sufficient resolutions (86–280 m/pixel) for their identification. All of the deposits are located within impact craters, and their sources appear to be crater wall material. The morphologies of the Callistan deposits are similar to terrestrial mass movements and rock glaciers. Rock glaciers involve ductile flow of ice and rock, but in the Callistan environment, water ice is likely to undergo brittle deformation. Consequently, rock glaciers are unlikely analogs for the features on Callisto. Three morphologies are observed: one blocky, one slump‐like, and nine lobate deposits. Blocky deposits are characterized by massive blocks on the crater floor, while slump‐like deposits appear as debris piles along the base of the crater wall. Lobate deposits are 1.7–9.9 km long, average ∼90 m in thickness, and have tapered or semicircular terminations. Callistan lobate deposits are morphologically similar to rapidly emplaced dry‐rock avalanches on Earth. Their significant thickness and steep frontal margins suggest that they may have behaved similarly to Bingham plastic material with a critical yield strength. The Callistan deposits have yield strengths similar to those estimated for terrestrial dry‐rock avalanches. The blocky deposit appears similar to terrestrial block glide deposits, in which large blocks detach and slide downslope. Individual blocks within the blocky deposits may be up to 1 km across and stand 30–60 m high. Callistan slump‐like deposits are similar to terrestrial slumps, which fail along rotational planes. Mass movements on Callisto are not confined to craters; they have also been observed along the base of scarps, knobs, and ridges. Three of the Callistan deposits may have been impact triggered because of the proximity of an impact crater near the source. First‐order analyses of the ground force generated by impacts show that two of the candidate deposits are capable of being impact triggered. The Callistan mass movements are probably set into motion by a combination of sublimation of ice, which undermines a near‐surface layer of “lag” material, and seismic “triggering” associated with nearby impact events.
Standardizing the nomenclature of Martian impact crater ejecta morphologies
Authors: Barlow Nadine G. , Boyce Joseph M. , Costard Francois M. , Craddock Robert A. , Garvin James B. , Sakimoto Susan E. H. , Kuzmin Ruslan O. , Roddy David J. , Soderblom Laurence A.
Published: February 2004

The Mars Crater Morphology Consortium recommends the use of a standardized nomenclature system when discussing Martian impact crater ejecta morphologies. The system utilizes nongenetic descriptors to identify the various ejecta morphologies seen on Mars. This system is designed to facilitate communication and collaboration between researchers. Crater morphology databases will be archived through the U.S. Geological Survey in Flagstaff, where a comprehensive catalog of Martian crater morphologic information will be maintained.
Rifts and large volcanoes on Venus: Global assessment of their age relations with regional plains
Authors: Basilevsky Alexander T. , Head James W.
Published: February 2004

We report on the results of synoptic global mapping of rifts and large volcanoes on Venus and detailed mapping in six areas illustrating the relationships among these features. Rifts and large volcanoes were subdivided into old, transitional (for volcanoes only), and young features. The geologic event separating old features from young ones is the formation of wrinkle ridges on the regional plains. Global mapping showed that the postplains rifts dominate in the population of rifts both in terms of number of mapped segments and in area. The postplains volcanoes are more abundant in terms of number and area occupied than those that are transitional in age, and these, in turn, dominate over the old ones. These data show that in post‐regional‐plains time rifting and formation of large volcanoes are significantly more important processes than in the immediately preceding period. The time period during which regional plains were emplaced, and old (and partly transitional) large volcanoes formed, was much shorter than the postplains time. Therefore the mean rate of formation of large volcanoes during this first period was significantly higher than in the subsequent time. Similarly, the relatively short time duration of the plains‐emplacement time indicates that the mean rate of old rifting was higher than in more recent time. This study shows prominent changes with time in the absolute rates and roles of rifting and large volcano formation. This supports the “directional” model of the geologic evolution of Venus during the last several hundred million years, as opposed to the “nondirectional” model, which predicts a more random occurrence of geologic processes throughout history.
Geology of Venus: Mapping of a global geotraverse at 30°N latitude
Authors: Ivanov Mikhail A. , Head James W.
Published: February 2004

