Saturn's Titan is the only moon in our solar system with a significant atmosphere. Its chemistry resembles a prebiotic Earth. And there are liquid natural gas pools in lakes on the moon's surface, which is home to more liquid than the entirety of the Great Lakes. If some form of primitive life or biochemistry hasn't arisen on the moon already, then many researchers suspect it will someday.

With so much to discover, many scientists eye potential missions to explore the surface of Titan. However, a joint team of NASA, Penn State, and Johns Hopkins University researchers have their sights on an idea that's more Jacques Cousteau than Juno probe. They want to design a submarine to go under the Kraken Mare, the greatest of Titan's great methane lakes. The team was awarded a Phase I NASA Innovative Advanced Concepts grant last week—the cornerstone toward a working prototype and, maybe, an eventual launch.

Traveling to Titan

Once on the moon, the submarine could measure the composition of the lakes through spectrometry, snap pictures of the area, measure waves and weather interactions, or map the floor of the lake bed. The orbiter spacecraft that carries the sub to Titan could act as a relay, sending radio transmissions from the explorer back to Earth (radio waves pass right through hydrocarbons, so Titan's thick atmosphere shouldn't be a problem).

And depending on the time it arrives, the craft may not even need an orbiter. If it arrives in the northern summer, it could potentially communicate directly with Earth.

"It may be possible to talk back to the orbiter and to send data without surfacing," Steven Oleson, lead of the COMPASS (Collaborative Modeling for Parametric Assessment of Space Systems) Concurrent Spacecraft Design Team, said. "On Earth, a submarine would have to surface to talk to (an orbiter.) This would give us the capability to stay submerged and send back data, and have scientists interact with the vehicle."

Now in the design stage, Oleson and his team of engineers are working on the big questions of what it will take to keep a submerged machine running so far from home. What's the power source? What instruments will be on board? What materials will be required? And how much autonomy should the explorer have?

That last one is a biggie. Because of the distance between Earth and Saturn, signals would take more than an hour to travel one way; the response will take just as long. So unlike with current Mars missions, the craft will need more onboard computers that can respond to the difficulties it encounters without phoning home.

"This thing has to go out and be smart enough to avoid collisions, avoid the bottom and do the jobs we give it," Oleson says. "We can talk to it periodically, but we won't be able to control it all the time."

Team member Ralph Lorenz, a research scientist at the Johns Hopkins University Applied Physics Lab, worked on the Huygens probe, the first probe to explore Titan. Thanks to that mission, he says, scientists know they can land on the Saturnian moon.

"Huygens confirmed the atmosphere profile and winds, which gives us assurance that we can parachute safely into the seas," Lorenz says via email. "It also showed that the Titan environment is overall rather benign—it's just cold!"

Like missions to Mars, a Titan explorer must take precautions to avoid contamination by Earth bacteria. Here, the sub may have a few advantages. For one, Oleson says the submarine may be self-contained and have few outward instruments, limiting the amount of sterilization work necessary. And as Lorenz points out, Titan's methane seas and ultra-cold temperatures make it inhospitable to even the most extreme Earth life.

Tight Window

Any potential vehicle is decades off, and still, the clock is already ticking. The mission would need to arrive in the more hospitable Titan summer, which would give it an eight-year window. While that may sound simple, consider that Cassini and Huygens took seven years to arrive at the Saturn system. Miss the launch window and it could be a decade before another is possible.

But that's not the concern right now. The team is working for the next nine months on the early proof-of-concept and design. If NASA approves the project for Phase II, the designer will make tweaks and conduct early lab experiments in a simulated Titan environment. If the concept works, they'll have multiple options beyond Titan. The sensors could be used for liquid natural gas transport and storage operations on Earth, or the craft could be modified for other environments like those of Enceladus or Europa.

"Once you figure out how to do these extraterrestrial submarines, you can figure out how to apply them to other liquid places," Oleson said.

And if the submarine probe reaches Titan someday?

"We know complex compounds form on Titan, although we don't know how complicated they can get," Lorenz says. "At some point on Earth, chemistry got complicated enough to permit functions of replication and information storage, and eventually life. Titan might help us understand how."

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