Golden dust in the afternoon

by George Wilkinson

Every year, the Earth captures thousands of tons of interplanetary particles, with objects in the micron size range striking every thirty microseconds. Because of their small size, some of them waft to the surface with minimal heat damage—thus their interiors preserved traces of their interplanetary origins. Although micrometeories fall in over the earth’s entire surface(and can be collected from rainspouts), scientists try to collect them in isolated environments, away from both natural and anthropogenic contaminants. An ongoing program in Antarctica makes use of the extremely clean, dry environment to collect micrometeorites from the icepack. Scientists melt ice by the ton and sieve the meltwater to recover a fine grit very rich in micrometeorites. Because of the heterogeneity of the collected particles, they have to be analyzed individually, by electron microscopy and specialized spectroscopy.

But how faithfully does the dust collected on earth reflect its interplanetary reservoir? The STARDUST space mission collected dust from the coma of the comet Wild-2, a Jupiter family comet recently deflected nearer to earth, making use of an aerogel as a high-tech butterfly net to capture dust particles with relative velocities approaching that of a rifle bullet. STARDUST returned to Earth bringing back thousands of small (<30 μm) solid particles. The examination of Wild 2 samples made it possible to explore the connection between micrometeorites recovered on earth and cometary and asteroidal objects as they exist in the interplanetary space.

A comparison of micrometeorites and Wild 2 samples recently concluded that the two populations have broadly the same bulk compositions, matching the solar system abundances for rock forming elements and for organic compounds. Their findings imply that well-preserved micrometeorites contain a record of the cold regions of our sun’s ancient proto-planetary disk. Further, the finding of chemical similarities between micrometeorites on the ground and interplanetary dust raises the possibility that early earth could have received a gentle rain of intact organic molecules, arising from elsewhere in the solar system and thus chemically distinct from the neighborhood of earth’s formation, through infall of micrometeorites. This settling dust could have delivered complex organics and biogenic elements such as carbon, nitrogen and amino acids, and thus played a role in the emergence of life on the early Earth. Whatever its contribution, it would have provided the same to the other planets.

Images: (top) Glacial sand mixture containing micrometorites collected from Cap Prud’homme, Antarctica. Credit Institut polaire français.

(bottom) Laboratory demonstration of aerogel capture of dust (in this case fired from an air gun). Each feathery track ends with a small dust particle. Stardust captured particles with the relative speed of rifle bullets. Credit NASA/JPL.