"One could imagine that kind of compositional layering in Titan's seas; you may have more ethane-rich liquid at the base and a 'fresher' methane rich layer at the surface. But maybe tidal and wind-driven currents are enough to stir everything up and mix it."
The $100,000 Niac study didn't identify instruments to be carried by the torpedo-shaped vessel. But sidescan sonar, a camera, and a seafloor sampling system are obvious candidates.
However, operating a sub in Titan's same-but-different environment presents unique challenges. For example, military submarines face a problem called cavitation, where the propellers cause bubbling that's audible to sonar. This can give away their presence to the enemy.
If Titan's seas are methane-rich, and have nitrogen dissolved in them (as scientists think), changing the temperature of the liquid hydrocarbon by just a few Kelvin could cause the nitrogen to come out of solution. This means the sub's system for getting rid of waste heat would cause fizzing that might interfere with sonar measurements.
But Dr Lorenz says tweaking the design of the heat rejection system, or using the sonar when the sub is at rest could help mitigate.
Communications are also a vital consideration. TiME would have taken advantage of a geometric window of opportunity when Titan's north pole was pointed towards Earth, allowing direct communication with our planet.
But as the end of the decade approaches, Earth gets lower and lower on the horizon - making it more difficult to send data directly. So the submarine mission is being targeted for 2040 - the next point when the direct mode becomes possible.
To save everyone the wait, an orbiting spacecraft could accompany the sub to Titan in order to relay data to Earth. This would enable the mission to launch at any time, but also add considerable cost.
Another crucial factor is power. Spacecraft that stay within the inner Solar System can use solar panels to generate electricity.
But missions venturing beyond the asteroid belt need radioactive power generators, usually fuelled by the decay of plutonium-238. However, the US stopped production in the 1980s and supplies have been running out, causing consternation within the planetary science community.
Without this radioactive fuel, outer Solar System destinations like Titan are shut down to exploration.
TiME was to have used a novel power system called an Advanced Stirling Radioisotope Generator, which would have made the most efficient use of valuable remaining plutonium-238.
But its development was hit by delays, and the project was eventually placed on ice in 2013 - when the US officially re-started plutonium production.
"If TiME had been selected, we'd be launching 13 months from now. It was on the path to implementation... it was a real thing," says Dr Lorenz.
He estimates that between $5m and $10m were spent on its development. But there's now the potential to send a lander that can dive as well as float.
Saturn's biggest moon will continue to fascinate and inspire, making a return inevitable. And when we do go back, it may just be with a submarine.
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