The ice of a distant moon

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On an unusually hot summer day in Wales, Sanjay Vijendran and colleagues aimed a rocket sled at an elephant-sized ice cube.

The sled rested on a raised metal track and carried what looked like a cartoon bundle of TNT to propel the contraption at the speed of sound. In front of it, a second sled held a bullet-shaped canister packed with scientific instruments.

Vijendran, a physicist at the European Space Agency, was ready to hurl the canister into a 6,200-kilogram block of ice, at the U.K.’s Ministry of Defense site in Pendine.

With chain saws on hand in case the canister got stuck, researchers watched the sleds hurtle down the track, launching the canister into the air at more than 340 meters per second. It flew the length of a school bus and then punched almost clear through the 3-meter-long frozen block, spraying geysers of snowy ice chips.

“It was all over in less than two seconds,” Vijendran says of the July 2013 test. “If you blinked, you missed it.”

Vijendran and his colleagues want to take this pyrotechnic spectacle to a place where no one will see it. They’re trying to design a device strong enough to pierce Jupiter’s fourth-largest moon, survive the impact and grab samples of ice.

The moon, named Europa, looks just as desolate and uninviting as any other place in the outer solar system. Its frozen façade is colder than the most frigid spot on Earth  by more than 100 degrees Celsius. Blasts of radiation sweep the surface. But underneath Europa’s inhospitable exterior, scientists think a vast ocean of liquid water flows. The moon’s seafloor might also bustle with activity from volcanoes and hydrothermal vents. If chemicals from the surface trickle down through the ice, as some scientists suspect, Europa could hold all the necessary ingredients for life.

Hydrothermal vents on Earth’s seafloor teem with life, says astrobiologist Kevin Hand of NASA’s Jet Propulsion Laboratory in Pasadena, Calif. “You’ve got incredible ecosystems of tube worms and crabs and fish and microbes,” he says. “It’s anybody’s guess whether or not you’d find tube worms on Europa.”

If they exist, scientists will have to peel away the moon’s frosty husk to find out. Research teams around the world are trying to figure out how to pierce, melt or drill through the ice. It’s not simple. Even if they could send a drill rig’s engineers and hulking machinery to Europa, the moon’s ice may be four times thicker than any ice found on Earth. Estimates vary, but the icy hull might reach more than 20 kilometers deep — about the height of 53 Empire State Buildings stacked one on top of another.

A mission to burrow down into the moon’s ocean could still be decades away. But if scientists found microbes in the ice or water, the implications would be Copernican.

“If we found life twice in the same solar system,” says physicist Dale Winebrenner of the University of Washington in Seattle, “it would tell us that when you look up at night, you’re seeing lots of places where there might be microbes.”

Breaching the shell

More than four centuries ago, Galileo Galilei pointed his telescope into the night sky and gazed upon Europa. He was aiming at Jupiter, but caught sight of four moons circling the gas giant. Scientists later named the littlest of these Europa, after one of Jupiter’s mythical lovers.

Modern astronomers got a good look at the icy moon in 1979 when the Voyager 2 spacecraft cruised past Jupiter. What they saw surprised them: Europa’s surface didn’t bear the typical marks of old age. Unlike Jupiter’s other moons, Europa’s ice was mostly unblemished by pockmarks from impact craters.

“That’s where it all started,” says planetary scientist Britney Schmidt of Georgia Tech in Atlanta. “That’s when people got really excited.”

Scientists suspected that Europa’s fresh, young skin might signal geologic activity underneath. As the moon travels around Jupiter, the giant planet’s gravity stretches and pulls Europa like a ball of taffy, NASA’s Hand says. This tug and pull may generate heat in the moon’s interior, which could help churn up the icy crust, erasing evidence of past craters. A roiling interior might also mean energy for life to tap into, he says.

Later data from the Jupiter-orbiting spacecraft Galileo suggested that the shell cloaks a salty ocean that may hold more than twice the volume of liquid water on Earth. The idea of exploring this vast ocean has launched a number of scientists on a quest for a space-ready ice drill.

Somehow, such a device has to breach the moon’s icy shell — perhaps with blazing hot metal or the jagged teeth of a drill bit — and carry enough power for the job. The device has to be simpler and more reliable than anything used to bore through ice on Earth, and it will have to take care of itself. There’s no way to send a team of engineers to the far edges of the solar system. And the entire ice-tunneling, power-toting, problem-free package needs to be light enough to launch beyond Earth’s gravitational grip.

It’s a tall order. So Vijendran’s team is taking a more moderate approach. The researchers just want to puncture Europa’s skin. Their “ice penetrator” would crash into the moon’s surface, and like a splinter buried in flesh, lodge within the ice shell itself. Sheltered inside the device, a microscope, mass spectrometer and electrodes could then analyze frozen slivers pulled from the crash site. “This very basic set of instruments would still give us quite an interesting picture of the astrobiology potential of the icy surface,” Vijendran says.

“It’s very different from a rover mission on Mars, where you’re taking your time, spending months and years doing experiments,” he says. “This would be a quick, one-shot thing.”

