bluuebunny/crossref_metadata_2025 huggingface dataset:
Role of convective scale momentum transport in climate simulation
Authors: Zhang Guang Jun , McFarlane Norman A.
Published: February 2004

This paper studies the effect of convective‐scale momentum transport in climate simulation using a comprehensive parameterization scheme. A unique feature of the scheme is the inclusion of the perturbation pressure field induced by convection and its effect on the cloud momentum transport. Through two experiments of seasonal simulations, it is shown that the perturbation pressure forcing on the cloud momentum transport accounts for a significant part of the total convective momentum source/sink, indicating that the cloud momentum field is substantially modulated by the convection‐induced pressure field. The overall effect of convective momentum transport is to reduce the vertical wind shear in both the zonal and the meridional directions. The response of the large‐scale circulation to convective momentum transport is very significant. The zonally averaged zonal wind decreases by as much as 5 ms−1 in a broad area in the upper tropical troposphere and the midlatitudes of the winter hemisphere. The Hadley circulation becomes stronger as a result of the zonal momentum transport. In general, inclusion of convective momentum transport leads to a much better simulation of the wind fields in both the upper and the lower troposphere. The temperature and moisture changes as a result of the inclusion of convective momentum transport are also examined in this study. The tropical troposphere is warmer and more moist due to the enhanced Hadley circulation. However, considering the uncertainties of the climatological analyses, most of these thermodynamic changes only make marginal improvement to the simulation.
Intercomparison between two satellite‐based products of net surface shortwave radiation
Authors: Li Zhanqing
Published: February 2004

This study compares two global satellite‐based surface radiation budget (SRB) data sets as a means of quality evaluation. One was developed from Earth Radiation Budget Experiment (ERBE) satellite data using the algorithm of Li et al. (ERBE/SRB), and the other was generated by applying the algorithm of Pinker and Laszlo to International Satellite Cloud Climatology Project (ISCCP/SRB) data. The comparison is limited to net surface shortwave radiation (NSSR). The global annual mean of NSSR obtained from the two data sets differ by 15 W m−2, and maximum regional differences exceed 100 W m−2. The differences are investigated in terms of discrepancies in both input data and algorithm. It was found that large regional differences in SRB are associated mainly with the discrepancies in the input parameters, namely, top‐of‐atmosphere (TOA) flux and precipitable water. Differences in TOA flux between ERBE and ISCCP are attributed to angular and spectral corrections for ISCCP data and to different sampling in time and space by ERBE and ISCCP. ISCCP/SRB underestimates NSSR by 20–30 W m−2 over some dry regions arising from excessive amounts of precipitable water from the TIROS operational vertical sounder. Deficient treatment of aerosol in the Li et al. algorithm results in too large NSSR for the major deserts. Systematic discrepancies are accounted for by different methods to compute water vapor absorption. According to the line‐by‐line results, water vapor absorption was significantly underestimated by the Lacis and Hansen parameterization (used by Pinker and Laszlo) and is moderately overestimated by LOWTRAN 6 (used by Li et al.). If both algorithms use the same water vapor absorption scheme and the same input data, their global annual mean of NSSR agree to within 1 W m−2. On the basis of these findings, some recommendations are made for future improvements of the two products. Overall, it appears that the quality of ERBE/SRB is superior to that of ISCCP/SRB (version 1.1).
Calculation of surface and top of atmosphere radiative fluxes from physical quantities based on ISCCP data sets: 1. Method and sensitivity to input data uncertainties
Authors: Zhang Y‐C. , Rossow W. B. , Lacis A. A.
Published: February 2004

Upwelling and downwelling, shortwave and longwave radiative fluxes are calculated at the top of the atmosphere and at the surface using a complete radiative transfer model and observations of the physical properties of the surface, atmosphere, and clouds based on the International Satellite Cloud Climatology Project (ISCCP) data sets. Results are obtained every three hours over the whole globe for every third month from April 1985 to January 1989. Sensitivity studies are conducted to assess the uncertainties in calculated fluxes caused by the estimated uncertainties in the measurement or specification of the input quantities. Except in the polar regions, uncertainties in cloud properties are no longer the predominant source of radiative flux uncertainty, even at the surface; rather they produce uncertainties similar in magnitude to those caused by atmospheric and surface properties. The largest uncertainty in upwelling shortwave (SW) fluxes (≈ 10–15 W/m2, regional daily mean) is caused by uncertainties in land surface albedo, whereas the largest uncertainty in downwelling SW at the surface (≈ 5–10 W/m2, regional daily mean) is related to cloud detection errors. The uncertainty of upwelling longwave (LW) fluxes (≈ 10–20 W/m2, regional daily mean) depends on the accuracy of the surface temperature for the surface LW fluxes and the atmospheric temperature for the top of atmosphere LW fluxes. The dominant source of uncertainty in downwelling LW fluxes at the surface (≈ 10–15 W/m2) is uncertainty in atmospheric temperature and, secondarily, atmospheric humidity; clouds play little role except in the polar regions. The uncertainties of the individual flux components and the total net fluxes are largest over land (15–20 W/m2) because of uncertainties in surface albedo (especially its spectral dependence) and surface temperature and emissivity (including its spectral dependence). Clouds are the most important modulator of the SW fluxes, but over land areas, uncertainties in net SW at the surface depend almost as much on uncertainties in surface albedo. Although atmospheric and surface temperature variations cause larger LW flux variations, the most notable feature of the net LW fluxes is the changing relative importance of clouds and water vapor with latitude. Uncertainty in individual flux values is dominated by sampling effects because of large natural variations, but uncertainty in monthly mean fluxes is dominated by bias errors in the input quantities.
Spatial and temporal variability of ClONO2, HNO3, and O3 in the Arctic winter of 1992/1993 as obtained by airborne infrared emission spectroscopy
Authors: Blom C. E. , Fischer H. , Glatthor N. , Gulde T. , Höpfner M. , Piesch C.
Published: February 2004

