bluuebunny/crossref_metadata_2025 huggingface dataset:
Petrologic evidence for low‐temperature, possibly flood evaporitic origin of carbonates in the ALH84001 meteorite
Authors: Warren Paul H.
Published: February 2004

High‐temperature models for origin of the carbonates in Martian meteorite ALH84001 are implausible. The impact metasomatism model, invoking reaction between CO2rich fluid and the host orthopyroxenite, requires conversion of olivine into orthopyroxene, yet olivine in ALH84001 shows no depletion in carbonate‐rich areas; or else conversion of orthopyroxene into silica, which should have yielded a higher silica/carbonate ratio. The impact melt model implies that the fracture‐linked carbonates, as products of melt injection, should appear as continuous planar veins, but in many areas they do not. Both vapor deposition and impact melting seem inconsistent with the zoned poikilotopic texture of many large carbonates. The popular hydrothermal model is inconsistent with the virtual absence of secondary hydrated silicates in ALH84001. Prior brecciation should have facilitated alteration. Hydrothermal fluids would be warm, and rate of hydration of mafic silicates obeys an Arrhenius law, at least up to ∼100°C. Most important, hydrothermal episodes tend to last for many years. Many areas of the ancient Martian crust show evidence for massive flooding. I propose that the carbonates formed as evaporite deposits from floodwaters that percolated through the fractures of ALH84001, but only briefly, as evaporation and groundwater flow caused the water table to quickly recede beneath the level of this rock during the later stages of the flood episode. The setting might have been a layer of megaregolith beneath a surface catchment of pooled floodwater, analogous to a playa lake. Carbonate precipitation would occur in response to evaporative concentration of the water. To explain the scarcity of sulfates in ALH84001, the water table must be assumed to recede quickly relative to the rate of evaporation. During the period when ALH84001 was above the water table, evaporation would have slowed, as the evaporation front passed beneath the surface of the debris layer, and possibly earlier, if the shrinking pool of surface water developed a porous sulfate crust. Alternatively, ALH84001 may have developed as a Martian form of calcrete, i.e., the evaporating flood(s) may have been entirely below ground as it (they) passed slowly through ALH84001. The greatest advantage of the flood evaporite model is that it exposes ALH84001 to carbonate precipitation without prolonged exposure to aqueous alteration. The model also seems consistent with the heavy and extremely heterogeneous oxygen isotopic compositions of the carbonates. However, this hypothesis seems no more than marginally consistent with the suggestion ofMcKay et al.[1996] that the carbonates are biogenic.
Antarctic paleolake sediments and the search for extinct life on Mars
Authors: Doran Peter T. , Wharton Robert A. , Des Marais David J. , McKay Christopher P.
Published: February 2004

Evidence of lakes in Mars history is substantial. The proposed similarities between the ancient Martian environment and certain modern environments on Earth have led exobiologists to study antarctic lakes as analogs to those purported to have existed on Mars. We have investigated modern sedimentation processes (especially with respect to δ13C of carbonate and organic matter) in lakes in the McMurdo Dry Valleys region of east Antarctica and assessed various paleolake deposits with respect to their utility as Martian analogs and targets for future Mars exobiology missions. Three main types of paleolake deposit were identified and assessed: strand lines, perched deltas, and lacustrine sand mounds. Deltas are usually identified as good targets, but our research shows that authigenic carbonates are not readily identifiable in the sediments. Large deltas, although most likely to attract attention through remote sensing, generally are difficult sites for discovery of paleobiological matter, and δ13C signals follow no coherent pattern. Lacustrine sand mounds, on the other hand, contain abundant authigenic carbonate and freeze‐dried organic matter and appear to be excellent records of paleolimnological conditions. The advantage of studying lake bottom deposits versus lake edge deposits is retrieval of a stable lake‐wide signal. Deltas are therefore most useful in that they are generally large‐scale features capable of drawing attention to a region of potential for the discovery of smaller lacustrine sand mounds.
The biological potential of Mars, the early Earth, and Europa
Authors: Jakosky Bruce M. , Shock Everett L.
Published: February 2004

The potential biomass that could have existed on Mars is constrained by the total amount of energy available to construct it. From an inventory of the available geochemical sources of energy, we estimate that from the time of the onset of the visible geologic record 4 b.y. ago to the present, as much as 20 g cm−2of biota could have been created. This is the same amount that could have been constructed on the early Earth in only 100 million years. This indicates that there likely was sufficient energy available to support an origin of life on Mars but not sufficient energy to create a ubiquitous and lush biosphere. Similar calculations for Europa suggest that even less geochemical energy would have been available there.
Nonlinear spectral mixture modeling of lunar multispectral data: Implications for lateral transport
Authors: Mustard John F. , Li Lin , He Guoqi
Published: February 2004

