Unmanned Robot and 25-Mile Tether Target Ocean Mysteries
Monday, June 15, 2009
Skimming past otherworldly tube worms and bizarre crustaceans as they traversed primordial sediments in inky darkness seven miles below the surface, an unmanned yellow robot two weeks ago became the world's deepest-diving unmanned submersible.
The craft, called Nereus, gave scientists on the surface their first long look at a portion of the Challenger Deep 35,768 feet (10,902 meters) down in the Mariana Trench. The trench is a gash in the Earth's crust in the volcanic Pacific Ring of Fire, where the Pacific tectonic plate collides with a smaller plate and plunges into the mantle.
Scientists hope Nereus will let them open new worlds of discovery in one of the last unexplored realms of Earth -- ocean depths below four miles (6,500 meters), which are home to a complex web of creatures that get their energy from methane rather than sunlight.
Two other vehicles have plumbed the Challenger Deep, but only for fleeting visits compared with the 10 hours Nereus spent there.
The first was the bathyscaphe Trieste, which carried Swiss oceanographer Jacques Piccard and Navy lieutenant Don Walsh to the deep in 1960. Trieste, now at the National Museum of the U.S. Navy in the District, was designed by Piccard and his father Auguste using the same buoyancy principles that helped Auguste set balloon-flight records. Between 1995 and 1998, Japan's robotic submersible Kaiko visited the Mariana Trench three times. It was lost at sea in 2003.
Nereus, developed and operated by the Woods Hole Oceanographic Institution on Cape Cod, Mass., is an engineering breakthrough because it can act as an autonomous underwater vehicle (AUV), controlled by onboard computers, or as a remotely operated vehicle (ROV) tethered to a mother ship by a fiber-optic cable. Named after a Greek god who was half man and half fish, Nereus is outfitted with a mechanical arm for collecting physical samples when it is under remote control.
In a typical operation, Nereus first roams an area autonomously, using sonar, chemical sensors and digital photography to record data such as temperature, acidity, salinity and sea-floor topography. When it returns to the ship, scientists analyze the information before deciding where to zero in using the tethered function to gather physical samples and more data.
During its maiden voyage from Guam into the Challenger Deep from May 24 to June 5, scientists tested only the tethered mode.
Unmanned, remotely controlled submersibles are widely used undersea for oil and gas operations, but until now they have been encumbered by weighty copper and steel cables providing energy from the mother ship.
Nereus is powered by 4,000 lithium-ion batteries, similar to those in a laptop computer, so most of its tether is a wispy glass-and-plastic fiber-optic cable. A conventional metal cable drops through the first several thousand meters of kelp and sea life, where the risk of a break is greatest, but from there the fiber-optic cable spools out of a small housing. Hundreds of small, hollow ceramic spheres in its double hull maintain its buoyancy.
"It's counterintuitive you could connect a vehicle weighing three tons to a surface vehicle weighing 3,000 tons and keep them connected by a thin fiber the diameter of a human hair with breaking strength of eight pounds," said Woods Hole engineer Andy Bowen, "but that's one of the tricks we've managed to perform."
Nereus's tether can stretch 25 miles, allowing it not only to reach places such as the Challenger Deep but also to move laterally over great distances. This will come in handy exploring underneath polar ice sheets, which is expected to be an important use of the vehicle.
The ramifications could go well beyond exploration. DNA analysis of deep-water specimens and geologic observations could lend insights into evolutionary processes and Earth's development. Nereus could also aid the search for life on other planets, both through findings in the Earth's oceans and by helping to refine technologies that could be used to explore extraterrestrial seas elsewhere in the solar system.
Chris German, chief scientist for deep submergence at Woods Hole, said Nereus's hybrid capabilities are ideally suited for seeking out and studying new hydrothermal vent sites, where volcanic activity causes chemicals to spew from deep inside the Earth and mix with seawater, often reaching more than 750 degrees Fahrenheit. Giant tube worms and other primitive, chemical-loving life forms frequent the vent sites, which are often highly acidic and rich in methane and hydrogen sulfide and could provide clues to how life on Earth began.
German will lead an expedition in October, sponsored by NASA, using Nereus in both its AUV and ROV modes to locate, study and sample hydrothermal vents along the Atlantic's deepest mid-ocean ridge, off the Cayman Islands.
A Nereus-like vehicle could be ideal for exploring Jupiter's moon Europa. It is covered in a thick sheet of ice, which scientists think caps a salty sea that may contain hydrothermal vents.
"If there are volcanoes at the bottom of the ocean that we know can sustain life, then that presents a whole other way that NASA can look for life in other places," German said. "If Nereus can be controlled to do all that remotely from a mother ship, then it's only one more leap, technologically, to do the same thing by remote control from right here on Earth."
Although useful data and samples were collected during Nereus's recent dive, the trip was mostly to test and fine-tune its technology.
"It's something that's not been done this way ever before," said Julie Morris, director of the ocean sciences division of the National Science Foundation, the major funder of the $8 million project.
Government agencies and research institutions will apply for funding for projects using Nereus. But its most significant contributions may be accidental discoveries. Woods Hole biologist Tim Shank noted that only in recent decades have scientists discovered such deep-ocean features as cold-water seeps, hydrothermal vents and methane hydrates trapped in marine sediments.
"When you think of what we've learned in the last 100 years, working mostly 4,000 meters and above, there's no reason to think discoveries of this magnitude won't happen between 6,000 and 11,000 meters," he said.
"When we were down there on the bottom, seeing different types of animals and habitat and rock formations, it was clear to me we're touching a new world we haven't touched before."