bluuebunny/crossref_metadata_2025 huggingface dataset:
A comparison of time domain electromagnetic and surface nuclear magnetic resonance sounding for subsurface water on Mars
Authors: Grimm Robert E.
Published: May 2003

The time domain electromagnetic (TDEM) method has enjoyed wide success in terrestrial groundwater exploration, and the contrast in electrical conductivity between dry overburden and groundwater containing even a small amount of dissolved solids on Mars will yield a robust response. However, moist clays or even ores (e.g., massive hematite) will also be electrically conductive and could be mistaken for aquifers on Mars if proper geologic context is lacking. Surface nuclear magnetic resonance (SNMR) is the only noninvasive geophysical method that responds nearly uniquely to water. As the measured EMF is proportional to the proton‐precession frequency, which in turn is proportional to the planet's static magnetic field, SNMR signals are comparatively weak. Using small systems of several kilograms and several watts, the exploration depth of SNMR is one to two orders of magnitude smaller than TDEM: the latter can detect water to depths up to a few kilometers, whereas the former is limited to depths of a few tens of meters. There is no improvement in SNMR signal‐to‐noise with increasing static field where penetration is controlled by aquifer salinity (skin depth). As reasonable integration times cannot substantially increase the exploration depth, much larger transmitter current or loop mass (either requiring system masses of tens to hundreds of kilograms) are the only way to implement SNMR for exploration to depths of at least hundreds of meters. In spite of some ambiguity in target identification, TDEM is recommended for the first generation of in situ active‐source EM measurements for groundwater on Mars.
Self‐affine behavior of Martian topography at kilometer scale from Mars Orbiter Laser Altimeter data
Authors: Orosei R. , Bianchi R. , Coradini A. , Espinasse S. , Federico C. , Ferriccioni A. , Gavrishin A. I.
Published: May 2003

We used Mars Orbiter Laser Altimeter (MOLA) data from the current Mars Global Surveyor mission to characterize the topographic roughness of the Martian surface and to provide a mean to evaluate clutter at long (10–300 m) wavelengths. Such wavelengths are relevant for the MARSIS and SHARAD subsurface radars, which will be flown in future missions to Mars. The method of analysis is based on the assumption that topography can be described as a self‐affine fractal: 30‐km‐long segments of MOLA altimetry profiles have thus been reduced to the topographic parameters RMS height, RMS slope, and Hurst exponent, this last related to the fractal dimension, which can be used as inputs to a near‐nadir radar scattering model. The values of the Hurst exponent are greater than 0.5 for most of the surface, meaning a scaling behavior which is almost self‐similar. Maps of RMS height and Hurst exponent show that these two parameters have very different spatial distributions: whereas the RMS height is patterned after the north‐south dichotomy, the Hurst exponent follows a latitudinal trend. We make use of a multivariate method called G‐mode to classify profiles in the three‐dimensional parameter space: we find several roughness units, some of which have a strong correlation with certain geologic units, while others are bound by latitude. However, at scales between 300 m and 3 km, large stretches of the surface of Mars share common statistical properties of topography, independent from the north‐south dichotomy.
Detection of near‐surface ice on Mars with electromagnetic techniques on board future surface vehicles
Authors: Hamelin Michel , Trautner Roland , Grard Réjean
Published: May 2003

After reviewing our knowledge about the presence of water on Mars and the potential advances that can be expected from the projects under development, we estimate the mission profile and potential targets of a future long‐range surface vehicle. Liquid water can be detected by electromagnetic techniques through its high dielectric constant and electrical conductivity, and cold ice can be identified without ambiguity through the variations of its permittivity versus frequency at a fixed temperature. We assume a superficial layer of 1–2 m of dry, nonconductive, regolith covering a half‐space mixture containing 20% of ice as a model of the subsurface at middle to moderately high latitudes. We show that a linear Wenner mutual impedance array of ∼6 m, trailed behind a vehicle should detect the presence of ice at a commensurate depth. We find that the optimal frequency range is centered around 2–20 Hz at ∼190 K. We discuss the synergy of these measurements with those of the ground‐penetrating radar technique, which can reach larger depths. We also simulate the performance of an array accommodated on a drill or a penetrator.
Hypervelocity impact cratering on water ice targets at temperatures ranging from 100 K to 253 K
Authors: Grey I. D. S. , Burchell M. J.
Published: May 2003

Surface temperatures on the icy satellites decrease substantially with distance from the Sun. Some of the main surface features on these bodies are hypervelocity impact craters. Understanding the influence of ice temperature on impact crater morphology is thus important. Previous laboratory impact crater data on water (H2O) ice targets was mostly at relatively high temperatures (255 K) and studies at low temperatures are limited. In addition, the few low‐temperature studies have been at much lower velocities than those considered hypervelocity. They are thus not representative of impact craters on icy satellites. Accordingly data is presented here studying the dependence on target temperature for hypervelocity impact cratering on H2O ice at velocities of 5 km s−1 using spherical, 1 mm diameter aluminium projectiles. In this study, the ice target temperature ranged from 100 K to 253 K. Results for crater diameter, depths and volume are given along with ratios of these data. The results showed strong dependencies with temperature for all the above parameters. The strongest dependencies were from crater depth and volume. However, not all these properties vary in the same way with temperature. As the temperature of the ice increased, volume and depth increased, but crater diameter decreased. While the overall size scale of the laboratory studies (mm–cm dimensions) is still not appropriate to planetary scales (m–km), the general behavior as a function of target temperature has been established.
Development of the Mars microbeam Raman spectrometer (MMRS)
Authors: Wang Alian , Haskin Larry A. , Lane Arthur L. , Wdowiak Thomas J. , Squyres Steven W. , Wilson Robert J. , Hovland Larry E. , Manatt Ken S. , Raouf Nasrat , Smith Christopher D.
Published: May 2003

