Lake Characteristics:

-Radar-dark patches are interpreted as lakes based on their very low radar reflectivity and morphological similarities to lakes, including associated channels and location in topographic depressions.

-Radar-dark surfaces are smooth and most likely liquid, rock, ice or organics. More than 75 radar-dark patches or lakes were seen, ranging from 3 kilometers (1.8 miles) to more than 70 kilometers (43 miles) across.

-Some lakes appear partly dry, while others seem liquid-filled. Some of the partly filled lakes may never have filled fully, or may have partly evaporated at some point in the past. The dry lakes have margins or rims and a radar brightness similar to the rest of the surrounding terrain, making them appear devoid of liquid.

-The varying states of how full the lakes are suggest that lakes in this region of Titan might be temporary on some unknown timescale.

-Approximately 15 of the dark patches seem filled and show no clear evidence of erosion. These dark patches resemble terrestrial lakes confined within impact basins (for example, Clearwater Lakes in Canada) or within volcanic calderas (for example, Crater Lake, Oregon). The nest-like nature of these lakes and their limited range of sizes make it unlikely that they originated from an impact. A volcanic origin for the depressions is possible, given their appearance.

-Some lakes have steep margins and very distinct edges, suggesting a topographic rim. These lakes are consistent with seepage or groundwater drainage lakes.

-Other lakes have diffuse, more scalloped edges, with a gradual decrease in radar brightness towards the center of the lake. These lakes are more likely to be associated with channels, and may be either drainage lakes or groundwater drainage lakes.

-Yet other lakes have curvy channel-like extensions, similar in appearance to terrestrial flooded river valleys (for example Lake Powell).

-Bright patches near the lake edges could be small islands peeking through the surface. Floating "icebergs" are unlikely because most materials would not float in liquid hydrocarbons.

Other Observations:

-Based on the lake characteristics, Cassini scientists think they are observing liquid-filled lakes on Titan today. Another possibility is that these depressions and channels formed in the past and have now been filled by a low-density deposit that is darker than any observed elsewhere on Titan. However, the absence of wind-blown features in this area makes the low-density hypothesis unlikely.

-These northern hemisphere lakes are the strongest evidence yet that Titan's surface and atmosphere have an active hydrological cycle, though with a condensable liquid other than water. In this cycle, lakes are filled through methane rainfall or intersect with a subsurface layer saturated with liquid methane.

-As Titan's seasons progress over the 29-year cycle of Saturn's orbit around the sun, lakes in the winter hemisphere should expand by steady methane rain, while summer hemisphere lakes shrink or dry up entirely.

To read more about the radar imaging from July 2006, go to Cassini Finds Lakes on Titan's Arctic Region (July 27, 2006) .

earlier related report
Lost Lakes Of Titan Are Found At Last
Paris (AFP) Jan 04 - Lakes of methane have been spotted on Saturn's largest moon, Titan, boosting the theory that this strange, distant world bears beguiling similarities to Earth, according to a new study. Titan has long intrigued space scientists, as it is the only moon in the Solar System to have a dense atmosphere -- and its atmosphere, like Earth's, mainly comprises nitrogen.

Titan's atmosphere is also rich in methane, although the source for this vast store of hydrocarbons is unclear.

Methane, on the geological scale, has a relatively limited life. A molecule of the compound lasts several tens of millions of years before it is broken up by sunlight.

Given that Titan is billions of years old, the question is how this atmospheric methane gets to be renewed. Without replenishment, it should have disappeared long ago.

A popular hypothesis is that it comes from a vast ocean of hydrocarbons. But when the US spacecraft Cassini sent down a European lander, Huygens, to Titan in 2005, the images sent back were of a rugged landscape veiled in an orange haze.

There were indeed signs of methane flows and methane precipitation, but nothing at all that pointed to any sea of the stuff.

But a flyby by Cassini on July 22 last year has revealed, thanks to a radar scan, 75 large, smooth, dark patches between three and 70 kilometers across (two and 42 miles) across that appear to be lakes of liquid methane, scientists report on Thursday.

They believe the lakes prove that Titan has a "methane cycle" -- a system that is like the water cycle on Earth, in which the liquid evaporates, cools and condenses and then falls as rain, replenishing the surface liquid.

As on Earth, Titan's surface methane may well be supplemented by a "table" of liquid methane that seeps through the rock, the paper suggests.

Some of the methane lakes seem only partly filled, and other depressions are dry, which suggests that, given the high northerly latitudes where they were spotted, the methane cycle follows Titan's seasons.

In winter, the lakes expand, while in summer, they shrink or dry up completely -- again, another parallel with Earth's hydrological cycle.

The study, which appears on Thursday in the British weekly journal Nature, is headed by Ellen Stofan of Proxemy Research in Virginia and University College London.

Titan and Earth are of course very different, especially in their potential for nurturing life. Titan is frigid, dark and, as far as is known, waterless, where as Earth is warm, light and has lots of liquid water.

But French astrophysicist Christophe Sotin says both our planet and Titan have been sculpted by processes that, fundamentally, are quite similar.

The findings "add to the weight of evidence that Titan is a complex world in which the interaction between the inner and outer layers is controlled by processes similar to those that must have dominated the evolution of any Earth-like planet," Sotin said in a commentary.

"Indeed, as far as we know," Sotin added, "there is only one planetary body that displays more dynamism than Titan. Its name is Earth."

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