Titan's greenhouse and antigreenhouse effects     

McKay CP  Pollack JB  Courtin R 《The Planetary report》1992,12(3):4-7

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The variety of lithologies in the Yamato-86032 lunar meteorite: Implications for formation processes of the lunar crust     

A. Yamaguchi  Y. Karouji  H. Takeda  D. Bogard  C.-Y. Shih  D. Garrison  G. McKay 《Geochimica et cosmochimica acta》2010,74(15):4507-4530

We performed a petrologic, mineralogical, geochemical, and isotopic study of several lithologies in the Y-86032 feldspathic breccia. This study leads us to conclude that Y-86032 likely originated on the lunar farside. Y-86032 is composed of several types of feldspathic clasts, granulitic breccias, and minor basaltic clasts set in a clastic matrix. We identify an “An97 anorthosite” that has An contents similar to those of nearside FANs. Mg′ (= molar Mg/(Mg + Fe) × 100) values vary significantly from ∼45 to ∼80 covering the ranges of both nearside FANs and the Mg′ gap between FANs and the Mg-suite. A light-gray feldspathic (LG) breccia making up ∼20% of the investigated slab (5.2 × 3.6 cm²) mainly consists of fragments of anorthosites (“An93 anorthosite”) more sodic than nearside FANs. LG also contains an augite-plagioclase clast which either could be genetically related to the An93 anorthosite or to slowly-cooled basaltic magma intruded into the precursor rock. The Na-rich nature of both An93 anorthosite and this clast indicates that the LG breccia was derived from a relatively Na-rich but incompatible-element-poor source. The Mg′ variation indicates that the “An97 anorthosite” is a genomict breccia of several types of primary anorthosites. Granulitic breccias in Y-86032 have relatively high Mg′ in mafic minerals. The highest Mg′ values in mafic minerals for the “An97 anorthosite” and granulitic breccias are similar to those of Mg-rich lithologies recently described in Dhofar 489. Basaltic clasts in the dark-gray matrix are aluminous, and the zoning trends of pyroxene are similar to those of VLT or LT basalts. The crystallization of these basaltic clasts pre-date the lithification age of the clastic matrix at ∼3.8 Ga. The low K contents of plagioclase in both the anorthositic and basaltic clasts and generally low incompatible element abundances in all the lithologies in Y-86032 indicate that KREEP was not involved during the formation of the precursor lithologies. This observation further suggests that urKREEP did not exist in the source regions of these igneous lithologies. All these facts support the idea that Y-86032 was derived from a region far distant from the PKT and that the lithic clasts and fragments are indigenous to that region. An An97 anorthositic clast studied here has distinct Sm-Nd isotopic systematics from those previously found for another An97 anorthositic clast and “An93 anorthosite”, and suggests either that An97 anorthosites come from isotopically diverse sources, or that the Sm-Nd isotopic systematics of this clast were reset ∼4.3 Ga ago. These lines of geochemical, isotopic, and petrologic evidence suggest that the lunar crust is geochemically more heterogeneous than previously thought.  相似文献   

A simple expression for vertical convective fluxes in planetary atmospheres   总被引：1，自引：0，他引：1  

Ralph D Lorenz  Christopher P McKay 《Icarus》2003,165(2):407-413

We explore the vertical convective flux F_c in a radiative-convective grey atmosphere. An expression of the form F_c=F_sτ_o/(C+Dτ_o) appears useful, where F_s is the shortwave flux absorbed at the base of an atmosphere with longwave optical depth τ_o and C and D are constants. We find excellent agreement with an idealized grey radiative-convective model with no shortwave absorption for D=1 and C=1∼2 depending on the surface-atmosphere temperature contrast and on the imposed critical lapse rate. Where shortwave absorption is correlated with longwave opacity, as in the atmospheres of Earth and Titan, C=2, D=2 provides an excellent fit, validated against the present terrestrial situation and the results of a nongrey model of Titan's strongly antigreenhouse atmosphere under a wide range of conditions. The expression may be useful for studying the energetics of planetary climates through time where there is insufficient data to constrain more elaborate models.  相似文献   

ON-LINE BIBLIOGRAPHIC SYSTEMS     

Duncan McKay 《地学学报》1991,3(3):325-325

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Some thoughts on the role of robust control theory in climate-related decision support     