Raman spectroscopy is a powerful tool for mineral characterization and for detection of water and organic and inorganic forms of carbon. The Mars microbeam Raman spectrometer (MMRS) is designed for close‐up analysis of rocks and soils in planetary surface exploration. The MMRS consists of a probe (in a flight unit to be deployed by a robotic arm) and a spectrograph, laser source, and electronics (in a flight unit to reside on a rover or lander). The Raman probe has a scanning optical bench that enables a 1‐cm linear traverse across a target rock or soil, both on target materials as encountered and on fresh surfaces of rocks exposed by abrasion or coring. From these spectra, one can identify major, minor, and trace minerals, obtain their approximate relative proportions, and determine chemical features (e.g., Mg/Fe ratio) and rock textural features (e.g., mineral clusters, amygdular fill, and veins). One can also detect and identify organic species, graphitic carbon, and water‐bearing phases. Extensive performance tests have been done on a brassboard model of the MMRS using a variety of geological materials (minerals, rocks, Martian meteorites, etc.). These tests show that a Raman spectrometer can be built that is suitably miniaturized, sufficiently robust, and low enough in power usage to serve as an on‐surface planetary instrument, yet the spectrometer can retain high detection sensitivity and yield near laboratory quality spectra over a broad wavelength range. These features are essential to provide definitive mineralogy in a planetary exploration.
Resurfacing history of the northern plains of Mars based on geologic mapping of Mars Global Surveyor data
Authors: Tanaka K. L. , Skinner J. A. , Hare T. M. , Joyal T. , Wenker A.
Published: May 2003

Geologic mapping of the northern plains of Mars, based on Mars Orbiter Laser Altimeter topography and Viking and Mars Orbiter Camera images, reveals new insights into geologic processes and events in this region during the Hesperian and Amazonian Periods. We propose four successive stages of lowland resurfacing likely related to the activity of near‐surface volatiles commencing at the highland‐lowland boundary (HLB) and progressing to lower topographic levels as follows (highest elevations indicated): Stage 1, upper boundary plains, Early Hesperian, <−2.0 to −2.9 km; Stage 2, lower boundary plains and outflow channel dissection, Late Hesperian, <−2.7 to −4.0 km; Stage 3, Vastitas Borealis Formation (VBF) surface, Late Hesperian to Early Amazonian, <−3.1 to −4.1 km; and Stage 4, local chaos zones, Early Amazonian, <−3.8 to −5.0 km. At Acidalia Mensa, Stage 2 and 3 levels may be lower (<−4.4 and −4.8 km, respectively). Contractional ridges form the dominant structure in the plains and developed from near the end of the Early Hesperian to the Early Amazonian. Geomorphic evidence for a northern‐plains‐filling ocean during Stage 2 is absent because one did not form or its evidence was destroyed by Stage 3 resurfacing. Remnants of possible Amazonian dust mantles occur on top of the VBF. The north polar layered deposits appear to be made up of an up to kilometer‐thick lower sequence of sandy layers Early to Middle Amazonian in age overlain by Late Amazonian ice‐rich dust layers; both units appear to have outliers, suggesting that they once were more extensive.
Lunar Gruithuisen and Mairan domes: Rheology and mode of emplacement
Authors: Wilson Lionel , Head James W.
Published: May 2003

The lunar steep‐sided Gruithuisen and Mairan domes are morphologically and spectrally distinctive structures and appear similar to terrestrial extrusive volcanic features characterized by viscous magma. We use the basic morphologic and morphometric characteristics of the domes to estimate the yield strengths (∼105 Pa), plastic viscosities (∼109 Pa s), and effusion rates (∼50 m3/s) of the magmas which formed them. These values are similar to those of terrestrial rhyolites, dacites, and basaltic andesites and support the hypothesis that these domes are an unusual variation of typical highlands and mare compositions. The dikes which formed them are predicted to have had widths of ∼50 m and lengths of about 15 km. The magma rise speed implied by this geometry is very low, ∼7 × 10−5 m/s, and the Reynolds number of the motion is ∼2 × 10−8, implying a completely laminar flow regime. Estimates of emplacement duration range from one to several decades. These new calculations confirm the unusual nature of these features and support previous qualitative suggestions that they were formed from magmas with significantly higher viscosity than those typical of mare basalts.
Mass spectrometer calibration of Cosmic Dust Analyzer
Authors: Ahrens Thomas J. , Gupta Satish C. , Jyoti G. , Beauchamp J. L.
Published: May 2003

