A formational mechanism for the large-scale polygons that are defined by intersecting fractures in the northern plains of Mars is proposed in this work. The presented model is based on Rayleigh convection of interstitial water driven by an unstable density/temperature gradient within a saturated, porous medium. Catastrophic emplacement of water-rich sediments into the Martian northern plain basins could have provided a thick sedimentary layer in which Rayleigh convection could have occurred. Convection within this active layer is proposed to have differentially thawed an underlying ice-rich permafrost layer that had been at the planet's surface before being rapidly buried. The resulting morphology of the permafrost subsurface is thought to have resembled the scalloped solid-liquid interface morphology produced in terrestrial convective flow visualization studies. The exact size and shape of the scallops on the frozen subsurface would have been controlled by the dimensions of the convection cells, which are estimated to have had width-to-depth ratios of between 3.4 and 4.5. Subsequent stresses (e.g., gravity and bending stresses) would have produced maximum tensile stresses in the overlying sediments preferentially above the subsurface topographic highs as described previously by McGill and Hills <1992>. Additional desiccation of the sediments would have required contraction of the sediment cover and would have produced large-scale polygonal fractures along the preexisting weaknesses located above the raised subsurface topography. Thus the underlying subsurface geometric morphology would have been translated to the surface and would be represented by the large-scale polygonal fractures visible in the Viking Orbiter and Mars Global Surveyor images of the Martian northern plains.