Edition: U.S. / Global

New Gauge for Shorter Day of Past

Published: July 9, 1996

BY counting microscopic stripes in sedimentary rock imprinted by the rise and fall of ancient tides, a team of planetary scientists and geologists has calculated that 900 million years ago, the length of a day was only about 18 hours and 10 minutes.

Their measurements, reported in the current issue of the journal Science, also imply that at a time when life on Earth was limited to single-cell organisms, Earth took 481 times as long to complete one orbit of the Sun as it does now, and the Moon was considerably closer to Earth.

Dr. Charles P. Sonett of the University of Arizona and his colleagues from the Universities of Arizona, Indiana and Utah, and Colorado State University, based their conclusion on a study of four ancient rock deposits dating from different periods in Earth's early history. The oldest, deposited some 900 million years ago, is at the Big Cottonwood Formation near Salt Lake City. The other sites are the Elatina Formation, which is near Adelaide, Australia, and is 650 million years old; the Pottsville Formation, in northern Alabama and some 312 million years old, and the Mansfield Formation, in Indiana and 305 million years old.

Like tree rings, tidal layers in rock, known as tidalites, can be interpreted as indicators of time intervals. Tidalites can be distinguished as daily markings and as the more prominent semimonthly "spring" and "neap" tides. (Spring tides are the highest high tides, occurring during the second and fourth quarters of the Moon. Neap tides are the lowest high tides and highest low tides, at the first and third quarters.)

Tidalites are created by the sedimentation of mudstone or sandstone, which proceeds at different rates during high and low tides. The semimonthly markings created by spring and neap tides are prominent and easier to identify and count than the daily tidal marks.

Scientists have long understood the tidal forces exerted by the Moon and sun that tend to slow down Earth's orbital rotation and increase the distance of the Moon from Earth. During the Apollo program of lunar landings, astronauts left a reflector device on the Moon; by periodically sending pulses of laser light from Earth to the reflector and measuring the time needed for light to make the round trip, scientists can calculate the changing distance between Earth and the Moon. The moon is receding at a steady rate of about 1.5 inches each year.

But determining tidal effects on Earth itself hundreds of millions of years ago has proved difficult. During the early 1960's, studies of the tidal rings left in the fossil shells of intertidal coral that lived between 345 million and 395 million years ago suggested that the Moon was retreating from Earth at an implausibly rapid rate.

"At that rate," Dr. Sonett said in an interview, "the moon would have been very close to the Earth at only half the estimated age of the Earth -- a seeming impossibility."

But the lunar recession rate determined by Dr. Sonett and his colleagues from the tidalite record appears to be more or less consistent with the rate implied by the Apollo laser measurements.

"I'm being asked whether we can infer anything about changes in the climate from these measurements," he said, "but I just can't say. With a more rapid rotation of the Earth, there would have been a stronger coriolis effect -- the effect that makes storms circulate counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere. We just don't know how that would have affected climate."

Aramais Zakharian, a member of Dr. Sonett's group, said that in another 15 billion years, the Moon would have receded from its present distance from Earth, 240,000 miles, to 336,000 miles. At that distance, the moon will be locked into a synchronous orbit, meaning it will be permanently over the same place on Earth.

But the scientists say that life is unlikely to survive to that distant time. By then, the expanding sun will have scourged the last vestige of life from the planet, even if Earth survives earlier catastrophes.