Branching river valleys show that Mars once had flowing water on its surface. These valleys formed about 3.6 billion years ago, shortly after heavy bombardment of the planets by asteroids and comets came to a close. As such, the valleys represent a moment in geologic time, and Mars has been mostly cold and dry since then. To tell whether Mars was more favorable for life earlier in its history, we need to know what the climate was like during heavy bombardment.
Our approach uses a computer simulation of erosion, deposition, and impact cratering on Mars, developed by Dr. Alan Howard of the University of Virginia. First, we determine the order in which craters formed in a study area, so the computer program will form the craters at about the right time and place. Second, we create a model of the initial landscape, before the visible craters formed (i.e., about 4 billion years ago). Third, we adjust the erosion rate by running water, earth movement, and wind. If the simulation includes the right erosion processes and rates, then the craters are eroded by about the right amount, and we don't form too many valleys or overflowed lake basins.