It was the size of a small house, weighed a million kilograms, and for a few seconds was as bright as the sun as it burst though Earth's atmosphere last September, releasing as much energy as a nuclear bomb. Unfortunately, nobody witnessed it. But the dust left by this once-in-a-decade shooting star drifted over an Antarctic climate research station, and scientists now have their first detailed measurements of a meteor cloud.
Like a tiny time capsule, cosmic dust contains material left over from the birth of the solar system. One source of the dust is meteoroids--chunks of rocky debris that become meteors if they enter Earth's atmosphere. Most meteors are small--the size of a grain of sand--and after they break up, the particles they leave behind can spend several weeks in the atmosphere before drifting down to Earth. Many people assumed these cosmic dust grains were only nanometers wide, but their size had never been measured because it is difficult to spot an incoming meteor soon enough to analyze the subsequent dust cloud.
Atmospheric scientist Andrew Klekociuk of the Australian Antarctic Division and his team were fortunate. They happened to be studying particles in the stratosphere around the hole in the ozone layer with lidar--a laser that sends light pulses into the stratosphere and records the reflection--when a meteor arrived on 3 September 2004. "It was a sheer stroke of luck," says Klekociuk. "We were in the right place at the right time."
Analysis of the meteor cloud revealed that the dust particles were as large as 20 micrometers across, the team reports tomorrow in Nature. The findings suggest that most cosmic dust on earth in the micrometer range has come from these large meteor events. Klekociuk and his team are currently analyzing Antarctic snow samples to look for the remains of the meteor and confirm its chemical make-up. This may give them a better understanding of what type of meteor it was and where it came from.
"[The study] makes a valuable contribution to our understanding of where this dust in our atmosphere is coming from," says Matt Genge, a planetary scientist from Imperial College London. Researchers will now know that anything that's extraterrestrial and in this size range has burned off larger objects, rather than just being interplanetary dust, he says.