Now that NASA has got the hang of planetary rovers, the space agency is looking at sending submarines into space around the year 2040. At the recent 2015 NASA Institute for Advanced Concepts (NIAC) Symposium in Cocoa Beach, Florida, NASA scientists and engineers presented a study of the Titan Submarine Phase I Conceptual Design, which outlines a possible mission to Saturn's largest moon, Titan, where the unmanned submersible would explore the seas of liquid hydrocarbons at the Titanian poles.
If you had to choose the odd man out of all the moons of the Solar System, Titan would be it. Larger than the planet Mercury, it's the only moon with a proper atmosphere. In this case, one composed largely of nitrogen and methane at a pressure one and half times that of Earth's, which is remarkable when you consider that the gravity is only 0.14 g. It is, however, unpleasantly cold at a nippy minus 290 °F (minus 179 °C).
As a result of the Voyager and Cassini probe flybys and the Huygens probe landing, it's been established that there are three large polar seas on Titan consisting of methane and ethane in a composition similar to that of liquified natural gas. The largest of these is Kraken Mare, which was discovered by the Cassini probe n 2007. It lies in the Titanian arctic between 60 and 80 degrees north latitude, covers 400,000 sq km (154,000 sq miles), and may be 160 m (525 ft) deep, though some estimates place it beyond 300 m (1,000 ft). It even has tides due to the pull of Saturn, a complex shoreline, and evaporite deposits, so it's of particular interest to scientists.
Unfortunately, as anyone who has peered over the side of a boat can tell you, there's only so much that can be learned by looking at the surface, so NASA is considering what kind of a submarine would be able to explore the depths of Kraken Mare.
NASA's conceptual Titan submarine is based on experience gained from the building and operations of drone submersibles on Earth. Weighing in at about one tonne (2,200 lb), it uses conventional electric propulsion modified for use on Titan for a 90-day mission covering 2,000 km (1,240 miles) of Kraken Mare.
Because of its elongated shape, the sub would need to be delivered to the surface of Titan using a winged spacecraft similar to US Air Force X-37 lifting body, which could survive a hypersonic entry into Titan's atmosphere, ditch on the surface of Kraken Mare, and then sink away, leaving the submarine floating on the surface. After orientation and testing, the sub would then begin its mission. Because of the great distance from Earth, the submarine would operate with a very high level of autonomy.
At its heart, the submarine would use a 1 kW radiothermal Stirling generator. This would not only provide power to propel the craft, but it would also keep the electronics from freezing. Unfortunately, Titan is so cold that it's almost a cryogenic environment, so the waste heat from the generator would cause the liquids around it to boil and this would need be taken into account when designing the sub to minimize interference. However, NASA estimates that the boat could do about one meter per second (3.6 km/h, 2.2 mph).
For economy and simplicity, the conceptual submarine would not use an orbiter as a relay because an orbiter would need to be nuclear powered and include a propulsion system, which would greatly increase the cost and complexity of the mission. Due to the large amount of data that needs to sent to Earth, the submarine needs a large dorsal fin that includes a planar phased-array antenna. While operating, the submarine would surface for 16 hours per day for Earth communications during which it would study its surroundings using a mast camera. This is a bonus because the high latitudes mean any break in the Titanian clouds would be rewarded with spectacular views of Saturn on the horizon.
Like an earthbound submarine, the Titan sub would use ballast tanks, but their design is still open to question because methane and ethane are not water and Titan is very different from Earth. The liquid density of different ratios of methane to ethane, for example, is very variable compared to that of fresh versus salt water, so something as basic as the size of the tanks has yet to be sorted out.
Titan's gravity is low, but if Kraken Mare is as deep as some theories indicate, and taking into account the composition and temperature of the Titanian atmosphere, it could also cause trouble because at great depths the nitrogen in the ballast tanks could condense into a liquid, which could result in a sudden loss of buoyancy. For this reason, the tanks would need to use a piston to allow in and expel liquid rather than relying on air pressure as in a conventional submarine.
NASA doesn't say much about the objectives of the Titan submarine, but it would probably be a full itinerary. This would likely include the study of the structure and composition of Kraken Mare in terms of both its liquid and its sediment. Also, since Titan has an overabundance of organic chemicals, the submarine would be tasked with looking for traces of prebiotic compounds that could give clues as to how life began on Earth.
The animation below shows the conceptual Titan submarine in action.
Source: NASA (PDF)