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Space Topics: Saturn

The Year in Pictures: 2007

Titan's North Polar Seas

Click to enlarge > Credit: NASA / JPL

This year, at long last, Cassini found unmistakable bodies of liquid methane and ethane hiding at Titan’s poles. In the decades since methane was detected in the smoggy moon’s atmosphere, scientists have suspected the presence of liquid methane on Titan’s surface. Any methane in Titan’s atmosphere should be destroyed over geologically short periods of a few million years, so its presence means either that we are seeing Titan at an unusual time in its history or that there is a reservoir of methane at and below the surface that replenishes the atmospheric supply.

Cassini arrived at Saturn in 2004 with a suite of a dozen instruments, a few of which could penetrate Titan’s clouds in infrared or radio wavelengths to see the surface. Throughout the first two years of the mission, no observations were made of areas that no one questioned were liquid-filled seas. Finally, during the summer of 2006, Cassini’s navigators began to fly the spacecraft above Titan’s north pole in maneuvers designed to increase the spacecraft’s orbital inclination. This maneuvering had the collateral benefit of allowing Cassini’s RADAR instrument to gather many thin swaths of images near Titan’s north pole through a technique known as synthetic aperture RADAR (SAR).

SAR images are in black and white, and the brightness is dependent in large part on the roughness of the surface and local topography. Surfaces that are rough on centimeter-size scales, as well as slopes facing the spacecraft, will be bright, whereas surfaces that are very smooth, as well as slopes facing away from the spacecraft, will be dark.

Click to enlarge > Possible Titan lakes and drainage channels In this segment of the July 22, 2006 RADAR swath on Titan, very dark features are possible lakes; some, less dark features could be lakebeds that are now dried up. The darkest lake-like feature near the top of this image appears to have a thin drainage channel emptying into it from the east. It may even be possible that this channel connects the lake feature to one of the possible dried-up features to its southeast. However, the channel disappears into the background noise of the RADAR data, so evidence that the two features are connected is not conclusive. Credit: NASA / JPL

The north polar SAR swaths instantly bore fruit, revealing the darkest surfaces yet seen on Titan, in circular to irregularly shaped depressions, many of which were clearly fed and/or emptied by sinuous, branching channels. Many other similarly shaped features were not quite so dark. Perhaps the darkest features are currently liquid-filled, and the less dark ones are dry. In some areas, dark features grade to a slightly lighter toned shore. Here, it is possible that Cassini’s broadcast radio waves are penetrating the shallowest parts of the seas to reflect from a rougher bottom, or even that the near-shore surface of the sea is roughened by Titanian breezes.

Polar passes continued through 2006 and 2007. Up to the time that this image was assembled in October 2007, the RADAR team had acquired SAR swaths at a variety of resolutions covering 60 percent of Titan's north polar region above 60 degrees north latitude. About 14 percent of the mapped region is covered by what are interpreted as liquid hydrocarbon lakes numbering more than 400 in various sizes. Most of the lake area is dominated by a few large “seas.”

Cassini has now moved on to an orbit that will more frequently cover Titan’s south pole and is beginning the search for similar seas there.

This image has been cropped and reduced in size by 50% from an even larger mosaic available here (JPEG format, 10 MB).