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Swimming to Europa

A robot designed to explore Mexican sinkholes is pointing the way to Jupiter’s watery moon

16 min read
Swimming to Europa
Explorer-inventor Bill Stone with his brainchild, DEPTHX.
Photo: Randi Silberman

It’s a hot late-spring Friday on a cactus-studded cattle ranch in Mexico, and nothing is happening. Nothing, in fact, has been happening for going on a week now, and it’s starting to get tedious.

Ordinarily, the group of scientists, engineers, and students who have gathered here might have enjoyed a respite from their otherwise crazy schedules. But they didn’t come here to catch up on their reading, play the guitar, or take long, leisurely walks. They came here to work.

Their goal is to field-test one of the most intelligent and agile underwater robots ever crafted, a possible predecessor of a machine that might someday swim the vast, ice-crusted ocean of Jupiter’s mysterious moon Europa. Called DEPTHX, for DEep Phreatic THermal eXplorer, the 1.3-metric-ton machine can maneuver freely, draw detailed, three-dimensional maps of its watery surroundings, and collect solid and liquid biological samples as it senses changing conditions in its environment. Most important, it does all that without any guidance from human operators.

Such autonomy would be essential if the robot ever does swim on Europa—which may be warm enough, thanks to geothermal activity, to have given rise to some sort of life. Human control of a robot sub that far away isn’t an option: radio waves don’t effectively penetrate water. Even if they did, a round-trip radio signal would take 2 hours or more, making remote control unlikely.

But today, on this sweltering retreat near the Gulf Coast of Mexico, with cicadas buzzing and a hazy sun beating down, Europa seems a long way off. At the moment, the robot is up on blocks, and the clock is ticking. Every project involving complex machinery experiences the odd delay out in the field. The weather won’t cooperate, a part breaks, software crashes. Such problems are expected and can be worked through. This delay, though, seems to defy rational remedy.

A permit from the Mexican government that will allow the crew to continue their activities has been caught up in a tangle of diplomatic grandstanding. For more than a week, phone calls, e-mails, and faxes have been flying back and forth in an attempt to extract the permit from the proper authorities. Many of the researchers will soon have to leave, as other work and family responsibilities call them home. If the team doesn’t get the go-ahead by 5 p.m. today, they’ll have no choice but to pack up the robot and leave.

It’s not looking good.

DEPTHX is the brainchild of Bill Stone. With a Ph.D. in structural engineering from the University of Texas at Austin, Stone worked for 27 years as a researcher at the National Institute of Standards and Technology, in Gaithersburg, Md., where he specialized in industrial automation. Since 2005, he’s also had his own company, Stone Aerospace, in Austin, which has been focused exclusively on building the DEPTHX robot.

Of the countless engineers who as children read the fictional adventures of Tom Swift and dreamed of becoming the fearless explorer-inventor, Stone is arguably the one who actually did it. Tall and lanky, with hawkish features and piercing blue eyes, he is probably best known for his exploits, chronicled in National Geographic and other magazines, in some of the world’s deepest and most dangerous caves. Not uncommonly, those expeditions revolved around sophisticated technology of his own design and construction.

He’d spend weeks underground, pushing to, and occasionally beyond, the limits of endurance. Many of the DEPTHX team members, in fact, are old caving buddies of his. Marcus Gary, the project manager and a geology Ph.D. candidate at UT Austin, is a caver. So are John Kerr, lab manager for Stone Aerospace, and Vickie Siegel, a geologist and part-time staffer.

Fascinated with caves since childhood, Stone learned to scuba dive just so he could explore water-filled caverns. Frustrated with the limits of diving gear, he spent years and most of his savings designing a rebreather, an intricate and ingenious piece of engineering that recycles a diver’s respired air, scrubbing out the carbon dioxide and adding oxygen and other gases as needed to let him stay submerged for up to 24 hours.

