The moons orbiting Jupiter and Saturn lie far from the sun’s warmth. Most have no atmosphere, and many are covered in an icy sheath miles thick. They’re also our best bet for finding life in our own solar system. Beneath the frozen crusts lie vast oceans, and space agencies in the United States and beyond are hard at work on the robots that will one day visit them.
“In the past we assumed there was this Goldilocks zone between Venus and Mars where you’d find liquid water, and…this was the only place in the solar system where you’d find life,” says Hari Nayar, leader of a robotics group focused on ocean worlds at NASA’s Jet Propulsion Laboratory in Pasadena, California. Yet Jupiter’s Europa and the Saturnian moon Enceladus appear to have the key ingredients for life—plenty of liquid water, food, and energy from deep-sea vents.
This life, if it exists, won’t be easy to reach. It’s most likely to be found swimming deep below the surface of frigid alien oceans. But once a spacecraft journeys to the outer solar system and manages to land on Europa or Enceladus, it will still be far above these waters. Robotic probes will have to delve into the ice, burrowing through an environment that is almost as fearsomely cold as liquid nitrogen.
There are a few different ways to breach this icy fortress. NASA recently announced that it is testing out new prototypes for robots that will explore frosty worlds, including a probe that will chop the ice up and heat the shavings in its toasty innards. Researchers in Germany have been developing a robot that will melt any ice in its path. And those are not the only ideas.
The engineers behind these robots won’t be satisfied with a probe that just digs straight down, either. Their creations will have to navigate and fire samples back up to the surface, on top of tunneling for months or more. Here’s how these intrepid probes will tackle ice worlds and search for life.
What lies beneath
If there is life on Europa or Enceladus, it will be microscopic. “There probably are not whales or giant squid or even little tubeworms or anything like that down there,” says Cynthia Phillips, a planetary geologist at the Jet Propulsion Laboratory. “We think there is not enough energy really to drive multicellular life.”
But vents on the seafloor would be a promising home for alien microbes (and are the same kind of ecosystem where life on Earth might have kicked off). And no matter where these vents are, if they harbor life, then traces of that life will have traveled far afield.
“In the Earth’s ocean, if you take any cubic meter of water in the ocean, it [probably] has genetic material from most of the organisms on Earth,” says Brian Wilcox, an aerospace engineer at the Jet Propulsion Laboratory. The same should hold true for the seas of Europa or Enceladus. So when our probes finally reach the sea, any drops of water they capture should be enlightening.
“If you have good enough instruments that can find things at very low concentrations, then you’re pretty much guaranteed of finding biological molecules if they exist,” Wilcox says.
However, this does put some limits on the probes we can send. Since the robot will by definition be looking for life, it must follow strict rules to avoid bringing Earth microbes along for the ride. Before touching down, it will be sterilized at such searing temperatures that nothing could survive—not even modern electronics. NASA is considering probes that rely on simple graphite and copper motors like those invented in the 19th Century. “You can make a motor of the type that was used 120 years ago, all out of stuff today that will survive this bake-out,” Wilcox says.
A lander can be spared this harshest cleansing, as it will never touch the ocean. So that’s where the electronics that actually control the probe and analyze the water it collects will likely reside. “The probe is kind of its like a marionette on the end of a string and it has no smarts of its own,” Wilcox says. “We have to put planetary protection front and center, because that’s really the hardest of all the problems.”
Slicing and dicing
On Earth, we delve into the thick ice in places like Antarctica and Greenland using drills or probes that sink ever deeper by heating the ice around them until it melts.
That won’t work on the moons of Jupiter and Saturn. “It’s almost impossible [to think] we could deliver drilling equipment to an icy moon,” says Bernd Dachwald, a professor of astronautical engineering at the Aachen University of Applied Sciences in Germany.
And the ice is hundreds of degrees below freezing. “It would essentially wick away all that heat,” Nayar says. The probe he, Wilcox, and their colleagues have in mind keeps its heat all on the inside, where it can’t leak away.
The probe uses a spinning buzzsaw to slice through the ice, and a pile driver to hammer itself deeper into the hole. The probe can steer by cutting deeper into the ice on one side than the other. The ice chips, meanwhile, are tossed into the probe’s insulated body to be melted. “The whole body of the probe is essentially a vacuum bottle, just like a thermos bottle that would keep your drink warm all day,” Wilcox says.
The heat will come from plutonium (the type that powers the Curiosity Rover and other spacecraft, not the type used to make nuclear weapons). Most of the water it melts will be pumped out the back. But the probe can also collect water samples in tiny canisters, and shoot them back to the surface through an aluminum tube inside its tether.
Once the melted water freezes back into ice, it will lock this tether in place. That means the probe will have to carry its own cable instead of tugging it from the surface. It also means that the probe can’t be dragged back up to the surface. “That’s another reason why it has to be absolutely sterilized beyond any shadow of a doubt, because it’s going to be down there forever,” Wilcox says.
Make like a mole
Another probe destined for frozen worlds is the IceMole, which is being developed for the German Space Agency’s (DLR) Enceladus Explorer project. At about 6.5 feet long it’s not quite as petite as its hairy namesake, although its designers are planning to make future generations shorter and lighter. They have already tested its burrowing prowess in Antarctica and other icy locales.
