California Institute of Technology | 15 July 2010
Transient Surface Liquid in Titan's Polar Regions from CassiniOn 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, Titan—the only other place in the solar system seen to have a hydrological cycle with standing liquid on the surface.
Using data gathered by NASA's Cassini spacecraft over a span of four years, the researchers—led by graduate student Alexander G. Hayes of the California Institute of Technology (Caltech) and Oded Aharonson, associate professor of planetary science at Caltech—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 Hayes. "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, Hayes, Aharonson, and their colleagues report 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. (One Titan year lasts 29.5 Earth years.)
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 backscatter 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 transect of Ontario Lacus in December 2008.
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Once the liquid's optical properties were known, the researchers could use the radar data to "see" the lake bed underneath the liquid—at least, down to the depth where the signal is completely attenuated. ... The researchers compared lake images obtained four years apart, and found that Ontario had shrunk.
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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. Over that period, the "apparent darkness" of the lakes—indicating the presence of a radar-attenuating liquid—either decreased or disappeared entirely, which means that their liquid levels had been reduced. The researchers were able to calculate the drop in lake depth, "and we got the same result: 1 meter per year of liquid loss," Aharonson says.
Lakes in Titan's northern hemisphere—which is now entering spring—have also been covered multiple times by radar instruments, but so far no analogous changes have been conclusively detected.
- Icarus (submitted)
- Journal of Geophysical Research (accepted)
NASA JPL Caltech | Cassini Equinox Mission | 15 July 2010
Ontario Lacus, the largest lake in the southern hemisphere of Saturn’s moon Titan, turns out to be a perfect exotic vacation spot, provided you can handle the frosty, subzero temperatures and enjoy soaking in liquid hydrocarbon.Flying Over Ontario Lacus (video) (Credit: NASA/JPL-Caltech/USGS)The Usual Suspects (video) (Credit: NASA/JPL-Caltech)
Several recent papers by scientists working with NASA’s Cassini spacecraft describe evidence of beaches for sunbathing in Titan’s low light, sheltered bays for mooring boats, and pretty deltas for wading out in the shallows. They also describe seasonal changes in the lake’s size and depth, giving vacationers an opportunity to visit over and over without seeing the same lake twice. (Travel agents, of course, will have to help you figure out how to breathe in an atmosphere devoid of oxygen.)
Using data that give us the most detailed picture yet of a lake on another world, scientists and animators have collaborated on a new video tour of Ontario Lacus based on radar data from Cassini’s Titan flybys on June 22, 2009, July 8, 2009, and Jan. 12, 2010. A web video explaining how scientists look to Earth’s Death Valley to understand places like Titan’s Ontario Lacus is also available.
“With such frigid temperatures and meager sunlight, you wouldn’t think Titan has a lot in common with our own Earth,” said Steve Wall, deputy team lead for the Cassini radar team, based at NASA’s Jet Propulsion Laboratory in Pasadena, Calif. “But Titan continues to surprise us with activity and seasonal processes that look marvelously, eerily familiar.”
Cassini arrived at Saturn in 2004 when the southern hemisphere of the planet and its moons were experiencing summer. The seasons have started to change toward autumn, with winter solstice darkening the southern hemisphere of Titan in 2017. A year on Titan is the equivalent of about 29 Earth years.
Titan is the only other world in our solar system known to have standing bodies of liquid on its surface. Because surface temperatures at the poles average a chilly 90 Kelvin (about minus 300 degrees Fahrenheit), the liquid is a combination of methane, ethane and propane, rather than water. Ontario Lacus has a surface area of about 15,000 square kilometers (6,000 square miles), slightly smaller than its terrestrial namesake Lake Ontario.
Cassini first obtained an image of Ontario Lacus with its imaging camera in 2004. A paper submitted to the journal Icarus by Alex Hayes, a Cassini radar team associate at the California Institute of Technology in Pasadena, and colleagues finds that the lake's shoreline has receded by about 10 kilometers (6 miles). This has resulted in a liquid level reduction of about 1 meter (3 feet) per year over a four–year period.
The shoreline appears to be receding because of liquid methane evaporating from the lake, with a total amount of evaporation that would significantly exceed the yearly methane gas output of all the cows on Earth, Hayes said. Some of the liquid could also seep into porous ground material. Hayes said the changes in the lake are likely occurring as part of Titan's seasonal methane cycle, and would be expected to reverse during southern winter.
This seasonal filling and receding is similar to what occurs at the shallow lakebed known as Racetrack Playa in Death Valley National Park, Hayes said. In fact, from the air, the topography and shape of Racetrack Playa and Ontario Lacus are quite similar, although Ontario Lacus is about 60 times larger.
"We are very excited about these results, because we did not expect Cassini to be able to detect changes of this magnitude in Titan's lakes,” Hayes said. “It is only through the continued monitoring of seasonal variation during Cassini's extended mission that these discoveries have been made possible.”
Other parts of the Ontario Lacus’ shoreline, as described in the paper published in Geophysical Research Letters in March 2010 by Wall, Hayes and other colleagues, show flooded valleys and coasts, further proof that the lake level has changed.
The delta revealed by Cassini radar data on the western shore of Ontario Lacus is also the first well-developed delta observed on Titan, Wall said. He explained that the shape of the land there shows liquid flowing down from a higher plain switching channels on its way into the lake, forming at least two lobes.
Examples of this kind of channel switching and wave-modified deltas can be found on Earth at the southern end of Lake Albert between Uganda and the Democratic Republic of Congo in Africa, and the remains of an ancient lake known as Megachad in the African country Chad, Wall said.
The radar data also show a smooth beach on the northwestern shore of Ontario Lacus. Smooth lines parallel to the current shoreline could be formed by low waves over time, which were likely driven by winds sweeping in from the west or southwest. The pattern at Ontario Lacus resembles what might be seen on the southeastern side of Lake Michigan, where waves sculpt the shoreline in a similar fashion.
“Cassini continues to take our breath away as it fills in the details on the surfaces of these far-off moons,” said Linda Spilker, Cassini project scientist based at JPL. “It’s exhilarating to ride along as it takes us on the ultimate cold-weather adventure."
