Meteorite holds clues to Earth’s cosmic roots

BY DR EMILY BALDWIN

ASTRONOMY NOW

Posted: 17 June, 2009

The interstellar stuff that became incorporated into the planets and life on Earth has younger cosmic roots than theories predict, according to new analysis of a meteorite that fell in 1969.

The Murchison meteor streaked through the skies near Murchison, Australia in September 1969, fragmenting in the atmosphere and delivering some 100 kilograms of debris to Earth. While it has long been thought that the grains contained in the meteorite had spent around 500 million years traveling through interstellar space, new analysis suggests a much younger transit time. University of Chicago postdoctoral scholar Philipp Heck and his international team of colleagues conducted high precision laboratory analysis on 22 of those grains to reveal that 17 of them spent somewhere between three million and 200 million years in interstellar space.

Grey-scale and false colour scanning electron microscope images of one of the silicon carbide grains studied in the new analysis. Images courtesy Phillipp Heck.

“Presolar grains are essentially the only material available that survived from the time before the Solar System formed and they are essentially the only solid samples of stars that are available to us,” Heck tells Astronomy Now. “Information retrieved from them provide a window into presolar times. Since the Murchison meteorite came from a parent asteroid that did not experience significant heating, it preserved the composition of the solar nebula material from which the Solar System objects formed.”

Grains of exotic material contained in the meteorite were formed outside our Solar System, flung into space by dying Sun-like stars in supernovae events, to eventually become incorporated into the meteoritic material that fell to Earth. Determining the length of time this material spent in space is essential to better constrain the timing of formation processes of the Solar System, especially since a supernova has been suggested to have triggered the collapse of the molecular cloud that led to the formation of the Solar System.

The theoretical 500 million year transit time is based on the effect of destructive events in the interstellar medium on the grains such as supernova shock waves, collision between grains and collisions of grains with gas. “To estimate the lifetimes one has to take into account the frequency of these events and how efficient these events ablate or destroy interstellar grains,” explains Heck. “This also depends on the material properties and grain sizes.” The estimate of 500 million years is for silicon carbide grains, the same type of grain that Heck and colleagues studied in the laboratory.

Philipp Heck with a sample of the Allende meteorite, which is of the same type as the Murchison meteorite. The dark portions of the meteorite contain dust grains that formed before the birth of the Solar System. Image: Dan Dry.

“The young ages we find for most grains imply that the Solar System formed from relatively fresh material,” says Heck. “Although the lifetimes of most grains are short, they are longer than the lifetime of the presolar molecular cloud. This implies that the grains did not form in this cloud and are older. This is likely also true for much of the material whose presolar signatures had been erased through alteration in the early Solar System and which is now part of our Solar System objects.”

The team plan to study more presolar grains from Murchison and other meteorites in order to see how representative their studied samples are and to include other grain types such as rare dust grains from supernovae. Further work in this area of cosmochemistry could help settle an important debate as to whether a nearby supernova injected material into the presolar molecular cloud core and also led to the cloud collapse to form the protosun and the solar nebula, or whether the supernova explosion occurred after the solar nebula disc has already formed and was not responsible for the cloud core collapse.

According to Heck’s team, a period of intense star formation that preceded the Sun’s birth may have produced large quantities of dust which could account for the timing discrepancy between the theoretically calculated values and the new experimental values.