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Andrija Mohorovicic showed us that the Earth is layered

Andrija Mohorovicic (1857-1936) was a pioneer of the developing science of seismology in the early twentieth century, whose close observations of seismic waves gave us the concept of the Earth's mantle. The Mohorovicic Discontinuity, named in his honor, is the boundary between the Earth's crust and the mantle.

Mohorovicic, a native of Croatia, was trained in mathematics and physics, but developed an interest in meteorology. He became head of the Meteorological Observatory in Zagreb in 1892, and eventually was the overseer of the meteorological services for all of Croatia and Slavonia. By the turn of the century, Mohorovic's curiosity had expanded from weather to geophysics, and he was able to acquire some advanced seismographs for the Observatory. These were deployed in time to capture readings from a strong 1909 earthquake in the Kulpa Valley. Mohorovicic was surprised to find that some readings reached his instruments faster than he had thought possible. He deduced that some of the seismic waves were traveling through a deeper, denser portion of the Earth, now called the mantle, while slower waves traveled through the crust. 

The crust is a thin layer, with only 1% of the earth's volume while the denser mantle constitutes about 84% of the volume (estimates vary). The mantle starts at about 35 km below the surface of the continents, but only about 7 km below the surface in some parts of the ocean. Below the mantle is the iron-nickel core, which contains a molten layer over a solid center.

The mantle ranges in temperature from about 500-900 degrees C., which would be sufficient to melt the rock but for the intense pressure. About 450 kilometers down, the mantle is thought to be populated by silicon and magnesium-rich minerals like wadsleyite and ringwoodite, known primarily from their presence in meteors. Below 600 meters, there may be exotic minerals like silicate perovskites and the magnesium-iron-oxide ferropericlase, also found sometimes in inclusions of natural diamonds. But, of course, we don't really know what the mantle consists of except by deduction. We have not yet been able to drill that far down. We know more about the surface of Mars.

Project Mohole was the first attempt to penetrate the Mohorovicic Discontinuity, but was abandoned in 1966 way short of the goal, after reaching a depth of 183 meters below the ocean floor. Deeper penetrations were made more recently by the drilling vessel Resolution operated by the Joint Oceanographic Institutions for Deep Earth Sampling, returning core samples from 1500 meters and more. The Russian Kola Borehole, started in 1970, is the deepest continental research effort at 12,260 meters, reached in 1989. Oil companies have now drilled deeper, but the wealth they were after was not scientific in nature.

The Japanese deep sea drilling vessel called Chikyu is setting records for the deepest sub-oceanic drilling for research purposes. One of the goals of the mission is to understand better the origin of life, which possibly happened under conditions of low oxygen and high temperature and pressure, similar to deeper parts of the Earth. The vessel has already obtained samples from ancient coal beds, formed 50 million years ago, from 2400 meters below the ocean floor in the Costa Rica Rift. That was the deepest sub-oceanic hole yet drilled for research purposes. The expedition includes microbiological sampling and DNA analysis of subsurface organisms.

The Chikyu also hopes eventually to reach the mantle through the sea floor, drilling a 30 cm hole to a depth 7000 meters below the ocean, while returning core samples. The project is expected to cost a billion dollars or more and be completed around 2020. The drilling is technologically difficult, but it may be harder still to obtain the funding.