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Porco CC Baker E Barbara J Beurle K Brahic A Burns JA Charnoz S Cooper N Dawson DD Del Genio AD Denk T Dones L Dyudina U Evans MW Fussner S Giese B Grazier K Helfenstein P Ingersoll AP Jacobson RA Johnson TV McEwen A Murray CD Neukum G Owen WM Perry J Roatsch T Spitale J Squyres S Thomas P Tiscareno M Turtle EP Vasavada AR Veverka J Wagner R West R 《Nature》2005,434(7030):159-168
Titan, the largest moon of Saturn, is the only satellite in the Solar System with a substantial atmosphere. The atmosphere is poorly understood and obscures the surface, leading to intense speculation about Titan's nature. Here we present observations of Titan from the imaging science experiment onboard the Cassini spacecraft that address some of these issues. The images reveal intricate surface albedo features that suggest aeolian, tectonic and fluvial processes; they also show a few circular features that could be impact structures. These observations imply that substantial surface modification has occurred over Titan's history. We have not directly detected liquids on the surface to date. Convective clouds are found to be common near the south pole, and the motion of mid-latitude clouds consistently indicates eastward winds, from which we infer that the troposphere is rotating faster than the surface. A detached haze at an altitude of 500 km is 150-200 km higher than that observed by Voyager, and more tenuous haze layers are also resolved. 相似文献
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Stimulation of insulin secretion by theophylline 总被引:16,自引:0,他引:16
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Thomas PC Armstrong JW Asmar SW Burns JA Denk T Giese B Helfenstein P Iess L Johnson TV McEwen A Nicolaisen L Porco C Rappaport N Richardson J Somenzi L Tortora P Turtle EP Veverka J 《Nature》2007,448(7149):50-53
Hyperion is Saturn's largest known irregularly shaped satellite and the only moon observed to undergo chaotic rotation. Previous work has identified Hyperion's surface as distinct from other small icy objects but left the causes unsettled. Here we report high-resolution images that reveal a unique sponge-like appearance at scales of a few kilometres. Mapping shows a high surface density of relatively well-preserved craters two to ten kilometres across. We have also determined Hyperion's size and mass, and calculated the mean density as 544 +/- 50 kg m(-3), which indicates a porosity of >40 per cent. The high porosity may enhance preservation of craters by minimizing the amount of ejecta produced or retained, and accordingly may be the crucial factor in crafting this unusual surface. 相似文献
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