In the winter of 1992/1993 the airborne Michelson interferometer for passive atmospheric sounding (MIPAS‐FT) was operated on board a German research aircraft (Transall C‐160) to record infrared emission spectra of the atmosphere inside and outside the Arctic vortex. Measurements were made during four campaigns between December 4, 1992, and March 29, 1993, in the European Arctic as well as over central and southern Europe (82°N–37.5°N). We present the retrieved zenith column amounts of the stratospheric trace gases ClONO2, HNO3, and O3 of this period. Inside the polar vortex, the column amounts of ClONO2 and HNO3 were considerably enhanced already in early December, up to 3.1×1015 cm−2 and 2.7×1016 cm−2, respectively. Around the end of January, low ClONO2 (1×1015 cm−2) and high HNO3 column amounts (up to 3.7×1016 cm−2) were observed inside the vortex, whereas a highly variable “collar” of ClONO2 had developed at the vortex edge. During March, after temperatures had been above the threshold for polar stratospheric clouds (PSCs) for several weeks, we measured lower HNO3 (below 2.5×1016 cm−2) and very high ClONO2 column amounts (up to 6×1015 cm−2) inside the vortex. Thus a major part of the reactive chlorine had been converted into ClONO2, and the potential for rapid ozone depletion was reduced markedly in the region observed. On March 10, when the polar vortex extended southward to the Mediterranean, ClONO2 column amounts as high as 4.6×1015 cm−2 were observed at 40°N. At the end of March, considerable amounts of ClONO2 (up to 3.4×1015 cm−2) were measured also far outside the vortex.
Spatial patterns of soil moisture for the FIFE study area derived from remotely sensed and ground data
Authors: Peck Eugene L. , Hope Allen S.
Published: February 2004

Soil moisture observations were made over the First International Satellite Land Surface Climatology Project (ISLSCP) Field Experiment (FIFE) study area in 1987 and 1989 using a variety of ground and airborne techniques. A procedure is described for deriving spatial patterns of soil moisture by combining these different data types. The spatial patterns of soil moisture are represented as both contour and grid values so that they can be readily incorporated in other FIFE studies. Two examples are given that illustrate the utility of these soil moisture maps for remote sensing and hydrometeorological studies over the FIFE study area.
Ozone and hydrogen peroxide during summer smog episodes over the Swiss Plateau: Measurements and model simulations
Authors: Dommen Josef , Neftel Albrecht , Sigg Andreas , Jacob Daniel J.
Published: February 2004

As part of the POLLUMET (Pollution and Meteorology in Switzerland) study, measurements of O3, NO2, and H2O2 were made at ground stations, by aircraft, and manned hydrogen balloons to characterize the photochemical processes prevailing during times of high ozone production. During summer smog episodes, elevated ozone concentrations were found over the whole area of the Swiss Plateau, which is a densely populated and industrialized region. However, NO2 concentrations were generally at a low level in the afternoon. An increase of hydrogen peroxide concentration in the mixing layer during photochemical episodes was observed. To understand better the photochemical processes which control ozone formation over the Swiss Plateau, calculations with the Harvard photochemical model and the Harwell photochemical trajectory model were conducted. A comparison of model results with measurements of the triad O3, NO2, and H2O2 indicates an ozone formation under low NOx conditions.
Seasonal transition from NOx‐ to hydrocarbon‐limited conditions for ozone production over the eastern United States in September
Authors: Jacob Daniel J. , Horowitz Larry W. , Munger J. William , Heikes Brian G. , Dickerson Russell R. , Artz Richard S. , Keene William C.
Published: February 2004

Concentrations of O3, CO, NO, total reactive nitrogen oxides (NOy), H2O2, and HCHO were measured from September 4 to October 1, 1990, at a mountain ridge site in Shenandoah National Park, Virginia. The data show evidence for a transition from NOx‐limited to hydrocarbon‐limited conditions for O3 production over the course of September. The transition is diagnosed by large decreases of the H2O2/(NOy‐NOx) and HCHO/NOy concentration ratios, weakening of the correlation between O3 and NOy‐ NOx concentrations, and decrease of the slope ΔO3/Δ(NOy‐NOx). A high‐O3 episode occurring in late September was associated with only 0.34 ppbv H2O2, indicative of hydrocarbon‐limited conditions. A seasonal transition in photochemical regime over the eastern United States in September would be expected from theory; the production rate of odd hydrogen radicals decreases by a factor of 2 over the course of the month, due to decreasing UV radiation and humidity, allowing HNO3 production to become the dominant sink for odd hydrogen in the boundary layer and resulting in hydrocarbon‐limited conditions for O3 production. Seasonal decline of isoprene emission can greatly accentuate the transition.
Table Mountain ozone intercomparison: Brewer ozone spectrophotometer Umkehr observations
Authors: McElroy C. T. , Kerr J. B.
Published: February 2004

