Impact cratering on Earth, Mars, and the icy satellites of the outer solar system is significantly affected by the presence of subsurface ice or water. The high volatility of H₂O modifies the crater formation process, resulting in more vapor production, higher ejection angles, fluidized ejecta blankets, and hydrothermal fracturing beneath the crater. We have been conducting experiments and modeling of impacts into ice-rock mixtures to quantify the effects of subsurface H₂O on observable crater features, such as the ejecta distribution, rampart and pedestal formation, and crater floor morphologies. Our model explains the formation of fluidized ejecta blankets on Mars as well as pedestal craters on Ganymede and Europa. Using our new understanding of cratering on icy materials and the location and morphologies of fluidized ejecta blankets on Mars, we are mapping the time-averaged volume fraction of subsurface ice on Mars.