Stone had also long dreamed of space travel. As a younger man, he applied repeatedly to enter NASA’s astronaut-training program and eventually made it far enough to be interviewed at the Johnson Space Center, in Houston. In his 2002 book Beyond the Deep, Stone identifies the point in that interview when his candidacy went south. He had just articulated his vision of establishing a permanent base on the moon. NASA at the time had no interest in such ventures, and one of the interviewers told him so. “Well, sir,” Stone blurted out, “then God help the United States of America.”

Robotics, exploration, and space travel: drawing on his three passions, Stone began thinking seriously about designing an underwater planetary probe about five years ago. It built on work he’d done in the late 1990s, when he had put together a diver-steered sonar apparatus called the Digital Wall Mapper, which successfully surveyed Florida’s vast freshwater Wakulla Springs. A planetary scientist named Dan Durda, of the Southwest Research Institute in Boulder, Colo., heard about that work and asked Stone if he could do the same for exploring Europa. “Piece of cake,” Stone told him, with customary bravado.

It took another year to fashion a proposal to NASA’s liking—unlike the Wakulla mapper, this robot would have to operate autonomously, and it would have to collect biological samples in addition to taking stock of its aquatic environs. In October 2003, the funding came through: US $5 million over three years.

“Then I had to figure out how to build it,” Stone says.

Though he didn’t yet know what the robot would look like, Stone knew exactly where he would test it out: in a series of deep sinkholes, or cenotes, at Rancho La Azufrosa, about 400 kilometers northeast of Mexico City. Scuba divers and geologists have long been fascinated with the site, but detailed studies were lacking.

The deepest cenote, called El Zacatón, held a particular significance. In 1994, Sheck Exley, one of the world’s premier cave divers and a good friend of Stone’s, died while attempting to reach 1000 feet (305 meters). When they pulled his body out of the water, wrapped in his descent line, his dive computer read 268 meters. No human since has succeeded in plumbing Zacatón’s depths.

Limestone cliffs reach up 25 meters above Zacatón’s smooth surface. Flurries of butterflies flit among the wildflowers at the cenote’s lip. Buzzards circle overhead, and the lime-green parrots that live in the cliffs screech at the intruders in their midst. In the afternoon sun, the warm water turns milky, the result of microbes that metabolize the sulfides in the water. After dusk, they’ll release the sulfides again, and powerful fumes will rise up into the night sky.

If Zacatón weren’t pulling double duty as a test bed for advanced robotics, it would be studded with tiny islands of tall grass, called zacate, that float freely on the water. For the robot’s safety, though, the zacates have been corralled at one end behind a yellow rope. A canopied dock festooned like a NASCAR racer with all the logos of the project’s participants sits nearby; during missions, the programmers and robot wranglers use the dock for monitoring the vehicle’s progress. A tall white construction crane perches on one cliff. When the time comes—if the time comes—to fire up the robot, the crane will gently lower the vehicle to the water’s surface.

During earlier runs in January and March, the robot had mapped La Pilita, a smaller, urn-shaped cenote just down the dirt road from Zacatón. The team had returned with high hopes of pushing DEPTHX to its limits, fully exploring Zacatón and perhaps some of the other nearby cenotes.

On its initial run at Zacatón just days earlier, before the snafu over the permit, DEPTHX had descended all the way to the bottom, registering a tentative depth of 318 meters. To put that in perspective: if you submerged New York City’s Chrysler Building in the sinkhole, just the last meter of its elegant spire would stick out of the water.

But the sonar map that DEPTHX drew was tantalizingly incomplete. There appeared to be a passage at the deepest point leading off to one side. Stone has been exploring deep caves in Mexico for decades, and he’s seen passages like this before. “If this is what I think it is,” he says, “it could go on for hundreds or thousands of meters.” The robot would need more time in Zacatón to tell them if the passage was real.

