Monte Carlo modeling of sodium in Mercury’s exosphere during the first two MESSENGER flybys     

Matthew H. Burger  Rosemary M. Killen  E. Todd Bradley  Menelaos Sarantos  Nelly Mouawad 《Icarus》2010,209(1):63-74

We present a Monte Carlo model of the distribution of neutral sodium in Mercury’s exosphere and tail using data from the Mercury Atmospheric and Surface Composition Spectrometer (MASCS) on the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft during the first two flybys of the planet in January and September 2008. We show that the dominant source mechanism for ejecting sodium from the surface is photon-stimulated desorption (PSD) and that the desorption rate is limited by the diffusion rate of sodium from the interior of grains in the regolith to the topmost few monolayers where PSD is effective. In the absence of ion precipitation, we find that the sodium source rate is limited to ∼10⁶-10⁷ cm⁻² s⁻¹, depending on the sticking efficiency of exospheric sodium that returns to the surface. The diffusion rate must be at least a factor of 5 higher in regions of ion precipitation to explain the MASCS observations during the second MESSENGER flyby. We estimate that impact vaporization of micrometeoroids may provide up to 15% of the total sodium source rate in the regions observed. Although sputtering by precipitating ions was found not to be a significant source of sodium during the MESSENGER flybys, ion precipitation is responsible for increasing the source rate at high latitudes through ion-enhanced diffusion.  相似文献   

Water ice cloud formation on Mars is more difficult than presumed: Laboratory studies of ice nucleation on surrogate materials     

Laura T. Iraci  Bruce D. Phebus  Bradley M. Stone  Anthony Colaprete 《Icarus》2010,210(2):985-991

The role of water ice clouds in the martian water cycle and climate depends on cloud properties such as particle size and number distribution. These properties, in turn, depend on heterogeneous nucleation parameters which are poorly understood. Here we report laboratory experiments performed under martian temperature and water partial pressure conditions (158–185 K, 9 × 10⁻⁷–1 × 10⁻⁴ Torr H₂O) to determine the critical saturation ratio for ice onset, S_crit, as a function of temperature and dust composition. Using infrared spectroscopy to monitor ice nucleation and growth, we find a significant barrier to ice formation, with a pronounced temperature dependence. Even on clay minerals which show uptake of non-crystalline water before ice nucleation, we find a saturation ratio of 2.5 or more (RH_ice > 250%) is needed to begin ice growth at temperatures near 160 K. These results could lead to changes of four orders of magnitude in the nucleation rate relative to the presumptions used currently in Mars microphysical models, which commonly set the contact parameter, m, to a single value of 0.95. Our results range from m = 0.84 to m = 0.98. For ice nucleation on Arizona Test Dust, the temperature dependence is described by m = 0.0046 * T_nucl + 0.1085, while m = 0.0055 * T_nucl + 0.0003 on a smectite-rich clay sample. Our findings suggest that cloud formation will be more difficult than previously thought, potentially leading to areas of increased near-surface humidity but generally drier conditions in the atmosphere of Mars, overall.  相似文献   

Preface   总被引：1，自引：0，他引：1  

Raymond S. Bradley 《Climatic change》1999,43(1):1-2

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Mississippi Valley-type lead–zinc deposits through geological time: implications from recent age-dating research   总被引：11，自引：0，他引：11  

David L. Leach  Dwight Bradley  Michael T. Lewchuk  David T. Symons  Ghislain de Marsily  Joyce Brannon 《Mineralium Deposita》2001,36(8):711-740

