Collisions on icy planetary bodies produce impact melt water, redistribute ground ice, and deposit thermal energy available for chemical reactions. The amount of melt generated from an impact is sensitive to the initial temperature, which ranges from the 273 K on Earth and Mars to 40 K on the surface of Pluto. Previous shock wave studies, centered at ∼263 K for terrestrial applications, had difficulty defining the onset of phase transformations on the ice Hugoniot, and consequently, the criteria for shock melting was poorly constrained. Because ices on most planetary surfaces exist at ambient temperatures much below 263 K, we conducted a detailed study of the shock response of solid ice at 100 K and ∼40 &percent; porous ice at ∼150 K to derive Hugoniots and critical pressures for shock‐induced melting that are applicable to most of the solar system. New Hugoniots for solid and ∼40 &percent; porous ice are defined, and the critical pressures required to induce incipient melting and complete melting upon isentropic release from the shock state are revised using calculated shock temperatures and entropy. The critical pressures required for incipient melting of solid ice are only 0.6 and 4.5 GPa for 263 and 100 K respectively, and pressures between 0.1–0.5 GPa initiate melting of porous ice between 250 and 150 K. Therefore, hypervelocity impact cratering on planetary surfaces, with peak shock pressures >100 GPa, and mutual collisions between porous cometesimals in the outer solar system, with peak pressures of ∼1 GPa, will generate prodigious amounts of shock‐induced melt water. © 2004 American Institute of Physics