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171.
Edgar L Andreas 《Icarus》2007,186(1):24-30
The strong hydrogen signal that the Lunar Prospector saw at the Moon's poles suggests that water ice may be present near the surface of the lunar regolith. A robotic mission to obtain in situ samples and to quantify the amount of this valuable resource must be designed carefully to avoid dissipating too much heat in the regolith during coring or drilling and, thus, causing the ice to sublimate before it is processed. Here I use new results for the saturation vapor pressure of water ice to extend previous estimates of its sublimation rate down to a temperature of 40 K, typical of the permanently shaded craters near the lunar poles where the water ice is presumed to be trapped. I find that, for temperatures below 70 K, the sublimation rate of an exposed ice surface is much less than one molecule of water vapor lost per square centimeter of surface per hour. But even if a small ice sample (∼4 ng) were heated to 150 K, it could exist for over two hours without sublimating a significant fraction of its mass. Hence, carefully designed sampling and sample handling should be able to preserve water ice obtained near the lunar poles for an accurate measurement of its in situ concentration. 相似文献
172.
M. R. Truss J. R. Murray G. A. Wynn R. G. Edgar 《Monthly notices of the Royal Astronomical Society》2000,319(2):467-476
We present a numerical scheme for the evolution of an accretion disc through a dwarf nova outburst. We introduce a time-varying artificial viscosity into an existing smoothed particle hydrodynamics code optimized for two- and three-dimensional simulations of accretion discs. The technique gives rise to coherent outbursts and can easily be adapted to include a complete treatment of thermodynamics. We apply a two-dimensional isothermal scheme to the system SS Cygni, and present a wide range of observationally testable results. 相似文献
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Affluence drives the global displacement of land use 总被引:2,自引:0,他引:2
Jan Weinzettel Edgar G. Hertwich Glen P. Peters Kjartan Steen-Olsen Alessandro Galli 《Global Environmental Change》2013,23(2):433-438
Increasing affluence is often postulated as a main driver for the human footprint on biologically productive areas, identified among the main causes of biodiversity loss, but causal relationships are obscured by international trade. Here, we trace the use of land and ocean area through international supply chains to final consumption, modeling agricultural, food, and forestry products on a high level of resolution while also including the land requirements of manufactured goods and services. In 2004, high-income countries required more biologically productive land per capita than low-income countries, but this connection could only be identified when land used to produce internationally traded products was taken into account, because higher-income countries tend to displace a larger fraction of land use. The equivalent land and ocean area footprint of nations increased by a third for each doubling of income, with all variables analyzed on a per capita basis. This increase came largely from imports, which increased proportionally to income. Export depended mostly on the capacity of countries to produce useful biomass, the biocapacity. Our analysis clearly shows that countries with a high biocapacity per capita tend to spare more land for nature. Biocapacity per capita can be increased through more intensive production or by reducing population density. The net displacement of land use from high-income to low-income countries amounted to 6% of the global land demand, even though high-income countries had more land available per capita than low-income countries. In particular, Europe and Japan placed high pressure on ecosystems in lower-income countries. 相似文献