  We report on the first results obtained with the IceTop air shower array on
the cosmic ray energy spectrum and mass composition in the range of 1 PeV to 80
PeV. IceTop is the surface detector of the IceCube neutrino telescope currently
under construction at the South Pole. A high sensitivity to the primary mass
composition was observed by reconstructing showers at different zenith angles.
Assuming only protons or iron nuclei as primary particles yields significantly
different energy spectra for different zenith angle ranges, while only models
with mixed composition, like the poly-gonato model, lead to the expected
isotropic flux. The prospects of composition measurements with different,
alternative methods using the full IceCube detector are also discussed.