Linear and nonlinear spectral mixture models applied to Clementine multispectral images of the Moon result in roughly similar spatial distributions of end‐member abundances. However, there are important differences in the absolute values of the predicted abundances. The magnitude of these differences and the implications for understanding geological processes are investigated across a geologic contact between mare and highland in the Grimaldi Basin on the western nearside of the Moon. Vertical and lateral mass transport due to impact cratering has redistributed mare and highland materials across the contract, creating a gradient in composition. Solutions to linear and nonlinear spectral mixture models for identical spectral end‐members of mature mare, mature highland, and fresh crater materials are compared across this simple geologic contact in the Grimaldi Basin. Formal estimates of uncertainty in the calculated fractions of the end‐members are ≈1%. Profiles of mare abundance across the contact are extracted and compared quantitatively. Profiles from the linear mixture models indicate that the geologic contact has an average mare abundance of 60% and the compositional boundary is asymmetric with more mare transported onto the highland side of the contact than highland onto the mare side of the contact. In contrast, the nonlinear abundance profiles indicate that the geologic contact has an average mare abundance of 50% and the compositional boundary is remarkably symmetric. Given the expectation that materials will be intimately mixed on the surface of the Moon and that the asymmetries implied by the linear model are not consistent with our understanding of lunar surface processes, the non‐linear spectral mixture model is preferred and should be applied whenever quantitative abundance information is required. The remarkable symmetry in the compositional gradients across this contact indicates that lateral mass transport dominates over vertical transport at this boundary.
Closed drainage crater basins of the Martian highlands: Constraints on the early Martian hydrologic cycle
Authors: Forsythe Randall D. , Blackwelder Clyde R.
Published: February 2004

One hundred forty four crater basins with closed drainage have been mapped within the Martian Noachian highlands that have influent but no outflow channels. The influent streambeds to these basins are characterized by steep gradients and elevation head losses on average of 1 km. These closed basins and influent fluvial channels provide a foundation to investigate the processes under which water was transferred among the early Martian atmosphere and groundwater reservoirs. Channel gradients reflect basin‐controlled water table conditions, and the channel head losses suggest sustained drawdowns within the basins due to evaporation and/or sublimation‐induced pumping of groundwater. Finite element modeling of groundwater flow permits estimation of minimum rates for evaporation/sublimation for a reasonable set of boundary conditions and hydraulic conductivities, and suggests a Noachian environment with a minimum of 10–20 cm/yr of evaporation and/or sublimation. The flow models are unable to be reconciled with channel gradients without introduction of local precipitation. Acceptable solutions are found with a minimum of 1 ‐2 cm/yr of groundwater recharge.
Lobateness of impact ejecta deposits from atmospheric interactions
Authors: Barnouin‐Jha Olivier S. , Schultz Peter H.
Published: February 2004

As an ejecta curtain advances through an atmosphere, it creates a vortex ring. By analogy with smoke rings, the curtain‐driven vortex ring develops instabilities that result in waves. The number of these waves depends upon the aspect ratio of the vortex ring (i.e., the ratio of the core vortex radius to the vortex radius) and the Reynolds number (or strength) of the flow in the vortex ring. In laboratory experiments the number of sinuous features at the edges of contiguous ejecta ramparts is consistent with the theoretical expectations for the origin of waves created in a curtain‐driven vortex ring. Observing the formation of these sinuous features provides direct evidence that they indeed result from instabilities in the curtain‐driven vortex ring. Scaling relations for curtain velocity, curtain size, and time of crater formation permit testing whether or not such instabilities explain the lobateness or sinuosity of distal ejecta faciès at broad scales on planets with atmospheres. Scaling relationships predict that the number of flow lobes observed for craters on both Venus and Mars should increase with increasing transient crater radius to the three‐fourths power, a prediction that is consistent with observation. Consequently, the curtain‐driven vortex may play an important role in controlling the morphology of ejecta on planets with atmospheres. Variations in the number of flow lobes for a given crater size probably reflect different impact conditions either in target properties (grain size, volatile content) and/or ambient atmospheric conditions.
Atmospheric loss since the onset of the Martian geologic record: Combined role of impact erosion and sputtering
Authors: Brain David A. , Jakosky Bruce M.
Published: February 2004

Chemical and geomorphic evidence suggests that Mars' atmosphere has undergone significant loss or modification since the onset of the Martian geologic record. Analyses of proposed loss processes have been unable to individually account for loss of enough atmosphere to have supported the presence of liquid surface water at the formation time of the oldest observed surface units. Here, we give a simple calculation of the combined effects of major atmospheric loss processes. Considering only impact erosion and using results from an analytical model byMelosh and Vickery[1989], we obtain an expression for surface pressure as a function of local crater density. Based on tabulated crater densities, the process of atmospheric cratering can account for a loss of 50–90% of the Martian atmosphere since the onset of the geologic record. Stable isotope fractionation measurements suggest that loss of ∼90% of atmospheric species to space has occurred via solar wind pick‐up‐ion sputtering [Jakosky and Jones, 1997]. Combined, ∼95–99% of Mars' atmosphere could have been lost to space. Adsorption of CO2in the regolith and sequestration in the poles as ice or clathrate could account for most of the remaining loss. These calculations suggest that the loss processes of impact erosion, sputtering, and sequestration are together capable of explaining the inferred transition from an early atmosphere on Mars capable of supporting stable liquid surface water to today's climate.
Mapping the Martian polar ice caps: Applications of terrestrial optical remote sensing methods
Authors: Nolin Anne W.
Published: February 2004

