Tidal Fossils Reveal 18-Hour Day in Earth's Past

Oscillating to a cosmic beat, tides do more than move sea water. They heat the Earth. They slow its spin. They shove the moon farther away. Scientists have known about these effects for many decades. But only now are they getting the tools to pin them down.

For example, the sun and moon cause tides in the solid Earth as well as in the air and sea. Using precision satellite data, scientists have isolated this body tide for the first time and estimated how much frictional heating it causes. It turns out to be about 83,000 megawatts or roughly 3 percent of average United States energy consumption.

Recent studies of fossil tides - yes, tides leave fossils - produced similar "gee whiz" numbers. They indicate that, 900 million years ago, the day was only about 18.2 hours long. Tidal interaction with the moon has slowed the day to 24 hours since then. In the process, Earth transferred enough energy to the moon to power the world for up to 90 million years.

The moon has "used" that energy to speed up its own orbital motion and move farther away. It does this because the bulge of material the lunar tide raises on Earth pulls on the moon gravitationally. This speeds up the moon's orbital motion and slows Earth's spin. The lunar orbit then expands to accommodate that added energy. Precision laser measurements of the earth-moon distance show the moon is receding at about 1.5 inches a year.

The highest tides - called spring tides - occur when sun and moon line up at new and full moon periods. The lowest - called neap tides - occur at half moon periods when the sun and moon are at right angles as seen from Earth. These cycles - two spring and two neap tides per lunar month - stand out in places where every tide leaves its mark in accumulating silt and sand.

In time, these sediments form siltstones and sandstones. These are the tidalites - fossil tides - that Charles Sonnett of the University of Arizona in Tucson and associates have been studying in Alabama, Indiana, Utah, and southern Australia. They published a paper in Science magazine last month explaining how they decode these fossil records to trace ancient tidal cycles. Some of them date back 900 million years.

Meanwhile, Richard Ray at the Goddard Space Flight Center in Greenbelt, Md., and his colleagues reported last month in Nature magazine how they have cracked the long-standing puzzle of detecting Earth's body tides. Small changes in the orbits of satellites reflect tidal deformations on the planet. These deformations are the total effect of tides in the air, sea, and solid Earth.

By subtracting the air and sea tides, Dr. Ray and his colleagues can extract the body tides from the satellite data. The breakthrough came with a radar map of sea-surface tides produced by the Topex/Poseidon satellite. These data on body tides will supplement studies of the earth's structure using seismic waves.

Tides are wide-ranging phenomena. They affect the earth-moon system in subtle ways. They influence the formation of planets and their moons. For example, tidal deformation of Jupiter's moon Io heats that moon to the point of inducing volcano eruptions. In distant galaxies, tidal forces help shape the distribution of gas and stars. Remember this when you look at a full moon and give a thought for what's going on beneath your feet. It has cosmic implications.

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