Resulting RMSD as a function of latitude when comparing the simulated microwave brightness temperatures with the MRM 37 GHz (top) and 19.35 GHz (bottom) data. Illustrated are the simulations assuming constant regolith properties (red line), applying a variation in the solar-incidence-dependent albedo (green stars), a poleward decrease in the deep layer density (blue crosses, calculated in steps of 5° latitude), or a poleward increase in the transition width (dashed orange, calculated in steps of 5° latitude), and the polynomial fit to the data (dash-dotted black).