SYDNEY: Astronomers have concluded that at least one of the large lakes observed on Saturn’s moon Titan contains liquid ethane. This makes it the only body in the Solar System beyond Earth known to have liquid on its surface.
Scientists made the discovery using data from an instrument aboard NASA’s Cassini spacecraft, currently in orbit around Saturn. The instrument identified chemically different materials based on the way they absorb and reflect infrared light.
Before Cassini, scientists thought Titan would have global oceans of methane, ethane and other light hydrocarbons. However, more than 40 flybys of Titan by Cassini show no such global oceans exist. They have revealed hundreds of dark lake-like features, but until now, we didn’t know whether these features were liquid or simply solid, reflective material.
“This is the first observation that really pins down that Titan has a surface lake filled with liquid,” said Bob Brown of the University of Arizona, Tucson. Brown leads a team of scientists controlling Cassini’s visual and mapping instruments, who report their find today in the U.K. journal Nature.
Ethane and several other simple hydrocarbons have been identified in Titan’s atmosphere, which consists of 95 per cent nitrogen, and five per cent methane and other hydrocarbons, including ethane. These are created by the breakdown of methane by sunlight, and some react further to form fine aerosol particles.
These hydrocarbons form a murky haze, which makes identifying materials on the surface difficult, but liquid ethane was identified using a technique that removed the interference from the atmospheric hydrocarbons.
“Confirming a long-held idea”
Cassini’s visual and mapping instrument observed a lake – Ontario Lacus – in Titan’s south polar region during a close Cassini flyby in December 2007. The lake is roughly 20,000 square kilometres in area, slightly larger than its namesake, North America’s Lake Ontario.
“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 interdisciplinary scientist with the U.S. Geological Survey in Flagstaff, Arizona.
“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 – an average of 179 ºC 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.
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.
“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 added.