During July and August 1989 a number of ozone profiling instruments were intercompared at Table Mountain Observatory in California. During the intercomparison a Brewer ozone spectrophotometer made observations of the total ozone amount and the vertical distribution of ozone. It was demonstrated that the Umkehr technique produced estimates of the ozone profile which were accurate to better than 15% in the 20‐ to 40‐km altitude range. Values determined for the 24‐ to 40‐km region were most accurate, within about 5% of the mean of the other instruments involved.
The annual cycle of stratospheric water vapor in a general circulation model
Authors: Mote Philip W.
Published: February 2004

The application of general circulation models (GCMs) to stratospheric chemistry and transport both permits and requires a thorough investigation of stratospheric water vapor. The National Center for Atmospheric Research has redesigned its GCM, the Community Climate Model (CCM2), to enable studies of the chemistry and transport of tracers including water vapor; the importance of water vapor to the climate and chemistry of the stratosphere requires that it be better understood in the atmosphere and well represented in the model. In this study, methane is carried as a tracer and converted to water; this simple chemistry provides an adequate representation of the upper stratospheric water vapor source. The cold temperature bias in the winter polar stratosphere, which the CCM2 shares with other GCMs, produces excessive dehydration in the southern hemisphere, but this dry bias can be ameliorated by setting a minimum vapor pressure. The CCM2's water vapor distribution and seasonality compare favorably with observations in many respects, though seasonal variations including the upper stratospheric semiannual oscillation are generally too small. Southern polar dehydration affects midlatitude water vapor mixing ratios by a few tenths of a part per million, mostly after the demise of the vortex. The annual cycle of water vapor in the tropical and northern midlatitude lower stratosphere is dominated by drying at the tropical tropopause. Water vapor has a longer adjustment time than methane and had not reached equilibrium at the end of the 9 years simulated here.
Validation of the NOAA/NESDIS satellite aerosol product over the North Atlantic in 1989
Authors: Ignatov Aleksandr M. , Stowe Larry L. , Sakerin Sergey M. , Korotaev Gennady K.
Published: February 2004

A validation experiment and resulting potential improvements to the operational satellite aerosol optical thickness product at the National Oceanic and Atmospheric Administration/National Environmental Satellite Data and Information Service (NOAA/NESDIS) are presented. An earlier paper described a set of Sun photometer measurements collected from the Soviet R/V Akademik Vernadsky during its cruise in the Atlantic Ocean and Mediterranean Sea from September to December 1989. The accuracy of the Sun photometer aerosol optical thickness was proven acceptable for use as a ground truth standard for validation of the NOAA product. This paper describes the validation methodology and the results of its application to the NOAA 11 satellite product. A systematic underestimation in the operational values by about 35%, relative to the ship truth, is found. Causes for this discrepancy are examined, emphasizing the importance of careful satellite instrument calibration, and a revision of the oceanic reflectance model used in the retrieval algorithm. It is shown that the remaining systematic underestimate in satellite aerosol optical thickness can be attributed only to the aerosol model used in the retrieval. Additional checks of this conclusion using independent data sets are underway. If confirmed, a fundamental revision of the presently used aerosol model would be required. An example of a simple adjustment to the present aerosol model which successfully removes the bias is given, based on the assumption of an absorbing aerosol.
Stability of the Martian atmosphere: Is heterogeneous catalysis essential?
Authors: Atreya Sushil K. , Gu Zhen Gang
Published: February 2004

A comprehensive homogeneous gas phase photochemical model is developed to study the problem of stability of the Martian atmosphere. The one‐dimensional model extends from the ground up to 220 km, passing through the homopause at 125 km. The model thus couples the lower (neutral) atmosphere to the ionosphere above which provides significant downward flux of carbon monoxide and oxygen atoms. It is concluded on the basis of currently accepted values for globally and seasonally averaged water vapor abundance, dust opacity and the middle atmospheric eddy mixing coefficient, as well as the relevant laboratory data (particularly the temperature dependence of CO2 absorption cross section and the rate constant for CO + OH reaction), that the rate of re‐formation of carbon dioxide exceeds its photolytic destruction rate by about 40%. Furthermore, it is found that this result is virtually independent of the choice of eddy mixing coefficient, unless its value in the middle atmosphere exceeds 108cm2s−1 or is far smaller than 105cm2s−1, or the dust opacity, unless it exceeds unity, or the water vapor mixing ratio at the surface, unless it is far smaller (≤1 ppm) or far greater (≥500 ppm) than the average value (∼150 ppm). Since none of these extremes represent globally and seasonally averaged conditions on Mars, we propose that the present model requires existence of a mechanism to throttle down the recycling rate of carbon dioxide on Mars. Therefore, it is suggested that a heterogeneous process which provides a sink to the species that participate in the recycling of CO2, i.e., H2O, H2O2, OH, CO or O, in particular, may be necessary to bring about the balance between the CO2 recycling rate and its photolytic destruction rate. Aerosols of dust or ice (pure or doped water or carbon dioxide ice present in the atmosphere of Mars) can provide the appropriate adsorption sites for the above heterogeneous process. Despite our conclusion that some heterogeneous process may be needed, it is important to recognize that one‐dimensional models can only provide first‐order results which, most likely, represent globally and seasonally averaged conditions. However, it is only after actual temporal, latitudinal and longitudinal variations of relevant atmospheric parameters are included in the model that one can determine fully whether the problem of atmospheric stability still continues to persist and whether some heterogeneous process is required to correct it.
Hydrous carbonates on Mars?: Evidence from Mariner 6/7 infrared spectrometer and ground‐based telescopic spectra
Authors: Calvin Wendy M. , King Trude V. V. , Clark Roger N.
Published: February 2004

