πF_n and P_s as functions of α for a model planet covered by ocean and with a center continent of sand between  ±22° longitude and  ±50° latitude (green, dashed-triple-dotted line). Also plotted: the functions for a homogeneous ocean planet (blue, dashed-dotted line), a homogeneous sand planet (orange, solid line) and a weighted sum of these with  ~16% sand and  ~84% ocean (gray, dashed line).