The time‐of‐flight (TOF) mass spectrometer (MS) of the Cosmic Dust Analyzer (CDA) instrument aboard the Cassini spacecraft is expected to be placed in orbit about Saturn to sample submicrometer‐diameter ring particles and impact ejecta from Saturn's satellites. The CDA measures a mass spectrum of each particle that impacts the chemical analyzer sector of the instrument. Particles impact a Rh target plate at velocities of 1‐100 km/s and produce some 10−8 to 10−5 times the particle mass of positive valence, single‐charged ions. These are analyzed via a TOF MS. Initial tests employed a pulsed N2 laser acting on samples of kamacite, pyrrhotite, serpentine, olivine, and Murchison meteorite induced bursts of ions which were detected with a microchannel plate and a charge sensitive amplifier (CSA). Pulses from the N2 laser (1011 W/cm2) are assumed to simulate particle impact. Using aluminum alloy as a test sample, each pulse produces a charge of ∼4.6 pC (mostly Al+1), whereas irradiation of a stainless steel target produces a ∼2.8 pC (Fe+1) charge. Thus the present system yields ∼10−5% of the laser energy in resulting ions. A CSA signal indicates that at the position of the microchannel plate, the ion detector geometry is such that some 5% of the laser‐induced ions are collected in the CDA geometry. Employing a multichannel plate detector in this MS yields for Al‐Mg‐Cu alloy and kamacite targets well‐defined peaks at 24 (Mg+1), 27(Al+1), and 64 (Cu+1) and 56 (Fe+1), 58 (Ni+1), and 60 (Ni+1) dalton, respectively.
Correlation of dielectric permittivity of volcanic units on Venus with age
Authors: Bondarenko N. V. , Kreslavsky M. A. , Raitala J.
Published: May 2003

Volcanic plains are the most common surface units on Venus. A survey of distinctive contrasts of the radiophysical properties associated with boundaries of volcanic units was carried out. Only those sites where the sign of the dielectric permittivity difference can be reliably inferred from radiophysical data were considered. The inferred contrasts in the dielectric permittivity across the unit boundaries were correlated with the relative stratigraphic positions of the units. It turned out that locally older units usually have locally lower dielectric permittivity. This phenomenon can be a result of a set of processes of surface alteration and mantling formation, which decrease the dielectric permittivity with time. The observations are not consistent, however, with a gradual and globally uniform decrease of the permittivity. Thus it is impossible to instantly use the radiophysical properties to date the surface material. The results show that further studies of the radiophysical data sets from the Magellan data collection are promising for understanding the surface mantles and mechanisms of their formation and evolution as well as for some inferences about ages.
High‐resolution photoabsorption cross section measurements of SO2, 2: 220 to 325 nm at 295 K
Authors: Rufus J. , Stark G. , Smith Peter L. , Pickering J. C. , Thorne A. P.
Published: May 2003

High‐resolution, ultraviolet SO2 photoabsorption cross section data are required for modeling density and temperature profiles of SO2 in solar system atmospheres. We report measurements of such cross sections on SO2 at 295 K for the 1B1 ‐ 1A1 and 1B2 ‐ 1A1 bands over the wavelength range 220 to 325 nm. Resolving powers of up to 550,000 were employed in order to study the congested spectrum of sharp SO2 features in parts of this region. Our data are compared to earlier values obtained with lower resolving power.
Extreme ultraviolet photolysis of CO2 ‐ H2O mixed ices at 10 K
Authors: Wu C. Y. Robert , Judge D. L. , Cheng Bing‐Ming , Yih Tai‐Sone , Lee C. S. , Ip W. H.
Published: May 2003

Experimental results on the spectral identification of IR absorption features and the production and depletion column densities produced through EUV photolysis of H2O + CO2 (4:1) mixed ices at 10 K are reported. A tunable intense synchrotron radiation light source was employed to provide the required high flux EUV photons. In this study the photon wavelengths used to irradiate the icy sample were selected to coincide with the prominent solar lines at 58.4 nm and 30.4 nm. A FTIR spectrometer was employed to obtain the in situ IR absorbance spectra produced through EUV photolysis of a given ice sample. Photon‐induced chemical reaction products were primarily H2CO3 and CO. While new molecular species were formed, the original reactants were depleted, as expected. We report the yields for producing H2CO3 and CO and the yields for depleting the parent CO2 molecule through EUV photolysis of the mixed ices. We have searched for possible intermediate products H2CO and HCO, but could not positively identify their existences in the spectra. The results presented in this work are particular relevant to our understanding of chemical evolution and synthesis in H2O‐rich icy satellites of the giant planets, comets, the interstellar medium, molecular clouds, and protostellar regions.
Volcanism on the Marius Hills plateau: Observational analyses using Clementine multispectral data
Authors: Heather David J. , Dunkin Sarah K. , Wilson Lionel
Published: May 2003

