NASA is in need of an ocean-going vessel. Not just any vessel: the agency requires a submarine to ply an ocean composed of methane and ethane. The sub must operate at temperatures approaching -300 F. Where is this frigid, gassy expanse? It’s on Titan, the largest of Saturn’s moons, where the hydrological cycle is ruled by methane.
Titan is unusual in that its surface includes oceans, rivers and clouds, and as on earth, it can rain. NASA plans to explore these potential resources with an autonomous submarine engineered to study atmospheric and ocean conditions, move around sea beds and hover at or below the surface. The design must also account for the varying concentrations of ethane and methane in Titan’s oceans which can change the liquid’s density properties.
To better understand design requirements and parameters, these unearthly conditions were simulated in a cryogenics laboratory at Washington State University. The Titan atmosphere was recreated with a liquid mixture maintained at cold temperatures in a test chamber. A two-inch, cylinder-shaped cartridge heater was included to approximate the heat generated by the submarine.
Bubbles posed a major challenge: nitrogen bubbles will likely form as the heat-producing sub is introduced to the cold seas. Too many bubbles would make it hard to maneuver the ship, take data and manage ballast systems.
Another problem was recording a video in extremely cold conditions and a pressure regime of 60 pounds per square inch. The researchers devised a solution using an optical device called a borescope and video camera that could withstand the low temperatures and high pressures to visualize what was going on within the test chamber.
Analysis of the freezing temperatures for methane and ethane lakes revealed that, because of a small amount of nitrogen in the liquid, the lakes freeze at lower temperatures than would be expected -- 75 Kelvin, or -324 degrees Fahrenheit, instead of 90.5 Kelvin. This implies that icebergs won’t be a navigation hazard.
The researchers are looking to continue the work with NASA to update the design of the Titan submarine, which could be launched within the next 20 years.