News flash: Lakes at Titan's south pole, too, on top of the land of lakes in the north

Posted by Emily Lakdawalla

2007/10/11 12:32 CDT

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This morning the RADAR team released the following image, from the "T36" flyby on October 2, 2007:

NASA / JPL

Lakes at Titan's south pole

Cassini's RADAR instrument acquired its first swath across the southern polar region on October 2, 2007, reaching to within 18 degrees of the south pole. Within the swath are small, irregularly shaped, radar-dark blobs much like the features that are now being interpreted as lakes in the northern regions, as well as many other features that share similarities with ones seen in the northern polar region. The similarity between the poles suggests that the surface features in the polar region are driven by Titan's climate.

Lakes have been spotted near the south pole before, in this image by the ISS team, which was considered compelling but not conclusive at the time:

NASA / JPL / Space Science Institute

A candidate lake on Titan?

The footprint-like feature in the upper left corner of this image is the unusual-looking feature that Cassini imaging scientists think may be a hydrocarbon lake. It is roughly 234 kilometers long by 73 kilometers wide (145 miles by 45 miles), about the size of Lake Ontario (a lake on the U.S.-Canadian border). The red cross below center identifies the location of Titan's south pole.

Because there are lakes at the north pole that are supposed to be fed by a methane cycle of evaporation, cloud formation, and precipitation, it has been suspected strongly that there should also be lakes in the south. However, it's winter in the north and summer in the south. It was possible that the methane rain only occurs in winter, so the summer pole might be dry, although it was also possible that the input of solar energy to the summer pole might drive a more active methane cycle. This one RADAR swath hasn't answered that question yet, but what it has done is show that the features visible to RADAR near Titan's south pole look a lot like the features visible to RADAR near Titan's north pole. And that, in turn, suggests that, as on Earth climate (rather than internal geologic forces or external forces like impact cratering) plays a large role in the formation of Titan's polar surface features, since climate, averaged over the year, is one thing that will be the same in the north and south.

Speaking of Titan's north pole, the RADAR team has released a mosaic of many, many swaths covering the northern polar regions. It is absolutely huge, so to fit it in here I had to crop out the most interesting portion and reduce it in size 50%; visit Photojournal to download the full-size version. The SAR swaths have a wide range of different native resolutions, so some areas of the mosaic are fuzzier than others. Even the fuzzy regions contain features decipherable as fields upon fields of lakes.

NASA / JPL

Cassini RADAR view of Titan's north pole, October 2007

This mosaic is composed of all synthetic-aperture-radar maps of Titan's polar regions acquired by Cassini to date. It has been cropped and reduced in size by 50% from an even larger mosaic available on NASA's Planetary Photojournal. Approximately 60 percent of Titan's northern polar region (poleward of 60 degrees north latitude) has been mapped as of October 2007, and of this area, about 14% appears to be covered with hydrocarbon lakes. The radar images are grayscale; they have been colored here with a color map that applies blue colors to the materials that are darkest to the RADAR instrument, and yellow colors to the materials that are brightest. This color scheme highlights the apparent lakes, but also shows that many lake-like features are not as dark as other lakes, and that darker channels appear to run down the interiors of less dark lakes.

The image is a polar projection, with zero longitude (the sub-Saturnian hemisphere) toward the bottom. The leading hemisphere (centered at 90 degrees W) is to the left, and the trailing hemisphere (centered at 270 degrees W) is to the right. The largest lakes are clustered in an area on Titan's trailing hemisphere.

I'm not entirely sure I approve of the way that the RADAR team chooses to color these images. By making dark stuff blue and bright stuff yellow, the images make our brains automatically interpret dark stuff as liquid in lakes and bright stuff as solid ground. But if you look around the mosaic, you can see a lot of things that look a lot like the lakes, but weren't quite dark enough to make the color table cutoff; and you see a lot of light blue lake-margin stuff that might be solid or might be liquid or might be a thin liquid layer or might be sludgy; we just don't know. But because it's blue, my brain wants to interpret it as liquid. I have the same reservation about the way the Mars Global Surveyor MOLA team always colored their Mars topographic maps, with blue at the lowest elevations, which makes the entire northern lowlands of Mars look like a great big ocean, when it's just as dry as the rest of Mars now.

We have to get below Titan's those polar clouds, someday, and get a close look at those putative lakes, to see what they really are!