With improvements in both instrumentation and algorithms, methods for mapping terrestrial snow cover using optical remote sensing data have progressed significantly over the past decade. Multispectral data can now be used to determine not only the presence or absence of snow but the fraction of snow cover in a pixel. Radiative transfer models have been used to quantify the nonlinear relationship between surface reflectance and grain size thereby providing the basis for mapping snow grain size from surface reflectance images. Model‐derived characterization of the bidirectional reflectance distribution function provides the means for converting measured bidirectional reflectance to directional‐hemispherical albedo. In recent work, this approach has allowed climatologists to examine the large scale seasonal variability of albedo on the Greenland ice sheet. This seasonal albedo variability results from increases in snow grain size and exposure of the underlying ice cap as the seasonal snow cover ablates away. With the current Mars Global Surveyor and future missions to Mars, it will soon be possible to apply some of these terrestrial mapping methods to learn more about Martian ice properties, extent, and variability. Distinct differences exist between Mars and Earth ice mapping conditions, including surface temperature, ice type, ice‐mineral mixtures, and atmospheric properties, so a direct application of terrestrial snow and ice mapping methods may not be possible. However, expertise in mapping and interpreting terrestrial snow and ice will contribute to the inventory of techniques for mapping planetary ices. Furthermore, adaptation of terrestrial methods will provide a basis for comparison of terrestrial and planetary cryospheric components.
Carbon dioxide crystals: An examination of their size, shape, and scattering properties at 37 GHz and comparisons with water ice (snow) measurements
Authors: Foster J. L. , Chang A. T. C. , Hall D. K. , Wergin W. P. , Erbe E. F. , Barton J.
Published: February 2004

On Earth, the temperature regime is such that water is generally fairly close to its freezing point, and thus relatively small differences in climate affect how much snow and ice are present and whether or not the snow covering will be seasonal or last from one year to the next. On Mars, as on Earth, the presence of ice also plays a role in large‐scale climate processes and it is important in controlling the abundance of atmospheric carbon dioxide (CO2) and water vapor. Passive microwave radiometry has been used to derive snow extent and snow depth on Earth, where scattering by snow (H2O) crystals is the dominant effect on the microwave radiation emanating from the ground and emerging from the snowpack. Microwave remote sensing may also prove to be useful for assessing the coverage and thickness of the frozen H2O and CO2 on Mars, but more exact information is needed on how both H2O crystals and frozen CO2 crystals scatter and absorb passive microwave radiation. In this study, CO2 crystals have been produced in a laboratory cold chamber with temperature conditions similar to those found on the polar caps of Mars, and detailed three‐dimensional images of their size and shape have been made with a low‐temperature scanning electron microscope. Unlike the much larger H2O snow crystals found on Earth, which typically range in size between 0.1 mm and 1.0 mm (radius), CO2 crystals are differently shaped and considerably smaller. Bipyramid crystals (base to base four‐sided pyramids) are commonly observed, some as small as 1.0 μm. A discrete dipole model was employed to calculate the passive microwave radiation scattered and absorbed by crystals of various sizes and shapes. Modeling results indicate that the shape of the crystal, whether for frozen CO2 or H2O, is of little consequence in affecting extinction efficiency. However, owing to their smaller size, frozen CO2 crystals are more emissive than the H2O crystals in the 37 GHz region of the microwave spectrum. For the larger sizes of the modeled crystals, scattering dominates over absorption since the particles approach the size of the wavelength. The scattering values are 2 orders of magnitude larger than absorption for the 900 μm size snow particles. For CO2 crystals of 3.0 μm in size, absorption is 7 orders of magnitude greater than scattering.
Hydrocode simulation of the Chicxulub impact event and the production of climatically active gases
Authors: Pierazzo Elisabetta , Kring David A. , Melosh H. Jay
Published: February 2004

We constructed a numerical model of the Chicxulub impact event using the Chart‐D Squared (CSQ) code coupled with the ANalytic Equation Of State (ANEOS) package. In the simulations we utilized a target stratigraphy based on borehole data and employed newly developed equations of state for the materials that are believed to play a crucial role in the impact‐related extinction hypothesis: carbonates (calcite) and evaporites (anhydrite). Simulations explored the effects of different projectile sizes (10 to 30 km in diameter) and porosity (0 to 50%). The effect of impact speed is addressed by doing simulations of asteroid impacts (vi= 20 km/s) and comet impacts (vi= 50 km/s). The masses of climatically important species injected into the upper atmosphere by the impact increase with the energy of the impact event, ranging from 350 to 3500 Gt for CO2, from 40 to 560 Gt for S, and from 200 to 1400 Gt for water vapor. While our results are in good agreement with those ofIvanov et al.[1996], our estimated CO2production is 1 to 2 orders of magnitude lower than the results ofTakata and Ahrens[1994], indicating that the impact event enhanced the end‐Cretaceous atmospheric CO2inventory by, at most, 40%. Consequently, sulfur may have been the most important climatically active gas injected into the stratosphere. The amount of S released by the impact is several orders of magnitude higher than any known volcanic eruption and, with H2O, is high enough to produce a sudden and significant perturbation of Earth's climate.
A multicentury stable isotope record from a New Caledonia coral: Interannual and decadal sea surface temperature variability in the southwest Pacific since 1657 A.D
Authors: Quinn Terrence M. , Crowley Thomas J. , Taylor Frederick W. , Henin Christian , Joannot Pascale , Join Yvan
Published: February 2004