We have studied the Marius Hills region and mapped the spectrally distinct flows present for the first time, using photographic and Clementine multispectral data. The basalts on the plateau are varied in age and composition but are dominated by a high‐titanium Eratosthenian basalt, most likely from the same source as the Flamsteed Basalt, further south. The thickness of the basalts across the plateau is consistently greater than 120 m and is considerably thicker in some areas. A lower limit of 5320 km3 of basalts has been erupted onto the plateau. The domes and cones in the region do not appear to be related to any specific basalt or volcanic episode but occur at all levels in the stratigraphy that can be derived in the region. The eruption conditions required to form these constructs indicate that they must represent a series of separate volcanic episodes occurring throughout the history of the plateau. Cones on the Marius plateau are dominated by a strong glassy signature and often have an associated microlitic structure. Examples are shown of localized microlitic or glassy materials in regions where no cone is evident in the photographic data. These features are proposed to represent short‐lived pyroclastic episodes that have deposited glassy and microlitic units on the plateau but have not been maintained for long enough to develop a cone. The volcanic history of the Marius Hills region is extremely complex, and most likely involved several separate episodes of volcanism with a large contrast in eruption styles and characteristics.
Aqueous flows carved the outflow channels on Mars
Authors: Coleman Neil M.
Published: May 2003

The role of water in carving the Martian outflow channels has recently been challenged by the hypothesis known as “White Mars.” This hypothesis claims that the channels were cut by CO2 gas‐supported debris flows that also resurfaced the northern plains. However, proposed analogs of “cryoclastic” flows are either inappropriate (i.e., submarine density flows) or are primarily depositional rather than erosional (i.e., pyroclastic flows). Subaerial mass movements on Earth do not carve long deep channels like those on Mars. I review runout efficiencies for mass movements on Earth and Mars. The efficiencies required for “cryoclastic” flows to resurface the northern plains of Mars are so large that they appear unattainable. White Mars seeks to resolve carbonate and floodwater “paradoxes” that probably do not exist. It is also doubtful whether reservoirs of CO2 could persist over geologic time in the crust. Overall, the CO2 hypothesis fails key tests and should be abandoned as a means to carve outflow channels. I present a new interpretation of the fluid source that created Aromatum Chaos and Ravi Vallis. Water, not liquid or gaseous CO2, was the causative fluid, and the source was an ice‐covered impoundment in ancestral Ganges Chasma. At that time the canyon had no eastern outlet, and groundwater flowed northward to discharge at Aromatum Chaos and Shalbatana Vallis. The presence of ice‐covered water bodies can help to calibrate models of volcanic‐hydrologic climaxes during Hesperian time. The outflow channels, like the spectacular landforms of the Channeled Scabland, are monuments to the erosive power of catastrophic aqueous floods.
Sonic anemometry of planetary atmospheres
Authors: Cuerva Alvaro , Sanz‐Andrés Angel
Published: May 2003

The application of a recently developed model of sonic anemometers measuring process has revealed that these sensors cannot be considered as absolute ones when measuring spectral characteristics of turbulent wind speed since it is demonstrated that the ratios of measured to real spectral density functions depend on the composition and temperature of the considered planetary atmosphere. The new model of the measuring process of sonic anemometers is applied to describe the measuring characteristics of these sensors as fluid/flow dependent (against the traditional hypothesis of fluid/flow independence) and hence dependent on the considered planetary atmosphere. The influence of fluid and flow characteristics (quantified via the Mach number of the flow) and the influence of the design parameters of sonic anemometers (mainly represented by time delay between pulses shots and geometry) on turbulence measurement are quantified for the atmospheres of Mars, Jupiter, and Earth. Important differences between the behavior of these sensors for the same averaged wind speed in the three considered atmospheres are detected in terms of characteristics of turbulence measurement as well as in terms of optimum values of anemometer design parameters for application on the different considered planetary atmospheres. These differences cannot be detected by traditional models of sonic anemometer measuring process based on line averaging along the sonic acoustic paths.
Stationary planetary waves in the atmosphere of Mars during southern winter
Authors: Hinson D. P. , Wilson R. J. , Smith M. D. , Conrath B. J.
Published: May 2003

We report new observations of stationary planetary waves in the Southern Hemisphere of Mars by Mars Global Surveyor (MGS). We focus on a period during midwinter (Ls = 134°–160°) when independent observations were acquired by two techniques. Radio occultation experiments sounded the atmosphere at essentially fixed latitude (∼68°S) and local time (∼1030), yielding profiles of geopotential and temperature between the surface and the 9 Pa pressure level. Observations by the Thermal Emission Spectrometer (TES) included systematic limb sounding at nine discrete latitudes and two local times (∼0200 and ∼1500), yielding temperature profiles at pressures of 1–100 Pa. We supplemented these data with a simulation by a Mars general circulation model (MGCM), which provides an accurate synthesis of the observations. These stationary planetary waves have significant amplitudes at zonal wave numbers s = 1 and 2. The s = 1 component propagates vertically, as reflected by a westward tilt with increasing height in the geopotential and temperature fields and a net poleward eddy heat flux. The peak amplitude at s = 1 is ∼1 km in geopotential height and ∼7 K in temperature. The geopotential field of the s = 2 component is “barotropic” in character, which results through hydrostatic balance in a distinctive temperature field. The peak amplitude at s = 2 is ∼700 m in geopotential height and 4–6 K in temperature.
Dissociation of N2 in capture and ionization collisions with fast H+ and N+ ions and modeling of positive ion formation in the Titan atmosphere
Authors: Luna H. , Michael M. , Shah M. B. , Johnson R. E. , Latimer C. J. , McConkey J. W.
Published: May 2003

