An artist’s impression of the surface of Titan, Saturn’s enigmatic moon
Credit: NASA/JPL    

NASA scientists have concluded that at least one of the large lake-like features observed on Titan, Saturn's largest moon, is filled with liquid hydrocarbons. They have also positively identified the presence of ethane. Therefore, Titan and Earth are the only members of the Solar System known to have liquids on their surfaces.

Scientists made the startling discovery using data from an instrument onboard the Saturn-bound Cassini spacecraft. The instrument identified various chemicals according to the way they absorb and reflect infrared light. Data from the twin Voyager spacecrafts, which flew by Saturn in the early 1980s, showed that large areas of liquid hydrocarbons could exist on Titan.

More than 40 close flybys of Titan by the Cassini spacecraft discovered hundreds of dark, that were interpreted as possible lakes in the moon’s southern hemisphere. It has not been known, however, whether these features were lakes or simply dark, solid material.

"This is the first observation that really pins down that Titan has a surface lake filled with liquid," said Bob Brown, leader of the Cassini visual and mapping instrument team. The results are due for publication in a scientific journal.

Ethane and several other simple hydrocarbons have been detected in Titan's atmosphere, which consists of 95 percent nitrogen, with methane making up nearly the other five percent. Ethane and other hydrocarbons are formed due to atmospheric chemistry caused by the breakdown of methane by solar radiation.

Some of the hydrocarbons react further and form fine aerosol particles, or haze, which obscures the surface of Titan for visual observations. Liquid ethane was identified using a technique that removed the interference from the atmospheric hydrocarbons.

The visual and mapping instrument observed a lake, termed Ontario Lacus, in Titan's south polar region, when Cassini flew near Titan in December 2007. The lake is roughly 20,000 km2 in area, slightly larger than the Terrestrial Lake Ontario, one of the Great Lakes of North America.

"Detection of liquid ethane confirms a long-held idea that lakes and seas filled with methane and ethane exist on Titan," said Larry Soderblom, a Cassini mission scientist. "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."

The ethane is in a liquid solution with methane, other hydrocarbons and nitrogen. At Titan's surface temperatures, approximately 170 C below zero, these substances can exist in liquid and gaseous states. Titan shows strong evidence of evaporation, rain, and fluid-carved channels draining into what, in this case, is a liquid hydrocarbon lake.

Earth has a hydrological cycle based on water, and Titan has a cycle based on methane. Scientists ruled out the existence of water ice, ammonia and other compounds in Ontario Lacus. The lake also seems to be evaporating. It is encircled by a dark shore, where the black lake merges with the bright shoreline. Cassini also observed a shelf and beach being exposed as the lake evaporates.

"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," Soderblom said.

Further Reading

NASA’s Jet Propulsion Laboratory Release
http://www.jpl.nasa.gov/news/news.cfm?release=2008-152
The Cassini-Huygens Mission to Saturn
http://saturn.jpl.nasa.gov/home/index.cfm

Aymen Mohamed Ibrahem
Senior Astronomy Specialist