NASA researchers have studied Saturn and its moons for almost two decades, collecting and analyzing data from the Cassini spacecraft. Cassini ended its run in 2017, but the space agency is already planning a more immersive return mission, aiming to plunge a submarine into the seas of Titan, the largest of Saturn’s many moons and the second largest in the solar system.

Titan is of particular interest to researchers because it is similar to earth in one important way, it holds liquid. Unlike almost anywhere else in the solar system, the moon’s surface includes oceans, rivers and clouds, and like on earth, it can rain. But, instead of water, the hydrological cycle is based on methane and the moon’s fraught seas flow at temperatures near -300°F.

The submarine that the agency is designing will operate autonomously to study atmospheric and ocean conditions, navigating around seabeds while hovering at or below the surface. The engineering is tricky, as the concentration of ethane and methane can vary dramatically in the Titan oceans and change the liquid’s density properties.

Washington State University researchers are working with NASA to determine how a submarine might work on Titan, and have published a new paper in the journal Fluid Phase Equilibria.

The team recreated a Titan ocean in WSU’s cryogenics lab, filling a test chamber with a supercooled methane/ethane mixture and then introducing a two-inch, cylinder-shaped cartridge heater to approximate the heat that a submarine would create.

One of the biggest challenges for researchers was understanding bubbles in the Titan seas. Adding a submarine powered by a heat-producing machine into the very cold Titan liquid causes nitrogen bubbles to form. Too many bubbles would make it hard to maneuver the ship, take data and manage ballast systems.

Another challenge, according to Ian Richardson, a former graduate student in the School of Mechanical and Materials Engineering who led the research, was getting video of conditions inside the test chamber. The study was conducted near -300°F with 60 pounds per square inch of pressure. Richardson’s group engineered a solution using an optical device called a borescope and a video camera that could withstand the low temperatures and high pressures to visualize what was going on. “Those aren’t the friendliest conditions. You have to come up with creative solutions,” he says.

The group also studied the freezing temperatures for the methane and ethane seas and determined that, because of a small amount of nitrogen in the liquid, the mixture freezes at 75 K, or -324°F, instead of 90.5 K, a lower temperature than the team expected. “That’s a big deal,’’ Richardson says. “That means you don’t have to worry about icebergs.”