Thursday, January 7, 2016

Lakes on Titan’s Southern Mid-Latitudes

Observations of Titan by the Visual and Infrared Mapping Spectrometer (VIMS) on NASA’s Cassini spacecraft reveal the presence of albedo-dark features on the southern mid-latitudes that could potentially be temperate lakes. If the albedo-dark features are indeed temperate lakes, they will be the first known persistent bodies of surface liquid on Titan that lie outside the areas southwards of 70°S latitude and northwards of 53°N latitude. The albedo-dark features are located near 40°S latitude and they are consistent with the presence of two potential lakes identified as Sionascaig Lacus and Urmia Lacus.

Figure 1: Image of Titan taken by NASA’s Cassini spacecraft. Image credit: NASA/JPL-Caltech/Space Science Institute.

Both Sionascaig Lacus and Urmia Lacus have lacustrine (i.e. associated with lakes) morphologies. For example, the surface of a lake is usually darker than its surrounding terrain. This is seen for both Sionascaig Lacus and Urmia Lacus, whereby the light-dark boundary outlining each potential lake is continuous, and appears quite similar to previously confirmed lakes on Titan, such as Ontario Lacus.

At the cold cryogenic temperatures on Titan, the seas and lakes on Titan are comprised of liquid hydrocarbons, mainly methane and ethane. When observed in the infrared, the surfaces of these seas and lakes have very low albedos (i.e. they appear very dark) because methane and ethane are strong absorbers in the 5 μm waveband. Sionascaig Lacus and Urmia Lacus are consistent with being bodies of liquid hydrocarbons due to their very low albedos. The surface albedos of Sionascaig Lacus and Urmia Lacus are 0.0070 and 0.0081, respectively. For comparison, the surface albedos of Kraken Mare and Ligeia Mare, two large northern seas on Titan, are 0.0114 and 0.0050 to 0.0089, respectively.

Figure 2: Artist’s impression of a lake on Titan.

The terrain surrounding Sionascaig Lacus and Urmia Lacus also exhibit low surface albedos, albeit not as low as the surfaces of the two potential lakes. It appears that the surrounding terrain could be interpreted as a wetland, with the ground soaked in liquid hydrocarbons. Sionascaig Lacus is by far the larger of the two potential lakes. Its surface area is estimated to be ~5000 km² and its average depth is estimated to be around 40 to 70 m. Sionascaig Lacus is also estimated to hold roughly 100 billion tons of liquid methane. Clouds have been observed on Titan’s southern mid-latitudes. The presence of temperate lakes may be driving cloud formation or be the result of cloud activity (i.e. precipitation); most likely both.

G. Vixie et al. (2015), “Possible temperate lakes on Titan”, Icarus Volume 257, 1 September 2015, Pages 313 to 323