A 335 year stable isotope record from a New Caledonia coral (22°S, 166°E) helps fill a large gap in historical climate reconstructions. Although the long‐term coral δ18O‐based sea surface temperature (SST) trend is one of warming, there are notable decadal fluctuations, especially in the early 18th and early 19th centuries. Mean annual SSTs between 1658 and 1900 are estimated to be ∼0.3°C lower than the 20th century average, with interdecadal excursions of 0.5°–0.8°C. Time series analyses of the coral isotope record reveals significant concentrations of variance in the El Niño band; an inderdecadal spectral peak is present, but its robustness requires additional statistical evaluation. A secular but irregular decrease in coral δ13C values begins in the mid‐1800s and may reflect the anthropogenic perturbation of the carbon reservoir. These and other results indicate that the New Caledonia coral isotope record is a valuable source of information on southwest Pacific climate history.
Direct correlation of millennial‐scale changes in western North American vegetation and climate with changes in the California Current System over the past ∼60 kyr
Authors: Heusser Linda
Published: February 2004

Terrestrial (pollen) and marine climate proxies from seven sediment cores taken on the continental margin of western North America between 32° and 43°N show systematically related short‐term changes superposed on Milankovitch‐scale oscillations of the past 60 kyr. During oxygen isotope stage 3, brief warming events in California and Oregon appear correlative with interstadial events in waters offshore, as do the rapid climate oscillations of the last glacial transition (∼14–∼10 ka). Estimated temperature depression of the coastal lowlands ranges from >10°C in southern California to ∼5°C in Oregon during the last glacial. The absence of long‐distance migration of Pacific coastal plant assemblages concurs with marine evidence that southward movement of the polar front during the last glacial maximum was limited. Correlative changes in terrestrial and marine ecosystems imply that broad reorganizations of circulation in the northeast Pacific Ocean were synchronous with reorganizations of atmospheric circulation over western North America.
Glacial‐age hydrography of the far northwest Pacific Ocean
Authors: Keigwin Lloyd D.
Published: February 2004

Stable isotope data on benthic foraminifera from more than 30 cores on the northern Emperor Seamounts and in the Okhotsk Sea are synthesized in paleohydrographic profiles spanning the depth range 1000–4000 m. Holocene (core‐top) benthic foraminiferal δ18O and δ13C data are calibrated to modern hydrographic properties through measurements of δ13C of ΣCO2 and δ18O of seawater. Cibicidoides stable isotope ratios are close to the δ13C and equilibrium δ18O of seawater, whereas Uvigerina δ18O and δ13C are variably offset from Cibicidoides. Glacial maximum δ13C of Cibicidoides displays a different vertical profile than that of the Holocene. When results are adjusted by +0.32‰ to account for the secular change in δ13C during the last glacial maximum, the data coincide with the modern seawater and foraminiferal curves deeper than ∼2 km. However, at shallower depths δ13C gradually increases by as much as 1‰ above the modern value. Furthermore, above 2 km the benthic δ18O decreases by ∼0.5‰. These results are consistent with a benthic front at ∼2 km in the North Pacific [see Herguera et al., 1992], but they differ from interpretations based on trace metal data which indicate a source of nutrient‐depleted deep water during glaciation. The isotopic data suggest that during glaciation there was a better ventilated watermass at intermediate depths in the far northwestern Pacific, it was relatively fresher than deep waters there, and deep waters were as nutrient‐rich as today.
Organic‐matter production and preservation and evolution of anoxia in the Holocene Black Sea
Authors: Arthur Michael A. , Dean Walter E.
Published: February 2004

