IMPACT-DERIVED SEDIMENTARY ROCKS: HOW COMMON ON MARS?

As on the Moon, much of the surface of Mars is covered with impact craters of various sizes and ages. Unlike on the Moon, Mars did and does have an atmosphere (albeit a dilute one), and the target areas of the impacts may have contained subsurface brines and salts in addition to ice (e.g., Knauth and Burt, 2002). The distinctive “rampart craters” of Mars have long been ascribed to the presence of an atmosphere and of volatile-rich targets. Nevertheless, the laminated sedimentary rocks of Mars (Malin and Edgett, 2000), have mainly been considered in terms of aqueous, aeolian, or volcanic deposition. We suggest that impact deposition cannot be excluded, especially given that the laminated, cross-bedded, sandy, salty sediments of the Opportunity landing site in Meridiani Planum more closely resemble distal impact surge deposits (i.e., deposits resulting from relatively low-density, ground-hugging, high-speed, heterogeneous flow), than they do any deposit produced by flowing or standing water (Burt and Knauth, 2004). Also, the uniformly shaped and sized Ni-rich hematitic spherules there more closely resemble accretionary lapilli or impact condensation spherules than they do typical concretions. If impact into a brine-, salt-, and ice-rich regolith, followed by later cold and dry weathering (including sulfide oxidation and wind deflation), produced the all of the geologic features seen in Meridiani Planum, then impact might have produced many the other finely layered, salt-encrusted sediments detected from orbit.

Of particular interest are the Br-rich rock interiors and layered deposits discovered by the Spirit rover in Gusev Crater. Given that the basalt-littered surface there appears never to have been immersed in water, heterogeneous dispersal caused by impacts into a brine-soaked basaltic regolith should also should be considered for those deposits.

By this hypothesis, the salty early hydrosphere of Mars would have dried up and frozen down (i.e., largely retreated into the regolith) BEFORE the end of major bombardment, allowing impact surge deposition of some (and conceivably all) of the laminated sediments exposed around the margins of the highlands. Concurrent and later sediment transport and deposition by explosive volcanism, brines, glaciers, and wind is not excluded.