page_title: Lakes of Titan
section_title: History
hierarchical_section_title: Lakes of Titan / History
caption_attribution_description: English: This image from NASA's Cassini spacecraft shows a vast river system on Saturn's moon Titan. It is the first time images from space have revealed a river system so vast and in such high resolution anywhere other than Earth. The image was acquired on Sept. 26, 2012, on Cassini's 87th close flyby of Titan. The river valley crosses Titan's north polar region and runs into Ligeia Mare, one of the three great seas in the high northern latitudes of Saturn's moon Titan. It stretches more than 200 miles (400 kilometers). Scientists deduce that the river is filled with liquid because it appears dark along its entire extent in the high-resolution radar image, indicating a smooth surface. That liquid is presumably ethane mixed with methane, the former having been positively identified in 2008 by Cassini's visual and infrared mapping spectrometer at the lake known as Ontario Lacus in Titan's southern hemisphere. Though there are some short, local meanders, the relative straightness of the river valley suggests it follows the trace of at least one fault, similar to other large rivers running into the southern margin of Ligeia Mare (see PIA10008). Such faults may lead to the opening of basins and perhaps to the formation of the giant seas themselves. North is toward the top of this image. The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and ASI, the Italian Space Agency. NASA's Jet Propulsion Laboratory, a division of the California Institute of Technology in Pasadena, manages the mission for NASA's Science Mission Directorate, Washington. The Cassini orbiter was designed, developed and assembled at JPL. The RADAR instrument was built by JPL and the Italian Space Agency, working with team members from the US and several European countries. JPL is a division of the California Institute of Technology in Pasadena. For more information about Cassini, visit http://www.nasa.gov/cassini and http://saturn.jpl.nasa.gov.
context_page_description: The lakes of Titan, Saturn's largest moon, are bodies of liquid ethane and methane that have been detected by the Cassini–Huygens space probe, and had been suspected long before. The large ones are known as maria and the small ones as lacūs.
context_section_description: The possibility that there were seas on Titan was first suggested based on data from the Voyager 1 and 2 space probes, launched in August and September 1977. 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 dryer 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 (48,649 sq. mi.) ( (slightly larger than Lake Michigan–Huron, the largest freshwater 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. 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 (1'4"-10.5'), and a maximum depth of 2.9–7.4 m (9.5'-24'4"). It may thus resemble a terrestrial mudflat. In contrast, the northern hemisphere's Ligeia Mare has depths of 170 m (557'9").
caption: Vid Flumina,[4] a 400 km long river emptying into Ligeia Mare (in lower right corner of top image).
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