Dating of sediments collected in gravity cores during Leg 1 of the 1988 R/V Knorr expedition to the Black Sea suggests that the onset of water‐column anoxia at ∼7.5 ka was virtually synchronous across the basin over a depth range of ∼200–2250 m. A finely laminated, organic carbon (OC) rich sapropel (unit II) was produced as a result of this anoxia. The trigger for increased OC production and development of anoxia was the spillover of saline waters through the Bosporus that probably began at ∼9.0 ka and peaked between ∼7.0 and 5.5 ka. This spillover enhanced vertical mixing and nutrient cycling and caused a short‐term (2–3 kyr) burst in surface‐water productivity during the early part of unit II deposition. Continued incursion of saline waters enhanced vertical stability and inhibited mixing of nutrients into surface waters, thus limiting primary production and decreasing the OC flux to sediments beginning ∼5.5 ka. Concentration, accumulation rate, and degree of preservation of organic matter all decreased in the upper part of unit II as a result of decreasing productivity, but anoxia persisted throughout most of the water column. The end of unit II sapropel deposition was synchronous across the Black Sea as the result of the first blooms of the coccolith Emiliania huxleyi, which presumably marked an increase in surface‐water salinity above 11 and the beginning of unit I deposition. The high coccolith‐carbonate fluxes that occurred during deposition of unit I diluted the OC concentration in the sediments, but OC accumulation rates are about the same as those in upper part of unit II.
New aspects of organic carbon deposition and its paleoceanographic implications along the Northern Barents Sea Margin during the last 30,000 years
Authors: Knies Jochen , Stein Ruediger
Published: February 2004

We studied variations in terrigenous (TOM) and marine organic matter (MOM) input in a sediment core on the northern Barents Sea margin over the last 30 ka. Using a multiproxy approach, we reconstructed processes controlling organic carbon deposition and investigated their paleoceanographic significance in the North Atlantic‐Arctic Gateways. Variations in paleo‐surface‐water productivity are not documented in amount and composition of organic carbon. The highest level of MOM was deposited during 25–23 ka as a result of scavenging on fine‐grained, reworked, and TOM‐rich material released by the retreating Svalbard/Barents Sea ice sheet during the late Weichselian. A second peak of MOM is preserved because of sorptive protection by detrital and terrigenous organic matter, higher surface‐water productivity due to permanent intrusion of Atlantic water, and high suspension load release by melting sea ice during 15.9–11.2 ka.
Glacial‐interglacial variability in upwelling and bioproductivity off NW Mexico: Implications for Quaternary paleoclimate
Authors: Ganeshram Raja S. , Pedersen Thomas F.
Published: February 2004

Sedimentary accumulation of biogenic components (organic carbon, opal, and biogenic barium) on the northwestern Mexican margin declined during every glacial interval of the past 140 kyr, indicating decreases in upwelling‐induced productivity during cold periods. The glacial‐interglacial contrasts in upwelling on this margin are attributed to reversals in land‐ocean thermal contrast, the waxing and waning of the Laurentide Ice Sheet, and consequent responses of the western hemisphere wind fields. This scenario is consistent with three independent lines of evidence: terrestrial paleoclimatic data, general circulation model results, and our marine records. This pattern of glacial‐interglacial variability in upwelling off NW Mexico is opposite to that observed in other low‐latitude and midlatitude upwelling areas, such as the eastern equatorial Pacific. These results add to a growing pool of observations that the response of oceanic upwelling to glacial climatic forcing has been regionally variable.
Propagation of electromagnetic fields in the coastal ocean with applications to underwater navigation and communication
Authors: Tyler Robert H. , Sanford Thomas B. , Unsworth Martyn J.
Published: February 2004

We examine the propagation of low‐frequency electromagnetic (EM) waves in the coastal ocean produced by controlled or motional impressed sources. Four important modes are the direct, up‐over‐down, down‐over‐up, and “beach” modes. The analyses of these modes are complicated by the varying bathymetry in the coastal region. We derive criteria to determine (1) which modes are important for given parameters; (2) a “matched phase” condition describing both when the up‐over‐down and down‐over‐up modes interfere constructively in the shallow zone and when the beach mode becomes important; and (3) a low‐frequency cutoff, below which the EM fields are not sensitive to the details of the coastal geometry. We verify the theoretically derived criteria with numerical examples and finally discuss the importance of our results in designing navigation and communications applications for subsurface vehicles and instruments.
Study of equatorial clutter using observed and simulated long‐range backscatter ionograms
Authors: Dandekar B. S. , Sales G. S. , Weijers B. , Reynolds D.
Published: February 2004

This synoptic study of equatorial clutter used over 6200 long‐range (8000 nautical miles, or 14,816 km) backscatter ionograms from the east coast over‐the‐horizon backscatter (OTH‐B) radar system at Bangor, Maine, obtained from 1100 to 0500 LT during the 22‐month period from October 1991 to July 1993. Data were collected in five 7.5°‐wide azimuthal sectors (radar beams 1‐6,1‐8, 2‐4, 2‐8, and 3‐5) from 58°T to 170°T. The backscatter clutter signatures can be identified with an equatorial clutter region covering a dip latitude range of ±40°. Equatorial clutter typically begins after sunset and continues through midnight, with a minimum of activity between 1700 and 2200 LT. The clutter range shows some dependence on solar activity, with a reduction in range from the radar from high to moderate solar activity periods. These data did not show any dependence of clutter on operating frequency or on magnetic activity. Synthetic backscatter ionograms were generated using the parameterized ionospheric model (PIM) for the ray trace technique. The analysis shows that the equatorial dome of the ionosphere favors the chordal mode of reflection from the south side (the farther side from the radar) of the dome, which produces horizontal traces as clutter signatures on the backscatter ionograms. The reduction in range at moderate solar activity is associated with the lowering of the mean height of the ionospheric dome. In beams 2‐8 and 3‐5 the ionospheric dome structure is associated with the equatorial (Appleton) anomaly. This dome structure geometry favors the chordal mode of reflection.
Coherent radar imaging using Capon's method
Authors: Palmer Robert D. , Gopalam Sridhar , Yu Tian‐You , Fukao Shoichiro
Published: February 2004

