HOW COMMON WAS IMPACT SEDIMENTATION ON EARLY MARS?

The short answer is that impact sedimentation should have been extremely common or even dominant on early Mars. The reason is that, unlike the Moon and many other planetary bodies, early Mars had an atmosphere and, by all indications, abundant water during its heavy bombardment stage. Whether the water was in ice or in liquid form makes little difference for the impact argument. The presence of an atmosphere and/or of vaporized water allows for the formation of turbulent impact-caused density currents that, for large impacts, can travel hundreds or even thousands of kilometers before depositing all their load of gravel, sand and dust. The main problem of interpretation is that bedded impact deposits can greatly resemble terrestrial deposits that were transported by water or wind, or were formed by explosive volcanism in the case of pyoclastic density currents. Interpretations based on orbital observation are especially problematic in this regard, although they dominate interpretations for Mars other than at the three rover lander sites (the two MER sites and the MSL site).

With regard to the rover sites, impact sedimentation has been completely neglected in all interpretations of basaltic bedded rocks to date, although we have maintained in our many publications since 2005 that can virtually all layered deposits studied by the three rovers can be interpreted in terms of impact sedimentation. The impact interpretation removes the numerous contradictions implied by purely terra-centric interpretations that we have called attention to at each site over the years.

During the many years after deposition and burial, many Martian sediments underwent limited diagenetic interaction with liquid water. Some was pervasive, and much fracture-controlled. It mainly involved the formation of primitive clays (smectites), iron oxides, and amorphous materials (although some of this could be inherited impact glass). In addition, the sediments were veined by various Ca-sulfates, and some paleo-surfaces and veins show evidence of post-depositional acid alteration (e.g., silica-rich, Fe-carbonate, or Fe-sulfate bearing compositions), probably representing acid fluids descending from the surface. The acids may reflect condensation of acidic vapor clouds caused by impacts or volcanism, or perhaps sulfide oxidation.