06 January, 2007

Titanian shores

Details of the proposed polar lakes on Titan first announced in July, have been published in Nature by Stofan et al. (see also the accompanying News and Views article). The “lakes” are found in topographic depressions, and are thought to be filled with liquid based on their very low radar backscatter, which means whatever is in them is very smooth and reflective. This might seem counter-intuitive until you realise the radar beam is being sent towards to surface at an angle, rather than just straight down. A smooth surface therefore acts as a tilted mirror, reflecting energy away at the incident angle rather than back towards Cassini; a rough surface will contain parts with the correct orientation to return radar energy.

These features are by far the smoothest and most reflective found on Titan, and the contrast with the surrounding regions is quite sharp, as the figure below shows: the top row are false colour radar data with and without noise correction, and the bottom row are horizontal and vertical profiles of the normalised radar cross section (NCRS – a measure of the returned radar energy) along the white lines shown in b. -25 dB is about the level of instrument noise.

What’s also clear in the radar images is a very lake-like morphology, with irregular edges and what look like channels draining into the main body. The similarity is really brought home by this lovely image from the JPL press release:

It’s important to note that radar can’t tell us for sure that what we are seeing here are liquid hydrocarbons, although it does indicate a material with a low dielectric constant (i.e. it’s not a polar liquid, like water), as you’d expect. The authors also note:

Our inference that the northern-hemisphere lakes discovered by Cassini radar are at least partly liquid methane is consistent with various other considerations. If such lakes cover at least 0.2–4% of Titan's surface … they will buffer the atmospheric methane's relative humidity at its observed value, removing the requirement for a putative steady drizzle at the equator. If the abundance of lakes seen in [these] data are typical of their coverage poleward of about 70° in both hemispheres, then the fraction of Titan's surface covered by lakes is within this range. More recent polar radar data from Cassini support this assertion.

It is also unclear how these lakes are filled: is it by methane rain, or do the depressions cut into a subsurface methane aquifer? If the former, you would expect seasonal variations in how large and full the lakes are. Hopefully Cassini will keep collecting data long enough for us to observe, or rule out, such variations.

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