A linearly constrained mathematical formulation is provided for the problem of coherent radar imaging. In contrast to studies of field‐aligned irregularities in the ionosphere, where the technique has previously been applied, lower atmospheric imaging is complicated by the fact that the scattering structures are not aligned along any single baseline. As a result, a two‐dimensional generalization of the brightness distribution was required. It is shown that Fourier‐based imaging is a special case of this general formulation. Furthermore, an imaging technique based on constrained optimization is introduced and shown to exhibit higher resolution and resistance to interfering signals. These techniques were applied to data from the middle and upper atmosphere radar in Shigaraki, Japan. The experiment was conducted during the Baiu season, which is characterized by significant precipitation events.
Radio wave phase scintillation and precision Doppler tracking of spacecraft
Authors: Armstrong J. W.
Published: February 2004

Phase scintillation caused by propagation through solar wind, ionospheric, and tropospheric irregularities is a noise process for many spacecraft radio science experiments. In precision Doppler tracking observations, scintillation can be the dominant noise process. Scintillation statistics are necessary for experiment planning and in design of signal processing procedures. Here high‐precision tracking data taken with operational spacecraft (Mars Observer, Galileo, and Mars Global Surveyor) and ground systems are used to produce temporal statistics of tropospheric and plasma phase scintillation. The variance of Doppler frequency fluctuations is approximately decomposed into two propagation processes. The first, associated with distributed scattering along the sight line in the solar wind, has a smooth spectrum. The second, associated principally with localized tropospheric scattering for X‐band experiments, has a marked autocorrelation peak at the two‐way light time between the Earth and the spacecraft (thus a cosine‐squared modulation of the fluctuation power spectrum). For X‐band data taken in the antisolar hemisphere the average noise levels of this process are in good agreement with average tropospheric noise levels determined independently from water vapor radiometer observations and radio interferometic data. The variance of the process having a smooth spectrum is consistent with plasma noise levels determined independently from dualfrequency observations of the Viking spacecraft made at comparable Sun‐Earth‐spacecraft angles. The observations reported here are used to refine the propagation noise model for Doppler tracking of deep space probes. In particular, they can be used to predict propagation noise levels for high‐precision X‐ and Ka‐band tracking observations (e.g., atmosphere/ionosphere/ring occultations, celestial mechanics experiments, and gravitational wave experiments) to be done using the Cassini spacecraft.
Ionospheric D region remote sensing using VLF radio atmospherics
Authors: Cummer S. A. , Inan U. S. , Bell T. F.
Published: February 2004

Lightning discharges radiate the bulk of their electromagnetic energy in the very low frequency (VLF, 3–30 kHz) and extremely low frequency (ELF, 3–3000 Hz) bands. This energy, contained in impulse‐like signals called radio atmospherics or sferics, is guided for long distances by multiple reflections from the ground and lower ionosphere. This suggests that observed sferic waveforms radiated from lightning and received at long distances (>1000 km) from the source stroke contain information about the state of the ionosphere along the propagation path. The focus of this work is on the extraction of nighttime D region electron densities (in the altitude range of ∼70–95 km) from observed VLF sferics. In order to accurately interpret observed sferic characteristics, we develop a model of sferic propagation which is based on an existing frequency domain subionospheric VLF propagation code. The model shows that the spectral characteristics of VLF sferics depend primarily on the propagation path averaged ionospheric D region electron density profile, covering the range of electron densities from ∼100 to 103 cm−3. To infer the D region density from observed VLF sferics, we find the electron density profile that produces a modeled sferic spectrum that most closely matches an observed sferic spectrum. In most nighttime cases the quality of the agreement and the uncertainties involved allow the height of an exponentially varying electron density profile to be inferred with a precision of ∼0.2 km.
The magnetopause position as an indicator of the ionization level in the dayside high‐latitude ionosphere
Authors: Makarova L. N. , Shirochkov A. V.
Published: February 2004

Experimental evidence of the strong connection between the near‐noon level of ionization in the high‐latitude ionosphere, expressed as the ƒoF2 values, and the subsolar distance between the magnetopause and the Earth is presented here. Preliminary studies made by the authors of the solar wind dynamic pressure influence on the high‐latitude ionosphere produced rather promising results. Since the solar wind dynamic pressure is an external force determining a subsolar distance between the magnetopause and the Earth, the former parameter was chosen as an indicator of the electromagnetic energy accumulated in the “magnetosphere‐ionosphere” system. Very high correlation (r > 0.85) has been found between the values of the near‐noon electron density derived from the data of the high‐latitude ionospheric stations located in different geophysical regions of both hemispheres and the magnetopause position. The periods when the magnetopause moved close to the Earth coincided with the dramatic drop of the near‐noon ionization level at the high‐latitude stations. However, only preliminary suggestions concerning the nature of this phenomenon can be made at the moment.
Constraints on the polarization purity of a Stokes microwave radiometer
Authors: Ruf Christopher S.
Published: February 2004