And more time, too, to complete DEPTHX’s scientific mission—namely, exploring the biology of the sinkhole. Zacatón’s walls are covered with a thick slimy coating of micro-organisms, while other microbes float freely in its sulfurous water. The plan is to have the robot gather solid and liquid samples at various depths. Later, DNA analyses and studies of the water’s chemistry would tell scientists much about life in the cenotes. This exercise would also be a vital step for exploring Europa, whose ocean may harbor life.

There’s never been an aqueous robot quite like DEPTHX. Most autonomous underwater vehicles look the same, Stone says. “Some have fat midsections, some are more elongated, but they pretty much all look like weird torpedoes.” We’re sitting in a cramped, airless room that’s become his temporary headquarters at the ranch. The temperature outside is climbing past 32 °C, and it’s even hotter in here, but Stone doesn’t seem to notice. He’s fired up, and he could go on for hours.

“Their design is dictated by their mission: traveling in straight lines at relatively high speed to survey the ocean floor or gather bathymetry data,” he continues. But for exploring uncharted territory, that shape can get you in trouble. You can back yourself into a tight spot where you can’t turn around.

Two years ago, a team at the University of Southampton, England, learned that lesson the hard way. Their autonomous robot, named Autosub, was exploring the ice shelf below Antarctica. “It was 17 kilometers in, under 200 meters of ice, when it got itself into a position where it couldn’t figure out which way was home,” Stone says. And so the $10 million robot parked itself, sent out an emergency signal, and waited. And waited. “The scientists back on the surface could still pick up its signal, but there was no way they could go in and rescue it.” It’s presumably still there, somewhere below the ice.

DEPTHX, by contrast, is designed not for high speed but for complicated maneuvering in unfamiliar environments. Hence its shape: a squashed sphere with no protruding parts to catch on things. “We used to have a Wi-Fi antenna on top,” Kerr says. ”But during one run we surfaced under the chase boat and it snapped off.” And if one of the robot’s six thrusters goes out, it can just rotate around and use the others.

With its top half encased in pebbly orange syntactic foam, for buoyancy, the robot looks kind of like a giant tangerine. The vehicle’s shape was also dictated by its mapping software, known as SLAM, for simultaneous localization and mapping.

SLAM “is designed to solve a chicken-and-egg problem,” says David Wettergreen, an associate research professor at Carnegie Mellon’s Field Robotics Institute, in Pittsburgh, which was responsible for DEPTHX’s software—all 100 000 lines of it. “To build a map, you have to know where you are, but to know where you are, you need a map.” SLAM does both things simultaneously, creating a 3-D map as it moves along and then positioning itself within the map.

Variations of SLAM are commonly used in robotics, and they typically rely on identifying distinct features in the surroundings, like a doorway or a tree, viewing those features from many points, and then triangulating the robot’s relative position. But underwater environments have few recognizable features—a slime-covered wall looks very different when viewed from the front and from the sides.

So Nathaniel Fairfield, a Ph.D. student at Carnegie Mellon who wrote the SLAM algorithm for DEPTHX, designed the software to look not at discrete objects but at the shape of the environment as a whole. To do that, the robot uses 56 sonars, mounted on two circular steel frames that intersect at the top and bottom of the vehicle. As the robot descends through the cenote at the leisurely pace of 1 meter per second, it also spins around about once per minute; each sonar fires up to four times per second, allowing the beams to “paint in” the surroundings.

Controlling the sonars and the 19 other subsystems on the machine are 36 onboard computers. Power is supplied by a pair of lithium-ion battery packs that will run for up to 5 hours between recharges. Much of the hardware is enclosed in pressure-resistant aluminum housings, to protect the contents from being crushed by the external water pressure. Other components are built into oil-filled housings, which balance the outside pressure while keeping water out.

DEPTHX’s other key piece of software gives the robot autonomy, allowing it to make decisions about when and where to move. The Carnegie Mellon programmers paid particular attention to how the robot deals with faults. “We have contingency plans for all kinds of failures, like all the sonars turning off at once, or one battery giving out, or the robot losing its way,” Wettergreen explains. “If something goes wrong, and it’s at the bottom of the cenote, with its batteries running low, it can’t just stop and wait. It has to do something sensible”—initiate a controlled ascent, for example.

