SEDIMENTS ON MARS DEPOSITED BY IMPACTS INSTEAD OF BY LAKES, RIVERS, AND OCEANS

Observations of ancient layered deposits on Mars by 3 surface rovers have mainly been interpreted in terms of deposition by liquid water (or wind), consistent with initial interpretations based on orbital imaging. This implies early Mars was comparatively warm and wet despite severe problems with modeling such a case. Indeed, some cratered terranes are lightly etched with presumed tributary networks, small sediment lobes appear at some canyon mouths, and surficial debris and dust can be slightly hydrous. However, there is relatively little erosional degradation of even old cratered terranes and the observed mineralogy, geochemistry, and sedimentology do not require (and even are inconsistent with) deposition from liquid water despite published claims.

Unlike the Moon, early Mars retained an atmosphere and unknown amounts of water, probably including subsurface ice and brines. Impacts on early Mars would therefore cause turbulent multiphase ground-hugging density currents capable of traveling hundreds to thousands of km and even globally. Smaller scale density currents (“base surges”) are widespread around terrestrial explosive volcanoes and nuclear test sites and are known to deposit the low-angle crossbeds observed by the 3 rovers. Hailstone-like accretionary lapilli grow to a maximum size and fall out into the layered deposits, and such have been observed in abundance by 2 of 3 rovers. Explaining these spherules as normal concretions at Meridiani Planum required possibly the most complex diagenetic scenario ever suggested for any sedimentary rock.

Rapid sediment deposition during Noachian impact bombardment followed by local hydration and alteration of sediment by surficial acid condensates and (especially in Gale Crater) by late neutral groundwater can explain all the observed features such as primitive basaltic compositions, persistent acidic salts, abundant amorphous materials, immature clays, low bulk densities, planar scoured unconformities, rounded cobbles from rock tumbling, and even lobes at canyon mouths where density currents emerge, fan out, and stop. We thus propose the outrageous hypothesis that many ancient sediments on Mars that superficially resemble terrestrial aqueous deposits could actually have resulted from a far more common geologic process in the early solar system.