Absorption features at 2.28 and 5.4 μm identified in Mariner 6/7 infrared spectrometer and terrestrial telescopic spectra are consistent with the spectra of hydrous magnesium carbonates such as hydromagnesite and artinite. Spectral characteristics of these hydrous carbonates are different from those of the anhydrous carbonates, as the former do not have the strong spectral features typically associated with anhydrous carbonates such as calcite and siderite. Theoretical mixing indicates that, depending on the type of hydrous carbonate, 10–20 wt % can be incorporated into the regolith without contradicting the spectral observations or the Viking x ray fluorescence chemical analysis. Hydrous carbonates form as weathering products of mafic minerals in the presence of H2O and CO2, even in the Antarctic. Their formation as evaporite minerals from either original magmas or hydrothermally altered rocks is consistent with the Martian environment, provided liquid water is or has been at least transiently present. On Earth, formation of hydrous Mg carbonates is associated with the production of amorphous iron oxides, which is consistent with both the environment and the inferred surface mineralogy of Mars. These minerals are about 60 wt % H2O, CO3, and OH; if they are abundant everywhere at the 10% level, then about 6% of the surface weight could be volatiles bound in this type of mineral. Although the stability of hydrous carbonates in a Martian environment is uncertain, there may be kinetic factors inhibiting the dehydration of these minerals, which may persist metastably in the current environment. Although the spectroscopic evidence for anhydrous carbonates is scant, the possible presence of hydrous carbonates provides an appealing mechanism for the existence of carbonates on Mars.
Near‐surface thermal gradients and their effects on mid‐infrared emission spectra of planetary surfaces
Authors: Henderson Bradley G. , Jakosky Bruce M.
Published: February 2004

We model the heat transfer by radiation and conduction in the top few millimeters of a planetary surface to determine the magnitude of near‐surface (≈100 μm) thermal gradients and their effects on mid‐infrared emission spectra for a number of planetary environments. The model is one‐dimensional and uses a finite difference scheme for ≈10‐μm layers. Calculations are performed for samples heated at the base and from above by sunlight. Our results indicate that near‐surface radiative cooling creates significant thermal gradients in the top few hundred microns of surfaces in which radiation is an important heat transfer mechanism. The effect is maximized in evacuated, underdense particulate media with sufficiently high temperatures. Near‐surface thermal gradients will be significant in fine‐grained particulate surfaces on the Moon (40–60 K/100 μm) and Mercury (≈80 K/100 μm), increasing spectral contrast and creating emission maxima in the transparent regions of the spectra. They will be of lesser importance on the surface of Mars, with a maximum value of around 5 K/100 μm in areas of low thermal inertia, and will be negligible on planets with more substantial atmospheres (<1 K/100 μm). We conclude that the effects that thermal gradients have on mid‐IR emission spectra are predictable and do not negate the utility of emission spectroscopy for remote determination of planetary surface composition.
Multispectral studies of western limb and farside maria from Galileo Earth‐Moon Encounter 1
Authors: Williams David A. , Greeley Ronald , Neukum Gerhard , Wagner Roland , Kadel Steven D.
Published: February 2004

New visible and near‐infrared multispectral images of the Moon obtained by the Galileo solid‐state imaging system, along with lunar orbiter images (for crater counts), and spectral mixing analyses were used to characterize western limb and eastern farside maria and determine compositional and age relationships in selected regions. Results indicate that (1) western limb mare deposits have less variability in titanium content (<2–7 wt% TiO2) and age (2.79–3.86 Ga) than areally extensive maria on the nearside; (2) areally extensive basin‐filling maria generally have higher titanium contents than smaller, crater‐filling mare patches and ponds; (3) ancient maria covered by highland material (cryptomaria) may be present in the Mendel‐Rydberg and South Pole‐Aitken basins; and (4) maria with compositional and age variations occur in the Grimaldi, Crüger, Mendel‐Rydberg, and Apollo regions. No extensive high‐titanium (>6 wt% TiO2) mare basalts were observed on the western limb and farside, which may reflect the inability of such denser magmas to penetrate the thicker farside crust.
Metalliferous asteroids as potential sources of precious metals
Authors: Kargel Jeffrey S.
Published: February 2004