Autonomy also means the robot has to decide on the fly where and whether to gather biological samples. The machine starts by characterizing its surroundings. Sensors continuously measure the water’s salinity, temperature, pressure, and chemistry. “Changes in any of these conditions are where we’d expect to find biological activity,” says Ernest Franke, an engineer from Southwest Research Institute, where the robot’s sampling arm and science autonomy system were designed and built.

The robot then “trains” itself by taking a baseline water sample. The liquid is inspected under an onboard microscope, and a subroutine counts any moving objects (likely micro-organisms), tracks their paths, and measures their speed. Another subroutine tells the robot’s video camera to take a baseline reading of the cenote’s slime-covered walls, measuring their color, intensity or saturation, and texture.

The result of each subroutine is a statistical classifier, a value that averages all the parameters, much as your credit score is computed from details such as your age, salary, and mortgage payment history. These two classifiers—one for water, the other for the walls—are then compared with subsequent readings the robot takes. When it spots a significant difference, suggesting a rise in biological activity, it takes a water or solid sample. For the latter, the robotic arm extends about 2 meters and punches a pinky-size chunk from the cenote wall. To gather liquid, a water sipper on the arm fills up plastic bags. After the robot surfaces, the team will remove the specimens and freeze them in liquid nitrogen for later analysis.

Any robotic probe that swims on Europa will have to do all that—and much more. Stone has no doubt that machines will eventually have the requisite smarts, though. Europa will still be there in 10 or 20 years, he notes. “We have time.”

Today, with the robot sitting on the back of a truck, all hands are scrounging around for projects to keep themselves occupied. Franke plugs an oil leak that’s sprung up in the sampling arm. Dominic Jonak and George Kantor, both of Carnegie Mellon, tweak the robot’s control and navigation software. Kerr replaces the sulfide sensor and mounts a pair of 100-watt lamps, for additional illumination at Zacatón’s murky depths, as well as a lead-acid battery pack to power them.

Throughout the day, local schoolchildren and picnickers arrive at the ranch to gawk at the robot, which has gotten wide coverage in the Mexican media. The Spanish speakers on the DEPTHX team obligingly give them the grand tour.

When they’ve run out of useful tasks, some people go for a dip in the cenotes, while others hang out in the shade of a palm-covered palapa, an open-sided gazebo that functions as the team’s computer room, dining hall, and after-hours jamming studio. Robin Gary, a geologist married to Marcus, cooks up a hearty meal. Over beers, people trade war stories about their favorite death-defying experiences in the field: lightning storms, carbide-lamp explosions, not showering for 62 days in a row. Someone breaks out a guitar. Nobody can seem to remember all the lyrics to “Hotel California,” but that doesn’t stop them from trying. As another day passes with no word on the permit, what else is there to do?

If you’re still not convinced that scoping out Mexican sinkholes is the logical jumping-off point for exploring one of Jupiter’s moons, just ask Richard Greenberg. He’s a professor of planetary sciences at the University of Arizona, in Tucson, and author of Europa, the Ocean Moon.

“I’m here as a sort of reality check,” Greenberg says, sitting on a beach chair in the late-afternoon heat. He’s been fascinated with Europa since at least 1977, when he joined the imaging team for NASA’s Galileo mission, which orbited Jupiter and its moons for nearly eight years, beginning in 1995. The space probe’s most intriguing discovery was the likely existence of a vast ocean beneath Europa’s icy crust.

“It’s the only other celestial body that we know of that has an ocean,” Greenberg says. The water covers the entire surface to a depth of perhaps 160 km. So even though Europa is only about the size of Earth’s moon, it has twice as much liquid water as all of the oceans of Earth combined.

