Mesospheric water ice clouds are a phenomenon frequently occurring on Earth at mid to high latitudes during summer. Surprisingly, similar clouds consisting of CO 2 ice were also detected at low latitudes in the mesosphere of Mars. Both types of clouds are believed to form by heterogeneous nucleation on nanometer size Meteoric Smoke Particles (MSPs). However, describing cloud formation is still flawed by high uncertainties due to a lack of experimental data regarding the nucleation process. We present measurements on heterogeneous nucleation and subsequent growth of H 2 O and CO 2 on sub 3nm silica, iron oxide and mixed iron silicates as analogues for MSPs. The experiments were performed at conditions which are relevant to the mesosphere of Earth and Mars. We show measured desorption energies and critical saturations of H 2 O and CO 2 for the different particle materials. Using Classical Nucleation Theory (CNT), we infer the contact parameter resulting in a full parameterization of the nucleation process. We conclude that the same parameterization may also be valid for Martian Dust Particles [MDPs] and predict conditions at which H 2 ... mehr O and CO 2 clouds form in the mesospheres of Earth and Mars. The results are discussed in comparison with atmospheric observations.