  The origin of the grains with diameters less than 4 micron from the
inclusions in Antarctic ice cores is discussed. It is proposed that these
grains were initiated by ions stopped in the upper atmosphere. The ions form
molecules, which coagulate and diffuse downwards. These processes give rise to
a characteristic mass spectrum of the small grains. Inclusions in ice were
abundant during cold periods of the Pleistocene. This supports a model in which
the influx of particles is large when Earth's orbit stays within a disk shaped
cloud around the Sun. The production of such a cloud may favor light atoms.
This should e.g. be apparent in the isotope distribution of Magnesium from the
small grains. The large grains, which have a variable mass distribution, are
terrestrial.
