The low value of the accelleration due to gravity on Mars causes shallow dykes to be systematically wider and longer than those on Earth and hence to represent a larger local reservoir of heat. Regions on either side of a dyke newly intruded into rocks containing water ice in their pore spaces will be heated above the melting point of the ice. It is straightforward to calculate the lateral and vertical extent of these regions and find the length of time for which the temperature will be maintained within a given range between the melting and boiling points. We assess the possibility that biologically desireable conditions may have been maintained for long periods of time in regions of appreciable size as a result of the repeated injection of dykes into the lateral rift zones of some martian shield volcanoes.