Tides Recorded The Moon's Retreat From Earth, Shorter Earth Days

July 05, 1996

Layers of sediment deposited by tides hundreds of millions of years ago at what today are sites in the United States and southern Australia show that 900 million years ago, a day on Earth was about 18 hours long. The moon has been moving away from Earth at a constant rate during the past 900 million years, according to the same evidence.

Charles P. Sonett, Regents Professor Emeritus of planetary sciences at The University of Arizona in Tucson, and Aramais Zakharian, a graduate student at the UA Lunar and Planetary Laboratory, collaborated with geologists in the analysis, published today (July 5) in Science. Their co-authors are Eric P. Kvale of the Indiana Geological Survey, Marjorie A. Chan of the University of Utah, and Timothy M. Demko of Colorado State University.

They studied sediments left by tides preserved in four exceptional formations: the Big Cottonwood Formation near Salt Lake City, Utah, deposited 900 million years ago; the Elatina Formation near Adelaide, Australia, deposited 650 million years ago; the Pottsville Formation of northern Alabama from 312 million years ago; and Indiana's Mansfield Formation from 305 million years ago. The ages of these formations are known by their geological context.

That Earth's tides are created by the pull of mostly the moon's gravity, and to a lesser degree by the sun's gravity, is well known. Scientists use the laws of celestial mechanics to calculate how this gravitational pull causes the Earth to spin slower on its axis -- lengthening the Earth day. They further calculate how the loss of Earth's rotational angular momentum and energy increases the size of the moon's orbit, lengthening the distance between Earth and moon.

But physical evidence that measures actual dynamics between Earth and moon has been hard to come by. Early modern-day astronomers realized from 2,000-year-old records that the timing of lunar eclipses was changing, so Earth-and-moon dynamics were altering, Sonett noted in an interview. In the 1960s and 1970s, there was a flurry of scientific interest in what physical evidence for changes in the length of Earth's day might exist in the fossil coral record. NASA's Apollo program focused further attention, especially when astronauts left a laser reflector on the moon that was used to measure the increasing distance between Earth and moon at 3.82 centimeters, or about 1.5 inches, each year.

The discovery of "tidalites" from southern Australia in the late 1980s alerted scientists like Sonett to a new way to examine how the moon's orbit has evolved through time.

Layers of tide-deposited sedimentary rock, called tidalites, are records of daily tides. Dark bands that periodically occur in the layers clearly mark the semi-monthly "neap" tides, or lower tides that form during the waxing and waning phases of the moon, i.e., when the moon is farthest from aligning with the sun and Earth. Lighter areas between the dark bands mark the semi-monthly "spring" tides, or the higher tides that form when sun, Earth and moon are most nearly aligned, at full moon and new moon.

The scientists used a binocular microscope fitted with a micrometer in counting the thicknesses of neap-spring intervals in cores taken from the formations. The record of neap-spring tide bands shows a lengthening monthly lunar cycle over the past 900 million years. Sonett and Zakharian used the record to calculate how much farther away from Earth the moon has moved since late Proterozoic times. The rate of the moon's retreat from Earth recorded in tidalites is consistent with the modern rate of lunar retreat measured by the Apollo experiments, the authors conclude.

It took only about 18 hours for Earth to make a complete rotation on its axis 900 million years ago, the authors also conclude. They analyzed how much energy the Earth has lost to the moon since that time, and how much energy the Earth has lost through tidal friction. Tidal friction is heat lost as oceans drag across sections of shallow ocean floor and layers of rock rub together as they rise and fall.

Years were 481 days long in Proterozoic times, they add.

Given its present-day rate of retreat, the moon eventually would reach synchronous orbit with Earth in about 15 billion years, Zakharian said in an interview. In synchronous orbit, the moon and Earth would orbit together as planet and satellite in fixed position, locked face-to-face, about 560,000 kilometers (336,000 miles) apart. The moon now is about 384,000 kilometers (240,000 miles) away.

Not that this is likely to happen, Sonett and Zakharian say: 15 billion years is older than the age of the universe. It's more probable that our sun will change into a red giant star, or that a major asteroid will strike Earth, before then.

University of Arizona

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