A massive bombardment of meteorites billions of years ago could have brought in enough water and carbon dioxide to jump-start the chemistry that let the Earth develop into the garden spot of our solar system.
By studying meteorites and other evidence from this bombardment, a team of researchers at Imperial College in England has calculated that the meteorites could have carried in as much as 10 billion tons of water vapor and carbon dioxide to the young Earth every year for millions of years.
That amount of water 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.
This isn't the first time scientists have theorized that the ingredients for life on Earth could have been delivered. Comets and asteroids both have been proposed as carriers of water as well as organic compounds to Earth.
For The Record
Los Angeles Times Sunday, June 14, 2009 Home Edition Main News Part A Page 4 National Desk 2 inches; 77 words Type of Material: Correction
Meteorite bombardment: In Section A on June 7, an article about a meteorite barrage that may have provided ingredients for life on Earth said that the meteorites may have been stripped of "oxygen- and water-rich outer layers" by "frictional heat" as they entered the atmosphere. It should have said carbon dioxide instead of oxygen; and the heat from such events is caused not by friction but by "ram pressure," in which compression heats air around the meteorite.
What distinguishes the new research, published 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.
"The amount delivered in this bombardment alone was enough to kick-start the Earth on its way to habitability," study co-author Richard Court, an Imperial College London professor of Earth science and engineering, said in an e-mail message.
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 of space rocks onto 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 used samples of ancient rocks left over from the bombardment, flash-heating them in the absence of oxygen to prevent combustion. The gases given off during the heating process were then measured.
The team found that, on average, the meteorites gave off as much as 12% of their masses as water and 6% as carbon dioxide, a potent greenhouse gas whose greater concentration in the atmosphere today is causing world temperatures to rise.
The atmosphere of the early Earth was so thin that it needed a lot of greenhouse gas to reach the point that it was thick enough to trap the sun's heat, allowing the Earth's surface to warm up and water to remain in liquid form over much of the planet.
Paul Warren, a planetary scientist at UCLA, said he was impressed by the British team's efforts to try to quantify the amount of water that could have been delivered by the Late Heavy Bombardment. But he said he's not convinced this one event could account for all the water on Earth.
He also said that far from giving life a running start, the bombardment could just as easily have caused "catastrophic heating" of the young planet's atmosphere.
"It actually could be an impediment to life," Warren said.
The British researchers believe Mars experienced the same unrelenting bombardment as the Earth. The Martian rovers Spirit and Opportunity have uncovered evidence that the Red Planet once featured standing seas and relatively mild weather. That ended billions of years ago. The Imperial College team believes the key difference between the planets' fates is the Earth's molten iron core, which allowed our planet to develop a magnetic field. That magnetic field prevents most harmful solar radiation from damaging the atmosphere.
Mars lost its magnetic shield long ago, allowing the atmosphere to be worn away by the sun's relentless radiation. Slowing of volcanic activity also cooled the planet, causing the water to retreat underground at the poles.