Robert Lempert  Shawn McKay 《Climatic change》2011,107(3-4):241-246

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New insights into the origin and evolution of Lake Vida,McMurdo Dry Valleys,Antarctica — A noble gas study in ice and brines     

Jessica L. Malone  M. Clara Castro  Chris M. Hall  Peter T. Doran  Fabien Kenig  Chris P. McKay 《Earth and Planetary Science Letters》2010,289(1-2):112-122

Unlike other lakes in the McMurdo Dry Valleys, Antarctica, Lake Vida has a thick (~ 19 m) ice cover sealing a liquid brine body of unusually high salinity (~ 245 g/L) from the atmosphere. To constrain the conditions under which the atypical Lake Vida ice cover formed and evolved, 19 ice samples were collected down to a depth of ~ 14 m, together with three brine samples trapped in the ice at ~ 16 m for analysis of helium, neon, argon, krypton, and xenon concentrations. The broad pattern of noble gas concentrations for Lake Vida samples is fundamentally different from that of air saturated water (ASW) at 0 °C and an elevation of 340 m for salinities of 0 (ice) and 245 g/L (brine). Overall, ice samples are enriched in He and depleted in Ne with saturation relative to ASW averages of 1.38 and 0.82, respectively, and strongly depleted in Ar, Kr, and Xe with relative saturations of 0.10, 0.06, and 0.05, respectively. By contrast, brine samples are generally depleted in He and Ne (relative saturation averages of 0.33 and 0.27, respectively) but enriched in Ar, Kr, and Xe, with relative saturation averages of 1.45, 3.15, and 8.86, respectively. A three-phase freezing partitioning model generating brine, ice and bubble concentrations for all stable noble gases was tested and compared with our data. Measured brine values are best reproduced for a salinity value of 175 g/L, a pressure of 1.1 atm, and a bubble volume of 20 cm³ kg^?1. Sensitivity tests for ice + bubble samples show an ideal fit for bubble volumes of ~ 1–2 cm³ kg^?1. Our results show that the conditions under which ice and brine formed and evolved at Lake Vida are significantly different from other ice-covered lakes in the area. Our brine data suggest that Lake Vida may be transitioning from a wet to a dry-based lake, while the ice + bubble data suggest at least partial re-equilibration of residual liquid with the atmosphere as ice forms at the top of Lake Vida ice cover.  相似文献   

Processes controlling rapid temperature variations on rock surfaces     

Jamie L. Molaro  Christopher P. McKay 《地球表面变化过程与地形》2010,35(5):501-507

In arid environments, thermal oscillations are an important source of rock weathering. Measurements of temperature have been made on the surface of rocks in a desert environment at a sampling interval of 0·375 s, with simultaneous measurements of wind speed, air temperature, and incoming shortwave radiation. Over timescales of hours, the temperature of the rock surface was determined primarily by shortwave radiation and air temperature, while rapid temperature variations, high dT/dt, at intervals of seconds or less, were determined by wind speed. The maximum values of temperature change and time spent above 2°C min^?1 increased at high measurement rates and were much higher than previously reported. The maximum recorded value of dT/dt was 137°C min^?1 and the average percentage time spent above 2°C min^?1 was ～70 ± 13%. Maximum values of dT/dt did not correlate with the maximum values of time spent above 2°C min^?1. Simultaneous measurements of two thermocouples 5·5 cm apart on a single rock surface had similar temperature and dT/dt values, but were not correlated at sampling intervals of less than 10 s. It is suggested that this is resulting from rapid fluctuations due to small spatial and timescale wind effects that are averaged out when data is taken at longer sampling intervals, ～10 s or greater. Published in 2010 by John Wiley & Sons, Ltd.  相似文献   

Origins of magnetite nanocrystals in Martian meteorite ALH84001     

K.L. Thomas-Keprta  S.J. Clemett  D.S. McKay  S.J. Wentworth 《Geochimica et cosmochimica acta》2009,73(21):6631-5335