Recent discoveries of near‐Earth asteroids (NEAs) and chemical analyses of fragments of asteroids (meteorites) suggest that there may be a gold mine, literally, in near‐Earth space. Judged from meteorite analyses, two types of asteroids offer particularly bright prospects for recovery of large quantities of precious metals (defined as Au, Pt, Ir, Os, Pd, Rh, and Ru), the ordinary LL chondrites, which contain 1.2–5.3% Fe‐Ni metal containing 50–220 ppm of precious metals, and metallic asteroids, which consist almost wholly of Fe‐Ni phases and contain variable amounts of precious metals up to several hundred ppm. The pulverized regolith of LL chondrite asteroids could be electromagnetically raked to separate the metallic grains. Suitable metallic asteroids could be processed in their entirety. Statistically, there should be approximately six metallic NEAs larger than 1 km in diameter that contain over 100 ppm of precious metals. Successful recovery of 400,000 tons or more of precious metals contained in the smallest and least rich of these metallic NEAs could yield products worth $5.1 trillion (US) at recent market prices. If marketed over 20 years, this would represent a 10‐fold increase over the recent global production rate of all precious metals combined. The market response to the hypothetical introduction of such large quantities of precious metals is difficult to predict, because these metals historically have not obeyed normal economic laws of the market. An empirical model suggests that the effects on market prices of a hypothetical increase in production of a given metal can be predicted given existing knowledge of the metal's present market value and production rate. This model suggests that the total value of 400,000 metric tons of precious metals, if marketed over 20 years, would decline to about $320 billion ($16 billion per year). Except for Au, for which production and prices would be marginally affected, the market prices of precious metals may decline by 1 to 2 orders of magnitude if one of the six asteroids were to be mined to depletion over 20 years. Less conservatively, there is a 50% chance that the richest metallic NEA contains at least 1.9 million metric tons of precious metals; this quantity, if marketed over 40 years, may be worth approximately $900 billion at collapsed market prices ($22.5 billion per year). The actual economic and technological impact of asteroidal metals may be considerably greater due to the increased availability and reduced prices of these resources. Despite this great potential, first‐order technological, scientific, and economic uncertainties remain before the feasibility of exploitation of asteroids for precious metals can be ascertained.
The role of rifting in the generation of melt: Implications for the origin and evolution of the Lada Terra‐Lavinia Planitia region of Venus
Authors: Magee Kari P. , Head James W.
Published: February 2004

A rift zone over 6000 km in total length runs along the border of Lada Terra, a highland in the southern hemisphere of Venus, and Lavinia Planitia, a basin that has been interpreted as a site of early‐stage mantle downwelling. Along the length of the rift are a number of volcanic centers of widely varying morphology and volcanic output. These include coronae, radially fractured domes, and large flow fields similar in scale to terrestrial flood basalts. We develop a model for the origin of extension related to passive rifting in response to stresses created by the adjacent downwelling. Volcanism and extension at other rifts on Venus, such as Devana Chasma, have been attributed to deep‐seated mantle plume activity. In contrast, we interpret the origin of extension and volcanism along the Lada rift to be linked to upwelling and decompression melting of mantle material due to rifting and, possibly, to counterflow associated with downwelling. Extension occurred generally prior to the formation of volcanic centers and the eruption of large‐scale flow fields, although most of the volcanic centers have been fractured by continued extension along the rift. Current debate over the formation of terrestrial flood basalts centers on the necessity of preexisting extension and stretched and thinned lithosphere to produce enhanced decompression melting within a large plume head or mantle thermal anomaly. Our studies of large‐scale flow fields associated with the Lada rift and coronae on Venus indicate that extension is a prerequisite for the formation of the majority of large‐scale flow units on Venus.
Asteroid orbit evolution due to thermal drag
Authors: Rubincam David Parry
Published: February 2004

Thermal drag, a variant of the Yarkovsky effect, may act on small asteroids with sizes from a few meters to a few tens of meters. Yarkovsky thermal drag comes from an asteroid's absorbing sunlight in the visible and reradiating it in the infrared. Since the infrared photons have momentum, by action‐reaction, they kick the asteroid when they leave its surface. The reradiation, which is asymmetric in latitude over the asteroid, gives a net force along the asteroid's pole. Due to the asteroid's thermal inertia, averaging this force over one orbital period produces a net drag if the spin axis has a component in the orbital plane. A regolith‐free basaltic asteroid 60 m in radius can shrink its semimajor axis by 2 AU (the distance from the asteroid belt to the Earth) over the age of the solar system. Regolith‐free iron asteroids evolve at about half the rate of basaltic ones. These calculations ignore planetary perturbations, collisions, erosion, etc. The rate of evolution varies inversely with the asteroid's radius for the size range considered here, so that smaller objects evolve faster than larger ones. The rate‐radius relation fails for objects smaller than a few meters because the thermal skin depth becomes comparable to the size of the asteroid. Basaltic asteroids covered by regoliths more than a few centimeters deep evolve much more slowly than regolith‐free ones. Thermal drag tends to circularize orbits. It can increase or decrease orbital inclinations. An object whose spin axis points in random directions over its lifetime displays little change in orbital inclination. Thermal drag appears to have little to do with the delivery of chondrites from the asteroid belt; the thermal drag timescale (108 years for meter‐sized objects) is long compared with their cosmic ray exposure ages, and aphelia in the asteroid belt are not expected for mature thermal drag orbits. However, Yarkovsky thermal drag may act on the recently discovered near‐Earth asteroids, which have radii of 10–30 m. Asteroid 1992 DA, for instance, might have its orbit shrunk by 0.1 AU in 3×107 years, removing it from an Earth‐crossing orbit. The near‐Earth asteroids also tend to have small to moderate orbital eccentricities, as expected for highly evolved thermal drag objects. However, the time needed to bring them in from the asteroid belt (about 109 years) is long compared with the collisional and dynamical lifetimes (both about 108 years) for Earth‐crossing objects, arguing against their emplacement by thermal drag.
Volatile transport on Venus and implications for surface geochemistry and geology
Authors: Brackett Robert A. , Fegley Bruce , Arvidson Raymond E.
Published: February 2004

