The Plan to Send a Submarine to Titan, Saturn's Largest Moon

Planetary scientists intend to send a submersible vessel to cruise the liquid hydrocarbon seas of Titan, Saturn's largest moon. The mission study is in its infancy, but its ambition and audacity harkens to the best of science fiction and the heady heights of the Space Race. As Ralph Lorenz of the Johns Hopkins University Applied Physics Laboratory (APL) explains, "The virtue of this study is that you just need to say those words—Titan submarine—and everyone kind of gets that it's out there, it's interesting, and there's a lot of exciting potential."

At the 47th annual Lunar and Planetary Science Conference held last month in The Woodlands, Texas, Lorenz—the Titan submarine's project scientist—led an open forum on the mission to solicit reactions from fellow space scientists to the mission's targets and objectives. The goal was to help the Titan sub researchers determine the best payload of scientific instruments for the craft.

Among the questions the scientists must eventually answer: How long should such a mission last? How far should the submarine go? How fast should it go? How much data might it try to return?

None of these questions are as simple as they might seem. Cruising speed and data transmission, for example, must be carefully balanced. Too much of one takes away from what little available power exists for the other. Shorter travel distances mean more data about fewer things; vice-versa for longer distances. If the vessel is going to be sticking around one area for a while, what instruments might be needed to really collect every scintilla of data possible? Then it's back to the drawing board with respect to balancing the use of available electricity. No problem in space exploration is trivial, and no decision can be made lightly. Add to all this the problems inherent to submersible vehicles—and that Titan's seas are cryogenic, or extremely cold—and you get some idea of how ambitious and exciting this mission really is.

"Titan lends itself to many mission configurations: orbiters, airplanes, floating capsules," Lorenz said at the forum. "What is it this enterprise could do that other platforms could not?"

WHY TITAN?

Of all the worlds in the solar system, why this particular Saturnian moon? Why not Enceladus, with its subsurface ocean? Why not Triton, orbiting Neptune—the size of our own moon, but with a troposphere and active ice volcanoes?

"There are two overarching scientific reasons to explore Titan," Lorenz told mental_floss. First is that Titan is rich in "process": It has an active meteorology and a complex climate history that is apparent both in its landscape of dunes and in the apparent mineral deposits left behind due to evaporation on the margins of its seas. He adds secondly that Titan "is a world amazingly rich in organic materials—the stuff of life." It has an internal water ocean (and occasional surface exposures of liquid water by way of impacts from meteorites), which can interact with the abundant photochemical carbon- and nitrogen-bearing compounds that make up its dunes.

"Titan can inform us on the chemical processes that lead to life (as we know it, based on liquid water)," Lorenz said. "There is also the possibility, albeit a remote one, of alternate chemical systems executing the functions of life—metabolism, information storage and replication, etc.—in a completely different solvent: liquid methane."

Lorenz also offers a third, more psychological reason: "It is such a familiar yet exotic place, that we can see many of the things—waves and tidal currents, beaches, rainfall—that are so much a part of the human experience on Earth, yet occur with quite different circumstances and materials on Titan." For this reason, he said, exploring Titan may resonate with people on a visceral level in a way that other worlds may not.

SENDING A SPACE SUBMARINE TO ANOTHER WORLD

Here is an actual problem that scientists have tackled, not as consultants for some sure-fire science fiction blockbuster, but rather, in order to put together a very real NASA mission: How do we launch a submarine into space, send it to another world, and drop it into an extraterrestrial lake?

As it turns out, a lot of work on the problem has already been done. The traditional shape of a submarine doesn't lend itself to the classic entry shell seen previously with the Mars landers. The Titan submarine team soon realized, however, that the submarine would fit quite nicely inside the cargo bay of a scaled-down space shuttle. Better still, DARPA—the Defense Advanced Research Projects Agency—has already built a scaled-down space shuttle, and it's flying today. It is called the X-37B—and the submarine would fit inside it.

The entry velocities for a mission to Titan would be the same as Earth orbital velocities, something the X-37B and its thermal protection can already handle. ("For [this phase of] the study, we just said, 'Sure, we could make that work,'" Lorenz explained at the forum.) Such an entry vehicle would be especially useful in that it could fly to a designated spot without dealing with the winds and consequent uncertainties that a typical parachute descent entry would have to overcome.

Next, the Titan team considered extracting the submarine from the back of the vehicle, much in the same way the U.S. Air Force pushes a MOAB from a C-130. They also looked at ditching tests conducted by NASA in the event that the space shuttle would ever have to land on water. A splashdown on Titan of their spacecraft, they found, would be quite forgiving, and if they attempted such a landing, they could simply flood the entry vehicle, let it sink, open the back, and let the submarine swim out into the sea. From there, the vehicle would conduct preliminary sea trials to discern maneuverability, and then get underway.

