We recognize Earth as the blue planet, but it’s not the only ocean world in our neighborhood. Oceans may be concealed beneath thick crusts of ice on moons orbiting Jupiter, Saturn, and Neptune, and on the dwarf planets Pluto and Eris. Saturn’s moon Titan even boasts liquid seas right on its surface, although they are full of methane rather than water.
If anywhere in our solar system holds signs of life, it is likely to be these frigid worlds. Scientists are determined to explore the distant seas of Titan and Jupiter’s moon Europa, and are designing ice-gripping rovers and submarines to take the plunge into their mysterious depths. They will have to contend with bitter cold, liquids that behave differently than the water we are used to on Earth, and other hostile conditions.
Here’s how these hardy robots will explore two very different types of alien seas.
Cruising gassy seas
Over the course of its mission to explore Saturn and its moons, the Cassini spacecraft discovered hundreds of small lakes on Titan’s surface, as well as three seas similar to the Great Lakes in size and depth.
Titan also has water ice on its surface and a water ocean likely buried beneath its crust. But its methane seas are intriguing because they are part of a process that resembles the water cycle we have here on Earth. As on our own planet, liquid on Titan evaporates from the seas, forms clouds, and rains back down. Researchers would like to find out more about how this methane cycle works. What’s more, carbon and nitrogen compounds that could support life are plentiful on Titan; scientists hope to investigate whether some form of life could have evolved to depend on liquid methane the way terrestrial life depends on water.
NASA has considered sending a buoy to drift through Titan’s seas. One drawback is that this capsule would be at the mercy of winds and currents. “Most likely a buoy encountering [the] shore would just get beached and might be refloated with the tide,” Ralph Lorenz, a planetary scientist at the Johns Hopkins University Applied Physics Lab in Laurel, Maryland, said in an email. There are no guarantees that it would make it back to sea, though.
A submarine, on the other hand, could set its own course and would be able to explore beneath the sea surface and sample sediments on the seafloor. NASA hopes to send a submarine to Titan in the next 20 years. Its generator—which would be powered by heat from the decay of radioactive materials such as plutonium—will be key to keeping the submarine’s electronics toasty in Titan’s seas, which are roughly -290 degrees Fahrenheit.
Dealing with the cryogenic conditions on Titan “needs careful engineering, but no physical miracles,” says Lorenz, who is the submarine’s lead designer. “The waste heat from a radioisotope power source is an essential part of that, together with…some judicious choices of foam insulation.”
Another challenge is that we don’t know the exact chemical makeup of Titan’s seas. They are mainly methane—similar to liquid natural gas found on Earth—and a smaller amount of liquid ethane and dissolved nitrogen gas. But the exact ratio in which these ingredients appear isn’t clear, and may vary quite a bit between Titan’s seas. So the project is designing a submarine that can navigate a liquid expanse whose density and viscosity are not yet firmly established.
Engineers are particularly concerned about the nitrogen in Titan’s seas, which could form bubbles that would interfere with the submarine’s navigation. This might happen when some of the waste heat from the vessel’s generator seeps into the environment. “That heat is not enough to boil the surrounding liquid, but we believe it’s enough to cause that dissolved nitrogen that’s in the liquid to come out,” says Jason Hartwig, a cryogenic propulsion engineer at NASA’s Glenn Research Center in Cleveland.
The propeller itself could also create effervescence as it slices through the liquid. Behind each blade is a little void, Hartwig says. This drop in pressure can give bubbles an opportunity to form, similar to how a can of soda fizzes when you open it.
All those tiny bubbles could cause two big problems. Firstly, they might get in the way of the submarine’s scientific equipment, making it more difficult to measure depth and other conditions. Even more worrying, the bubbles might prevent the submarine’s propellers from working properly.
“If we’re trying to move from one location in the seas to another, are all those bubbles going to coalesce at the back end of the submarine?” Hartwig says. “You try to spin the propellers and the vehicle won’t move, it will just sit and spin.”
“This whole effervescence issue is a non-factor for submersibles on Earth because the amount of air that can get dissolved in water is very, very low,” Hartwig continues. “The pressure is higher on Titan and because the liquid is colder more gas is dissolved in the liquid, which means more gas can come out.”
Since we don’t know the exact chemical composition of Titan’s seas, it’s not certain how much nitrogen they hold. To get a better sense of what a submarine might encounter, Hartwig and his colleagues at Washington State University have recreated Titan’s seas here on Earth. They filled a test chamber with different mixtures of methane, ethane, and dissolved nitrogen at temperatures and pressures similar to those on Titan, then added a little heater in to mimic the heat that would radiate from an actual submarine.
The good news: if a submarine has stopped moving in order to collect samples, it might not have to worry. It probably wouldn’t emit enough heat to create the amount of fizz needed to stymie its instruments, the team reported in February in the journal Fluid Phase Equilibria. However, “We still haven’t ruled out the propeller issue,” Hartwig says. He intends to repeat the experiment with a propeller instead of a heater to find out how much bubbling the vessel might face as it journeys through Titan’s seas.
The submarine we send to Titan could have the same long, slender shape we are used to seeing on Earth. This kind of vehicle would weigh around 2,600 pounds and be about 20 feet long, Hartwig says. However, it would have to return to the sea surface in order to communicate with Earth. It would also have to wait until the 2040s to arrive, when Earth will be high enough above Titan’s horizons to give the submarine a direct line of sight (and communication) back to our planet.
