WIND, WAVES, AND SHORELINE DEVELOPMENT ON MARS

Understanding the history of water on Mars is fundamental to the search for evidence of life on that planet. It is not just the presence of water, however, that is key. It is also important to understand the durations over which the water was present. To date, more than 200 different lake basins have been identified on Mars, many characterized by inflowing channels, deltas and outflowing channels. What are less common are positively identified shoreline features. Locating and characterizing Martian shorelines is important because they can record the duration lakes were stable at particular elevations.

We first addressed the question of, did waves on non-frozen Martian water bodies have sufficient wave energy to transport sediment and thus form shorelines? To determine the velocities of waves impacting a shore, we employed physically-based wave models, adjusted for Mars gravity, and used reasonable wind speeds and fetch lengths to simulate wave formation, propagation, and shoaling. We then combined the simulated wave velocities with sediment transport formulae to determine the potential particle-size distributions of beaches on Mars. Our results indicate that winds ranging from 5-35 m/s blowing across fetch distances from 10 – 100 km would have generated breaking wave velocities ranging from about 1 – 4 m/s. These velocities, in turn, would have been capable of transporting basalt boulders up to about 50 cm.

Next, we performed a global survey of basins identified in the literature to look for shorelines. The features most easily interpreted as shorelines found thus far, are located in the northern ocean basin in a restricted bay (10-15 km fetch) in the Cerberus Fossae region at elevations ranging from -2920 – -2930 m. Our interpretation of HiRISE imagery and a derivative DTM indicate that these bouldery features represent beach barriers, spits, tombolos, and wave cut escarpments. There are several shoreline levels represented here, with each individual level occupying a narrow elevational band of 1 to 3 m over distances of 10-15 km, similar to the natural variability of shoreline heights on Earth. The existence of these shorelines demonstrates that shorelines did form and were preserved on Mars, even in places of restricted fetch. Ongoing research is quantifying the duration of time each shoreline feature took to form.