Late in the cold war, the United States Navy decided it would be a good idea to survey the altitude of the ocean surface, all over the world, to within a few inches. The point was not to measure waves. The ocean is not flat even where it is calm: it has hills and valleys that depart by as much as a few hundred feet from what we think of as sea level. The slopes of these features are so gentle--they extend over tens or even hundreds of miles--that no ship ever feels them. Yet the Navy decided that submarine commanders, of all people, would benefit from precise measurements of this imperceptible topography.
Why? Because the study of bumps on the ocean surface is a reliable kind of phrenology: it reveals deeper truths about the ocean. Small, shifting bumps are created by the shifting fronts between water masses--between the warm Gulf Stream and the cold Atlantic, say--and those same fronts scatter sound, thus creating sonar shadows that can hide a Red October. The larger and more permanent hills and valleys are created by something else entirely: by Earth’s gravity field, which varies slightly from place to place. Knowing those variations helps a submarine stay on course when it is underwater and sailing blind. And when the time comes to launch a missile at Minsk, knowing the precise direction of gravity at the launch site--it does not always point straight toward the center of Earth-- is essential. If the missile starts out on a slightly wrong heading, it will miss its target, thousands of miles away.
So in 1985 the Navy launched Geosat, a satellite that measured the height of the sea surface by bouncing a radar beam off it. In a near- polar orbit, 500 miles high, Geosat circled the spinning Earth, painting it with a tight mesh of densely packed radar tracks. The satellite worked flawlessly, and it yielded the most comprehensive set of gravity measurements ever. For the Navy, the payoff was a substantial reduction in a missile’s margin of error--which meant a better chance of hitting Minsk. The Navy was not very interested in making a beautiful map of Earth’s gravity field; and even less was it interested in using such a map to chart Earth’s most remote frontier--the unseen topography of the seafloor. But David Sandwell and Walter Smith were interested in precisely that. And when the Navy finally declassified the Geosat data last summer, Sandwell and Smith wasted no time in creating the map you see here.
Let us be perfectly clear about one thing: this is a map of gravity, not of seafloor topography. Where the map is blue-green, the rate at which Earth’s gravity accelerates a falling object (little g in the equations of physics) has more or less its average value of 9.8 meters per second squared--or 980 gals, as physicists say, in honor of Galileo, who first measured the acceleration. In the bright orange areas of the map, gravity is at least 60 milligals--about 60 parts per million--stronger than average. In the darkest purple areas it is at least 60 milligals weaker.
