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Monday, June 18, 2012

Tropical lakes on Titan

The existence of oceans or lakes of liquid methane on Saturn's moon Titan was predicted more than 20 years ago. But with a dense haze preventing a closer look it has not been possible to confirm their presence. Until the Cassini flyby of July 22, 2006, that is. Radar imaging data from the flyby, published in the journal Nature, provide convincing evidence for large bodies of liquid. Intensity in this colorized image is proportional to how much radar brightness is returned, or more specifically, the logarithm of the radar backscatter cross-section. The colors are not a representation of what the human eye would see. The lakes, darker than the surrounding terrain, are emphasized here by tinting regions of low backscatter in blue. Radar-brighter regions are shown in tan. Smallest details in this image are about 500 meters (1,640 feet) across.
Credit: NASA/JPL-Caltech/USGS

NASA's News release (June 13, 2012)

NASA’s Cassini spacecraft has spied long-standing methane lakes, or puddles, in the “tropics” of Saturn’s moon Titan. One of the tropical lakes appears to be about half the size of Utah’s Great Salt Lake, with a depth of at least 3 feet (1 meter).

The result, which is a new analysis of Cassini data, is unexpected because models had assumed the long-standing bodies of liquid would only exist at the poles. The findings appear in the June 14, 2012 issue of the journal Nature.

Where could the liquid for these lakes come from? “A likely supplier is an underground aquifer,” said Caitlin Griffith, the paper’s lead author and a Cassini team associate at the University of Arizona, Tucson. “In essence, Titan may have oases.”

Understanding how lakes or wetlands form on Titan helps scientists learn about the moon’s weather. Like Earth’s hydrological cycle, Titan has a “methane” cycle, with methane rather than water circulating. In Titan’s atmosphere, ultraviolet light breaks apart methane, initiating a chain of complicated organic chemical reactions. But existing models haven’t been able to account for the abundant supply of methane.

“An aquifer could explain one of the puzzling questions about the existence of methane, which is continually depleted,” Griffith said. “Methane is a progenitor of Titan’s organic chemistry, which likely produces interesting molecules like amino acids, the building blocks of life.”

Global circulation models of Titan have theorized that liquid methane in the moon’s equatorial region evaporates and is carried by wind to the north and south poles, where cooler temperatures cause methane to condense. When it falls to the surface, it forms the polar lakes. On Earth, water is similarly transported by the circulation, yet the oceans also transport water, thereby countering the atmospheric effects.

The latest results come from Cassini’s visual and infrared mapping spectrometer, which detected the dark areas in the tropical region known as Shangri-La, near the spot where the European Space Agency’s Huygens probe landed in 2005. When Huygens landed, the heat of the probe’s lamp vaporized some methane from the ground, indicating it had landed in a damp area.

Areas appear dark to the visual and infrared mapping spectrometer when liquid ethane or methane are present. Some regions could be shallow, ankle-deep puddles. Cassini’s radar mapper has seen lakes in the polar region, but hasn’t detected any lakes at low latitudes.

The tropical lakes detected by the visual and infrared mapping spectrometer have remained since 2004. Only once has rain been detected falling and evaporating in the equatorial regions, and only during the recent expected rainy season. Scientists therefore deduce the lakes could not be substantively replenished by rain.

“We had thought that Titan simply had extensive dunes at the equator and lakes at the poles, but now we know that Titan is more complex than we previously thought,” said Linda Spilker, the Cassini project scientist based at NASA’s Jet Propulsion Laboratory, Pasadena, Calif. “Cassini still has multiple opportunities to fly by this moon going forward, so we can’t wait to see how the details of this story fill out.”

The map of the Shangri-La region on Titan.
Credit: NASA/JPL/Space Science Institute

Nature article (June 13, 2012)

Besides Earth, Titan is the only solid object in the Solar System to circulate liquids in a cycle of rain and evaporation, although on Titan the process is driven by methane rather than water.