Electron capture and ionization cross sections for protons and nitrogen ions incident on N2 are measured in the energy range 10–100 keV using time of flight (TOF) coincidence counting techniques. In the case of proton impact the formation of N2+ ions dominates for both electron capture and ionization channels at all energies, whereas for N+ ions, the N2+ formation dominates for electron capture and the dissociative processes for ionization channels. The energy distribution of the fragment products at 20 and 100 keV have also been measured for the first time using the TOF method. These cross sections are useful in the simulation of energetic ions and atoms interacting with Titan's N2‐rich atmosphere. Titan resides primarily within Saturn's magnetosphere where H+ and N+ ions are the major ions present along its orbit. It is found that the neutralization of these ions by charge exchange does not occur efficiently above Titan's exobase, so energetic particles with large gyroradii penetrate primarily as ions. The ionization rate and energy deposition in Titan's atmosphere by the energetic H+ ions observed by the Voyager spacecraft are explained with the help of the present measurements.
Carbon dioxide clouds in an early dense Martian atmosphere
Authors: Colaprete Anthony , Toon Owen B.
Published: May 2003

We use a time‐dependent, microphysical cloud model to study the formation of carbon dioxide clouds in the Martian atmosphere. Laboratory studies by Glandorf et al. [2002] show that high critical supersaturations are required for CO2 cloud particle nucleation and that surface kinetic growth is not limited. These conditions, which are similar to those for cirrus clouds on Earth, lead to the formation of carbon dioxide ice particles with radii greater than 500 μm and concentrations less than 0.1 cm−3 for typical atmospheric conditions. Within the current Martian atmosphere, CO2 cloud formation is possible at the poles during winter and possibly at high altitudes in the tropics. In both cases, temperature perturbations of several degrees below the CO2 saturation temperature are required to nucleate new cloud particles, suggesting that dynamical processes are the most common initiators of carbon dioxide clouds rather than diabatic cooling. The microphysical cloud model, coupled to a two‐stream radiative transfer model, is used to reexamine the impact of CO2 clouds on the surface temperature within a dense CO2 atmosphere. The formation of carbon dioxide clouds leads to a warmer surface than what would be expected for clear sky conditions, but it also warms the atmosphere. The amount of surface warming is sensitive to the presence of dust and water vapor in the atmosphere, both of which act to dampen cloud effects. The radiative warming of the atmosphere associated with cloud formation, as well as latent heating, work to dissipate the clouds when present. In these simulations, clouds never last for periods much longer than several days, limiting their overall effectiveness for warming the surface. The time average cloud optical depth is approximately unity leading to a 5–10 K surface warming, depending on the surface pressure. The surface temperature does not rise above the freezing point of liquid water even for pressures as high as 5 bars, at a solar luminosity of 75% the current value. Our model shows that warming of the surface‐atmosphere system by carbon dioxide clouds is self‐limiting, since by heating the air the clouds cause themselves to dissipate. However, further analysis of the climatic effects of carbon dioxide clouds considering their global distribution and properties is warranted.
Cavi Angusti, Mars: Characterization and assessment of possible formation mechanisms
Authors: Ghatan Gil J. , Head James W. , Pratt Stephen
Published: May 2003

Cavi Angusti represent a series of large irregular depressions localized in part of the south circumpolar area previously mapped as the Hesperian‐aged Dorsa Argentea Formation. Their origin has primarily been interpreted to be due to eolian deflation or subglacial melting. We use MGS MOLA and MOC data to analyze the largest of these features (∼100 × 50 km, and up to about 1500 m deep). These data reveal terraced interiors, centrally located equidimensional and elongated edifices, and lava‐flow‐like structures that strongly suggest that this basin formed as a result of magmatic intrusion and extrusion, causing heating and melting of a volatile‐rich substrate and drainage and loss of the liquid water. Volume estimates and heat transfer calculations are consistent with a mechanism involving a combination of intrusion and extrusion very similar to that observed to be responsible for Icelandic subglacial eruptions and meltwater generation. Mounds and ridges in the floors of other depressions suggest that this mechanism may have operated in at least several other features of the Cavi. Eolian activity, sublimation, and solution are also likely to have played a role in further modification of these features. Meltwater from basin formation appears to have drained laterally and may also have reentered the regional subsurface groundwater system.
Chemical markers of possible hot spots on Mars
Authors: Wong Ah‐San , Atreya Sushil K. , Encrenaz Thérèse
Published: May 2003

