Ancient Asteroid That Hit Earth Had a Chilling Effect, Scientists Surmise

An asteroid at least four-tenths of a mile in diameter fell into the southeast Pacific Ocean 2.3 million years ago and may have triggered the most recent ice age, UCLA geophysicists reported in a paper to be published today in Science magazine.

Debris from the asteroid, the largest to strike the Earth in recent history, is scattered over the ocean floor for a distance of at least 360 miles, according to Frank T. Kyte, Lei Zhou and John T. Wasson.

Water vapor sent into the upper atmosphere by the impact, which had the explosive force of 12 billion tons of TNT, could have blocked sunlight and pushed an Earth already teetering on the edge of an ice age into an abrupt climatic shift, they speculated.

“We have a fairly good impact at a time that was critical in terms of the Earth’s climate,” Kyte said in a telephone interview. “The timing may be sheer coincidence, but we think there may be some cause-effect relationship.”

The asteroid probably struck in about 2.5-mile-deep water roughly 1,000 miles west of the southern tip of South America. There is no evidence of a crater in the area, Kyte said, although the ocean there “is one of the most poorly explored areas on the face of the Earth.”

The trio detected the asteroid impact by studying core samples drilled from the ocean floor 20 years ago by the research vessel Altinin, a converted Navy icebreaker operated by Florida State University. At a core depth that has been accurately dated at 2.3 million years ago, they found both melted and non-melted metallic particles containing iridium, a metal that is rare on the Earth’s surface but common in meteors.

By measuring the amount of iridium-containing particles at various locations around the presumed impact site, they were able to estimate the minimum diameter of the asteroid at about 0.4 miles. It could have been several times larger than that, Wasson said, but if it had been very much larger, it would have left a crater.

By comparison, the asteroid that is thought to have hit the Earth 65 million years ago at the time the dinosaurs became extinct was about 1,000 times as heavy as the 200-million-ton South Pacific asteroid, Wasson said. The South Pacific impact is not thought to be associated with any species extinctions, he added.

“Now that we knew how big it was, we had to start considering what effects it had on the environment,” Kyte said. The asteroid would have injected at least 10 times its own weight of water into the upper atmosphere, the scientists calculated. But if the water had just been injected into the stratosphere in the area of the impact, it would have rained out of the atmosphere immediately, he said.

“The force of the impact would have had to eject the water out of the atmosphere on a ballistic trajectory that would spread it around the world,” Kyte said. “It could have been sufficient to saturate the stratosphere and form clouds that would block out sunlight.”

The Earth had already been cooling slowly over a period of about 15 million years with the ice pack in the Antarctic slowly increasing in size. Cooling of the Earth caused by the stratospheric clouds could have sharply increased glaciation in the Northern Hemisphere and thereby plunged the Earth into the ice age within a year, he said.