The strongest evidence for a Martian origin of the SNC meteorites is the strong correlation between the rare gas abundances and isotopic compositions in shock-induced melt pockets, and those measured for the Martian atmosphere. However, the formation of melt pockets and the entrapment of atmospheric gases remain poorly understood. Here we report the presence in the melt pockets of three Martian meteorites of the hollandite-structured high-pressure polymorph of feldspar. These occurrences set constraints on the continuum pressure (21--25 GPa), the local temperature increase (2000--2500 K) and the energy delivered during shock. We then test a mechanism for melt pocket formation by compaction of pre-existing porous precursors. The model explains the local temperature increase required for melting and the presence of an atmospheric component in pores that were connected with the Martian atmosphere prior to the shock event.