Although there is no evidence of active volcanism on Mars today, “localized” outgassing sources, the hot spots, may not be ruled out. If outgassing does occur somewhere on Mars, water, carbon dioxide, sulfur species, methane, and to a lesser extent, halogens would be the likely molecules of outgassing, based on terrestrial analogs. The sulfur species, methane, and halogens have not been detected in the Martian atmosphere, but the observations were averaged over large areas, which could result in substantial dilution in abundances. If the interpretation of certain Mars Global Surveyor images indicating recent ground water seepage and surface runoff [Malin and Edgett, 2000] is correct, it may imply that Mars could still be active internally in some places from time to time, and outgassing of the abovementioned species may occur with or without the water seepage. Moreover, if the tentative detection of formaldehyde (CH2O) in the equatorial region of Mars [Korablev et al., 1993] is confirmed by future observations, it would imply at least local outgassing of methane, whose oxidation results in the formation of CH2O. Considering the possibility of outgassing from some localized hot spots, we have developed a one‐dimensional photochemical model that includes methane (CH4), sulfur dioxide (SO2), and hydrogen sulfide (H2S), starting with their current “global average” upper limits of 0.02, 0.1, and 0.1 ppm at the surface, respectively, and then progressively increasing their abundances above possible hot spots. We find that the introduction of methane into the Martian atmosphere results in the formation of mainly formaldehyde, methyl alcohol (CH3OH), and ethane (C2H6), whereas the introduction of the sulfur species produces mainly sulfur monoxide (SO) and sulfuric acid (H2SO4). The effect of outgassed halogens on the Martian atmosphere is found to be negligible. Depending upon the flux of outgassed molecules from possible hot spots, some of these species and the resulting new molecules may be detectable locally, either by remote sensing or in situ measurements.
Atmospheric composition, radiative forcing, and climate change as a consequence of a massive methane release from gas hydrates
Authors: Schmidt Gavin A. , Shindell Drew T.
Published: May 2003

The massive perturbation to global climate and the carbon cycle during the Paleocene/Eocene Thermal Maximum (PETM) (approx. 55.5 Ma) may have been forced by a catastrophic release of methane gas (CH4) from hydrate deposits on the continental slope. We investigate whether reported PETM paleotemperature and paleo‐CO2 proxies are consistent with this hypothesis by considering the impact of large increases in CH4 emissions to the atmosphere. Significant effects on atmospheric chemistry and CH4 lifetime are seen for a range of plausible emission rates (1500 Gt carbon over 500–20,000 years). The resulting peak anomalous radiative forcing is 1.5–13.3 W/m2 depending on the emission scenario. The scenarios most closely matched to the PETM carbon isotope excursion have peak forcing of around 3 W/m2, which translates to peak temperature changes as a function of latitude that are a reasonable match to derived estimates. High CH4 levels and enhanced stratospheric water vapor amounts persist for as long as do the emissions and are responsible for more of the peak radiative forcing than CO2 levels, although results are sensitive to the background climate state and base CH4 concentration.
Northwest African Continental Margin: History of sediment accumulation, landslide deposits, and hiatuses as revealed by drilling the Madeira Abyssal Plain
Authors: Weaver P. P. E.
Published: May 2003

ODP drill sites in the Madeira Abyssal Plain reveal sequences of organic‐rich turbidites derived from the northwest African margin, in which each turbidite has a volume of tens to hundreds of cubic kilometers. The frequency of turbidite emplacement has been combined with core and seismic data to show the volume of redeposited sediment. The basin began to fill about 22 Ma with numerous small turbidites, up to 100 per million years, each with volumes of a few cubic kilometers. The total volume of turbidites deposited increased between 16 and 11 Ma, as did their individual volumes, and then declined to 7 Ma. At 7 Ma, there was a dramatic increase in the amount of turbidite input to 768 km3/Myr and a rise in the average volume of each unit to 59 km3. These high values have been maintained to the present day. The variations in the amount of redeposited sediment most likely reflect the rates of sedimentation on the northwest African margin since high sedimentation leads to oversteepening of the slopes and eventual mass wasting. The dramatic changes at about 7 Ma may be due to a large increase in upwelling off northwest Africa caused by circulation changes associated with increased glaciation of the poles. Up to 20% of sediment may be remobilized by landslides, with each event leaving a hiatus. Each of these hiatuses extends over an average area of ∼4800 km2 and represents removal of sediment layers several tens of meters thick and of several hundred thousand years duration.
Reorganization of Miocene deep water circulation in response to the shoaling of the Central American Seaway
Authors: Nisancioglu Kerim H. , Raymo Maureen E. , Stone Peter H.
Published: May 2003

The response of ocean circulation to the shoaling of the Central American Seaway (CAS) is investigated using the Massachusetts Institute of Technology (MIT) Ocean General Circulation Model (OGCM). In contrast to earlier model studies, it is found that significant amounts of deep water are formed in the North Atlantic prior to the closure of the CAS. However, the circulation pattern is fundamentally different from the modern ocean. In the upper layers of the CAS, there is a relatively strong geostrophic flow from the Pacific to the Atlantic, controlled by the pressure difference across the seaway. However, when the CAS is deeper than the level of North Atlantic Deep Water (NADW) outflow, a significant amount of NADW passes through the CAS to the Pacific Ocean. In the Pacific, the deep water traverses the basin from east to west in a relatively narrow zonal jet, and becomes a southward flowing western boundary current, before it joins with the Antarctic Circumpolar Current (ACC) to the south. This implies that deep sea sediment records from the Miocene Pacific Ocean could have been influenced by relatively young NADW and provides a new framework for the interpretation of geochemical tracer data. Diversification of benthic foraminifer fauna suggests that the CAS shoaled to a depth of about 1000 m toward the end of the middle Miocene. This would have prevented the passage of NADW to the Pacific and established the modern deep water circulation pattern at that time.
Comparison of glacial and interglacial conditions between the polar and subpolar North Atlantic region over the last five climatic cycles
Authors: Helmke Jan P. , Bauch Henning A.
Published: May 2003

