Scientists reach most precise estimates on age of 2 groups of meteorites

March 02, 1992|By Douglas Birch | Douglas Birch,Staff Writer

Scientists, including a geochemist at the University of Maryland, say they have made the most precise estimates so far of the age of two groups of meteorites that are thought to be fragments of a pair of long-vanished planets.

Writing in this week's edition of the journal Science, Dr. Richard J. Walker of College Park and three colleagues from the U.S. Geological Survey report that one group of meteorites formed and cooled within 100 million years of the creation of the solar system, which is an estimated 4.56 billion years old.

Although collected from sites scattered around the world, the meteorites are believed to be pieces of the metallic core of a single planet.

A meteorite is any part of a meteor that survives the fiery descent through the atmosphere to Earth.

The Science article also suggests that a second group of chemically similar meteorites may have been part of the core of a larger planet that was created around the same time as the first but took up to 178 million years longer to cool.

Dr. Walker, in an interview, cautioned that this second point is still speculative because of the limited number of samples analyzed to date.

Dr. Walker and USGS geologists Mary F. Horan, John W. Morgan and Jeffrey N. Grossman examined the meteorites last summer using an advanced mass spectrometer at the College Park campus.

Deciding when the molten liquid cores of some of these early planets cooled to the point where they became solid iron is critical to understanding the planets' formation, development and ultimate demise, the authors write in Science.

Dr. Walker added that the research eventually could lead to a better understanding of how the current array of planets, including Earth, emerged from the debris of interplanetary collisions.

Scientists have long thought that, as the gases of the solar nebula condensed into solid matter, they gradually spawned a swarm of "protoplanets" that no longer exist.

Scientists believe a few of these early planets may have been quite massive, but most were smaller than the moon.

At least some of the smaller planets were large enough to possess molten cores -- the kind that Earth has and that Mars and Venus probably have, Dr. Walker said.

As the protoplanets aged, their cores slowly cooled.

Scientists think the protoplanets gradually broke up and part of their cores became what now are called iron-nickel meteorites, which make up about 10 percent of meteorites found.

Nine of 10 meteorites are made of stony silicates rather than metals. Among these meteorites, called chondrites, are the apparent remnants of the first material to form in the solar system.

Scientists have long been able to date chondritic meteorites because they contain measurable amounts of long-lived radioactive isotopes of the elements uranium, thorium or samarium.

These isotopes decay into other elements at a known rate. By comparing the amounts of these isotopes that must have been present when the meteorites were formed with the amounts measured in them now, scientists can deduce the meteorites' age.

But iron meteorites don't contain measureable amounts of uranium, thorium or samarium, and couldn't be dated accurately until recently.

New techniques have been developed to measure precisely the isotopes of rhenium and osmium, two heavy metals that are extremely rare on Earth but relatively abundant in iron meteorites.

For example, in terrestrial rocks, osmium -- a metal similar to platinum -- is generally present in only one to 10 parts per trillion.

Gold, on the other hand, is present on the average in about 500 parts per trillion.

Dr. Walker said the rhenium-osmium dating technique showed that planets with metallic cores probably developed very early.

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