ABSTRACT Layered deposits on Mars imaged by the three rovers are generally inferred to have been deposited by liquid water (or wind or volcanism), consistent with interpretations based on orbital imaging. This interpretation implies early Mars was warm and wet, despite long-standing problems with modeling this case. As an alternative hypothesis, rapid sediment deposition during Late Noachian impact bombardment followed by local hydration and alteration of sediment by surficial acid condensates and (at least in Gale Crater) by chemically neutral groundwater can explain all the observed sediment features, such as ubiquitous low-angle cross-bedding, primitive basaltic compositions, persistent acidic salts, abundant amorphous materials, immature clays, high friability with low bulk densities, planar scoured unconformities, and rounded cobbles from rock tumbling. In other words, the ground-observed mineralogy, geochemistry, and sedimentology do not require and even are inconsistent with deposition from liquid water. Unlike the Moon, early Mars is believed to have had an atmosphere and water, perhaps mostly frozen. If so, impacts should have formed turbulent ground-hugging impactoclastic density currents capable of traveling hundreds of kilometers, and even globally. As terrestrial analogs, smaller-scale density currents are widespread around explosive volcanoes and nuclear test sites, whereas terrestrial impact analogs are lacking. Steam condensation on particles causes accretionary lapilli to form, grow to a maximum size, and fall out on layered deposits, and similar spherules have been observed by two of three rovers. Explaining these spherules as normal sedimentary concretions at Meridiani Planum required ignoring some of the observations. Ancient sediments on Mars that superficially resemble terrestrial aqueous deposits could therefore actually have resulted from impact cratering, the dominant geologic process in the early solar system.

Sulfate-rich mineral deposits have been discovered in many locations on Mars through observations by orbiters, landers, and roving spacecraft. It appears that in most cases, these minerals are produced by acid-sulfate weathering of igneous rocks, which may have been a widespread process for the first billion years on Mars. The origin of life on Earth may have occurred in iron-sulfur hydrothermal settings, and early Mars likely had similar environmental conditions. An excellent terrestrial analog for acid-sulfate weathering of Mars-like basalts exists at Cerro Negro, Nicaragua, where acidic sulfur-bearing gases interact with recently erupted basaltic ash in numerous active fumaroles. We investigated the chemistry and mineralogy of the pristine basalts and their chemically weathered products, and we studied the associated microbiological communities as an analog for potential early life on Mars. Measured pH values of condensed volcanic vapors range from −1 to 5, and near-surface temperatures in the fumaroles range from 40 to 400 °C. In a few years, fresh basalt can weather to amorphous silica and gypsum, along with lesser amounts of other sulfates (natroalunite and jarosite), Fe-hydroxides, and clays. Altered rocks have up to 35 wt% SO 3 equivalent, similar to the amounts of sulfur reported for Meridiani Planum bedrocks and inferred in other sulfate-bearing bedrock on Mars. Heavily weathered rocks have silica contents up to 80 wt%, similar to silica-rich soils at Gusev crater that possibly formed in hydrothermal environments. Here, we provide preliminary results of our studies and outline the logistics for a field excursion to this Mars analog system.