Asteroid Hit 3.5 BYA Ago

[Richard W Travsky <rtravsky@uwyo.edu>]

http://www.washingtonpost.com/wp-dyn/articles/A49897-2002Aug22.html

WASHINGTON  Nearly 3.5 billion years ago, when the Earth was just a
billion years old, a huge asteroid slammed into the young planet and
produced a global rain of glass droplets and towering tidal waves that
raced around the world, according to an analysis of ancient rocks.

Researchers at Stanford University and Louisiana State University say
samples collected from deposits in South Africa and Australia show that a
space rock about 12 miles wide smashed into the Earth and sent into the
atmosphere millions of tons of dust and vaporized rock.

The rock samples have been age-dated at 3.47 billion years old and are the
oldest known evidence of an asteroid impact on the Earth, said Donald R.
Lowe, a researcher at Stanford University and a senior author of the study
appearing Friday in the journal Science.

Lowe said that although evidence of the impact was found in both Australia
and South Africa, the movement of the Earth crust in the billions of years
since has wiped out any crater created by the asteroid. It did leave a
concentrated layer of iridium, a chemical common in space rocks but rare
on Earth.

"This is the oldest record of an asteroid impact ever found on Earth,"
said Gary R. Byerly, a professor of geology at Louisiana State University
and co-author of the study.