NASA is also considering sending a smaller, turtle-shaped submarine. This “Titan Turtle” would be paired with an orbiter to relay information to Earth. It could communicate while submerged and could potentially launch a few years earlier because it wouldn’t need to rely on Earth’s position in the sky.
Titan is unique among ocean worlds, Hartwig says. Nowhere else in the solar system are liquid seas so easy to reach. But a Titan submarine could inspire designs for future vessels that will explore seas hidden beneath the ice crust on other bodies. “I’ve always looked at Titan as a pathfinder,” Hartwig says.
Under the ice
One of these less accessible seas can be found on Europa. While the moon’s surface is lashed with radiation from Jupiter’s magnetic field and is a bleak -280 degrees Fahrenheit, the ocean underneath is protected by a wall of ice that averages 5 to 15 miles thick. Because this ocean is made of water, it’s a tantalizing place look for life and study what chemical conditions might be needed for it to form.
To get through Europa’s icy barricade, scientists are testing robots that would melt or cut their way down to the ocean. These robots could carry submarines, rovers to drive along the underside of the ice, or even landers that would sink to the seafloor. Once they hit the water, these probes would likely encounter balmy temperatures around 32 degrees Fahrenheit. “It’s actually a fairly comfortable environment for our electronics,” says Andy Klesh, an engineer at NASA’s Jet Propulsion Laboratory in Pasadena, California. “What is a little bit more troublesome is the saltwater.”
Europa’s seas could be as briny as or own and may even be significantly saltier, says Kevin Hand, a planetary scientist at the Jet Propulsion Laboratory. “As an added challenge, there is likely some sulfuric acid in the mix,” he says. This means a probe’s electronics will be in danger of corrosion. The robot would also face formidable pressures as it ventured deeper into the ocean. On the seafloor, the pressure would be akin to that at the bottom of the Mariana Trench on Earth.
All in all, a journey to Europa’s seas is going to be a pretty rough ride. “We have the challenges of deep space exploration along with the challenges of deep ocean exploration,” Klesh says. “We have a vacuum in space; we have high pressure under the ice. And we have radiation on the way out there…we’re protected from that underneath the ice but then the environment’s trying to corrode us along the way.”
The easiest place to visit will be the underside of Europa’s ice shelf, he says. Klesh, Hand, and their colleagues are working on a rover that would drive around on the bottom of the ice. A rover or deep-sea probe wouldn’t be buffeted by currents, as would likely happen with a submarine. “These rovers [and] these landers may be the best way to explore in a very controlled matter and not be tossed about or knocked into other things,” Klesh says. The underside of the ice is also a particularly good place to search for life. On Earth, algae and microbes like to anchor themselves beneath the ice. If life exists on Europa, it may be drawn to this kind of terrain as well.
Klesh and his team are eager to begin the hunt. They’ve been testing their rover—currently known as BRUIE (Buoyant Rover for Under-Ice Exploration)—in Alaska’s freezing lakes. Its wheels sport both circular sawblades and little panels that act like snowshoes, distributing the rover’s weight across the ice. This prevents BRUIE from cutting into the ice and getting stuck in place.
The version of BRUIE that will eventually visit Europa will likely be carried through the ice by the robot responsible for digging down to the sea. That means it has to be pretty small, likely 18 inches or less in length, Klesh says. In two weeks, he plans to test collapsible wheels on BRUIE, which would allow it to become more compact and portable. The team also hopes to send BRUIE on its most ambitious mission yet—a trip nearly 1,000 feet beneath sea ice without a tether.
Ideally, a rover on Europa would also be untethered. “We have had a tendency to end up slicing the tether at least once during almost every trip when we’ve been out there,” Klesh says. But despite the risk of tangling it represents, a cord may be needed to provide power and communicate with equipment on the surface.
Klesh and his team are also looking for easier ways to reach the ocean. They have used submersibles to explore moulins—steep, flooded shafts that form inside glaciers—in Alaska. Last summer, their robot traveled about 160 feet deep beneath the ice, and was able to find connecting points between different tunnels. It’s possible that Europa also has moulins in its ice shell that a rover or submersible could use to travel part of the way down to the water.
There’s still a long way to go before a rover is ready to make the jaunt to Jupiter’s moons or beyond. Hopefully, Klesh says, the planned Europa Clipper and Europa Lander missions will pave the way for a probe to be sent beneath the ice.
In the meantime, the rovers that will one day explore Europa could teach us a few things about our own planet. BRUIE is already being used to scope out how thawing permafrost is releasing methane in Arctic lakes. And this fall, the intrepid rover will take a three or four-month-long sojourn in those chilly waters.
“The scientific goal is to leave the rover underneath the lake ice to watch the seasonal changes as the ice forms and thickens above these methane-rich lakes as the sun sets and the winter goes dark,” Hand says. This could reveal how a rover might become frozen in place as the ice around it thickens. It’s also a chance to learn more about an environment too punishing for humans to observe on our own.
“The largest ocean world of all is right here on Earth,” Klesh says. “The techniques we’re using to start to look at exploring Europa, we’re applying here to explore…regions we have never been able to go to before.”