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Earth's Mantle: Drilling To Science's Deep Frontier

Holly K. Given
Deputy to President,
IODP Management International Inc.

Fifty years after the U.S. government canceled Project Mohole, a Sputnik-era attempt to recover a complete section of the sediments and rocks that form the ocean crust, scientists empowered by 21st-century technology are discussing a renewed deep quest. The Integrated Ocean Drilling Program (IODP) is considering a proposal by an international group of more than 60 researchers to drill through the oceanic crust into the mantle with the Japanese deep-sea drilling vessel Chikyu at one of three sites in the Pacific Ocean.

Nearly all knowledge about the mantle, which composes 85 percent of the Earth's volume and 68 percent of its mass, is inferred from remote observation and modeling, or geologic clues left on the surface. In addition to answering basic questions about planetary evolution, geochemistry and rock physics, mantle samples will address how much carbon and water are stored in the deep Earth, and the pressure, temperature and chemical conditions under which life can exist. Scientific ocean drilling has previously discovered chemosynthetic microbial communities flourishing deep within the oceanic crust, which have implications for how life evolved on Earth and how it might survive on other planets.

Getting to the mantle won't be easy or cheap at any of the sites under consideration. While greater depths have been drilled, none has been attempted in this combination of water depth (4,000 meters), penetration depth (6,000 meters of ocean crust), hard-rock geology and extreme formation temperatures. Ultradeep oil and gas wells in the Gulf of Mexico are in 3,000 meters of water at most, and the target is softer and cooler sediments. Two ultradeep scientific boreholes drilled from land—more straightforward than a ship-based platform—have reached 9,000 and 12,000 meters, too shallow to reach the mantle through the thick continental crust.

An initial feasibility study by Blade Energy Partners in 2011 concluded that the mantle goal is now within the reach of the Chikyu, which deploys a riser system to enable deeper drilling. The riser provides a closed circulation system for drilling mud engineered for the varying conditions in the deep borehole: to cool and lubricate the drill bit, equalize pressure, stabilize the borehole and return cuttings to Chikyu's deck for environmental capture and analysis.

The study estimates between 480 to 750 days of operation will be needed, depending on site conditions and whether scientists are simply raising cuttings or taking more complete sections of rock cores. More than 40 percent of time could be consumed by trips to replace conventional drill bits as they wear out against the hard formation. Longer-lived drill bits and rock coring systems that incorporate harder, thermally stable cutting materials could save the project tens of millions of dollars, and are being investigated as one of the top planning priorities. Logging tools will need to operate reliably and stably at 250° C or perhaps even 300° C.

Scientists plan to leave quasipermanent environmental, chemical and geophysical sensors in place as the world's deepest observatory, requiring further thought about instrument resilience similar to applications in space exploration or warfare. Sensor components (circuit boards, digital clocks, memory, batteries) are only beginning to become available in the anticipated temperature range and will need to be customized for constraints on size, power and communications.

A nontechnical challenge, of course, is funding. Chikyu's operator, the Japan Agency for Marine-Earth Science and Technology (JAMSTEC), is studying how the mantle quest might be financed, and whether it could be included in Chikyu's next long-range plan, with a start date as early as 2017. The project would not exceed the cost of a simple planetary mission, and technology is finally ready for another try at this bold mission, considered by some as the earth science analogy of Apollo, bringing rocks back from an as-yet unsampled part of the solar system.

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