Numerical modeling of coarse-grained ash dispersal in current and ancient atmospheres on Mars: Implications for the origin of the Nili Fossae olivine‑carbonate unit

Mafic explosive volcanism is inferred to have been widespread on early Mars. The Nili Fossae olivine- and carbonate-bearing unit is a Noachian-aged series of mafic/ultramafic rocks with a grain size of 0.5–1 mm, for which a pyroclastic origin has been proposed. To investigate the plausibility of this origin, we developed a comprehensive theoretical model for the settling of ash from Mars eruption plumes, incorporating particle size, density, and shape parameters based on terrestrial analog datasets, and applying shape-dependent drag formulae. This model also incorporates evolving ambient atmospheric properties, such as density, temperature, viscosity, and wind, to simulate particle descent. We examined the influence of particle parameters as well as a range of plausible past and present atmospheres, and applied the model results in the context of the regional olivine‑carbonate unit. The results show that the adoption of an appropriate inverse particle size–density relationship has a significant effect on predicted dispersal distance, such that ash was likely less widely dispersed than previously inferred for both current and past atmospheric conditions on Mars. For the Nili Fossae olivine‑carbonate unit to be a mafic/ultramafic pyroclastic fall deposit, we infer that multiple sources, as well as potentially a higher-pressure atmosphere and significant plume heights, would be required to account for the unit's spatial extent. This implies that a primary pyroclastic fall origin for the majority of the Nili Fossae olivine‑carbonate unit is unlikely.