We mapped a swath of terrain on Venus that extends circumferentially completely around the globe at 30°N latitude representing about 11% of the surface of the planet in order to address the following questions: Do presently defined stratigraphic units prove useful for broad application to regional and global mapping? Do units described in one area occur in other regions of the planet and how laterally continuous are they? If similar sequences of units are found in different parts of the planet, do they correlate temporally (e.g., time‐correlative), or do they represent similar sequences formed at different times (e.g., time‐transgressive or independently locally repetitive)? We found that similar sequences of units are seen in different parts of the global geotraverse and that key stratigraphic units could be traced across almost the entire circumference of Venus at this latitude, providing evidence that these units are not only global in extent, but also laterally contiguous. This gives confidence that the stratigraphic sequence can be extended to other parts of Venus for further testing and refinement. On the basis of this mapping, we find that several areas, including tessera massifs and Beta Regio, show a concentration of older units and appear to have been topographically high over much of the visible history of Venus. Tessera terrain is not uniformly distributed in some areas where these older units are concentrated. Ridge belts and fracture belts are not randomly distributed throughout the area, suggesting that they represent regional patterns of deformation. The altitude distribution of the stratigraphic units and structures shows that the regional plains tend to be the lowest, with older units (tessera and earlier plains) at intermediate to high elevations, consistent with regional plains emplacement following the formation of the most significant regional topography. Sixty corona and corona‐like features were found in the geotraverse, and the vast majority show evidence of being initiated in the earliest posttessera time (early Guineverian) in an extensional tectonic environment; few show evidence of postregional plains volcanism, and no positive evidence was found for the initiation of coronae in the postregional plains period of the history of Venus. Regional mapping shows that shield plains represent a unit occurring primarily prior to the emplacement of regional plains with wrinkle ridges (early part of the middle Guineverian), perhaps overlapping somewhat with its lower member. Evidence is found for changes in styles of volcanism with time, and tectonic activity changed in time in terms of pervasiveness, style, and areal distribution. On the basis of mapping of this globally continuous swath representing about 11% of the planet, we find that our analysis supports the presence of changes in the style and relative importance of different types of geological activity as a function of time, rather than the “nondirectional” history proposed by Guest and Stofan [1999].
Radio occultation measurements of forced atmospheric waves on Mars
Authors: Hinson D. P. , Tyler G. L. , Hollingsworth J. L. , Wilson R. J.
Published: February 2004

Mars Global Surveyor performed a series of radio occultation experiments in December 1998, resulting in 36 profiles of the neutral atmosphere in late northern spring (Ls = 74.1°–77.3°). The measurements are confined in latitude (64.6°–67.2°N) and local time (0321–0418), but their distribution in longitude is fairly uniform. We used least squares spectral analysis to characterize the zonal structure of the atmosphere and constructed longitude‐height cross sections of both temperature and geopotential. Zonal variations of temperature exceed 12 K near the surface but are much smaller (2–3 K) at higher altitudes. Zonal variations of geopotential are ∼200 m throughout the vertical range of the measurements. These patterns of temperature and geopotential appear to be stationary relative to the surface with little day‐to‐day variation within the 7‐sol span of the measurements. We relied heavily on Mars general circulation models (GCMs) for guidance in understanding these data. Stationary planetary waves are responsible for some aspects of the temperature and geopotential fields, particularly at pressures exceeding 100–200 Pa. On the basis of strong similarities between a GCM simulation and the observations, we conclude that the disturbance takes the form of a planetary wave train excited by Alba Patera. The data also include the signature of non‐Sun‐synchronous thermal tides, which produce a pattern that appears to be stationary when sampled at fixed local time. Comparison between a GCM simulation and the measured geopotential field provides evidence for the presence of the resonantly enhanced, diurnal, wave‐1 Kelvin mode.
Overview of the Mars Global Surveyor mission
Authors: Albee Arden L. , Arvidson Raymond E. , Palluconi Frank , Thorpe Thomas
Published: February 2004

The Mars Global Surveyor spacecraft was placed into Mars orbit on September 11, 1997, and by March 9, 1999, had slowly circularized through aerobraking to a Sun‐synchronous, near‐polar orbit with an average altitude of 378 km. The science payload includes the Mars Orbiter Camera, Mars Orbiter Laser Altimeter, Thermal Emission Spectrometer, Ultrastable Oscillator (for Radio Science experiments), and Magnetometer/Electron Reflectometer package. In addition, the spacecraft accelerometers and horizon sensors were used to study atmospheric dynamics during aerobraking. Observations are processed to standard products by the instrument teams and released as documented archive volumes on 6‐month centers by the Planetary Data System. Significant results have been obtained from observations of the interior, surface, and atmosphere. For example, Mars does not now have an active magnetic field, although strong remanent magnetization features exist in the ancient crust. These results imply that an internal dynamo ceased operation early in geologic time. Altimetry and gravity data indicate that the crust is thickest under the south pole, thinning northward from the cratered terrain to the northern plains. Analysis of altimetry data demonstrates that Mars is “egg‐shaped” with gravitational equipotential contours that show that channel systems in the southern highlands drained to the north, largely to the Chryse trough. A closed contour in the northern plains is consistent with the existence of a great northern ocean. Emission spectra of low‐albedo regions show that basaltic rocks dominate spectral signatures on the southern highlands, whereas basaltic andesites dominate the northern lowlands. The bright regions show nondiagnostic spectra, similar to that of dust in the atmosphere. Signatures of aqueous minerals (e.g., clays, carbonates, and sulfates) are noticeably absent from the emission spectra. High spatial resolution images show that the surface has been extensively modified by wind and that layering is nearly ubiquitous, implying that a complex history of events is recorded in surface and near‐surface materials. Altimetry data imply that both permanent caps are composed of water ice and dust, with seasonal covers of carbon dioxide frost. Finally, the altimetry data, coupled with thousands of atmospheric profiles, are providing new boundary conditions and dynamic controls for the generation and testing of more realistic dynamic models of the global circulation of the atmosphere.
Martian dust storms: 1999 Mars Orbiter Camera observations
Authors: Cantor Bruce A. , James Philip B. , Caplinger Michael , Wolff Michael J.
Published: February 2004