A multiparameter‐based interpretation of sediment records from the northeast Atlantic and the western Nordic seas suggests that during the last 500,000 years only in marine isotopes stage (MIS) 11, 5e, and 1 were there somewhat comparable interglacial boundary conditions in both regions, i.e., strongly reduced occurrence of iceberg‐rafted debris (IRD) and high carbonate bioproductivity. Although the northeast Atlantic experienced such conditions during all peak interglaciations, with the exception of MIS 7, planktic foraminiferal δ18O from this region would still indicate that significantly colder sea surface temperatures (SST) prevailed during MIS 11 than during MIS 9, 5e, and 1. This assumption is corroborated by a continuous input of IRD into the western Nordic seas during MIS 11, implying a much steeper SST gradient between the polar and subpolar region and an overall reduced thermohaline activity in the polar latitudes. The iceberg proxy also reveals that maximum IRD discharge always happened during the final phase of glaciation and into early deglaciation (terminations). As these IRD records from the two regions are characterized by a high time coherency, it is concluded that short‐term variability is a persistent feature of the glacial climate system.
Early Cenozoic benthic foraminiferal isotopes: Species reliability and interspecies correction factors
Authors: Katz Miriam E. , Katz David R. , Wright James D. , Miller Kenneth G. , Pak Dorothy K. , Shackleton Nicholas J. , Thomas Ellen
Published: May 2003

Oxygen and carbon isotope records are important tools used to reconstruct past ocean and climate conditions, with those of benthic foraminifera providing information on the deep oceans. Reconstructions are complicated by interspecies isotopic offsets that result from microhabitat preferences (carbonate precipitation in isotopically distinct environments) and vital effects (species‐specific metabolic variation in isotopic fractionation). We provide correction factors for early Cenozoic benthic foraminifera commonly used for isotopic measurements (Cibicidoides spp., Nuttallides truempyi, Oridorsalis spp., Stensioina beccariiformis, Hanzawaia ammophila, and Bulimina spp.), showing that most yield reliable isotopic proxies of environmental change. The statistical methods and larger data sets used in this study provide more robust correction factors than do previous studies. Interspecies isotopic offsets appear to have changed through the Cenozoic, either (1) as a result of evolutionary changes or (2) as an artifact of different statistical methods and data set sizes used to determine the offsets in different studies. Regardless of the reason, the assumption that isotopic offsets have remained constant through the Cenozoic has introduced an ∼1–2°C uncertainty into deep sea paleotemperature calculations. In addition, we compare multiple species isotopic data from a western North Atlantic section that includes the Paleocene‐Eocene thermal maximum to determine the most reliable isotopic indicator for this event. We propose that Oridorsalis spp. was the most reliable deepwater isotopic recorder at this location because it was best able to withstand the harsh water conditions that existed at this time; it may be the best recorder at other locations and for other extreme events also.
Physiological impacts on alkenone paleothermometry
Authors: Prahl F. G. , Sparrow M. A. , Wolfe G. V.
Published: May 2003

We conducted isothermal (15°C) batch culture experiments with the coccolithophorid Emiliania huxleyi (strain NEPCC 55a) to evaluate the extent to which nutrient and light stress contribute to variability in the alkenone unsaturation index U37K′. Alkenone content and composition were constant throughout exponential growth in both experiments when nutrients (nitrate and orthophosphate) were replete. Stationary phase (nutrient‐starved) cells continued to produce alkenones, amassing concentrations (ΣAlk) ≥ 3 times higher than those dividing exponentially (1.5–2 pg cell−1), and the U37K′ of “excess” alkenone dropped by 0.11 units. In contrast, 5 days of continuous darkness resulted in a 75% decrease in cellular ΣAlk and a significant U37K′ increase (+0.11 units). Given an established 0.034 unit/°C response for exponentially growing cells of this strain, the observed range of U37K′ variability at 15°C corresponds to an uncertainty of ±3.2°C in predicted growth temperature. This level of variability matches that of the global U37K′ annual mean sea surface temperature calibration for surface marine sediments, begging the question: What is the physiological condition of alkenone‐producing cells exported to marine sediments? Comparison of our laboratory results for a strain of E. huxleyi isolated from the subarctic Pacific Ocean with depth profiles for alkenones in surface waters from two contrasting sites in the northeast Pacific Ocean suggests that the answer to this question depends on the ocean regime considered, a possibility with significant bearing on how stratigraphic U37K′ records in marine sediments are to be interpreted paleoceanographically.
Paleocurrent reconstruction of the deep Pacific inflow during the middle Miocene: Reflections of East Antarctic Ice Sheet growth
Authors: Hall Ian R. , McCave I. Nicholas , Zahn Rainer , Carter Lionel , Knutz Paul C. , Weedon Graham P.
Published: May 2003