The high vapor pressure of volatile metal halides and chalcogenides (e.g., of Cu, Zn, Sn, Pb, As, Sb, Bi) at typical Venus surface temperatures, coupled with the altitude‐dependent temperature gradient of ∼8.5 K km−1, is calculated to transport volatile metal vapors to the highlands of Venus, where condensation and accumulation will occur. The predicted geochemistry of volatile metals on Venus is supported by observations of Cu, Zn, Sn, Pb, As, Sb, and Bi minerals around terrestrial volcanic vents, spectroscopic observations of CuCl in volcanic gases at Kilauea and Nyiragongo, and large enrichments of these and other volatile elements in terrestrial volcanic aerosols. A one‐dimensional finite difference vapor transport model shows the diffusive migration of a thickness of 0.01 to >10 μm/yr of moderately to highly volatile phases (e.g., metal halides and chalcogenides) from the hot lowlands (740 K) to the cold highlands (660 K) on Venus. The diffusive transport of volatile phases on Venus may explain the observed low emissivity of the Venusian highlands, hazes at 6‐km altitude observed by two Pioneer Venus entry probes, and the Pioneer Venus entry probe anomalies at 12.5 km.
Western boundary currents in the Martian atmosphere: Numerical simulations and observational evidence
Authors: Joshi M. M. , Lewis S. R. , Read P. L. , Catling D. C.
Published: February 2004

Western boundary currents (hereafter WBCs) are an intensification of meridional fluid flow along the eastward facing flank of a boundary. They occur in the Earth's oceans, the best known instance being the Gulf Stream, and in the terrestrial atmosphere, an example of which is the East African Jet. We have investigated WBCs in numerical simulations of the Martian atmosphere, where they occur in the presence of large longitudinal topographical gradients, combined with the β effect. We suggest that WBCs have already been simulated by other Martian atmospheric models, although not identified as such. The intensity of these currents is dependent on basic model parameters, notably surface drag. We show that for physically reasonable values of drag, frictional forces dominate the WBCs' behavior. The requirement of zero net cross‐equatorial time‐mean mass flow produces a “return flow” in simulations with relatively low surface drag, which acts in opposition to the Tharsis WBC. We also show that the intense flow associated with WBCs advects potential vorticity across the equator, although the presence of strong radiative damping inhibits low‐level inertial instabilities that might result from this. Slope winds are found to have a profound effect on WBC structure, especially where they have a component parallel to the jet. In these cases, slope winds can cancel out or reinforce WBCs, depending on their direction. Enhanced low‐level winds associated with WBCs may be a factor that influences the locations of dust storm generation. The high vertical shear associated with WBCs may also play a role in the maintenance and subsequent decay of these dust storms. Some observational evidence may exist for WBCs in the form of the alignment of bright depositional dust streaks, wave clouds, and in wind measurements taken during the parachute descent phase of Viking Lander 1 through the planetary boundary layer.
The sensitivity of the Martian surface pressure and atmospheric mass budget to various parameters: A comparison between numerical simulations and Viking observations
Authors: Hourdin Frédéric , Forget François , Talagrand Olivier
Published: February 2004

The sensitivity of the Martian atmospheric circulation to a number of poorly known or strongly varying parameters (surface roughness length, atmospheric optical depth, CO2 ice albedo, and thermal emissivity) is investigated through experiments performed with the Martian version of the atmospheric general circulation model of Laboratoire de Météorologie Dynamique, with a rather coarse horizontal resolution (a grid with 32 points in longitude and 24 points in latitude). The results are evaluated primarily on the basis of comparisons with the surface pressure records of the Viking mission. To that end, the records are decomposed into long‐period seasonal variations due to mass exchange with the polar caps and latitudinal redistribution of mass, and short‐period variations due to transient longitudinally propagating waves. The sensitivity experiments include a 5‐year control simulation and shorter simulations (a little longer than 1 year) performed with “perturbed” parameter values. The main conclusions are that (1) a change of horizontal resolution (twice as many points in each direction) mostly affects the transient waves, (2) surface roughness lengths have a significant impact on the near‐surface wind and, as a matter of consequence, on the latitudinal redistribution of mass, (3) atmospheric dust optical depth has a significant impact on radiative balance and dynamics, and (4) CO2 ice albedo and thermal emissivity strongly influence mass exchange between the atmosphere and the polar caps. In view of this last conclusion, an automatic procedure is implemented through which the albedo and emissivity of each of the two polar caps are determined, together with the total (i.e., including the caps) atmospheric CO2 content, in such a way as to get the closest fit of the model to the Viking pressure measurements.
A slightly more massive young Sun as an explanation for warm temperatures on early Mars
Authors: Whitmire Daniel P. , Doyle Laurance R. , Reynolds Ray T. , Matese John J.
Published: February 2004