IceMole is primarily a melt probe, which means it will heat its way through the ice. This takes a lot of energy, so the probe would probably draw its power from a refrigerator-sized nuclear generator up on the surface. However, the mechanical mole is also fitted with an ice screw. “This force presses the melting head firmly against the ice, so that you have always a very good heat contact,” says Dachwald, who has spent years designing and refining the probe.
One problem with regular melt probes is that dust or sand embedded in the ice can sink to the bottom of the melted water in front of the robot and build up. Eventually, the probe is faced with a plug of mud it can’t heat its way through and gets stuck. IceMole would avoid this catastrophe because its ice screw can drag it through dirty ice. Its designers have tested the probe in soil and sediment-rich ice in Antarctica’s Lake Hoare—IceMole slowed down, but it did not stop. The handy ice screw is also hollow so it can slurp up samples.
Like NASA’s proposed robot, IceMole can switch course. By directing more heat to one side of its melting head, IceMole can be forced into a curve. “They are not as good as a real mole, but we have a turn radius of about 10 meters and this should be sufficient to avoid large obstacles,” Dachwald says.
It will use a few different instruments to navigate, and can even melt upwards. That means that, perhaps, IceMole could find its way back up the surface.
A hostile environment
Europa and Enceladus are not welcoming places, even aside from the punishing cold. Robots that roam the surface will bear the brunt of these extreme conditions.
For one thing, they will be too far from the sun to rely on solar power. And the ice may not be easy to drive over. Europa and Enceladus are thought to shoot plumes of water vapor that freeze and fall to the ground as tiny grains. “This material would behave like sand dunes in the desert that do not stick together, and so you could easily sink,” says Nayar, whose team is designing a lightweight rover that resembles a dune buggy.
Europa is blasted with radiation from Jupiter’s magnetic field that would kill an unprotected human in 10 minutes. It’s not so great for robots, either. “The surface is basically just pummeled with charged particles of radiation that would be very damaging to any kind of surface spacecraft,” Phillips says.
Any robots on the ground will need shielding to protect them from this assault. This might not seem like a problem for the probes far beneath, sheltered by the ice. But they will still rely on equipment back at the surface, which will have to endure while the probes slowly penetrate miles of ice.
And the ice itself will present its own trials. It probably won’t just be pure water. “The problem is we do not know what the real composition of that material is going to be,” Nayar says. A robot could have to steer around rocks or crevasses, or encounter corrosive chemicals such as sulfuric acid.
“To have something that works for several months or even years through the ice, through an unknown environment and the slightest failure can lead to a loss of the mission, this is challenging,” Dachwald says.
NASA can send probes out for field tests in places like Antarctica or Greenland to make sure they’re up to snuff. But compared to Europa or Enceladus, these icy wilds are a cakewalk. Engineers will have to mimic some of the harshest conditions of icy worlds in the lab, using special cold and vacuum chambers and beds of super-cold ice.
Despite the hassles, journeying through ice has its upsides. A probe can’t easily melt its way through solid rock. “We do want to actually melt the ice, because it’s so easy to handle liquid water,” Wilcox says.
And any samples the probe collects on its travels will be simple to sift through. “Somewhere like Mars or the Moon where it’s really a rocky sample, you have to…break down the rock so you can study what’s in it,” Phillips says. Icy samples can just be heated up. “That’s an easy way to separate out the ice from the non-ice materials.”
Ripe for exploration
As tantalizing as Europa and Enceladus are, we needn’t confine our explorations to these two moons. There are many potentially watery bodies beyond Earth—Mars, large asteroids, Pluto, and other moons like Saturn’s Titan or the Jovian moons Ganymede and Callisto. “There’s dozens of worlds in the outer solar system where you can use a very similar architecture,” Phillips says.
The same kinds of landers, rovers, and eventually probes may bring all of these worlds within our grasp. How exactly this technology will look is still uncertain. “We don’t have a proven solution that’s better than every other solution,” Nayar says. “This is a very different environment from any other that we’ve been to.”
We’ll learn more about these distant worlds as new information comes in from missions like Cassini and the planned Europa Clipper. This will make it easier to design the probes that will one day delve below their ice surfaces.
It will be quite a few years before these robots alight on Europa or Enceladus; a probe to Europa probably wouldn’t launch before 2028. But that doesn’t mean space-bound probes can’t help closer to home—there’s plenty of ice research we can do right here on Earth. “It would be a little sad if you invest money in just technical demonstrations without any science,” Dachwald says. He and his crew have already used IceMole to sample bacteria at Blood Falls in Antarctica, where an englacial brine reservoir is full of poorly-understood bacteria that have been secluded from the outside world for more than 1 million years.
And we will have plenty of other chances to test a probe’s space-worthiness and put it to work at the same time. Beneath the ice sheets of Antarctica, there are lakes yet to be explored. We still don’t know how far life extends into the ice, or how well it can survive in these frozen wildernesses. “If we want to know whether life can exist in the ice on other planets and moons we have to find out the birth conditions…here on Earth for life in the ice,” Dachwald says.