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41.
Grant JA Arvidson R Bell JF Cabrol NA Carr MH Christensen P Crumpler L Des Marais DJ Ehlmann BL Farmer J Golombek M Grant FD Greeley R Herkenhoff K Li R McSween HY Ming DW Moersch J Rice JW Ruff S Richter L Squyres S Sullivan R Weitz C 《Science (New York, N.Y.)》2004,305(5685):807-810
The Mars Exploration Rover Spirit has traversed a fairly flat, rock-strewn terrain whose surface is shaped primarily by impact events, although some of the landscape has been altered by eolian processes. Impacts ejected basaltic rocks that probably were part of locally formed lava flows from at least 10 meters depth. Some rocks have been textured and/or partially buried by windblown sediments less than 2 millimeters in diameter that concentrate within shallow, partially filled, circular impact depressions referred to as hollows. The terrain traversed during the 90-sol (martian solar day) nominal mission shows no evidence for an ancient lake in Gusev crater. 相似文献
42.
Spielhagen RF Werner K Sørensen SA Zamelczyk K Kandiano E Budeus G Husum K Marchitto TM Hald M 《Science (New York, N.Y.)》2011,331(6016):450-453
The Arctic is responding more rapidly to global warming than most other areas on our planet. Northward-flowing Atlantic Water is the major means of heat advection toward the Arctic and strongly affects the sea ice distribution. Records of its natural variability are critical for the understanding of feedback mechanisms and the future of the Arctic climate system, but continuous historical records reach back only ~150 years. Here, we present a multidecadal-scale record of ocean temperature variations during the past 2000 years, derived from marine sediments off Western Svalbard (79°N). We find that early-21st-century temperatures of Atlantic Water entering the Arctic Ocean are unprecedented over the past 2000 years and are presumably linked to the Arctic amplification of global warming. 相似文献
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44.
Gilbert MT Kivisild T Grønnow B Andersen PK Metspalu E Reidla M Tamm E Axelsson E Götherström A Campos PF Rasmussen M Metspalu M Higham TF Schwenninger JL Nathan R De Hoog CJ Koch A Møller LN Andreasen C Meldgaard M Villems R Bendixen C Willerslev E 《Science (New York, N.Y.)》2008,320(5884):1787-1789
The Paleo-Eskimo Saqqaq and Independence I cultures, documented from archaeological remains in Northern Canada and Greenland, represent the earliest human expansion into the New World's northern extremes. However, their origin and genetic relationship to later cultures are unknown. We sequenced a mitochondrial genome from a Paleo-Eskimo human by using 3400-to 4500-year-old frozen hair excavated from an early Greenlandic Saqqaq settlement. The sample is distinct from modern Native Americans and Neo-Eskimos, falling within haplogroup D2a1, a group previously observed among modern Aleuts and Siberian Sireniki Yuit. This result suggests that the earliest migrants into the New World's northern extremes derived from populations in the Bering Sea area and were not directly related to Native Americans or the later Neo-Eskimos that replaced them. 相似文献
45.
Arkin IT Xu H Jensen MØ Arbely E Bennett ER Bowers KJ Chow E Dror RO Eastwood MP Flitman-Tene R Gregersen BA Klepeis JL Kolossváry I Shan Y Shaw DE 《Science (New York, N.Y.)》2007,317(5839):799-803
Na+/H+ antiporters are central to cellular salt and pH homeostasis. The structure of Escherichia coli NhaA was recently determined, but its mechanisms of transport and pH regulation remain elusive. We performed molecular dynamics simulations of NhaA that, with existing experimental data, enabled us to propose an atomically detailed model of antiporter function. Three conserved aspartates are key to our proposed mechanism: Asp164 (D164) is the Na+-binding site, D163 controls the alternating accessibility of this binding site to the cytoplasm or periplasm, and D133 is crucial for pH regulation. Consistent with experimental stoichiometry, two protons are required to transport a single Na+ ion: D163 protonates to reveal the Na+-binding site to the periplasm, and subsequent protonation of D164 releases Na+. Additional mutagenesis experiments further validated the model. 相似文献
46.
Christensen NL 《Science (New York, N.Y.)》1973,181(4094):66-68
Analysis of soils from burned and unburned chaparral indicates that high nitrate concentrations following fire are due to the addition of ammonium and organic nitrogen in the ash. Inhibition of mineralization in unburned chaparral results in low nitrate concentrations. Fluctuations in the amount of soil nitrate in unburned chaparral are the direct result of foliar leaching. 相似文献
47.
48.
Hoefen TM Clark RN Bandfield JL Smith MD Pearl JC Christensen PR 《Science (New York, N.Y.)》2003,302(5645):627-630
We have detected a 30,000-square-kilometer area rich in olivine in the Nili Fossae region of Mars. Nili Fossae has been interpreted as a complex of grabens and fractures related to the formation of the Isidis impact basin. We propose that post-impact faulting of this area has exposed subsurface layers rich in olivine. Linear mixture analysis of Thermal Emission Spectrometer spectra shows surface exposures of 30% olivine, where the composition of the olivine ranges from Fo30 to Fo70. 相似文献
49.
Danish agriculture is facing a turning point. Centuries of struggle to increase arable land area as well as its productivity is about to be reversed, due to overproduction and environmental problems. Some land will probably be turned back to nature, and the use of chemicals in agriculture in general will be reduced, perhaps with a lower productivity and a better environment as a consequence. This paper mainly describes some of the political actions taken in Denmark to influence environmental-agricultural issues. First, however, some of the recently perceived environmental problems and over-production problems connected with Danish farming are outlined. 相似文献
50.
To rigorously test the in vivo cell fate specificity of bone marrow (BM) hematopoietic stem cells (HSCs), we generated chimeric animals by transplantation of a single green fluorescent protein (GFP)-marked HSC into lethally irradiated nontransgenic recipients. Single HSCs robustly reconstituted peripheral blood leukocytes in these animals, but did not contribute appreciably to nonhematopoietic tissues, including brain, kidney, gut, liver, and muscle. Similarly, in GFP+:GFP- parabiotic mice, we found substantial chimerism of hematopoietic but not nonhematopoietic cells. These data indicate that "transdifferentiation" of circulating HSCs and/or their progeny is an extremely rare event, if it occurs at all. 相似文献