A robotic vacuum has retrieved thousands of particles that journeyed from the chill of outer space to the frigid depths of a water well at the South Pole. The tiny meteorites, less than a millimeter across, make up the most precisely dated collection of the debris that drifts through the inner solar system, researchers report in tomorrow's issue of Nature. The team's analysis suggests that about 90% of these objects burn up before reaching the ground.
About 40,000 tons of dust from comets, asteroids, and interstellar space collides with Earth every year. However, planetary scientists weren't sure how much of that dust withstands the searing heat of streaking through Earth's atmosphere. Some dust incinerates completely, while the remnants of larger particles fall to the ground. Some of the surviving particles land on Antarctic ice, which traps them for hundreds or thousands of years. Vast numbers of these extraterrestrial bits have settled to the bottom of a teardrop-shaped cavity under the ice at the South Pole, which provides water for the Amundsen-Scott research station.
Scientists at the U.S. Army Cold Regions Research and Engineering Laboratory in Hanover, New Hampshire, devised a robot that gently suctioned particles from the bottom. Graduate student Susan Taylor combed through the debris to find thousands of cosmic spherules: round grains of rock that melt in the atmosphere but fall to Earth largely intact. Based on the volume of the well, which extends through 110 meters of ice that has accumulated during the last 4 centuries, Taylor and her colleagues deduced that about 1600 tons of spherules flutter to the surface of the globe each year. They were unable to measure some kinds of extraterrestrial dust that resembled iron oxide contamination in the well, but they filled this gap with data from other studies. That still left 90% of the total cosmic flux of particles unaccounted for, which the researchers concluded must have been incinerated in the atmosphere.
As the well melts through progressively older layers of ice, Taylor says, the team might see changes in the flux of dust during earlier centuries. That could help resolve whether cosmic dust contributes to climate fluctuations, says planetary scientist Donald Brownlee of the University of Washington, Seattle. "This may be one of the clearest records we have of extraterrestrial material coming onto the Earth" during the last millennium, Brownlee says.