Wrinkle ridges are a manifestation of horizontal shortening in planetary lithospheres, for which deformation is localized on faults that underlie individual ridges. In ridged plains of Mars, such as Solis Planum or Lunae Planum, wrinkle ridges are spaced ∼40 km apart, whereas in the Martian northern lowlands, where ridges are identified only in Mars Observed Laser Altimeter (MOLA) altimetric data, the ridge spacing is at least ∼80 km. We attribute ridge spacing to an instability of the lithosphere under horizontal compression. The localization instability, which results in periodically spaced faults [ Montési and Zuber, 2003a ], links the difference of ridge spacing in the northern lowlands and in the highland ridged plains to the difference of crustal thickness via the depth of the brittle-ductile transition (BDT). In Solis and Lunae Plana, where the crust is 50 to 60 km thick, the crust may be ductile at depth, limiting faulting to the BDT of crustal rocks. In the lowlands, the crust is only about 30 km thick and may be brittle throughout. Thus the depth of faulting may be controlled by the BDT of mantle rocks, which is roughly a factor of two deeper than that of crustal rocks. The geotherm can be identical in both regions, at 12 ± 3 K.km⁻¹, although differences of a few K.km⁻¹ can be accommodated within this model. The heat flux implied by this geotherm is similar to the heat produced by radiogenic decay 3 Gyr ago. Our analysis provides a rheological explanation for the difference in spacing between ridges in the highlands and the lowlands, in contrast to the suggestion of Head et al. [2002] , who proposed that alternating lowlands ridges are buried by sediments. In addition, finite element models that use the lithospheric structure deduced from ridge spacing show that modest gradients of crustal thickness or heat flux across a ridged plains favor the formation of faults dipping toward high-elevation areas, as may be the case in Solis Planum [ Golombek et al., 2001 ].