This cycle is expected to form liquid bodies near the moon's poles, but not at its dune-covered equator, where Cassini measurements show that humidity levels are low and little rain falls to the surface. "The equatorial belt is like a desert on Earth, where evaporation trumps precipitation," says astrobiologist Jonathan Lunine of Cornell University in Ithaca, New York.

Any surface liquid there should evaporate and be transported to the cooler poles, where it should condense as rain. "Lakes at the poles are easy to explain, but lakes in the tropics are not," says Caitlin Griffith. Indeed, Cassini has spotted hundreds of lakes and three seas in Titan's polar regions.

Now Griffith and her colleagues think they have found a tropical lake — some 60 kilometres long and 40 kilometres wide, and at least 1 metre deep — in Cassini observations made between 2004 and 2008. It appears as a black splotch at seven near-infrared wavelengths that can travel relatively unimpeded through the moon's thick atmosphere, which blocks visible light.

The team also found four smaller, brighter splotches, which Griffith says may be "shallower ponds similar to marshes on Earth, with knee-to-ankle-level depths". Because tropical lakes on Titan should evaporate over a period of just a few thousand years, the researchers argue that these ponds and lakes are being replenished by subsurface oases of liquid methane.

That would expand the number of places on the moon where life could potentially originate. Methane, which is made up of one carbon and four hydrogen atoms, is the source of more complicated organic molecules found on Titan. "There may be organic chemical processes that occur in liquid hydrocarbons that could lead to compounds analogous to proteins and information-carrying molecules," says Lunine, who was not involved in the work. "There might be a kind of life that works in liquid hydrocarbons."

Lunine and Griffith are members of a proposed NASA mission to look for such complex chemistry, called the Titan Mare Explorer (TiME). The TiME probe would spend three months bobbing around Ligeia Mare, a sea near Titan's north polar region, measuring its chemistry with a mass spectrometer.

But should that mission, the fate of which will soon be decided by NASA, land on a tropical lake instead? No, says Lunine. He points out that a number of lines of evidence — including telltale radar signatures — show that the polar regions are filled with liquid hydrocarbon lakes and seas. So far there is less evidence for the tropical features. "Something else that just happens to be dark at those wavelengths", such as a solid organic compound, might mimic a lake, he says.

Only 17% of the equatorial region's surface area has been analysed at the high resolutions required to spot these small features, but Lunine says that lower-resolution observations suggest tropical lakes are relatively few and far between. Still, the idea of oases on Titan appeals to him. "There's a place on Titan named Xanadu, and if you go back to the Coleridge poem on Xanadu, he talks about 'caverns measureless to man',” Lunine says. He adds that he would love to find such caverns filled with methane on Titan.

Saturn's rings lie in the distance as the Cassini spacecraft looks toward Titan and its dark region called Shangri-La, east of the landing site of the Huygens Probe. This view looks toward the anti-Saturn side of Titan (5,150 kilometers, or 3,200 miles across). North on Titan is up (The moon's north polar hood is also visible). The image was taken with the Cassini spacecraft narrow-angle camera on Aug. 9, 2011 using a spectral filter sensitive to wavelengths of near-infrared light centered at 938 nanometers. The view was acquired at a distance of approximately 1.4 million kilometers (870,000 miles) from Titan and at a Sun-Titan-spacecraft, or phase, angle of 35 degrees. Image scale is 8 kilometers (5 miles) per pixel.
Credit: NASA/JPL-Caltech/Space Science Institute

Cosmos Magazine (June 14, 2012)

A lake of liquid methane has appeared near the equator of Titan, which hints at an underground source of methane, feeding the lakes and atmosphere.

Liquid methane lakes were known to exist in the polar regions of Saturn’s largest moon for over a decade. However, previous models of Titan’s methane cycle — thought to be similar to Earth’s water cycle — have held that lakes would not be stable at tropical latitudes.

“This discovery was absolutely not expected,” said Caitlin Griffith at the University of Arizona, who led the study, published in the journal Nature. “Lakes at the poles are easy to explain, but lakes in the tropics are not.”

Researchers detected the lake by measuring sunlight reflected from Titan’s surface and its atmosphere. Similar to the way oceans show up as dark blue images in satellite images of Earth, lakes on Titan can show up as dark near-infrared regions.

