Digging through the discontinuity
In the late 1950s and early 1960s, some of the last hurdles in human exploration of the globe were overthrown, notably the scaling of Mt. Everest and the plumbing of the depths of the Marianas Trench. They paved the way for planting a flag on the Moon. But one notable project went underfunded and eventually forgotten.
Project Mohole was the first effort to dig down to the Mohorovi?i? Discontinuity, where exists an unsolved mystery. Named after a Croatian seismologist, the Discontinuity is where seismic waves abruptly change in frequency. The change can be attributed to a difference in material as the basalt of Earth’s crust gives way to perioditic materials of the more plastic mantle. The switchover is so sudden, however, that this theory clashed with the expectation that the interface between the mantle and the crust is not well defined. As a result, other explanations have been offered, such as the idea that the waves undergo a phase switch as the result of passing through temperature gradients in mantle.
After digging about 600 feet into the crust (in nearly 12,000 feet of water), Project Mohole’s original support mechanism, the American Miscellaneous Society, dissipated and funding from Congress dissipated after control was passed to the National Science Foundation. However, the project was considered successful, and the techniques developed proved that deep holes could be dug in deep water.
Digging through the ocean floor, of course, is easier in that the crust is just 3 miles thick in many places, letting researchers avoid many miles of extra drilling. That didn’t stop the Soviet Union, of course, from drilling their monstrous 40,000-foot Kola Superdeep Borehole.
In a commentary in the March 24 issue of Nature, Damon Teagle of the National Oceanography Centre at the University of Southampton, UK, and Benoît Ildefonse of the University of Montpellier in France say we can now drill into the Earth’s mantle. I cannot vouchsafe their argument as I do not have an online access to Nature’s articles, but perhaps it’s best to say that we can finally do so without having to spend quite so much time and money. Nature, in fact, has published several articles about mantle-deep boreholes, dating back to a 1958 piece by T.F. Gaskell, a physicist at British Petroleum.
The technology to reach the mantle has probably been available since Project Mohole, but whether anyone is willing to commit to the effort is another question. We have some precedent for what to expect at these depths. Temperatures in the Russian borehole achieved 360 degrees C, hot enough to spur a hoax about the discovery of Hell. In addition to the Kola Superdeep, another 34,000 foot hole was drilled in the U.S. in search of oil. Crushing pressures of 25,000 psi were encountered before molten sulfur melted the drill bit.
Undoubtedly drilling will be very expensive and fraught with setback. The drill team will probably need to design an entirely new drill setup that can do without the traditional riser that vents explosive gases. And some method of forcing drilled material to the surface will need to be invented. Insights derived from the Integrated Ocean Drilling Program will probably be crucial to the effort.
But is it worth it? Absolutely. In the 1950s, the excitement of Project Mohole was enough to prompt a gushing article from Popular Science. I think we can consider it unfinished business.