The valley network channels on the heavily cratered ancient surface of Mars suggest the presence of liquid water ∼3.8 Gyr ago. However, the implied warm climate is difficult to explain in the context of the standard solar model, even allowing for the maximum CO2 greenhouse heating. In this paper we investigate the astronomical and planetary implications of a nonstandard solar model in which the zero‐age, main‐sequence Sun had a mass of 1.05±0.02 M⊙. The excess mass was subsequently lost in a solar wind during the first 1.2+0.4−0.2 Gyr of the Sun's main sequence phase. The implied mass‐loss rate of 4−2+3 × 10−11 M⊙yr−1, or about 103× that of the current Sun, may be detectable in several nearby young solar type stars.
The albedo spectrum of Europa from 2200 Å to 3300 Å
Authors: Noll Keith S. , Weaver H. A. , Gonnella A. M.
Published: February 2004

We obtained a spectrum of Europa from 2224 to 3302 Å with high signal to noise and a resolution of 6.3 Å with the Hubble space telescope faint object spectrograph. We detect a broad absorption feature centered at 2800 Å, seen previously at lower signal to noise in International Ultraviolet Explorer spectra of Europa. No additional absorption features are detected. We compare the observed spectrum to laboratory spectra of sulfur ions in water ice and sulfur dioxide ice on water ice. The absorption in Europa is a closer match to sulfur dioxide ice. The poor fit of ice irradiated by sulfur ions may indicate that a direct source of SO2 is required to explain the observed absorption.
Milankovitch band forcing of sub‐Milankovitch climate variability during the Pleistocene
Authors: Hagelberg Teresa K. , Bond Gerard , deMenocal Peter
Published: February 2004

Climate variability at periods from 10 kyr to 12 kyr that originates from Milankovitch band forcing is quantified at three locations for the late Pleistocene (eastern equatorial Pacific Ocean Drilling Program site 846, eastern equatorial Atlantic ODP site 663, and northeastern Atlantic Deep Sea Drilling Project site 609). Variability at these periods is not present in the primary Milankovitch forcing, so no linear linkage to Milankovitch band variations is possible. However, these periods are equal to harmonics of precession band oscillations. The magnitude of interactions between processes that occur at different timescales can only be resolved in time series data through application of higher‐order statistics. Through such an application, we demonstrate that up to 75% of the variance in the 10‐ to 12‐kyr band in the sediment records is nonlinearly transferred from precession band (19–23 kyr) variations. Within the millennial to sub‐Milankovitch band, defined as the band of variance ranging from ∼15 to ∼2 kyr, approximately 1/3 of the variability in the records studied is consistent with a low‐frequency, Milankovitch band origin. This variability may derive from high sensitivity of the tropics to summertime insolation in both hemispheres relative to wintertime insolation. A mechanism having equatorial origin and related to low‐latitude precession variations appears consistent with the observations. Because the phase coupling between 10‐ to 12‐kyr oscillations and precession is resolved, this result has implications for development of models which seek to explain global climate variations on this timescale.
Late Cenozoic sediment drift complex, northeast Rockall Trough, North Atlantic
Authors: Howe J. A. , Stoker M. S. , Stow D. A. V.
Published: February 2004

Seismic reflection profiles and two shallow cores have revealed a sediment drift complex in the northeastern Rockall Trough.The drift complex consists of an elongate drift with associated sediment waves, a broad sheeted drift with a larger field of sediment waves, and smaller, moat‐related, isolated drifts. The internal reflection configuration of these features indicates consistent upslope migration throughout the late Cenozoic. The larger field of sediment waves has been described previously with a southerly migration; a reevalution of these waves suggests a migration direction to the east, conforming with the other drift complex features. The most vigorous current activity probably occurred during the Miocene, with a more reduced current flow prevailing during the Pliocene to Holocene interval. Core evidence from the elongate drift and the moat has revealed upper Pleistocene and lower Holocene glaciomarine sediments reworked to produce muddy‐silty and sandy contourites. Sedimentation rates for the early Holocene (pre‐7.5 ka) are up to 4‐times greater on the drift compared to the moat. It is suggested that the sediment drift complex has formed through the interaction of a northward flowing slope current, of North Atlantic Deep Water origin, with an area of complex bathymetry at the northeastern end of the Rockall Trough, where the Wyville‐Thomson Ridge intersects the Hebridean Margin.
Abyssal sediment waves in the Gulf of Mexico: An enigma
Authors: Behrens E. William
Published: February 2004

A sediment wave field in the western Gulf of Mexico is similar to many other fields that are located on continental rises and related to thermohaline contour currents, especially western boundary undercurrents. Such currents should not occur in the Gulf, because it is silled. At present, the only postulated source of deep currents in the Gulf is the Loop Current (in the eastern half), eddies detached from it, and Rossby waves but no thermohaline currents. If any of the postulated processes cause the mud waves, it is a new mode of formation. If they do not, much more needs to be learned about deep Gulf circulation. These questions are important, because the sediment waves represent paleoceanographic conditions that have persisted for over 5 million years.
Timescale and paleoceanographic implications of a 3.6 m.y. oxygen isotope record from the northeast Indian Ocean (Ocean Drilling Program Site 758)
Authors: Chen Jiajie , Farrell John W. , Murray David W. , Prell Warren L.
Published: February 2004