Titan’s miasmic atmosphere — 10 times thicker than our own — makes the moon’s surface nearly indiscernible at visible wavelengths. However, there are seven minuscule wavelengths at which the atmosphere is transparent enough for Titan’s surface to be seen.

Through these seven ‘windows’, said Griffith, “we see a uniform black surface with an area of 2400 square kilometres” in addition to several smaller areas. The surface reflectivity, or lack thereof, indicates the presence of liquid methane at least one metre deep.

Current models hold that any liquid methane in Titan’s tropical regions should evaporate and be carried back to the poles before it has the opportunity to condense into lakes. But onsite measurements from the Huygens Probe reveal a landscape that is damp and has features consistent with erosion from an underground source.

“Our detection of a lake suggests that a subterranean source of methane may flood the surface and dampen the ground,” Griffith said. “In essence, Titan may have oases.”

Their presence and replenishment from an underground source would help answer the question of where the methane in Titan’s atmosphere and lakes come from.

Shane Walsh, a Magellan Fellow at the Australian Astronomical Observatory who was not involved in the research, praised the study, calling it “robust”.

“Most people consider methane an unfortunate by-product of digestion,” he said. “But on Titan, where it undergoes similar processes to water on Earth, it may be the organic solvent for the chemistry of life — but probably not as we know it.”

“If the conclusions of this study hold true, then Titan just got a lot more interesting.”

With its atmosphere and its surface lakes of stable liquid, Titan is as close a relative to Earth as exists in our Solar System. Understanding its atmosphere is thus linked with our understanding — or lack thereof — of how life originates.

“Since the methane supplies the atmosphere with a diversity of carbon-based molecules, we are also asking where most of the constituents in Titan's atmosphere came from,” Griffith said. Amino acids — the building blocks of life — are believed to be among those constituents.

All of the methane on Titan, however, will eventually wind up in its atmosphere, where the Sun’s ultraviolet rays breaks it up in order to form other organic molecules. Research predicts that all of the methane in the lakes and the atmosphere of Titan will be gone in 300 million years at the latest, leaving the barren moon to stand for the rest of its 4500 million-year lifespan as a symbol of what might have been.

Titan's atmosphere makes Saturn's largest moon look like a fuzzy orange ball in this natural color view from the Cassini spacecraft. Titan's north polar hood is visible at the top of the image, and a faint blue haze also can be detected above the south pole at the bottom of this view. This view looks toward the anti-Saturn side of Titan. North is up. Images taken using red, green and blue spectral filters were combined to create this natural color view. The images were obtained with the Cassini spacecraft wide-angle camera on Jan. 30, 2012 at a distance of approximately 119,000 miles (191,000 kilometers) from Titan. Image scale is 7 miles (11 kilometers) per pixel.
Credit: NASA/JPL-Caltech/Space Science Institute

Saturn's largest moon, Titan, looks small here, pictured to the right of the gas giant in this Cassini spacecraft view. Titan is in the upper right. Saturn's rings appear across the top of the image, and they cast a series of shadows onto the planet across the middle of the image. The moon Prometheus (53 miles, or 86 kilometers across) appears as a tiny white speck above the rings in the far upper right of the image. The shadow cast by Prometheus can be seen as a small black speck on the planet on the far left of the image, between the shadows cast by the main rings and the thin F ring. The shadow of the moon Pandora also can be seen on the planet south of the shadows of all the rings, below the center of the image towards the right side of the planet. Pandora is not shown here. The image was taken with the Cassini spacecraft wide-angle camera on Jan. 5, 2012 using a spectral filter sensitive to wavelengths of near-infrared light centered at 752 nanometers. The view was acquired at a distance of approximately 426,000 miles (685,000 kilometers) from Saturn and at a Sun-Saturn-spacecraft, or phase, angle of 20 degrees. Image scale is 23 miles (37 kilometers) per pixel on Saturn.
Credit: NASA/JPL-Caltech/Space Science Institute