Asteroid Dust May Influence Weather, Study Finds

In a study to be published this week in the journal Nature,scientists from the Australian Antarctic Division, the University ofWestern Ontario, the Aerospace Corporation, and Sandia and Los Alamosnational laboratories found evidence that dust from an asteroid burningup as it descended through Earth's atmosphere formed a cloud ofmicron-sized particles significant enough to influence local weather inAntarctica.

Micron-sized particles are big enough to reflect sunlight,cause local cooling, and play a major role in cloud formation, theNature brief observes. Longer research papers being prepared from thesame data for other journals are expected to discuss possible negativeeffects on the planet's ozone layer.

"Our observations suggest that [meteors exploding] in Earth'satmosphere could play a more important role in climate than previouslyrecognized," the researchers write.

Scientists had formerly paid little attention to asteroid dust,assuming that the burnt matter disintegrated into nanometer-sizedparticles that did not affect Earth's environment. Some researchers(and science fiction writers) were more interested in the damage thatcould be caused by the intact portion of a large asteroid strikingEarth.

But the size of an asteroid entering Earth's atmosphere issignificantly reduced by the fireball caused by the friction of itspassage. The mass turned to dust may be as much as 90 to 99 percent ofthe original asteroid. Where does this dust go?

The uniquely well-observed descent of a particular asteroid and its resultant dust cloud gave an unexpected answer.

On Sept. 3, 2004, the space-based infrared sensors of the U.S.Department of Defense detected an asteroid a little less than 10 metersacross, at an altitude of 75 kilometers, descending off the coast ofAntarctica. U.S. Department of Energy visible-light sensors built bySandia National Laboratories, a National Nuclear SecurityAdministration lab, also detected the intruder when it became afireball at approximately 56 kilometers above Earth. Five infrasoundstations, built to detect nuclear explosions anywhere in the world,registered acoustic waves from the speeding asteroid that were analyzedby LANL researcher Doug ReVelle. NASA's multispectral polar orbitingsensor then picked up the debris cloud formed by the disintegratingspace rock.

Some 7.5 hours after the initial observation, a cloud of anomalousmaterial was detected in the upper stratosphere over Davis Station inAntarctica by ground-based lidar.

"We noticed something unusual in the data," says Andrew Klekociuk, aresearch scientist at the Australian Antarctic division. "We'd neverseen anything like this before - [a cloud that] sits vertically andthings blow through it. It had a wispy nature, with thin layersseparated by a few kilometers. Clouds are more consistent and lastlonger. This one blew through in about an hour."

The cloud was too high for ordinary water-bearing clouds (32 kilometersinstead of 20 km) and too warm to consist of known manmade pollutants(55 degrees warmer than the highest expected frost point ofhuman-released solid cloud constituents). It could have been dust froma solid rocket launch, but the asteroid's descent and the progress ofits resultant cloud had been too well observed and charted; thepedigree, so to speak, of the cloud was clear.

Computer simulations agreed with sensor data that the particles' mass,shape, and behavior identified them as meteorite constituents roughly10 to 20 microns in size.

Says Dee Pack of Aerospace Corporation, "This asteroiddeposited 1,000 metric tons in the stratosphere in a few seconds, asizable perturbation." Every year, he says, 50 to 60 meter-sizedasteroids hit Earth.

Peter Brown at the University of Western Ontario, who wasinitially contacted by Klekociuk, helped analyze data and didtheoretical modeling. He points out that climate modelers might have toextrapolate from this one event to its larger implications. "[Asteroiddust could be modeled as] the equivalent of volcanic eruptions of dust,with atmospheric deposition from above rather than below." The newinformation on micron-sized particles "have much greater implicationsfor [extraterrestrial visitors] like Tunguska," a reference to anasteroid or comet that exploded 8 km above the Stony Tunguska river inSiberia in 1908. About 2150 square kilometers were devastated, butlittle formal analysis was done on the atmospheric effect of the dustthat must have been deposited in the atmosphere.

The Sandia sensors' primary function is to observe nuclearexplosions anywhere on Earth. Their evolution to include meteorfireball observations came when Sandia researcher Dick Spaldingrecognized that ground-based processing of data might be modified torecord the relatively slower flashes due to asteroids and meteoroids.Sandia computer programmer Joe Chavez wrote the program that filteredout signal noise caused by variations in sunlight, satellite rotation,and changes in cloud cover to realize the additional capability. TheSandia data constituted a basis for the energy and mass estimate of theasteroid, says Spalding.

The capabilities of defense-related sensors to distinguishbetween the explosion of a nuclear bomb and the entry into theatmosphere of an asteroid that releases similar amounts of energy - inthis case, about 13 kilotons - could provide an additional margin ofworld safety. Without that information, a country that experienced ahigh-energy asteroid burst that penetrated the atmosphere might provokea military response by leaders who are under the false impression thata nuclear attack is underway, or lead other countries to assume anuclear test has occurred.

More detailed papers are slated for the Journal of Geophysical Researchand the Journal of Meteoritics and Planetary Science, Pack says.

Sandia is a multiprogram laboratory operated by SandiaCorporation, a Lockheed Martin company, for the U.S. Department ofEnergy's National Nuclear Security Administration. With main facilitiesin Albuquerque, N.M., and Livermore, Calif., Sandia has major R&Dresponsibilities in national security, energy and environmentaltechnologies, and economic competitiveness.