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The lakes of Titan, Saturn's largest moon, are bodies of liquid ethane, methane, and propane that have been detected by the ''Cassini–Huygens'' space probe, and had been suspected long before. The large ones are known as maria (seas) and the small ones as lacūs (lakes). ==History== The possibility that there were seas on Titan was first suggested based on Voyager 1 and 2 data. The data showed Titan to have a thick atmosphere of approximately the correct temperature and composition to support them. Direct evidence was not obtained until 1995 when data from the Hubble Space Telescope and other observations had already suggested the existence of liquid methane on Titan, either in disconnected pockets or on the scale of satellite-wide oceans, similar to water on Earth. The ''Cassini'' mission affirmed the former hypothesis, although not immediately. When the probe arrived in the Saturnian system in 2004, it was hoped that hydrocarbon lakes or oceans might be detectable by reflected sunlight from the surface of any liquid bodies, but no specular reflections were initially observed. The possibility remained that liquid ethane and methane might be found on Titan's polar regions, where they were expected to be abundant and stable.〔 In Titan's south polar region, an enigmatic dark feature named Ontario Lacus was the first suspected lake identified, possibly created by clouds that are observed to cluster in the area. A possible shoreline was also identified near the pole via radar imagery. Following a flyby on July 22, 2006, in which the ''Cassini'' spacecraft's radar imaged the northern latitudes (which were at the time in winter), a number of large, smooth (and thus dark to radar) patches were seen dotting the surface near the pole. Based on the observations, scientists announced "definitive evidence of lakes filled with methane on Saturn's moon Titan" in January 2007. The ''Cassini–Huygens'' team concluded that the imaged features are almost certainly the long-sought hydrocarbon lakes, the first stable bodies of surface liquid found off Earth. Some appear to have channels associated with liquid and lie in topographical depressions.〔 Channels in some regions have created surprisingly little erosion, suggesting erosion on Titan is extremely slow, or some other recent phenomena may have wiped out older riverbeds and landforms. Overall, the Cassini radar observations have shown that lakes cover only a few percent of the surface and are concentrated near the poles, making Titan much drier than Earth. The high relative humidity of methane in Titan's lower atmosphere could be maintained by evaporation from lakes covering only 0.002–0.02% of the whole surface. During a ''Cassini'' flyby in late February 2007, radar and camera observations revealed several large features in the north polar region interpreted as large expanses of liquid methane and/or ethane, including one, Ligeia Mare, with an area of 126,000 km2 (slightly larger than Lake Michigan–Huron, the largest lake on Earth), and another, Kraken Mare, that would later prove to be three times that size. A flyby of Titan's southern polar regions in October 2007 revealed similar, though far smaller, lakelike features. During a close Cassini flyby in December 2007 the visual and mapping instrument observed a lake, Ontario Lacus, in Titan's south polar region. This instrument identifies chemically different materials based on the way they absorb and reflect infrared light. Based on this instrument's observations, scientists concluded that at least one of the large lakes observed on Saturn's moon Titan does in fact contain liquid, that liquid being hydrocarbons, and have positively identified the presence of ethane.〔(NASA - Sunlight Glint Confirms Liquid in Titan Lake Zone )〕 This makes Titan the only object in the Solar system other than Earth known to have stable ambient-temperature liquid on its surface (Io has long-lasting surface bodies of lava at elevated temperatures).〔(NASA Confirms Liquid Lake on Saturn Moon - NASA Jet Propulsion Laboratory )〕 Radar measurements made in July 2009 and January 2010 indicate that Ontario Lacus is extremely shallow, with an average depth of 0.4–3.2 m, and a maximum depth of 2.9–7.4 m. It may thus resemble a terrestrial mudflat. In contrast, the northern hemisphere's Ligeia Mare has depths of 170 m. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「lakes of titan」の詳細全文を読む スポンサード リンク
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