It may be enough even to tap into Europa’s water. Pockets of water may linger within the icy surface itself, Schmidt and colleagues reported in Nature in 2011 (SN: 12/17/11, p. 5). And plumes of water vapor may erupt from the shell, researchers suggested in January in Science (SN: 1/25/14, p. 6). If these pockets or plumes carry tiny microbes, the ice penetrator may be able to detect them, which could hint at more complex organisms living far below.

“We might not have to get all the way down to the ocean to look at whether anything is living in Europa’s water,” Schmidt says.

What’s more, at around 150 total kilograms — about the weight of a football lineman — the team’s penetrator system is small enough to send into space. It’s too big, however, to ride along on the European Space Agency’s upcoming mission, called JUICE for JUpiter ICy moons Explorer. That spacecraft is set to launch in 2022 and reach the Jovian system in 2030.

Vijendran hefts his team’s prototype from the window ledge of his office, and cradles the canister in his arms. About the size of a 2-liter soda bottle, but heavier, it’s in remarkably good shape after last year’s trial run. A few tiny scuffs in the blue paint expose glimpses of shiny steel.

The test in Wales proved that the design could take a beating while still protecting equipment stashed inside. This summer, the team hopes to create a battery and a communication system that can withstand impact with the ice.

Compared with devices that tunnel beneath the ice, “a penetrator project is going to be cheaper, and there’s a better chance of everything going right,” says Victoria Siegel, a researcher at Stone Aerospace near Austin, Texas.

Melting down

Siegel and others, however, are still aiming for the ocean. “That’s where the big stuff is going to be happening,” she says. Getting to that big stuff is much more complicated than scratching the moon’s surface.

Siegel and colleagues at Stone Aerospace have been working since 2011 on a device that would melt its way through Europa’s ice, shooting jets of heated meltwater to clear the way ahead and to steer. Once through the ice, the device, called a cryobot, would launch a miniature submarine from its belly into the ocean.

“It’s the equivalent of a Mars rover, but in an underwater vehicle form,” Siegel says.

The plan is ambitious, but scientists and engineers at the company have already created a prototype of the cryobot and are in the early phases of building the underwater vehicle. The vehicle is part of a NASA-funded project Schmidt leads called SIMPLE, which could help calibrate ice-penetrating radar technology so spacecraft could one day form a clearer picture of Europa’s ice. Together, the cryobot and the vehicle will weigh about 180 kilograms.

The cryobot, a tube about as long as a compact car, holds wires coiled within a sleek aluminum frame and five jets arranged in a domed head. By heating aluminum blocks within the head, the cryobot can melt ice, and then suck in the water and shoot out hot streams. To thaw the ultracold ice of Europa, the bot will need to carry some sort of onboard nuclear reactor. Siegel and colleagues are testing their device on Earth using laser light pumped down a fiber optics wire connected to the machine.

When the team ran the idea by fiber optics and laser manufacturers, “they all said, ‘You’re going to burn the fiber and everything’s going to go up in a big poof of smoke,’ ” Siegel says.

But the power source worked, and last October the team used it to send the cryobot through a 2-meter-tall block of ice.

At the company’s large warehouse-style workshop, half a dozen people gathered around the cryobot. A nearby desk held computer monitors displaying the bot’s temperature readings, and team members watched closely to make sure the machine didn’t overheat. If something went wrong, they could reach over and press a button. “It’s like a big red Wile E. Coyote emergency stop button,” Siegel says. “It knocks the laser off right away.”

The team hoisted the robot on top of the ice block with a crane and then fired up the laser. Orange safety lights flashed and the laser’s chiller system kicked into gear, rumbling like a loud refrigerator.

Light flowed through the fiber to the cryobot, and then, Siegel says, “Lo and behold, it actually started to descend.”

The machine melted through the block at about 1 meter per hour — an impressive feat considering that one of the two pumps running the jets malfunctioned. Siegel reported the success in December in San Francisco at a meeting of the American Geophysical Union.

Now, the team is working on the bot’s steering jets, and trying to incorporate sensing tools into the machine’s body. This June, the group will travel to Alaska’s Matanuska glacier and attempt to melt through about 30 meters of ice. On the way down, the team will test the bot’s tools and collect data about organisms living within the ice, Siegel says.

A dangling drill

Kris Zacny, an engineer at Honeybee Robotics in Pasadena, Calif., says drilling is an easier way to go — and it won’t require as much power as the cryobot.

For the last century, the oil and gas industry has used segmented drills to reach deep beneath the Earth’s surface. As engineers tunnel deeper and deeper, they add segments to extend the length of the drill, and pump in fluids to clear the hole.

These rigs aren’t the obvious choice for a mission to Europa: Lots of segments would make the drill heavy, and drilling fluids could freeze. So Zacny and colleagues teamed up with Jet Propulsion Laboratory scientists to design something different.

They came up with a wireline drill called the Auto-Gopher, a 22-kilogram, 2-meter-long tube about the diameter of a soda can, that’s suspended from a wire tether.

