Experiments may back theory that Earth formed closer to sun
When the Earth was born, it may have orbited far closer to the sun than it now does. This scenario - for some time a matter of speculation among the small community of scientists who theorize about the origins of the solar system - has been given some solid backing from laboratory experiments that involved blasting pieces of primordial rock with a variety of pellets.
The research, and associated computer analysis, was performed by Thomas J. Ahrens of the California Institute of Technology and Manfred A. Lange of the Alfred Wegener Institute for Polar Research in West Germany. It was reported this week at the annual meeting of the American Geophysical Union.
The purpose of their laboratory ballistics was to throw some quantitative light on how the Earth was formed more than 3.5 billion years ago.
Since the 17th century, scientists have thought that the planets somehow formed, along with the sun, from a disk of dust and gas called the solar nebula. But all the complicated steps involved in the formation of large, solid planets from billions of particles of interstellar dust and gas was extremely difficult to visualize. It was not until the last two decades that information from spacecraft dispatched to other planets has stimulated significant advances in unraveling the planet-formation process.
This process appears to have proceeded in a number of steps. As the giant, interstellar dust cloud collapsed from its own weight, it began to spin faster and faster like a twirling ice skater tucking in her arms. As a result, the cloud was gradually reshaped into a thin and increasingly dense disk.
As the dust thickened, the grains collided more and more frequently, gradually clumping into millions of asteroid-sized objects dubbed ''planetesimals.'' (To visualize this, picture a solar-system-sized version of Saturn and its rings.)
In the early solar system, however, the sun formed at the center. With the ignition of the sun's nuclear fires, a second physical process began. Between the frigid, outer limits of the cloud and its fiery core, a strong temperature gradient formed.
Much like a petroleum refinery, where temperature differences are created to separate crude oil into a spectrum of products ranging from heavy oil to lightweight gasoline, the material in the solar nebula was chemically fractionated.
In the high temperatures near the core, lighter materials like water vaporized, so the planetesimals that formed were rich in heavier elements like iron. In the frigid conditions in the outer reaches, however, even extremely light compounds like ammonia and carbon dioxide were present as ices and so were incorporated into the planetary building blocks.
Against this backdrop, the planetesimals continued to collide and stick, forming ever larger objects. The larger the body, the stronger its gravity. This enabled the larger planetesimals to attract the smaller objects in their vicinity. As these ''protoplanets'' grew bigger, their greater gravity also meant that they were colliding with greater severity. The course of planet formation grew increasingly violent until the solar system was swept clean of debris.
It was to study the collisional implications of this process that Drs. Ahrens and Lange performed their recent experiments.
They started with centimeter-sized targets of the mineral serpentine. This is found in the most primitive of meteorites and is thought to be the basic material from Earth was formed. To simulate the tremendous range of velocities with which planetesimals would have collided over the entire process of the Earth's formation (all the way from zero to more than 10,000 miles per hour), the scientists fired pellets of materials ranging from plastic to stainless steel at the rock and carefully observed the results.
The underlying principle, Ahrens explains, is that ''when a pillow hits a target while traveling at 100 feet per second, it has the same impact as a brick going one foot per second.''
The scientists found that as the collisions became increasingly violent, a point was reached where the energy released was great enough to create the temperature required for the iron, water, and silicate in the serpentine to react chemically to form iron silicate and hydrogen gas.
This is important, because hydrogen is an essential ingredient in water. But it is so light that it would have rapidly seeped out into space from the proto-Earth. And these considerations lead to the conclusion that it is ''virtually impossible'' for Earth to have formed from a homogeneous material, as is commonly assumed, Ahrens says.
Our world has several distinct features that must be explained by theories detailing its formation. First, there is its sizable core of metallic iron. Second, there is a thick mantle thought to consist primarily of iron-silicate rock. Finally, most of its surface is covered with copious amounts of water.
Incorporating their laboratory results, the scientists wrote a computer program that simulated the planet-building process. But, using homogeneous material, they could not create a planet that matched even the gross features of Earth. The best they could come up with was a bleak, desert world.
On the other hand, postulating a two-stage formation process made it possible to create an Earth-type planet, they found. If, for the first 100 million years, the material forming the protoplanet was rich in iron, it would explain the metallic iron core. Then, when the embryonic world was about half its current size, if the chemical composition of the planetesimals changed so that they contained more water and less iron, it would account for the mantle and oceans.
The most straightforward way to get such a two-stage development is to place Earth much nearer the sun at the beginning and then, a hundred million years later, move it out to its present-day position.
According to current theory, an origin somewhere between the current orbits of Mercury and Venus would have provided the iron-enriched ingredients called for during the initial stage, while the lighter, water-enriched planetesimals thought to have been present at Earth's current location are appropriate to finish the process, Ahrens says.