Scientists believe there’s a good chance that something is alive in that watery world. “It’s one of the most likely places for us to find extraterrestrial life,” Greenberg says. “All the basic ingredients that life might need are there.” Ingredients like sunlight, organic molecules delivered by asteroid or comet, and radiation from Jupiter’s magnetosphere. If life exists, he says, “it could just be micro-organisms, but with an ocean that deep it could be much more complex.”

To get to that ocean, though, means somehow penetrating the several-kilometer-thick ice sheet that encapsulates the moon. Fortunately, the ice is networked with cracks, some of which extend deep below the surface. “If you landed ‘smart,’ and you got next to a crack in the ice that went down to the ocean, you could sample oceanic material that had sloshed up to the surface—or even send a probe down through that crack and into the ocean,” Greenberg suggests. DEPTHX is the first incarnation of what such a probe would look like.

John Spear, for one, wouldn’t mind if life on Europa turned out to be entirely microscopic. He’s a microbiologist by training, and, it must be said, by disposition. “There are 600 kinds of micro-organisms living in your mouth,” he informs me when we first meet at the ranch. I hadn’t even considered the possibility, but brushing my teeth is now a whole new experience.

Based at the Colorado School of Mines, in Golden, Spear searches for microbes that live in so-called extreme environments—geysers in Yellowstone National Park, volcanic hot springs in Kamchatka, and hypersaline lagoons in Baja California.

Show him a warm, sulfurous cenote and he is a happy man. “I knew that when we explored these water-filled limestone holes, we wouldn’t be looking for big fish or bugs or plants,” Spear says. “We’re looking for the microbes, because those are the most likely things to be found throughout the cenote, on the walls from top to bottom and also within the water column.”

Gathering samples in the field is mostly done by hand, he adds, but at Zacatón, the robot would have to serve as his proxy. “I wanted the machine to be me—to be a field microbiologist,” Spear says. “When I’m in Yellowstone, I smell things, I look at things, I touch things—you can even taste them if you want.”

But building a machine that can replicate even a single human sense is pretty hard. So DEPTHX was designed to act as a sort of simple-minded assistant. It can collect samples in the cenote, but the actual analysis would happen later.

Spear and his students will look for DNA in the samples, with the hope of discovering new types of bacteria. They’ve good reason to be optimistic. Two years ago, a legendary deep diver named Jim Bowden descended to about 90 meters in Zacatón and brought back some specimens. Spear discovered six new divisions of bacteria in the haul.

If you’re not a biologist, you may not realize how huge that was. To put it in perspective, you have to think of organisms not by the old Linnaean taxonomy of animals, plants, and such, but by the newer three-domain system: eukaryotes, which include all complex life forms, from us to protozoa; bacteria; and archaea, which are sort of like bacteria. “Within the domain of bacteria, there are about 110 main kinds, or divisions,” Spear says. “We found six new ones.”

That kind of diversity in micro-organisms exists pretty much everywhere, he adds. “It could be garden soil, it could be your ear canal. But to find six here was a good start. We think we’re going to find more.”

The 5 p.m. deadline comes and goes, as the orange sun dips toward the palm trees. The more fragile components have already been stripped off the robot, which now sits strapped to its flatbed trailer, ready for the journey back to Austin. Some members are packing gear into their pickup trucks. Others hang out beneath the palapa, where some consolatory tequila swilling and singing go on. Stone heads out to one of the cenotes for a swim. “Heard anything?” somebody calls out. “Not a goddamn thing,” he says, shaking his head as he walks off. The mood is somber: this wasn’t how things were supposed to end.

Then, at 2 minutes before 6, a shout and a whoop emerge from the vicinity of Marcus Gary’s room. Seconds later, he’s out on the lawn, delivering the good news: the permit has finally come through. There’s a brief moment of stunned incredulity, followed by some quick mental recalibration—after several weeks on the site, many of the researchers had been getting used to the idea of returning home. But there’s no question that they will go back to work.