Today the deep western boundary current (DWBC) east of New Zealand is the most important route for deep water entering the Pacific Ocean. Large‐scale changes in deep water circulation patterns are thought to have been associated with the development of the East Antarctic Ice Sheet (EAIS) close to the main source of bottom water for the DWBC. Here we reconstruct the changing speed of the southwest Pacific DWBC during the middle Miocene from ∼15.5–12.5 Ma, a period of significant global ice accumulation associated with EAIS growth. Sortable silt mean grain sizes from Ocean Drilling Program Site 1123 reveal variability in the speed of the Pacific inflow on the timescale of the 41 kyr orbital obliquity cycle. Similar orbital period flow changes have recently been demonstrated for the Pleistocene epoch. Collectively, these observations suggest that a strong coupling between changes in the speed of the deep Pacific inflow and high‐latitude climate forcing may have been a persistent feature of the global thermohaline circulation system for at least the past 15 Myr. Furthermore, long‐term changes in flow speed suggest an intensification of the DWBC under an inferred increase in Southern Component Water production. This occurred at the same time as decreasing Tethyan outflow and major EAIS growth between ∼15.5 and 13.5 Ma. These results provide evidence that a major component of the deep thermohaline circulation was associated with the middle Miocene growth of the EAIS and support the view that this time interval represents an important step in the development of the Neogene icehouse climate.
Diatom‐inferred salinity records from the Arctic Siberian Margin: Implications for fluvial runoff patterns during the Holocene
Authors: Bauch Henning A. , Polyakova Yelena I.
Published: May 2003

Diatom assemblages were employed to study temporal changes of Siberian river runoff on the Laptev Sea shelf. Using a correlation between freshwater diatoms (%) in core‐top sediments and summer surface water salinities from the inner Kara Sea, salinity conditions were reconstructed for a site northeast of the Lena River Delta (present water depth 32 m) since 9 calendar years (cal) ka. The reconstruction indicate a strong, near‐coastal, and river‐influenced environment at the site until about 8.6 cal ka. Corroborated by comparison with other proxy records from further to the east, surface salinities increased from 9 to 14 until about 7.4 cal ka, owing to ongoing global sea level rise and synchronous southward shift of the coastline. Although riverine water became less influential at the site since then, salinities still varied between 12.5 and 15, particularly during the last 3.5 kyr. These more recent salinity fluctuations agree well with reconstructions from just north of the Lena Delta, emphasizing the strong linkage between shelf hydrography and riverine discharge patterns in Arctic Siberia.
Mode separation and direction of arrival estimation in HF links
Authors: Arikan F. , Yilmaz N. , Arikan O. , Miled M. K. B. H.
Published: May 2003

Estimation of arrival angles and incoming signals is a challenging problem for HF channels where the signals are correlated and the separation between the signals can be as low as a couple of degrees. In this paper, a new algorithm, Multipath Separation‐Direction of Arrival (MS‐DOA), is developed to estimate both the arrival angles in elevation and azimuth and the incoming signals at the output of the reference antenna with very high accuracy. The MS‐DOA algorithm provides reliable angle and signal estimates even with small separation of arrival angles and for low SNRs. The minimum number of antennas that are required by the algorithm is only one more than the number of incoming signals. In a narrowed down region of interest and for a few incoming signals, the computational search time for MS‐DOA is only a couple of minutes in a standard PC.
Electromagnetic whistler‐mode radiation from a dipole in the ionosphere
Authors: James H. G.
Published: May 2003

Dipole‐antenna radiation theory has been tested for electromagnetic whistler‐mode waves in the frequency range 0.1–1.3 MHz using data from the two‐point rocket experiment Observations of Electric‐field Distributions in the Ionospheric Plasma; a Unique Strategy (OEDIPUS) C. Waves were emitted from a double‐V dipole on a transmitting subpayload and received at a distance of about 1200 m on a similar dipole connected to a synchronized receiver. The magnitudes of transmitted signals predicted by a theory for short dipoles have been found to be in good agreement with observations. The agreement is very good in the top two thirds of the frequency range where the dispersion relation and the geometry provide only one saddle‐point solution with the required group direction. In this part of the parameter space studied, the transmitter‐receiver separation is at least 10 wavelengths. This is not always true at the lowest frequencies, where the expected effects of interference of multiple saddle‐point rays and of the oblique resonance cone are observed.
Comparison of iteration convergences of SIE and VSIE for solving electromagnetic scattering problems for coated objects
Authors: Lu Cai‐Cheng , Luo Chong
Published: May 2003

The surface integral equation approach and the hybrid surface‐volume integral equation approach are compared for solving the problem of electromagnetic scattering by conducting objects with dielectric coating. The surface integral equation is formulated by the PMCHW method, and it is efficient for modeling bulk material scattering. The hybrid surface‐volume integral equation approach is better conditioned for the volume scattering problems. For a special class of problems in which the dielectric coating is electrically thin (one to two grid sizes), the numbers of unknowns needed by the two approaches are of the same order, and the surface‐volume integral equation approach converges significantly faster than the pure surface integral equation approach. Several numerical examples are given, and the results showed good agreements with the above statement.