In the past couple of weeks, I've posted three entries on the proposed Titan Mare Explorer that would float a probe for months on the surface of a Titan lake.
I learned yesterday from a Decadal Survey white paper that Titan lake floaters are small dreams. A white paper that apparently is based on a SwRI study proposes a Titan lake submersible. Details on the implementation are sketchy, but it appears that the probe would first float on the surface and then fill ballast tanks (or alternatively detach from a flotation device) to sink to the lake bottom. It's not clear whether the ballast tanks could be blown to resurface. There's also no information on how the craft would communicate to Earth once below the surface. Would the lake liquids be transparent to radio waves or would the craft periodically resurface?
The white paper includes the most succinct summary of why chemical measurements of Titan's lakes are a high priority: "the mixing ratios of minor constituents (hydrocarbons, nitriles, noble gases) dissolved in the ethane-methane fluid of Titan’s lakes are expected to be higher than in the atmosphere, enabling spectrometric measurements of significantly higher sensitivity than were achievable with the Huygens GCMS or the Cassini INMS. Measurements of the dissolved species will provide information needed to better constrain models of the formation and evolution of Titan and its atmosphere." In addition, "The relative deficiency of Titan’s atmosphere in oxygen gives rise to the question whether prebiotic organic chemistry at Titan (1) is terrestrial in nature, occurring in cryovolcanic ammonia-water flows or in melt pools resulting from impacts, or (2) represents an altogether different chemistry, “where ammonia substitutes for water, and Nchemical groups substitute for O-chemical groups” [Raulin and Owen, 2002, p.383; Raulin, 2008b]. With its hydrocarbon lakes and ammonia-water cryomagma, Titan affords a unique laboratory for the investigation of alternative—“weird”—biochemical processes involving nonwater polar solvents (ammonia) or nonpolar solvents (hydrocarbons) [cf. NRC, 2007]."
Submersion through the lake depth to the bottom would allow the examination of changes in pressure, temperature, and composition along the descent profile. The paper mentions possible sampling of the lake bottom sediments. The paper also talks about the measurement of lake tides from the bottom (using upward viewing sonar).
In addition to studies below the surface, the white paper emphasizes the importance of studies at the surface to study Titan's hydrological (methane-ological?) cycle. Apparently the craft would spend considerable time on the surface.
The paper briefly mentions that it might be possible to build a dual probe that includes a submersible and a floater. It doesn't say which or both might include the RTG power source. (Retaining it on the floater for long-term meteorological and hydrological studies would seem more important to me; the submersible might be battery powered and relay its findings through the floater.)
The paper emphasizes that this would be a New Frontiers (~$650M) class mission.
Editorial Thoughts: An intriguing idea. I suspect that the design problems are greater than suggested by the paper ("...no major technical drivers that must be overcome..."). Just the problem of keeping the craft warm in a dense, extremely cool fluid would seem a challenge to me. And might the craft's temperature (it has to be kept warm inside and that heat will leak out) cause problems?
Whatever the technical challenges, this is an intriguing idea that seems to be exploration at its best. Imagine having the descent camera return images from the bottom of an alien sea (okay, it will probably just be dull mud flats, but still...).
Note: There's also been a lively discussion of Titan lake probes here at Unmanned Spaceflight that you might want to check out.
Titan Lake Probe white paper
SwRI Concept for a Titan Submersible
Abstract for a the Titan Mare Explorer