Nature, Published online: 26 February 2025; doi:10.1038/s41586-024-08565-9Emplacement of the north polar cap of Mars is investigated by combining viscoelastic deformation calculations and observations, showing that it formed over the last 1.712.0Myr atop a stiff lithosphere and high-viscosity mantle (1022 Pa s), and that glacial isostatic adjustment could be further constrained.