PROBLEMS WITH TERRESTRIAL ANALOGS FOR IMPACT SEDIMENTATION ON MARS

In keeping with prior expectations and terrestrial analogs, the cross-bedded clastic rocks on Mars studied by the first 3 rovers were all interpreted to have been deposited by flowing or standing water or by wind, implying that Mars was habitable for a very long time. Beginning at the 2004 GSA Meeting, Paul Knauth and I reinterpreted the spherule-bearing cross-bedded rocks as probably being caused by large-scale impactoclastic density currents (IDCs), analogous to the pyroclastic density currents (PDCs) associated with terrestrial volcanoes and nuclear blasts. These were likely because early Mars had both an atmosphere and abundant water (perhaps mainly ice) while being bombarded. Later alteration took place owing to neutral groundwater (in Gale Crater), surficial acid condensates, and wind, although the Martian atmosphere clearly appears to have been much thinner and colder than Earth’s, especially after about 3.5 Ga.

Recognized examples of impact sedimentation are rare to absent on Earth, except as ancient marine layers of impact spherules, or as coarse surviving breccias, and these are poor analogs for the Martian surface. Mars itself has had relatively rapid (in multi-million-year terms) wind erosion of finer-grained distal impact sediments, including most of those resulting from young rampart cratering (the ramparts represent the coarsest fractions). Also, unlike the terrestrial continental crust, the Martian crust (including sediments) is almost entirely basaltic, and the basalt is twice as Fe-rich. Mars also is rich in Cl and S, implying relatively acid and Fe-rich impact devolatilization and condensation. Fumarolic specularite flakes condensing out and accreting as Fe-rich spheroids in IDCs, as at Meridiani Planum, thus became possible.

Terrestrial PDC (volcanic) deposits have also been cited as IDC analogs, but the related volcanic (and also nuclear) blasts were orders of magnitude less energetic and compositionally wrong, plus their finer fractions have been eroded. Nevertheless, they provide interesting textural analogs in terms of cross-bedding and accretionary lapilli.

Given the above shortcomings, perhaps the best place in the Solar System to study impact sedimentation is planet Mars itself. Making interpretations based wholly on terrestrial analogs would be a serious mistake.