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  Geotimes - June 2007 - Trends and Innovations

A Hydrobot’s View Into the Abyss

The DEPTHX probe can decide where to search for life in the murky underwater depths. The hydrobot uses its arm to bore through mats of biomass and retrieve samples from the rocks lining sinkhole walls. Such samples (including those taken this past February from this Mexican sinkhole, known as La Pilita) have contained unique life forms.

Cave divers know Sheck Exley as one of their sport’s pioneers. He was also a man who pushed the limits of the human body. In 1994, after more than 29 years of diving, his risk taking caught up with the daredevil in central Mexico, where he drowned during an attempt to dive more than 300 meters (about 1,000 feet) down into Zacatón — the deepest water-filled sinkhole in the world.

Today, Zacatón’s exact depth, structure, geochemistry and inhabitants at its lowest levels are still a mystery because all exist below 287 meters (920 feet), the current record of human underwater exploration at Zacatón. A sounding line, or a weighted rope used to measure depths, revealed the sinkhole to be at least 329 meters (1,145 feet) deep, more than enough to submerge the Eiffel Tower. Nobody knows the exact depth, as the line may have hung up on a ledge, or if the sinkhole’s bottom slopes and perhaps even connects to nearby sinkholes.

Although humans must stay in the shallower depths, a new autonomous robot called DEPTHX (pronounced “Depth X”) could soon provide answers, following its scheduled May plunge into Zacatón. The 2.5-meter (8.2-foot)-wide hydrobot resembles a squished tangerine with eyes, but it houses much more than pulp. Innovative software and technology enables DEPTHX to explore without continually receiving human commands, allowing it to decide where to collect samples within the sinkhole. If successful, the technology could one day provide the means to explore other remote regions of Earth, as well as other planets.

Marcus Gary, a geoscientist at the University of Texas at Austin, first dove into Zacatón in the 1990s as a commercial diver and underwater cave explorer. He describes the sinkhole as “spectacular,” with clear water, and magenta and green bacterial mats covering the walls. Sometimes he encountered white “clouds” of sulfur and calcite, produced by bacteria or algae in the sinkhole. “It’s like you’re in an airplane flying through the clouds, and you can drop through the bottom of the clouds and the water is crystal clear,” he says. “It’s just a really beautiful, amazing place.”

Most of Mexico’s sinkholes, known as cenotes, exist in the Yucatán. But those associated with the Zacatón system are more than 1,500 kilometers (900 miles) away, located on private property in Tamaulipas near the northeastern coast of Mexico. Such karst systems typically form sinkholes when acidic rain or ocean water percolates through porous rock and erodes it away. In the Zacatón system, however, magma intrusions near the surface have caused the local groundwater to become more acidic, carving out the world’s deepest sinkhole.

Intrigued by features he observed at Zacatón, Gary enrolled in graduate school at the University of Texas at Austin to study geology. During every class, he says, he asked himself, “How would this apply to Zacatón?” In 2001, Gary and some of his colleagues gathered to brainstorm about how to safely explore and map such seemingly endless depths. The idea for DEPTHX was born.

DEPTHX, which stands for “DEep Phreatic THermal eXplorer,” caught the attention of NASA, which now provides the funding for the project. Researchers at the University of Texas at Austin, the Robotic Institute at Carnegie Mellon University in Pittsburgh, Pa., the Southwest Research Institute in San Antonio, Texas, and the Colorado School of Mines in Golden, as well as William Stone, the president of Stone Aerospace in Del Valle, Texas, made the robot come to life.

DEPTHX can appear curiously lifelike. The robot is autonomous: It is programmed to collect data without human help, and it then uses this data to make decisions about where to move and collect samples. John Spear, a geomicrobiologist at the Colorado School of Mines, helped give the robot the ability to interpret its surroundings. He is familiar with looking for life in extreme environments, from the geothermal landscape of Yellowstone National Park to the volcanic terrain of Kamchatka, Russia, where microbes have been found to thrive in and around the hot and chemically unusual conditions of thermal pools and geysers. “When I walk into Yellowstone, I use my own senses: vision, touch and smell,” he says. “We needed to give this machine senses, and the ability to interpret those senses.”

Mark Airhart
Botanist Antonio Fegroso (left) and geologist Marcus Gary (right) sometimes dive with the DEPTHX hydrobot during test missions to help it in and out of the water, or to search for it if it fails to return to the surface.

Designing a robot to look at a feature and use sonar to create 3-D maps is not new. But DEPTHX can also measure the sinkholes’ properties, such as its water’s acidity or alkalinity, and fluctuations in temperature and oxygen levels — all to determine where life could most likely exist.

To test DEPTHX’s performance, the team took the robot to the Zacatón region for two testing sessions, one in February and another in March. DEPTHX mapped, measured and sampled the walls of La Pilita, the system’s second-largest sinkhole. The researchers wanted to know what might be living in the porous rocks behind the microbial mats covering the walls. Light and occasional water trickling into these pores make such rock “a great place to live,” Spear says. Exploring at depths of about 100 meters (about 330 feet), the robot was able to retrieve uncontaminated samples by extending an arm and firing a spring-loaded piston device that penetrated the microbial mat, removed a core sample and swiftly sucked the sample back into its sterile arm. First results indicate the La Pilita samples are turning up novel kinds of single-celled organisms and bacteria.

DEPTHX “performed exceptionally well,” Gary says, and has generated “beautiful” 3-D models of the sinkhole’s morphology. However, the robot is not without issues. For example, it could not determine the source of the sinkhole’s water. The robot can only map features greater than 1 meter (about 3 feet) in diameter, so small passages from La Pilita to neighboring sinkholes might easily go undetected. “Determining how water flows in a karst system, which is like a giant sponge, is always a trick,” Spear says. And although the robot’s independence is beneficial, it can also be “pretty scary,” he says. One test mission left the team particularly worried: After the mission’s scheduled four hours were up, DEPTHX failed to reappear. In the end, a diver was sent to retrieve the AWOL robot, which had become wedged under a ledge only about 15 meters (50 feet) from the surface.

Gary’s primary interest is to better understand karst systems and sinkholes. However, if the team’s next mission to Zacatón is successful, the technology could be used to explore other regions. It could be used to characterize the geothermal processes at Yellowstone, or determine the stability of the bases of oil rigs and dams. Stone is also interested in taking DEPTHX to Antarctica, and NASA could one day send a miniaturized version of the hydrobot to Europa, one of Jupiter’s moons, to map and sample its ocean. Like Zacatón, the opportunities for this mechanized explorer seem endless. Even the adventurous Exley might have been impressed by what DEPTHX may do next.

Kathryn Hansen

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