“It’s like a fishing rod,” Zacny says, “and at the end of the fishing line, you have a drill.”

The team’s rig takes ice fishing to the extreme: It would rest on Europa’s surface, dangle the drill from a slim tether and lower it into the shell as the bit chewed through ice.

At the tip of the Auto-Gopher, eight sharp, pointy teeth jut out from the bit. They’re made of tungsten carbide — a material that scientists can design into nearly unbreakable barbs to bite through rock and ice. When researchers switch on the motors, a rotary system cranks the bit around, drilling the teeth downward.

Tungsten carbide is tough, but gnawing through kilometers of ice may dull the drill’s teeth, or the drill could run into a particularly hard patch. So Zacny’s team packed a percussive system into the Auto-Gopher’s body that can pound the bit into the rock like a hammer.

Researchers don’t need a lot of power to chip away at ice, Zacny says. Even with the drill bit spinning and the hammer system thumping full time, the Auto-Gopher runs on about 350 watts, less energy than a typical hair dryer uses. Rovers like Curiosity can carry plenty of juice to run such a drill plus scientific instruments, Zacny says.

Using the Auto-Gopher, he and colleagues drilled about 3 meters into a gypsum quarry in Southern California. The rock is similar in strength and uniformity to ice, he says. At top speed, the drill moved a bit faster than Stone Aerospace’s cryobot — about 1.6 meters per hour, the team reported last March at the 2013 IEEE Aerospace Conference in Big Sky, Mont. But the researchers ran into one big stumbling block: They had to stop periodically to clear out rock cores from the borehole.

“You’d think the core would always be a nice cylinder that you can capture and pull out,” Zacny says. Not so. “Sometimes the core gets broken up into pieces.”

Now the team is working on a new and improved drill, the AMNH Deep Drill, named after its funding source, the American Museum of Natural History. This drill will shuttle rock or ice cuttings to a container inside the tube instead of creating cores that have to be pulled from the borehole. And researchers plan to pack electronics and equipment, such as a microscope and sensors, inside the tube.

Zacny sees two big advantages for using his team’s drill system to explore Europa. First, by capturing ice chips as the drill breaks through the shell, “you can systematically get data on every single foot of ice as you go all the way down,” he says.

Second, by suspending the drill from a wire, the team avoids the bulk of added drill segments that weigh down earthly equipment.

But even at a dainty 10 grams or so per meter, 10 kilometers of tether (Kevlar cable wrapped around fiber optics wire) would weigh 100 kilograms. Using lighter materials, such as carbon nanotubes, could trim the tether’s weight, Zacny says.

He and colleagues plan to test the AMNH Deep Drill at the gypsum quarry in the fall. This time, they’re aiming for a depth of 30 meters.

Zacny is optimistic about the team’s invention: “It’s feasible to deploy this drill in the next decade,” he says.

Getting there

Even if some kind of technology to puncture Europa’s ice is ready to go in 10 years, prospects for a U.S. trip to the Jovian moon are far from certain.

Schmidt has helped devise a $2 billion mission concept for a spacecraft called the Europa Clipper. The Clipper would orbit Jupiter and send back data about Europa from a series of flybys to try and figure out whether the moon is habitable. “It’s a gangbuster concept that gets the science done and is as cost-constrained as you can be,” she says.

In March, President Obama proposed funding studies for a Europa mission — the first time the distant moon has made it into the President’s budget. But NASA wants a mission for half the cost of the Clipper.

“It’s kind of a slap in the face,” Schmidt says.

She’s thrilled that the President’s budget now includes Europa, but frustrated by less-than-ideal funding. Scientists have been studying possible Europa missions for 15 years, Schmidt says, and NASA keeps bouncing back and forth between high and low price tags.

NASA astrobiologist Christopher McKay thinks the new budget is plenty for an exciting mission to Europa — just one that’s small and focused.

“It makes so much sense to start small and build up,” he says. “I see all sorts of opportunities in $1 billion.” He’d like to land a camera and microphone on Europa’s surface and take pictures of the landscape and listen to the ice.

“Ice talks,” he says. “It’s always creaking and groaning and cracking and chatting. The ice is telling a story — all we have to do is put our ear to the ground and listen.”

Plus, he adds, less expensive missions may help build support for a more expensive ride later. “Imagine what pictures of Europa from the ground could do to motivate interest.”

Surface-piercing technologies — whenever they are eventually used — could also be useful elsewhere in the solar system. Scientists have flagged other potentially habitable spots, such as Saturn’s hydrocarbon-rich moon Titan and its watery moon Enceladus (SN: 5/3/14, p. 11).

But to Schmidt and other scientists, Europa is the top target: It’s got water, a supply of potential nutrients and a possible source of energy bubbling beneath the surface.

“We think geologic activity is a really big part of what makes a planet habitable,” she says. “That’s why Europa is so exciting.”

Discovering whether creatures live within or beneath the moon’s ice strikes at “some of the biggest scientific questions that we have,” says University of Washington’s Winebrenner. “Is anybody out there? And if so, how widespread might life be?”