Meteors may have brought building blocks of life to Earth billions of years ago

That amount of water, about 10 times the daily outflow of the Mississippi River, and carbon dioxide would have been enough to set off a greenhouse effect that eventually made the Earth warm and wet enough to harbor plants and creatures. Meanwhile, the other planets entered existences of torture by fire and ice.

This isn't the first time scientists have theorized that the ingredients for life on Earth could have been delivered by a kind of cosmic mail-order system once the Earth was move-in ready. Comets and asteroids both have been proposed as cosmic Fed-Exes ferrying water as well as organic compounds to Earth.

What distinguishes the new research, published this week in the earth and planetary science journal Geochimica et Cosmochimica Acta, is its suggestion of when and how the Earth received its shipment of life-giving material.

"It is fair to say that the amount delivered in this bombardment alone was enough to kick-start the Earth on its way to habitability," study author Richard Court, an Imperial College professor of Earth science and engineering, said in an e-mail.

"This may have been a pivotal moment in our early history where Earth's gaseous envelope finally had enough of the right ingredients to nurture life on our planet," Court's colleague, Mark Sephton, said in a statement.

The incident at the heart of the study is known as the Late Heavy Bombardment, a time about 4 billion years ago, not long after the Earth was formed from the dust and debris swirling around the young sun. According to the scientists, the Late Heavy Bombardment lasted 20 million years and rained millions and millions of space rocks onto the surfaces of the Earth, moon and Mars.

Because of the Earth's rapidly changing geology, evidence of this event was mostly lost, except for the leftover meteorites themselves. But the moon, with no atmosphere, has little erosion, allowing closer study of the bombardment period's effects on the lunar surface. Court said that at least 6,000 craters greater than 14 miles across remain from this period. On Earth, the bombardment might have produced 22,000 of these huge craters, he said.

According to the scientists' theory, the frictional heat of passing through the thin atmosphere that surrounded the Earth at that time would have been enough to strip the oxygen- and water-rich outer layers from the meteorites as they plunged toward the planet. That process would slowly have caused a buildup of oxygen and water in the atmosphere.

To find out how much of those critical compounds could be carried by each meteorite, the team heated samples of the ancient rocks left over from the bombardment in a pro