Measurement of the third and fourth Stokes parameters of microwave thermal emission can be degraded by nonideal radiometer characteristics. Of particular importance is the level of polarization purity and the knowledge of polarization impurity on the part of the instrument. An explicit relationship is developed between the true Stokes parameters and the measurements, as functions of nonideal polarization characteristics. In principle, knowledge of the characteristics allows the degradation to be removed by a postprocessing calibration procedure. However, errors in the knowledge are possible, resulting from inexact initial testing and/or subsequent drifting of the instrument. The effects of these errors on overall calibration are considered. It is found that there is considerably greater tolerance to the errors when incoherent detection is used (differencing ±45° linear and left‐ or right‐hand circular polarizations), as opposed to coherent detection (direct cross correlation of vertical and horizontal linear polarizations). This is particularly true when cross‐polar leakage is imbalanced. The level of nominal polarization purity that is required is also more relaxed in the incoherent case. On the other hand, coherent detection has the capability for superior performance with regard to precision.
More than one way to stretch: a tectonic model for extension along the plume track of the Yellowstone hotspot and adjacent Basin and Range Province
Authors: Parsons Tom , Thompson George A. , Smith Richard P.
Published: February 2004

The eastern Snake River Plain of southern Idaho poses a paradoxical problem because it is nearly aseismic and unfaulted although it appears to be actively extending in a SW‐NE direction continuously with the adjacent block‐faulted Basin and Range Province. The plain represents the 100‐km‐wide track of the Yellowstone hotspot during the last ∼16–17 m.y., and its crust has been heavily intruded by mafic magma, some of which has erupted to the surface as extensive basalt flows. Outside the plain's distinct topographic boundaries is a transition zone 30–100 km wide that has variable expression of normal faulting and magmatic activity as compared with the surrounding Basin and Range Province. Many models for the evolution of the Snake River Plain have as an integral component the suggestion that the crust of the plain became strong enough through basaltic intrusion to resist extensional deformation. However, both the boundaries of the plain and its transition zone lack any evidence of zones of strike slip or other accommodation that would allow the plain to remain intact while the Basin and Range Province extended around it; instead, the plain is coupled to its surroundings and extending with them. We estimate strain rates for the northern Basin and Range Province from various lines of evidence and show that these strains would far exceed the elastic limit of any rocks coupled to the Basin and Range; thus, if the plain is extending along with its surroundings, as the geologic evidence indicates, it must be doing so by a nearly aseismic process. Evidence of the process is provided by volcanic rift zones, indicators of subsurface dikes, which trend across the plain perpendicular to its axis. We suggest that variable magmatic strain accommodation, by emplacement and inflation of dikes perpendicular to the least principal stress in the elastic crust, allows the crust of the plain to extend nearly aseismically. Dike injection releases accumulated elastic strain but generates only the small earthquakes associated with dike propagation. The rate of dike emplacement required to accommodate the estimated longitudinal strain rate of the plain is roughly a composite width of 10 m every 1000 years for the geologically youngest and most active part of the plain. The locus of most rapid intrusion and strain has migrated toward Yellowstone and is now in the northeastern 100–150 km of the plain. Reduced magmatic input in the transition zone of the plain causes the transitional expression of seismicity and faulting there.
Crustal deformation of the Lhasa terrane, Tibet plateau from Project INDEPTH deep seismic reflection profiles
Authors: Alsdorf Douglas , Brown Larry , Nelson K. Douglas , Makovsky Yizhaq , Klemperer Simon , Zhao Wenjin
Published: February 2004

International Deep Profiling of Tibet and the Himalaya (INDEPTH) deep reflection data in the Yangbajain‐Damxung graben of southern Tibet yield evidence of magmatism and deformation beneath the southern Lhasa terrane and Yarlung‐Zangbo suture. Shallow reflections and low‐velocity first arrivals indicate a thin, Quaternary graben fill of generally less than a few hundred meters thickness. Underlying stratified reflections, extending to a maximum of about 11.5 km depth, likely originate from deformed Paleozoic‐Cenozoic supracrustal strata of the Lhasa terrane. A prominent, undulatory band of reflections within the crystalline basement extends beneath the length of the Yangbajain‐Damxung graben (depth ranges from ∼12 km to ∼18 km). This horizon has been interpreted to mark the top of a midcrustal partial‐melt zone underlying southern Tibet. The undulatory character of this horizon additionally suggests that it may be tectonically deformed. A somewhat deeper, subhorizontal, wide‐angle reflection extends southward beneath the outcrop of the Yarlung‐Zangbo suture, indicating that the suture is cut off or superposed by a younger structure at depth. Three gently north dipping reflections at 19 km, 24 km, and 27 km depth beneath the Gangdese batholith are suggestive of a midcrustal duplex and may mark the northward (downdip) extension of the late Oligocene‐early Miocene Gangdese thrust system. A prominent ∼40° north dipping reflection imaged in the deep crust beneath the Gangdese batholith, between 40 and 60 km depth, might mark the downdip expression of the Yarlung‐Zangbo suture or, alternatively, a younger reverse fault in the lower crust (or both). Viewed in aggregate, the reflection data are suggestive of moderate, postcollisional shortening of the upper crust of the Lhasa terrane, accompanied by melting of the middle crust. Although the data are permissive of wholesale underthrusting or fluid injection of Indian continental crust beneath the Lhasa terrane, they show no direct evidence for this having occurred.
Foreland normal fault control on northwest Himalayan thrust front development
Authors: Blisniuk Peter M. , Sonder Leslie J. , Lillie Robert J.
Published: February 2004

