Taking a vacuum of extraterrestrial dust.

Taking a vacuum to extraterrestrial dust

While cleaning dust out of the corners of a room may be a loathsome part of everyday existence, scientists standing on Greenland's glacial ice cap are only too glad to do a little vacuuming. From the bottom of shallow lakes that form on the ice, they are collecting black dust that is helping to answer some cosmic questions, including how the early solar system developed.

Several years ago, researchers discovered that this black dust actually contains micrometeorites measuring about one-tenth of a millimeter across. In fact, the annual thawing cycle on the ice cap concentrates the extraterrestrial grains at the lake bottoms, making these the richest known deposits of micrometeorites on the earth's surface (SN: 8/30/86, p. 133).

This year, Michel Maurette of the University of Paris and his colleagues finished a thorough analysis of the sizes and compositions of the micrometeorites-- an analysis that is yielding unexpected results. "In addition to families of grains never reported before,' write the researchers in the Aug. 20 NATURE, "we have found a suprisingly high abundance of unmelted chondritic fragments.'

While chondritic fragments are the most abundant type of micrometeorite, scientists previously had thought that unmelted grains were extremely rare. However, the researchers found that about one-fourth of the total micrometeorites were unmelted and many were relatively large. They propose that a "very effective cooling mechanism' protected the grains from a fiery demise as they soared through earth's atmosphere.

Moreover, when Maurette's group compared the micrometeorites taken from Greenland with those collected from outside the earth's atmosphere by satellite, they found that each sample had a similar distribution of sizes. Such a correlation suggests that the grains found on earth come from a population of micrometeorites that inhabits the inner solar system.

Many believe that these micrometeorites come from comets, the so-called dirty snowballs that vaporize as they orbit the sun. Since comets are thought to have developed before the sun and planets, scientists study micrometeorite composition as a representative of the primordial matter that eventually coalesced into the solar system, says Ian Mackinnon, a cosmochemist at the University of New Mexico in Albuquerque. With Greenland yielding much larger grains than other sources--such as the deep sea-- scientists will now be able to perform a wider range of physical and chemical tests on this cosmic dust.