HOW DO WE TALK TO THE SUB ONCE IT'S ON TITAN?

The submarine must obviously be able to communicate with Earth. For the purposes of this preliminary phase of a potential mission, Lorenz and his team have assumed direct communications from the submarine to the Earth—that is: pointing the Deep Space Network at Titan, blasting signals to the submarine, and listening closely for a response. This was the plan for the Titan Mare Explorer, a boat mission proposal that came close to being approved by NASA in 2012.

Envisioning a direct communications system—as opposed to a relay satellite around Titan (akin to a floating cell phone tower)—has allowed the team to focus for now on the submarine's technical details. "Everything is easier when you have an orbiter as a relay," said Lorenz, "but then you have a second element that's expensive."

But direct communication brings with it problems of its own. Because Titan's seas are near its poles, Earth is always low in the Titan sky. The idea of doing direct Earth transmissions imposes a restriction on when a submarine mission can actually launch, Lorenz said. "As we go into the mid-2020s and 2030s, the Earth is below the horizon of the Titan seas."

This means there's no line of sight between the Deep Space Network and the vehicle. A relay orbiter, not bound by horizons, would have no such problem.

ROAMING IN THE DEEP

"Oceanography is no longer just an Earth science," Lorenz said. Already, his colleagues are adapting terrestrial oceanographic models to Titan's seas. This involves taking those seas and making informed guesses about the bathymetry, or study of the sea beds; adding in Titan's orbit and tides; applying the winds from global circulation models and convection currents from solar heating of the sea; and evaluating what kinds of ocean currents develop. Such things are enormously difficult to model without in-situ data. But for the scientists, adapting the models seems not a question of if, but when.

This phase of the Titan submarine study is funded by NASA's Innovative Advanced Concepts (NIAC) program, and costs approximately $100,000. The team is preparing to carry forward a subset of this study's findings to a more comprehensive, half-million-dollar "phase II" analysis. NIAC emphasizes low TRL stuff—that is: "technology readiness level." That means NIAC mission concepts can proceed under the assumption of reasonable advances in technology (e.g., more efficient power sources) that will be available by the time such missions actually fly.

So when might this mission happen? If the Titan submarine is indeed built for direct communications (as opposed to an orbital relay), it will need a line of sight between the Titan sea and the Earth. That means 2040 at the earliest, when Earth again appears over the horizon of Kraken Mare. (Travel times to Titan will depend on the type of rocket used to launch the mission.) On the other hand, if the mission builds serious momentum and money is promised by NASA for a communications relay orbiter, the timetable might look much more favorable to a Titan splashdown years earlier.

A lot of that depends on NASA's budgetary environment. The agency passed over a Titan watercraft (the Titan Mare Explorer) in 2012, to the dismay of many. Would they do so again? As exciting as rovers are on Mars, the sound of methane waves lapping against a surfaced submarine, and the sight of Saturn, massive and hanging near in the sky, its rings reaching across the horizon, might be even more exciting. One imagines our species finally ready to zip away from the Earth the way we once leapt down from trees and, before that, clawed out of oceans.

LABELING THE MAP

The mission as currently conceived has the submarine splashing down in Kraken Mare, whose watery footprint is more than 154,000 square miles and thought to be nearly 1000 feet deep, to explore over the span of 90 Earth days. As it tours the coastline for some 1100 miles in all, it will collect samples, spectral data, and imagery.

Different regions would be conducive to different lines of scientific investigation. Ligeia Mare, for example, is a large lake to the north of the upper Kraken Mare. In the same way that the Baltic Sea (on Earth) drains into the North Sea, and the Black Sea drains into the Mediterranean, so too might Ligeia drain into Kraken. This would allow scientists to determine if the compositions of the two seas are different. The sub could cruise to the channel and "sniff" the water from Ligeia to check. Once the primary mission is complete, the submarine could travel the channel connecting the northern Kraken sea (Kraken-1) to its southern body (Kraken-2). After crossing through Seldon Fretum ("the Throat of Kraken-2"), it would begin a possible second mission.

NASA Glenn Research Center

The "tour" design of the mission motivated the designation of inlets and islands in Titan's seas. "Nobody ever needed to name them," Lorenz said, "but when you start talking about, 'Oh, the inlet next to the thing that connects Kraken and Ligeia,' it gets awkward, so we came up with names."  

The convention established by the team has Titan's seas named after sea monsters (e.g. Kraken Mare); lakes after Earth lakes ("I can see some confusion arising from that in the future," joked Lorenz); islands after mythical islands; and channels after characters in the Foundation series by Isaac Asimov. 

This is a small thing, and yet glorious. We have to name tiny islands on Titan to make its exploration easier and shed light on the moon's liquid mysteries. As Asimov himself has been quoted, "There is a single light of science, and to brighten it anywhere is to brighten it everywhere."