Reliable measurements of cosmic dust abundances have been obtained by ionization detectors during particle impact and by collectors controlled either by inflight shadowing or by penetration-hole identification. A description of the techniques used is given.
Crater-number densities observed on the lunar surface and on lunar samples represent an important source of information on cosmic dust fluxes. The related results from the Apollo 11 and 12 missions are reviewed. The overall knowledge gained from these measurements leads to the following flux model: The cumulative flux Φ vs mass m follows the extrapolation from larger meteoroid-size range (Watson’s Law) and can be described by
The Pioneer 8 dust experiment and lunar samples indicate a depletion of the flux at approximately 10-8g. However, cosmic dust particles exist in interplanetary space at least down to 0.3 μ. diameter. They are interpreted as nonmetallic particles in the solar system.
The atmosphere shows an enhancement in particles of about one order of magnitude compared to the flux in interplanetary space at 1 AU. No depletion or cutoff could be detected. These particles are interpreted as lunar debris or as disintegrated products from fireballs.
The numbers of large lunar craters (>140 m diameter) in Mare Tranquillitatis and in Oceanus Procellarum are compared with the meteoroid flux. These comparisons lead to a time-variable flux of Φ.e-Bt, with B = 2.6 and t = time in 109 yr. Thus, the meteoroidflux at the formation of the lunar maria was approximately 4 orders of magnitude higher than today.