Hydrologic Cycles On Earth And On Titan : A Comparable Paradigm

"Fluvial Channels on Titan: Initial Cassini Radar Observations"

The Cassini-Huygens mission has been instrumental in unmasking the surface of Saturn's mysterious moon, Titan. Radar observations of this Earth-like moon have revealed that catastrophic downpours of methane rain are carving the landscape, and resemble washes of the Sonoran deserts of the Southwestern United States.

A braided fluvial network of dry washes on Earth observed from a commercial airliner near Parker, Arizona. Image is approximately 10 km across -- the morphology may be compared with the eastern T3 flyby network

Dr. Ralph Lorenz of the Johns Hopkins University Applied Physics Laboratory compares processes on Titan to similar processes on Earth. As the lead author of "Fluvial Channels on Titan: Initial Cassini Radar Observations," a paper published in the June issue of Planetary and Space Science, Lorenz contends that Titan and Earth share many characteristics, including similar hydrological cycles. Studying these climate-related processes on Titan can help scientists understand and predict changes in Earth's hydrological cycle.

According to Lorenz, "It's a very familiar process under very different conditions. But the interval between big storms is longer-- maybe hundreds or thousands of years. Titan's hydrological cycle is like Earth's taken to extremes," he says.

Titan provides scientists with a natural laboratory for understanding processes like drought and flood, as well as addressing one NASA's key science objectives, which is to better understand the origin and evolution of celestial bodies throughout the Solar System.

A section of the T3 radar swath (a), showing fluvial networks at left and right.

Earth may be experiencing longer droughts and larger, more severe storms with uncertain implications for the future. Studies of these types of phenomena on Titan could help scientists better understand climate-driven changes that are occurring in Earth's storm systems.

In the Southwestern desert of the United States for example, torrential rains occur in the heat of the summer. Clouds of evaporated water from the Pacific and the Gulf of California blow into the Sonora Desert and precipitate in heat-driven thunderstorms. Although the details of seasonal changes in weather on Titan are very different from Earth's -- how methane humidity, clouds and rainfall vary with space and time is still being learned -- the idea of occasional storms in what is usually a dry region seems to be common to areas on both bodies.

"The biggest clues to the power of the rainstorms is in the braided channels," says Lorenz. The flow that makes these braided, or 'anabranching' channels, is so energetic, that it blasts forcefully out of existing channels and carves out new channels. Titan's channels can run several hundred kilometers in length."

Meandering dark channel draining into an apparent lake, observed on T25. Radar illumination from bottom. Arrow points where channel can be traced in dark lakebed.

Cassini radar observations show that in the geologically recent past, if not the present, liquid -- almost certainly methane -- has flowed energetically across Titan's surface, much as water flows in rivers on Earth. A variety of channel networks, including braided, shallow washes, as well as more dendritic and incised valleys, and meandering and deposition channels, have formed as a result. These channels have been observed up to 400 kilometers (about 250 miles)long and a few kilometers (miles) wide.

"Through continuing observations of this area," Lorenz says, "Titan gives us a new laboratory in which to test ideas about how liquid flows shape an Earthlike surface and affect the evolution of its landscape."

On Titan, the gravity is different, the material on the ground is different and the fluid is different, yet, according to Lorenz, "any theory of geomorphologic change must now accommodate the very similar landforms that result in this exotic environment."

As outlined in the paper, intervening flybys of Titan in February 2007, (T16, T18) detected hydrocarbon lakes at latitudes above 70 degrees N on Titan.

For the full online version of this paper, go to www.sciencedirect.com.

For further reading about Titan, please refer to the following:

Lorenz, Ralph, Mitton, Jacqueline. Titan Unveiled -- Saturn's Mysterious Moon Explored.
Princeton, N.J., Princeton University Press, c2008.

Titan's Hidden Ocean

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