The Martian meteorite ALH84001 preserves evidence of interaction with aqueous fluids while on Mars in the form of microscopic carbonate disks. These carbonate disks are believed to have precipitated 3.9 Ga ago at beginning of the Noachian epoch on Mars during which both the oldest extant Martian surfaces were formed, and perhaps the earliest global oceans. Intimately associated within and throughout these carbonate disks are nanocrystal magnetites (Fe₃O₄) with unusual chemical and physical properties, whose origins have become the source of considerable debate. One group of hypotheses argues that these magnetites are the product of partial thermal decomposition of the host carbonate. Alternatively, the origins of magnetite and carbonate may be unrelated; that is, from the perspective of the carbonate the magnetite is allochthonous. For example, the magnetites might have already been present in the aqueous fluids from which the carbonates were believed to have been deposited. We have sought to resolve between these hypotheses through the detailed characterization of the compositional and structural relationships of the carbonate disks and associated magnetites with the orthopyroxene matrix in which they are embedded. Extensive use of focused ion beam milling techniques has been utilized for sample preparation. We then compared our observations with those from experimental thermal decomposition studies of sideritic carbonates under a range of plausible geological heating scenarios. We conclude that the vast majority of the nanocrystal magnetites present in the carbonate disks could not have formed by any of the currently proposed thermal decomposition scenarios. Instead, we find there is considerable evidence in support of an alternative allochthonous origin for the magnetite unrelated to any shock or thermal processing of the carbonates.  相似文献   

The role of organic haze in Titan's atmospheric chemistry: II. Effect of heterogeneous reaction to the hydrogen budget and chemical composition of the atmosphere     

Yasuhito Sekine  Sébastien Lebonnois  Takafumi Matsui  Christopher P. McKay  Seiji Sugita 《Icarus》2008,194(1):201-211

One of the key components controlling the chemical composition and climatology of Titan's atmosphere is the removal of reactive atomic hydrogen from the atmosphere. A proposed process of the removal of atomic hydrogen is the heterogeneous reaction with organic aerosol. In this study, we investigate the effect of heterogeneous reactions in Titan's atmospheric chemistry using new measurements of the heterogeneous reaction rate [Sekine, Y., Imanaka, H., Matsui, T., Khare, B.N., Bakes, E.L.O., McKay, C.P., Sugita, S., 2008. Icarus 194, 186-200] in a one-dimensional photochemical model. Our results indicate that 60-75% of the atomic hydrogen in the stratosphere and mesosphere are consumed by the heterogeneous reactions. This result implies that the heterogeneous reactions on the aerosol surface may predominantly remove atomic hydrogen in Titan's stratosphere and mesosphere. The results of our calculation also indicate that a low concentration of atomic hydrogen enhances the concentrations of unsaturated complex organics, such as C₄H₂ and phenyl radical, by more than two orders in magnitude around 400 km in altitude. Such an increase in unsaturated species may induce efficient haze production in Titan's mesosphere and upper stratosphere. These results imply a positive feedback mechanism in haze production in Titan's atmosphere. The increase in haze production would affect the chemical composition of the atmosphere, which might induce further haze production. Such a positive feedback could tend to dampen the loss and supply cycles of CH₄ due to an episodic CH₄ release into Titan's atmosphere.  相似文献   

Experimental impact shock chemistry on planetary icy satellites     

Delphine Nna-Mvondo  Bishun Khare  Tomoko Ishihara  Christopher P. McKay 《Icarus》2008,194(2):822-835

We present new experimental results on impact shock chemistry into icy satellites of the outer planets. Icy mixtures of pure water ice with CO₂, Na₂CO₃, CH₃OH, and CH₃OH/(NH₄)₂SO₄ at 77 K were ablated with a powerful pulsed laser—a new technique used to simulate shock processes which can occur during impacts. New products were identified by GC-MS and FTIR analyses after laser ablation. Our results show that hydrogen peroxide is formed in irradiated H₂O/CO₂ ices with a final concentration of 0.23%. CO and CH₃OH were also detected as main products. The laser ablation of frozen H₂O/Na₂CO₃ generates only CO and CO₂ as destruction products from the salt. Pulsed irradiation of water ice containing methanol leads also to the formation of CO and CO₂, generates methane and more complex molecules containing carbonyl groups like acetaldehyde, acetone, methyl formate, and a diether, dimethyl formal. The last three compounds are also produced when adding ammonium sulfate to H₂O/CH₃OH ice, but acetone is more abundant. The formation of two hydrocarbons, CH₄ and C₂H₆ is observed as well as the production of three nitrogen compounds, nitrous oxide, hydrogen cyanide, and acetonitrile.  相似文献