In 30 minutes flat, the robot is reassembled and towed down to the lip of Zacatón. The Carnegie Mellon programmers run further diagnostics on the control software, and just before 7:30, with the light fading and the parrots screaming, the crane slowly maneuvers the robot into the water. Looking on, Stone wonders aloud which of the many favors he called in finally shook the permit loose. ”I’m going to be writing thank-you notes for months,” he says happily.

The crew will spend the next 5 hours, and then the next three days, exploring Zacatón and its sister cenotes, Caracol and Posa Verde. In all, they squeeze in six shifts, running around the clock, with just enough time between shifts to recharge the vehicle’s batteries. They send the sphere down to Zacatón’s bottom and confirm the 318-meter depth. Alas, what had looked like an opening to the side was just noise on the earlier sonar map. The sampler grabs microbial specimens at three depths; Spear is already preparing for a busy summer.

“We did everything we set out to do,” Wettergreen says, back in his office in Pittsburgh in early June. “Of course, we would’ve liked to do more dives, take more samples. But the robot continued to improve. We hit all the bullets.”

For all its accomplishments, DEPTHX won’t enjoy a long senescence in the National Air and Space Museum, or anywhere else. Before long, Stone and company will begin scavenging its parts for the next big step on the road to Europa. Sometime in 2008, a robot called ENDURANCE will be plunked down into Antarctica’s icy Lake Bonney and start creating the first detailed 3-D map of the 2-km-long body of water.

The project’s name is, of course, an acronym: Environmentally Non-Disturbing Under-ice Robotic ANtarctic Explorer, if you must know. But it also harkens back to Ernest Shackleton’s ill-fated ship, which became hopelessly mired in the Antarctic ice in 1915. Modern transportation and permanent base stations have made exploring the polar continent somewhat easier, but the robot ENDURANCE and its team will still face deprivation and nasty weather.

Like Europa, Lake Bonney is covered with ice year-round, so just getting the robot into the water will require melting a hole through several meters of ice, a process that takes about three days. All of the robot’s components will have to be sterilized beforehand; otherwise the lake and its microbial inhabitants might be inadvertently exposed to foreign elements. The lake’s varying levels of salinity—which fluctuate from freshwater at the surface to four times that of seawater at the bottom—will create weird refraction and reflection patterns for the sonars. “It’s like shining a laser through a bunch of glass of different densities,” Stone explains. “So we’ll have to develop an algorithm that takes into account the layering.”

He currently envisions a tetherless vehicle about 1.4 meters long and weighing in at 80 kg. In addition to onboard sonars and sensors, the machine will have a robotic doppelgänger, which Stone calls a “dropsonde spooler.” At various points of interest, the dropsonde will feed out from a fiber-optic spool of cable, take measurements, and get reeled back in like a fish.

Somehow, Stone and his engineers will need to pull all that together by next winter, when the team plans to test the machine in a lake in Wisconsin. They’ll then ship the craft to Lake Bonney for a first run in late fall of 2008 and a final run the following year. NASA is funding the project, led by Peter Doran of the University of Illinois at Chicago, at less than half the level that DEPTHX got. “Budgetwise it’s about as tight as you can get,” Stone says. “We’ll be working flat out” to make the 2008 expedition.

But if they succeed at Lake Bonney, the next, and penultimate, stop on the Europa express will be Lake Vostok. This enormous freshwater lake, some 600 meters deep and 4 km below the Antarctic ice, would be “in all respects the stage test for Europa,” Stone says.

And then on to Europa. “That mission will be the greatest intellectual accomplishment of all time,” Stone says, and he seems to mean it. ”It’s something we should strive for.”

To Probe Further

Field notes, photos, and other information about DEPTHX are available at the following sites:

Stone Aerospace, https://www.stoneaerospace.com/news-/news-latest.php; the Field Robotics Center at Carnegie Mellon University, https://www.frc.ri.cmu.edu/project/depthx; and the University of Texas, https://www.geo.utexas.edu/zacaton/depthx.

To see our slide show and video clips from the expedition in May, go to /sep07/depthx.

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