Antarctic large micrometeorites: State-of-art and preliminary results.

The Earth's accretion rate of meteoritic material in a single typical year is dominated by submillimeter particles with the peak of the micrometeoroid mass distribution occurring at particles size between 0.1 and 1.0 mm, as evidenced by spacecraft meteoroid detectors and meteor radar techniques. Micrometeors and micrometeorites 50 mu m to 1 mm in diameter constitute an intermediate population between interplanetary dust particles (IDPs) and meteorites,and represent almost 80% of all the meteoritic mass (excluding rare, large impactors with mass m greater than or equal to 10(11)kg) accreting onto Earth. Deceleration in the Earth's atmosphere causes frictional heating in micrometeorites, as a function of the entry angle, mass and initial velocity of the particle. The atmospheric entry of these large cosmic particles gives evidence that only those with very low geocentric velocities survive partially melted and unmelted without vaporising, suggesting that the large micrometeorites arrive to Earth by non-gravitational effects, as the Poynting-Robertson (P-R) effect and the solar pressure radiation, from asteroids of different belts and low-inclination comets. In a preliminary test study, wind-blown Antarctic gross-particles (up to 300-400 micron) have been trapped during December 1994-January 1995 in collectors inside meteorological containers and aerosol samplers nearby Campo Icaro (74 degrees 42' 43 '' S; 164 degrees 06' 58 '' E) in Antarctica. Mineralogical analysis shows that the large micrometeorites are conspicuously rare and the collected particles are constituted by single crystal fragments of feldspars, quartz, biotite and amphibole as observed in order of abundance. A mechanism of concentration of cosmic dust in polar regions by sedimentation processes occurring into seasonal meltwater lakes on glaciers close to coastal regions, is thought to be a decisively more efficient mechanism than the eolian transport of these large micrometeorites.