Space erosion of meteorites and the secular variation of cosmic rays (over 109 years)

The space erosion of stony meteorites has been determined to be 650 μm 106 y-1, while that of iron meteorites has been determined to be 22 μm 106 y-1. The erosion rates are based on flux and size distributions of small particles in the solar system, meteoroid orbitals and the relation, determined by laboratory experiments, between excavated volume due to a collision and the size and velocity of the impacting small particle. Neither multiple collision or space erosion can explain the difference in cosmic ray exposure ages based on 40K and those based on 36Cl, 39Ar and 10Be. It is concluded that there is a long term cosmic ray variation. (author)