The heterosphere is the layer of an atmosphere where the gases are separated out by molecular diffusion with increasing altitude such that lighter species become more abundant relative to heavier species. The heavier molecules and atoms tend to be present in the lower layers of the heterosphere while the lighter ones are present higher up. The exact boundaries between the different molecules vary according to temperature and solar activity.[1] The heterosphere extends from the turbopause to the edge of a planet's atmosphere and lies directly above the homosphere.[2]