Numerous studies have shown that δ18O records from benthic and planktonic foraminifera, primarily a proxy of global ice volume variations, reflect Milankovitch periodicities. To study climatic response to orbital forcing at Ocean Drilling Program site 758, we have generated continuous δ18O and δ13C records from a single benthic foraminiferal species Cibicides wuellerstorfi for the last 3.6 m.y. and extended the planktonic foraminiferal isotope records of Farrell and Janecek (1991) (0‐2.5 Ma, based on Globigerinoides sacculifer) to 3.6 Ma (Chen, 1994). We then constructed an age model by matching, correlating and tuning the benthic δ18O record to a model simulation of ice volume (Imbrie and Imbrie, 1980). The filtered 41‐ and 23‐kyr signals based on the resultant astronomically tuned age model are highly correlated to obliquity (r=0.83) and precession (r=0.75), respectively. Although derived with methodology different from Shackleton et al. (1990) and Hilgen (1991a, b), our results generally agree with their published astronomical timescales for the time interval from 0 to 3.0 Ma, providing additional support for the newly emerging chronology based on orbital tuning. Slight discrepancies exist in the time interval from 3.0 to 3.6 Ma, suggesting several possibilities, including differences in the approaches of orbital tuning and the relatively low amplitude of δ18O variations in our record. However, even if the discrepancies are due to the relatively low amplitude of the isotope signals in our record at 3.0–3.6 Ma, our resultant timescale as a whole does not adversely affect our evaluation of the paleoclimatology and paleoceanography of the Indian Ocean, such as the evolution of the 100‐, 41‐ and 23‐kyr cycles, and variation of global ice volume and deepwater temperature during the past 3.6 m.y.
Magnetic susceptibility variations in Upper Pleistocene deep‐sea sediments of the NE Atlantic: Implications for ice rafting and paleocirculation at the Last Glacial Maximum
Authors: Robinson Simon G. , Maslin Mark A. , McCave I. Nicholas
Published: February 2004

Magnetic susceptibility (MS) variations are used to intercorrelate 17 Upper Pleistocene sediment cores taken from the NE Atlantic, between 40° and 60°N. The MS‐based correlation depends on regionally consistent patterns of variation in the deposition of ice‐rafted detritus (IRD) in response to Pleistocene glaciations, and especially to high‐frequency ice‐rafting episodes referred to in recent studies as “Heinrich events.” The sedimentological and rock‐magnetic basis for the apparent relationship between the MS signal and IRD content of NE Atlantic sediments is examined by (1) comparing the MS profiles of selected cores with their records of coarse fraction (> 150 µm) lithic fragment abundance and Neogloboquadrina pachyderma (sin) percentages, and (2) normalizing MS by expressing it both on a carbonate‐free basis, and as a quotient with anhysteretic remanent magnetization (a parameter sensitive to magnetic mineral grain size variations). These comparisons show that variations in bulk‐sediment MS are only partly driven by simple carbonate dilution (± productivity and dissolution) effects. Changes in both the concentration and grain size of magnetic minerals within the lithogenic noncarbonate fraction also impose a significant influence on bulk MS values. In particular, horizons rich in IRD are associated with significant increases in the relative proportion of coarse grained (multidomain) ferrimagnetic particles in the sediment. This is because ice‐rafting, in contrast to most other mechanisms capable of transporting detrital magnetic minerals to pelagic environments, has a high potential for delivering large ferrimagnetic grains as components of sand‐sized, polycrystalline lithic fragments. This fundamental linkage between the IRD content and MS signal of NE Atlantic sediments is used to reconstruct the patterns of variation in IRD deposition and, by inference, surface currents of the last glacial maximum (LGM, ∼18–19 ka) relative to the present‐day NE Atlantic, using the time‐slice mapping approach developed by the CLIMAP project group. Our LGM/Holocene MS ratio map, based on sample pairs from over 80 deep‐sea cores, confirms that there was a weak, cyclonic gyre north of the polar front in the LGM North Atlantic. The gyre comprised a sluggish warm current in the NE Atlantic flowing north between latitudes 47° and 62°N, partly fed by subtropical waters from south of the polar front, and carrying large numbers of icebergs derived from several sources, most of which melted between Latitudes 45° and 52°N. The warm current probably continued its flow into the Iceland Basin, where it fed into a south‐flowing current which transported melting icebergs from Iceland and Scandinavia along the western flank of the Reykjanes Ridge.
Comparison of plasma drift modeling to DE 2 data
Authors: Carter L. N. , Anderson D. N.
Published: February 2004

Nighttime zonal plasma drifts in the equatorial region are calculated from a simplified expression involving Pedersen conductivity, geomagnetic field strength, and zonal neutral wind, integrated along flux tubes between apex altitudes of 250–1500 km. The ambient conditions correspond to solar maximum at equinox over the American sector with vertical electron density profiles derived from average drift measured over Jicamarca. Plasma drift outputs from four neutral wind models are evaluated for overall consistency and compared to similar data from the DE 2 satellite collected during solar cycle 21 from 1.5 to 3 years after the solar maximum peak. Drift velocities are displayed to compare model output against the DE 2 measurements. Model inputs of solar maximum with vertical electron drift reduced to one‐half average strength and a heuristically derived wind are found to give very good agreement with the DE 2 results. The implicit conclusion is that this final model is most representative of actual thermospheric winds for the conditions of near‐solar maximum at equinox in the nighttime sector.