The Mars Orbiter Camera on board the Mars Global Surveyor from March 9, 1999 (Ls = 107°, where Ls is the areocentric longitude of the Sun measured in degrees from Mars' northern spring equinox), to December 31, 1999 (Ls = 274°), has obtained 7.5 km pixel−1 daily global maps of the Martian surface in two wavelength bands: blue (400–450 nm) and red (575–625 nm). Visual inspection of these maps during the 1999 dust storm season has resulted in the detection of 783 dust storms, ranging in size from “local” (>102 km2) to “regional” (>1.6×106 km2). No global storms were observed. The difference in the numbers of dust storms occurring in the two hemispheres (north: 343; south: 440) was not statistically significant. Fifty percent of the regional size dust storms formed as a result of the “merger” of two or more smaller local storms. The regional storms from Ls = 207° to Ls = 223° were observed to move from the northern hemisphere into the southern hemisphere between 30°W and 50°W longitude. This cross‐equatorial transport of dust suggests the location of the low‐lying southward flowing branch of the Hadley circulation. Observations show that dust storms occur in several regions on Mars: at the two polar cap edges, at the base of high elevation regions in the northern hemisphere, near the polar hood during northern fall, and at mid‐latitudes in both hemispheres. Specific regions such as Solis Planum and Hesperia, which were regions of significant dust activity in the past, showed almost no dust storm activity in 1999, suggesting that regional dust sources are variable over a 20‐year time frame. One region of exceptional activity not noted previously was the Arcadia‐Amazonis border. However, other regions, such as near Elysium Mons, Acidalia, Chryse, Hellas, Noachis, Argyre, Cimmeria, and Sirenum, again were active dust storm regions. Relative dust opacities were modeled for 117 red wavelength events and ranged from 0.54 “diffuse haze” to 2.13 “concentrated local events.” From the dust opacities we estimated the north polar sedimentation rate to be 6.0–10×10−4 g cm−2 yr−1, a factor of 40 smaller than previous estimates from the two 1977 global dust storms [Pollack et al., 1979], along with the global dust mass loading (2.6–4.0×10−4 g cm−2) and cross‐equatorial dust mass loading (3.6×10−4 g cm−2). The 1999 cross‐equatorial mass dust loading suggests that the southern hemisphere subtropical latitudes require at least 2–3 Martian years to replenish their dust sources before global storms can form.
Mars: Evidence for geologically recent advance of the south polar cap
Authors: Head James W.
Published: February 2004

An impact crater near the south pole characterized by pristine‐appearing secondary crater chains is partly covered with more than a kilometer of polar layered terrain deposits continuous with the main polar deposit. The observed relationships strongly suggest that the crater formed near the edge of the cap in Amazonian time, was buried by polar cap advance, and that some retreat occurred. These data appear to indicate that the south polar cap has been active in the geologically recent past.
Mars Global Surveyor observations of Martian fretted terrain
Authors: Carr Michael H.
Published: February 2004