In the Trans‐Indus Ranges along the western part of the northwest Himalayan thrust front, unconformities, changes in paleocurrent directions, and locally derived conglomerates in synorogenic foreland basin deposits provide evidence for major local deformation at ≥3.5 Ma. The tectonic history of the Trans‐Indus Ranges has previously been described in terms of a single episode of major thrusting at ≤1 Ma, thus our work implies that there were two distinct phases of deformation. In conjunction with published evidence in the Salt Range to the east for two phases of deformation (∼6 to 5 Ma, and ∼2.5 Ma to present), this study demonstrates that these two phases of deformation are regionally significant and probably correlative along the entire present‐day NW Himalayan thrust front. Reconstruction of possible source areas for the locally derived conglomerates shows that the earlier deformation is probably related to normal faulting. These results suggest that the tectonic evolution of the area along the present‐day thrust front is characterized by (1) latest Miocene to early Pliocene formation of north dipping normal fault zones (total throw ≥ 600 m) within the foreland basin, related to syn‐orogenic flexure of the Indian plate, and (2) late Pliocene to early Pleistocene initiation of south directed thrusting along the present‐day thrust front, related to outward growth of the NW Himalayan thrust wedge. The location of the present‐day thrust front appears to be controlled by north dipping normal faults and monoclines that formed during the earlier deformation and subsequently localized structural ramps during later thrusting.
Tectonically versus climatically driven Cenozoic exhumation of the Eurasian plate margin, Svalbard: Fission track analyses
Authors: Blythe Ann E. , Kleinspehn Karen L.
Published: February 2004

Apatite and zircon fission track (FT) thermochronology, as well as regional geologic data, are used to assess the tectonic and climatic mechanisms responsible for exhumation of Svalbard during the last 70 million years. Four stratigraphic profiles in the Paleogene Central basin spanning ∼0.6–1.1 km vertical relief yield apatite FT ages ranging from 28.3 ± 2.3 Ma to 41.8 ± 3.7 Ma, with a mean age of ∼36 Ma and no apparent vertical trend. Apatite ages from the Paleogene Forlandsundet and Kongsfjorden basins and the Late Cretaceous‐Tertiary Svalbard Orogen range from 26.8 ± 3.7 to 55.7 ± 4.7 Ma, with Kongsfjorden data suggesting cooling earlier than the Central basin. Although 10 detrital zircon samples were analyzed from both the Forlandsundet and Central basins, only one sample (from the basal Paleogene deposits in the northwesternmost Forlandsundet basin) had a reset age of 42.4 ± 1.8 Ma. Nonreset detrital zircons elsewhere retain Caledonian and Proterozoic detrital ages. Thermal‐history models derived from apatite track‐length distributions of six Central and Kongsfjorden basin samples indicate a strikingly consistent five‐phase thermal history. Initial uplift of sediment sources to the north and west is recorded as a 70–50 Ma cooling signature. Following deposition from 63 to 49 Ma, the samples were heated until ∼35 Ma, probably as the result of continued sediment burial. This basin filling coincided with a dextral continental transform connecting spreading ridges in the Arctic and Norwegian‐Greenland oceanic basins. The thermal models are consistent with ∼40°–50°C of cooling from ∼35 to 25 Ma. This cooling, which is consistent with deposition patterns both onshore and offshore, appears to be the result of rift‐related uplift and erosion as the Atlantic spreading ridge propagated northward to separate Greenland from Svalbard. Track‐length models are consistent with thermal stasis from ∼25 Ma to 5 Ma, despite sedimentation in offshore Miocene basins suggesting that continued erosion was occurring. We suggest that an increase in the geothermal gradient (by the “Yermak Hot Spot” and associated onshore volcanism) offset cooling expected at this time as the result of continued erosion. A final rapid post‐5 Ma cooling phase of ∼70°C is attributed to ∼2.1 km of erosion in response to intensified glacial denudation. Pliocene‐Holocene cooling, which occurred despite a persistently elevated geothermal gradient, is consistent with offshore evidence for first ice cover in the Norwegian‐Greenland Sea at 5.5 Ma and a substantially increased sediment flux to continental‐slope basins from 5.0 to 0.44 Ma.