Lakes on Titan

Titan, is the largest moon of Saturn, the only natural satellite known to have a dense atmosphere, and the only object other than Earth for which clear evidence of stable bodies of surface liquid has been found. On Earth, lake levels rise and fall with the seasons and with longer term climate changes, as precipitation, evaporation, and runoff add and remove liquid. Now, for the first time, scientists have found compelling evidence for similar lake level changes on Saturn's largest moon showing that is possesses a similar change cycle.

The atmosphere of Titan is largely composed of nitrogen; minor components lead to the formation of methane and ethane clouds and nitrogen rich organic smog. The climate (including a form of wind and rain)creates surface features similar to those of Earth, such as sand dunes, rivers, lakes and seas (likely of liquid methane and definitely not water) and shorelines.

With its liquids (both surface and subsurface) and robust nitrogen atmosphere, Titan is viewed as similar to the early Earth, although at a much lower temperature.

Titan's clouds, probably composed of methane, ethane or other simple organics, are scattered and variable, punctuating the overall haze. The findings of the Huygens probe indicate that Titan's atmosphere periodically rains liquid methane and other organic compounds onto the moon's surface.

Using data gathered by NASA's Cassini spacecraft over a span of four years, the current research (California Institute of Technology have obtained two separate lines of evidence showing roughly a 1 meter per year drop in the levels of lakes in Titan's southern hemisphere. The decrease is the result of the seasonal evaporation of liquid methane from the lakes—which, because of Titan's frigid temperatures (roughly minus 300 degrees Fahrenheit at the poles), are composed largely of liquid methane, ethane, and propane.

"It's really exciting because, on this distant object, we're able to see this meter-scale drop in lake depth," says Alexander Hayes, one of the authors. "We didn't know Cassini would even be able to see these things."

One of the lakes—Ontario Lacus (named after Earth's Lake Ontario, which is of comparable size) —is the southern hemisphere's largest lake, and was the first lake to be observed on the moon. In a paper submitted to the journal Icarus, it is reported that the shoreline of Ontario Lacus receded by about 10 kilometers (6 miles) from June 2005 to July 2009, a period of time that represents mid-summer to fall in Titan's southern hemisphere. A Titan year is very long compared to the Earth so the climate cycles are longer.

Ontario Lacus and other southern hemisphere lakes were analyzed using Synthetic Aperture Radar (SAR) image data from the Cassini spacecraft. In radar data, smooth features (such as lakes)appear as dark areas, while rougher features (such as mountain belts)appear bright. The intensity of the radar back scatter provides information about the composition and roughness of surface features.

In addition to the SAR data, radar altimetry ( which measures the time it takes for microwave signals bouncing off a surface to arrive back at the spacecraft) was collected across a transit of Ontario Lacus in December 2008.

"The combination of SAR and altimetry measurements across the transect gave information about the absorptive properties of the liquid, and argues that the liquids are relatively pure hydrocarbons made up of methane and ethane and not a gunky tar," Oded Aharonson (another author) says.

"The liquid is not highly attenuating," explains Hayes, "which means it is fairly clear to radar energy—that is, transparent, like liquid natural gas." Because of this, radar can see through the liquid in Titan's lakes to a depth of several meters. "Then the radar hits the floor, and bounces back," he says. "Or, if the lake is deeper than a few meters, the radar is completely absorbed, producing a 'black' signature."

Once the liquid's optical properties were known, the researchers could use the radar data to "see" the lakebed underneath the liquid—at least, down to the depth where the signal is completely attenuated. From this data, slopes can be determined.

The lake is shallowest and most gently sloped along its southern edge, in areas where sediment is accumulating. Along its eastern shore, the slope of the lake is somewhat steeper. "This is what we are calling the 'beachhead,'" Hayes says. The slope is very steep along the lake's northern boundary, where it butts up against a range of mountains.

"The extent to which the lake has receded is related to the slope—i.e., where the lake is shallow, the liquid will have receded more," Hayes says. "This allows us to deduce the vertical height by which the lake depth has dropped, which is about 1 meter per year."

The researchers also analyzed the evaporation of methane from nearby lakes by comparing the radar signatures of these lakes as measured in December 2007 with data obtained in May 2009.

The researchers were able to calculate the drop in lake depth from the change in light intensity. The same change in lake depth of one meter was then deduced.

Lakes in Titan's northern hemisphere (which is now entering spring)have also been covered multiple times by radar instruments, but so far no similar changes have been conclusively detected.

For further information: http://www.eurekalert.org/pub_releases/2010-07/ciot-csm061610.php