The Martian fretted terrain between latitudes 30° and 50°N and between 315° and 360°W has been reexamined in light of new Mars Orbiter Camera (MOC) and Mars Orbiter Laser Altimeter (MOLA) data from Mars Global Surveyor. Much of the terrain in the 30°–50° latitude belt in both hemispheres has a characteristic stippled or pitted texture at MOC (1.5 m) scale. The texture appears to result from partial removal of a formerly smooth, thin deposit as a result of sublimation and deflation. A complex history of deposition and exhumation is indicated by remnants of a former, thicker cover of layered deposits. In some hollows and on some slopes, particularly those facing the pole, are smooth textured deposits outlined by an outward facing escarpment. Throughout the study area are numerous escarpments with debris flows at their base. The escarpments typically have slopes in the 20°–30° range. At the base of the escarpment is commonly a deposit with striae oriented at right angles to the escarpment. Outside this deposit is the main debris apron with a surface that typically slopes 2°–3° and complex surface textures suggestive of compression, sublimation, and deflation. The presence of undeformed impact craters indicates that the debris flows are no longer forming. Fretted valleys contain lineated fill and are poorly graded. They likely form from fluvial valleys that were initially like those elsewhere on the planet but were subsequently widened and filled by the same mass‐wasting processes that formed the debris aprons. Slope reversals indicate that downvalley flow of the lineated fill is minor. The ubiquitous presence of breaks in slope formed by mass wasting and the complex surface textures that result from mass wasting, deflation, and sublimation decreases the recognizability of the shorelines formerly proposed for this area.
Ancient Martian volcanoes in the Aeolis region: New evidence from MOLA data
Authors: Stewart Emily M. , Head James W.
Published: February 2004

Mars Orbiter Laser Altimeter (MOLA) altimetric data confirm the volcanic origin of a Noachian‐aged hilly feature (Zephyria Tholus) in the Aeolis area of Mars. Similar characteristics shared by at least one nearby topographic high suggest that volcanic activity represented by such features may have been more widespread than previously thought. Morphological structures and morphometric data for Zephyria are consistent with a stratovolcano origin, in which the edifice formed by mixed explosive and effusive eruptions, and was altered by subsequent flank erosion, to produce a distinctive truncated cone with concave upward flanks. Criteria are outlined to help detect and distinguish the origin of such features in other ancient regions of Mars.
Estimates of Martian crustal thickness from viscous relaxation of topography
Authors: Nimmo F. , Stevenson D. J.
Published: February 2004

Isostatically compensated crustal thickness variations and associated topographic contrasts at the surface of a planet result in lateral pressure gradients, which may cause the lower crust to flow and reduce the relief. Areas of thicker crust are generally associated with more rapid relaxation of topography. On Mars, topographic features such as impact basins and the hemispheric dichotomy have survived for 4 Gyr. We use a finite difference representation of depth‐dependent, non‐Newtonian lower crustal flow to investigate how topography decays with time. For a dry diabase rheology, total radiogenic concentrations ≥80% of terrestrial values, and crustal radiogenic concentrations similar to terrestrial basalts, we find that an upper bound on the mean planetary crustal thickness is ∼100 km. In the probably unrealistic case where all the radiogenic elements are in the crust, this maximum crustal thickness can be increased to ∼115 km. The main uncertainty in these results is the total radiogenic abundances on Mars. Comparing our results with the observed shape of the crustal dichotomy provides no evidence that this slope is primarily the result of lower crustal flow. Both Hellas and the dichotomy are isostatically compensated; if the mechanism is Airy isostasy, then the lower bound on mean crustal thickness is ∼30 km. Crustal thicknesses of 30–100 km on Mars can be produced by mid‐ocean ridge spreading at potential temperatures of 1350°–1600°C. However, for such crustal thicknesses the lithosphere is likely to be positively buoyant, making subduction difficult.
Wavelet analyses of Mars polar topography
Authors: Malamud Bruce D. , Turcotte Donald L.
Published: February 2004

We have analyzed topography tracks over the Mars northern polar cap (composed of H2O ice, CO2 ice, and sediments) and the Antarctic ice sheet on Earth using a Mexican hat wavelet transform. The great utility of the wavelet transform is that it gives both spatial and spectral resolution. We have used the variance of the wavelet output in order to quantify the spectral content of the topography tracks. At short wavelengths, for both the Mars northern polar cap and the Antarctic ice sheet, we find a power law dependence of the wavelet transform variance on wavelength, with a power law exponent of β ≈ 3.5–3.7. This compares with a power law exponent of ∼2.0 (Brownian motion), typical of topography on both Mars and Earth. There is a power law smoothing of the ice topography at short wavelengths on both planetary bodies. At long wavelengths we infer a similar power law dependence with β ≈ 1.5–2.0, typical of planetary topography. A transition is observed between the smooth short‐wavelength behavior (high β) to rougher long‐wavelength behavior (low β). This fairly sharp transition occurs at a wavelength of ∼24 km for Mars and ∼11 km for Antarctica. The transitions appear to scale with ice thicknesses and suggest a relaxation of short‐wavelength topography by ice flows or surface processes.