Earth's Bulges

The earth is generally considered a slightly flattened sphere, its flatness at the poles resulting from plastic response to its spinning motion. Last week Dr. John A. O'Keefe, assistant director of the Theoretical Division of the National Aeronautics and Space Administration, gave evidence before the American Physical Society that the earth is very slightly pear shaped. If its continents are evened out and its spin-flattening allowed for, it has a faint bulge around the North Pole, a faint depression around the South Pole, and a depressed ring in the north mid-latitudes (see diagram).

The discovery was made by careful observation of the Vanguard satellite Beta 1958. Last summer Ann Eckels, also of the Theoretical Division, noticed slight peculiarities about Vanguard Beta's orbit. Sometimes it moved a little slower than expected; sometimes it moved faster. Since a satellite's speed reflects the earth's gravitational pull, the changes of pace were interpreted to mean that gravitation varies over broad areas in an unexpected way.

Once the gravitational variations had been measured, the NASA scientists could calculate their effect on the shape of the earth. The excess of gravitation around the North Pole, for instance, indicates an extra 200-ft. bulge of rock over an area equivalent to the Atlantic Ocean. This extra mass would attract enough sea water to raise sea level about 50 ft. above the theoretical curve of an ideally plastic earth. None of the newfound bulges are large compared to the polar spin-flattening (about 13 miles), but they may cast new light on the earth's mysterious interior.

Some unknown force must support the bulges, which could not exist if the earth were a simple, spinning mass of plastic material. One possibility: the earth's mantle (the 1,800-mile layer below the crust) may not be as plastic as has been thought. It may have mechanical strength, like brickwork, that keeps the earth out of shape. Another possibility: the bulges are supported by slow currents in the mantle, which push up the surface like massive bubbles in a spring.

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