How Bits of Red Planet Got to Earth : Asteroid Suspected in Martian Rock Mystery

Caltech scientists, using a 106-foot-long, 35-ton Space Age stun gun that shoots one-ounce plastic bullets at speeds up to 16,000 miles an hour, believe they have figured out how pieces of Mars wound up on Earth millions of years ago.

If their calculations are correct, they will have answered a question that has bedeviled scientists since the first of eight meteorites, all believed to have come from Mars, was discovered just before the turn of the century.

The geochemical evidence that the meteorites did indeed come from Mars is “very convincing,” according to geophysicist Thomas J. Ahrens, but no one until now has come up with a plausible explanation.

Asteroid Theory

In a report in the current issue of Science magazine, Ahrens and John D. O’Keefe, a planetary scientist at Caltech, said they have determined that if an asteroid hit the Red Planet at just the right angle, it could have sent debris flying fast enough to escape the gravity of Mars. The debris then could have spun around the solar system, and some of it could have collided with Earth.

Other scientists have suggested similar explanations in the past, but it had been thought that an asteroid either would have had to plunge straight into the planet or barely graze it in order to blast material into space.

Both those scenarios have fallen out of favor because either would have produced material quite different from the meteorites found on Earth. A glancing blow could have kicked up a lot of dust, but it would not have sent boulders flying through space; if it hit perpendicular to the surface, it would have produced so much heat that the rocks would have melted, the Caltech team concluded.

Ahrens and O’Keefe came up with data for a computer model that they believe provides the answer to how the meteorites got here. They used the huge gun to accelerate bullets to speeds fast enough to replicate the dynamic pressures of violent collisions in the solar system. The gun is so powerful that it shakes the building on the Caltech campus every time it is fired.

By using data from firing the gun repeatedly at various materials, the scientists came up with a computer model showing that if the asteroid hit Mars at an angle of between 25 and 60 degrees from the horizontal, it would have produced a jet of hot gas powerful enough to sweep up rocks three feet in diameter.

The rocks would have been hurled into space at more than 11,300 miles an hour--fast enough to escape the gravity of Mars, but not the solar system. The rocks would have gone into orbits of their own, subject to the gentle tug of the various planets. Some of them would have fallen eventually back onto Mars, but some could have been captured by Earth’s gravity, Ahrens said.

The Martian meteorites are in laboratories and museums around the world. “They look like Earth rocks,” he said.

But over the years scientists have learned that they are, indeed, different not only from Earth rocks but from the 10,000 or so other meteorites that came from asteroids.

For one thing, they apparently crystallized from molten rock only 1.3 billion years ago, and thus could not have come from asteroids. That led scientists to conclude that they must have been of volcanic origin, and since volcanic activity ended on the moon 4.2 billion years ago, they had to have come from one of the planets.

Evidence From Viking

The Viking space probe to Mars produced evidence that pointed to the Red Planet.

When the asteroid hit Mars 180 million years ago, the rocks would have melted slightly from the heat of the impact, thus absorbing atmospheric gases. And chemical analysis of the meteorites reveals gases that are consistent with what was learned about the Martian atmosphere through instruments on the Viking landers.

All the evidence points directly to Mars, Ahrens said, where the rocks formed from lava 1.3 billion years ago, only to be knocked into space by an asteroid more than a billion years later.

The Martian meteorites have been found in India, Egypt, France, Antarctica, Nigeria, Brazil and Indiana.