A DIRECT MEASUREMENT OF THE TERRESTRIAL MASS ACCRETION RATE OF COSMICDUST

The mass of extraterrestrial material accreted by the Earth as submill imeter particles has not previously been measured with a single direct and precise technique that samples the particle sizes representing mo st of that mass. The flux of meteoroids in the mass range 10(-9) to 10 (-4) grams has now been determined from an examination of hyper-veloci ty impact craters on the space-facing end of the Long Duration Exposur e Facility satellite. The meteoroid mass distribution peaks near 1.5 x 10(-5) grams (200 micrometers in diameter), and the small particle ma ss accretion rate is (40 +/- 20) x 10(6) kilograms per year, higher th an previous estimates but in good agreement with total terrestrial mas s accretion rates found by geochemical methods. This mass input is com parable with or greater than the average contribution from extraterres trial bodies in the 1-centimeter to 10-kilometer size range.