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Thursday, 31 July 2008 21:11

First outside of Earth: NASA finds liquid lake on Titan

NASA announced on Wednesday, July 30, 2008, that it has observed—for the first time ever in the solar system (other than on Earth)—a liquid body on the surface of a celestial object--in this case, Saturn’s moon Titan.

NASA scientists have concluded that at least one of Titan’s lakes contains liquid hydrocarbons (an organic compound solely consisting of hydrogen (H) and carbon (C)).

Thus, Titan contains the only large, stable body of surface liquid known to exist anywhere in our solar system, other than good-old planet Earth.

In addition, the NASA scientists also identified the presence of ethane, a specific type of hydrocarbon, on Titan.

Ethane (an alkane) is a chemical compound of carbon and hydrogen, with a chemical formula of C2H6--that is, two carbon atoms and six hydrogen atoms. On Earth is is a colorless, ordorless gas at standard temperature and pressure.

Specifically, the liquid-methane-and-ethane lake is called Ontario Lacus. Located in the south polar region of Titan, the largest moon of Saturn, Ontario Lacus was first discovered in December 2007 during a flyby of the Cassini spacecraft.

Ontario Lacus is approximately 7,800 square miles (20,200 square kilometers) in area, which is comparable to the surface area of Lake Ontario in North America of Earth. Ontario Lacus was named after Lake Ontario because of its similar size.

Cassini interdisciplinary scientist Larry Soderblom (U.S. Geological Survey, Flagstall, Arizona) states, “Detection of liquid ethane confirms a long-held idea that lakes and seas filled with methane and ethane exist on Titan. The fact we could detect the ethane spectral signatures of the lake even when it was so dimly illuminated, and at a slanted viewing path through Titan's atmosphere, raises expectations for exciting future lake discoveries by our instrument." [NASA news release, see page two]

Soderblom adds, "During the next few years, the vast array of lakes and seas on Titan's north pole mapped with Cassini's radar instrument will emerge from polar darkness into sunlight, giving the infrared instrument rich opportunities to watch for seasonal changes of Titan's lakes.” [NASA news release, page two]

See an artist’s drawing of Ontario Lacas lake on the next page.

Described as “a mirror-smooth lake on the surface of the smoggy moon Titan,” Lake Ontario Lacas is drawn by an artist on NASA’s website “Titan's Ethane Lake.”

According to the NASA news release “NASA confirms liquid lake on Saturn moon,” “The ethane is in a liquid solution with methane, other hydrocarbons and nitrogen. At Titan's surface temperatures, approximately 300 degrees Fahrenheit below zero, these substances can exist as both liquid and gas. Titan shows overwhelming evidence of evaporation, rain, and fluid-carved channels draining into what, in this case, is a liquid hydrocarbon lake.”

It continues, “Earth has a hydrological cycle based on water and Titan has a cycle based on methane. Scientists ruled out the presence of water ice, ammonia, ammonia hydrate and carbon dioxide in Ontario Lacus. The observations also suggest the lake is evaporating. It is ringed by a dark beach, where the black lake merges with the bright shoreline. Cassini also observed a shelf and beach being exposed as the lake evaporates.”

The discovery was made using a visual and mapping instrument, called the Visual and Infrared Mapping Spectrometer (VIMS), onboard the Cassini spacecraft. It has been known that Titan contains numerous lakes—in fact, some scientists thought huge oceans exist—however, what wasn’t known was whether these bodies contained liquid or solid matter.

The Cassini instruments bounced infrared radiation off the surface of Titan. When the radiation returned to Cassini the instruments were able to confirm the chemical composition of different materials and the presence of liquid by the way the materials absorbed and reflected the radiation (light)

The team leader of Cassini’s visual and mapping instrument, Bob Brown (University of Arizona, Tucson, U.S.A.), stated "This is the first observation that really pins down that Titan has a surface lake filled with liquid.”

Titan’s atmosphere is a mixture of about 95% nitrogen and 5% methane. This research study also found ethane and several other simply hydrocarbons in the atmosphere. It is stated in the NASA news release that “Ethane and other hydrocarbons are products from atmospheric chemistry caused by the breakdown of methane by sunlight.”

Read more about the Cassini-Huygens mission and the Nature article discussing the discovery on Titan, all on page three.

The Cassini-Huygens mission is a hugely successful joint effort of Italy (Italian Space Agency, ASI), the United States (National Aeronautics and Space Administration, NASA), and the European Union (European Space Agency, ESA).

Additional information on the wealth of information learned from the Cassini-Huygens mission to Saturn is found on the NASA Jet Propulsion Laboratory website “Cassini-Huygens: Mission to Saturn and Titan.”

The conclusion of the study of Titan is published as a July 31, 2008 article in the journal Nature. Its title is “The identification of liquid ethane in Titan’s Ontario Lacus.”

The abstract to their paper states, “Titan was once thought to have global oceans of light hydrocarbons on its surface, but after 40 close flybys of Titan by the Cassini spacecraft, it has become clear that no such oceans exist. There are, however, features similar to terrestrial lakes and seas, and widespread evidence for fluvial erosion, presumably driven by precipitation of liquid methane from Titan's dense, nitrogen-dominated atmosphere."

It concludes, "Here we report infrared spectroscopic data, obtained by the Visual and Infrared Mapping Spectrometer (VIMS) on board the Cassini spacecraft, that strongly indicate that ethane, probably in liquid solution with methane, nitrogen and other low-molecular-mass hydrocarbons, is contained within Titan's Ontario Lacus.”

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