Martian cratering 7. The role of impact gardening

Viking-era researchers concluded that impact craters of diameter D < 50 m w ere absent on Mars, and thus impact gardening was considered negligible in establishing decameter-scale surface properties. This paper documents marti an crater populations down to diameter D <similar to> 11m and probably less on Mars, requiring a certain degree of impact gardening. Applying lunar da ta, we calculate cumulative gardening depth as a function of total craterin g. Stratigraphic units exposed since Noachian times would have experienced tens to hundreds of meters of gardening. Early Amazonian/late Hesperian sit es, such as the first three landing sites, experienced cumulative gardening on the order of 3-14 m, a conclusion that may conflict with some landing s ite interpretations. Martian surfaces with less than a percent or so of lun ar mare crater densities have negligible impact gardening because of a prob able cutoff of hypervelocity impact cratering below D similar to 1 m, due t o Mars' atmosphere. Unlike lunar regolith, martian regolith has been affected, and fines remove d, by many processes. Deflation may have been a factor in leaving widesprea d boulder fields and associated dune fields, observed by the first three la nders. Ancient regolith provided a porous medium for water storage, subsurf ace transport, and massive permafrost formation. Older regolith was probabl y cemented by evaporites and permafrost, may contain interbedded sediments and lavas, and may have been brecciated by later impacts. Growing evidence suggests recent water mobility, and the existence of duri-crust at Viking a nd Pathfinder sites demonstrates the cementing process, These results affect lander/rover searches for intact ancient deposits. The upper tens of meters of exposed Noachian units cannot survive today in a p ristine state. Intact Noachian deposits might best be found in cliffside st rata, or in recently exhumed regions, The hematite-rich areas found in Terr a Meridiani by the Mars Global Surveyor are probably examples of the latter . (C) 2001 academic Press.