Tracing evolution of our double planet. Fossils record time when moon was closer, and year had 435 days
Where was the moon a billion years or more ago? How fast was Earth spinning? And how far over did the planet tilt toward the plane of its orbit -- the tilt which gives Earth its seasons? A variety of indications, including computer simulations of the Earth/moon system's evolution, suggest that the answers are, respectively, ``closer,'' ``faster,'' and ``farther.'' But scientists would like more direct evidence of how our double planet has evolved into the system we know today; and one of Earth's oldest types of living structure may help provide it.
It's the stromatolites. These are stony columns built by microscopic algae and bacteria which make or trap sediment particles and cement them together. Fossil stromatolites date back some 3.5 billion years.
Their descendants live today in such places as geyser and hot spring outflows in Yellowstone National Park. Some of these now have been found to grow in such a way as to point toward the sun and to keep a running total of successive days in their growth segments, which are laid down daily.
This holds out the promise that at least some fossil stromatolites may also have preserved a running count of days and a record of how the sun moved across the sky billions of years ago. The angle of the sun above a stromatolite's horizon would reveal the tilt of the planet. The number of days it took for the sun to run through its annual cycle from, say, one summer solstice to the next, would reveal the rate of Earth's rotation.
Moreover, knowing Earth's ancient spin rate, geophysicists can estimate the Earth/moon distance at that time. When the moon raises tides on Earth, this slows Earth's rotation. But, because the tidal bulge on Earth exerts a small gravitational force on the moon, it pulls the moon along as Earth rotates and increases the moon's speed along its orbit. The process, in effect, takes momentum out of Earth's rotation and adds it to the moon's orbital motion. As the moon speeds up, it moves farther out from Earth just as a spacecraft moves into a higher orbit when a rocket engine gives it more energy. Thus, if the earth were spinning faster billions of years ago, scientists could conclude that the moon was much closer than it now is.
The key to extracting this kind of information from stromatolites is the concept of heliotropism -- the tendency of stromatolite-building organisms to orient their structures toward the sun on a day-by-day basis. Until recently, interested scientists had doubted this would happen. They believed the orientation of the structures would be so strongly influenced by flowing water and other local environmental factors that any heliotropic tendency would be overwhelmed.
While this seems to be true for most stromatolites ancient and modern, Stanley M. Awramik and James P. Vanyo of the University of California at Santa Barbara suspected some of these structures might grow in quiet enough conditions for sun-seeking tendencies to dominate. So they went looking for sun-orienting stromatolites and found them.
Awramik found north-leaning, sun-oriented structures in Hamelin Pool at Shark Bay in Australia. Working with United States National Park Service geologist R. A. Hutchinson, they have also found examples of south-leaning, sun-oriented stromatolites in Yellowstone National Park.
From these studies, they find that they can trace the sun's orientation and its annual cycle in these living stromatolite structures. This gives them confidence that at least some ancient stromatolites will yield such information also. In fact, they have been able to analyse 850-million-year-old stromatolite columns from Australia, known as Anabaria juvensis, to reveal that Earth was indeed spinning faster at that remote epoch. There were 435 days in the year. And the moon probably was only nine-tenths as distant as it is today.
Finding suitable stromatolite fossils to study conditions billions of years ago will be difficult. Vanyo says a global search is needed -- a search that could be carried out by scientists in a number of countries. If enough specimens can be found around the world, dating from different ages, it may be possible to reconstruct at least an outline of the evolution of the Earth/moon system and, perhaps, of the drifting of Earth's continents for much of our planet's history.
This would be useful information for scientists who are trying to understand how our planetary environment became what it is today and what it may evolve into in the future.
A Tuesday column. Robert C. Cowen is the Monitor's natural science editor.