Remarkable advances in age dating Mississippi Valley-type (MVT) lead-zinc deposits provide a new opportunity to understand how and where these deposits form in the Earth's crust. These dates are summarized and examined in a framework of global tectonics, paleogeography, fluid migration, and paleoclimate. Nineteen districts have been dated by paleomagnetic and/or radiometric methods. Of the districts that have both paleomagnetic and radiometric dates, only the Pine Point and East Tennessee districts have significant disagreements. This broad agreement between paleomagnetic and radiometric dates provides added confidence in the dating techniques used. The new dates confirm the direct connection between the genesis of MVT lead-zinc ores with global-scale tectonic events. The dates show that MVT deposits formed mainly during large contractional tectonic events at restricted times in the history of the Earth. Only the deposits in the Lennard Shelf of Australia and Nanisivik in Canada have dates that correspond to extensional tectonic events. The most important period for MVT genesis was the Devonian to Permian time, which corresponds to a series of intense tectonic events during the assimilation of Pangea. The second most important period for MVT genesis was Cretaceous to Tertiary time when microplate assimilation affected the western margin of North America and Africa-Eurasia. There is a notable paucity of MVT lead-zinc ore formation following the breakup of Rodinia and Pangea. Of the five MVT deposits hosted in Proterozoic rocks, only the Nanisivik deposit has been dated as Proterozoic. The contrast in abundance between SEDEX and MVT lead-zinc deposits in the Proterozoic questions the frequently suggested notion that the two types of ores share similar genetic paths. The ages of MVT deposits, when viewed with respect to the orogenic cycle in the adjacent orogen suggest that no single hydrologic model can be universally applied to the migration of the ore fluids. However, topographically driven models best explain most MVT districts. The migration of MVT ore fluids is not a natural consequence of basin evolution; rather, MVT districts formed mainly where platform carbonates had some hydrological connection to orogenic belts. There may be a connection between paleoclimate and the formation of some MVT deposits. This possible relationship is suggested by the dominance of evaporated seawater in fluid inclusions in MVT ores, by hydrological considerations that include the need for multiple-basin volumes of ore fluid to form most MVT districts, and the need for adequate precipitation to provide sufficient topographic head for topographically-driven fluid migration. Paleoclimatic conditions that lead to formation of evaporite conditions but yet have adequate precipitation to form large hydrological systems are most commonly present in low latitudes. For the MVT deposits and districts that have been dated, more than 75% of the combined metal produced are from deposits that have dates that correspond to assembly of Pangea in Devonian through Permian time. The exceptional endowment of Pangea and especially, North America with MVT lead-zinc deposits may be explained by the following: (1) Laurentia, which formed the core of North America, stayed in low latitudes during the Paleozoic, which allowed the development of vast carbonate platforms; (2) intense orogenic activity during the assembly of Pangea created ground preparation for many MVT districts through far-field deformation of the craton; (3) uplifted orogenic belts along Pangean suture zones established large-scale migration of basin fluids; and (4) the location of Pangea in low latitudes with paleoclimates with high evaporation rates led to the formation of brines by the evaporation of seawater and infiltration of these brines into deep basin aquifers during Pangean orogenic events.  相似文献   

High resolution flow modelling in hydrology and geomorphology edited by P.D. Bates and S.N. Lane. John Wiley,Chichester, 2000. No. of pages: 374. Price: £45·00. ISBN 0 471 97875 2.     

C. Bradley 《地球表面变化过程与地形》2001,26(4):457-457

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The Matuyama-Brunhes reversal at Tecopa basin, southeastern California, revisited again     

Edwin E. Larson  Penny E. Patterson 《Earth and Planetary Science Letters》1993,120(3-4):311-325

Numerous records of the Matuyama-Brunhes geomagnetic transition have been obtained from paleomagnetic studies. Because few of the reversal records are of acceptable reliability, however, the exact behavior of the field during the transition has remained enigmatic. To provide confirmation of one of the more reliable records, we have re-examined the transition at two sites, 150 m apart, in lake sediments of Tecopa basin, southeastern California. The two sites are geographically very close to that of Valet et al. [10], who previously obtained a record from that site indicating that the transitional field was non-dipolar and axisymmetric.

The Matuyama-Brunhes reversal is recorded differently at each of our two sites and at that of Valet et al. [10]. Zones of mixed polarities and/or intermediate directions occur at all three sites but they differ greatly in polarity character, thickness and stratigraphic position. It appears that all three sites have provided mutually contradictory records of the transition. It is unlikely, therefore, that any of the records is acceptable for establishing the nature of the transition at this locality.

Obliteration of the transition is apparently the result of acquisition of a stable, normal-polarity overprint that appears to consist of two remanence components, one acquired during post-depositional compaction and dewatering, and one during later sediment diagenesis.  相似文献   

Solar noble gases in the Earth: The systematics of helium-neon isotopes in mantle derived samples   总被引：2，自引：0，他引：2  

Masahiko Honda  Ian McDougall  Desmond Patterson 《Lithos》1993,30(3-4):257-265

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Acceptance speech for the V. M. Goldschmidt medal     

Clair C. Patterson 《Geochimica et cosmochimica acta》1981,45(8):1385-1388

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Turbulence Reynolds number and the turbulent kinetic energy balance in the atmospheric surface layer     

E. F. Bradley  R. A. Antonia  A. J. Chambers 《Boundary-Layer Meteorology》1981,21(2):183-197

The relation between the turbulence Reynolds numberR and a Reynolds numberz* based on the friction velocity and height from the ground is established using direct measurements of the r.m.s. longitudinal velocity and turbulent energy dissipation in the atmospheric surface layer. Measurements of the relative magnitude of components of the turbulent kinetic energy budget in the stability range 0 >z/L 0.4 indicate that local balance between production and dissipation is maintained. Approximate expressions, in terms of readily measured micrometeorological quantities, are proposed for the Taylor microscale and the Kolmogorov length scale .  相似文献   

Secondary flows in the lee of porous shelterbelts     

P. J. Mulhearn  E. F. Bradley 《Boundary-Layer Meteorology》1977,12(1):75-92

The results of wind-tunnel experiments indicate that the flow close to the surface behind shelterbelts is quite sensitive to the orientation of the barrier to the mean flow direction, and that orientation is more important for short than long windbreaks. The implications of these findings are discussed in the context of field experiments to validate mathematical models of shelter effects.  相似文献