The poorly understood mantle accounts for about two thirds of the planet's mass and is key in the unseen convection processes linked with tectonic plate motion. For Japan, an archipelago straddling the fractious intersection of at least three crustal plates, the issue is also earthquakes. "Japan is situated on these active planetary processes, and 30 million people actually live on one of the most dangerous or active places on the earth," says Asahiko Taira, director general of the Center for Deep Earth Exploration (CDEX) of the Japan Agency for Marine-Earth Science and Technology, which operates the ship.
To study the mantle, geophysicists have had to rely on indirect methods, such as looking at seismic signals and measuring gravitational field variations. They can examine mantle rocks that have been brought to the surface via volcanism or faulting, but because this material has undergone massive amounts of heating, cooling and other processes, many argue it is not truly representative of the mantle. Breaking through the border between the crust and hotter mantle--known as the Mohorovicic discontinuity, or Moho--would give scientists a direct, fresh sample of mantle as well as the fluid, gas, temperature and pressure conditions of its environment (including possible microorganisms) that are lost by the time the rock arrives at the surface naturally. Researchers from 18 countries working on the U.S. drill ship JOIDES Resolution recently tried to reach the mantle at the Mid-Atlantic Ridge, but they missed by less than an estimated 300 meters.
In July technicians in a Nagasaki port completed the final outfitting of the Chikyu (Japanese for planet "Earth") and handed over the colossal 57,500-ton, 210-meter-long white ship to CDEX. The Chikyu, to start crew training around Hokkaido this fall, is being deployed as part of the Integrated Ocean Drilling Program, a long-term effort begun in 2003 and whose main participants are Japan, the U.S. and the European Union.
Besides being the most sophisticated laboratory on the seas, the science vessel boasts the tallest drilling derrick at 112 meters above the waterline and a drill pipe that is 9.5 kilometers long--22 times the height of the Empire State Building. This borer is expected to cut through some 7,000 meters of crust when the Chikyu, which cost about $540 million, is floating in seas up to 2,500 meters deep. Target drilling sites include areas where the mantle has been brought closest to the surface by tectonic action or where the crust is relatively thin, such as the Nankai Trough off Japan's Pacific coast.
To beat the current record drill depth of 2,111 meters, the Chikyu brings technology proven in the oil industry to bear. Its drilling system uses a 380-ton protective casing over the wellhead that is about the size of a six-story office building. It shields the vessel against eruptions of methane gas and pressurized fluids and allows for the secure retrieval of nine-meter-long core samples.
Another vital technology is the Chikyu's dynamic positioning system, an automatic, satellite-guided location fixer that corrects against wind, wave and current forces with six 360-degree thrusters under the hull, keeping the ship over the borehole. The Chikyu will be involved in multiple drilling and coring projects, and some of the holes it creates could be used in the future to house on-site crust monitors that would improve quake warning systems, according to CDEX scientist Shinichi Kuramoto.
One risk in drilling to the mantle--or any seafloor drilling--is tapping into a pocket of gas hydrates. If a plume blows out and rises, it can sink the ship. Escaping gas can also spark catastrophic explosions and fires. But geophysicists believe that the risk is worth taking. Directly sampling and monitoring the mantle, asserts CDEX scientist Daniel Curewitz, "will greatly expand our understanding and could open new avenues of inquiry into the nature, history and future of the planet we call home."
