Ancient volcanism may have influenced patterns of hydrated regolith on Mars

The hydration of the martian regolith at decimeter depth scales varies regionally, suggestive of geologically sustained processes, independent of diurnal soil-atmosphere exchange. A combination of factors, including residual hydration from ancient processes in the hydrosphere/cryosphere, seasonal exchange between the crust and atmosphere in the martian critical zone, or structural water in minerals or salts sourced via hydrothermal processes can contribute to such lateral changes in bulk regolith hydration. Here, we consider the theoretical extent of regolith hydration from explosive volcanic eruptions and subsequent hydration. While magmatic degassing has previously been considered as a contributing factor to regolith hydration, the relationship between hydrated regolith and the depositional footprint of volcanic eruption columns and associated atmospheric dispersal remains poorly known. We run simulations of eruptions in an ancient atmosphere over a martian year to account for seasonality and produce theoretical ash deposition maps which are then input into a geographic weighted regression model to predict hydrated regolith. We find that ash deposited via explosive volcanism can help explain certain patterns of hydration enrichment in the martian regolith, including those near Apollinaris Patera Arabia Terra, and Sabea Terra. While the global patterns of regolith hydration cannot be explained by one process alone, we found that hydrated ash likely contributed to hydrated regolith at a commensurable level to other leading hydration mechanisms. Plain language summary: The surface of Mars is blanketed by a pervasive layer of regolith, a mixture of unconsolidated sediment of multiple grainsizes that range from very fine dust to larger pebbles. This regolith contains significant amounts of water that are stored in the chemical bonds that make up the individual regolith grains. The amount of water bound in the regolith is spatially inconsistent throughout the planet; some areas contain more water, while others contain less. Several different geologic processes have likely contributed to these inconsistencies, including volcanic processes. While gentle volcanic outgassing has already been considered, here we specifically looked at how explosive volcanism and the associated spreading of volcanic ash in the martian atmosphere may have contributed to hydrated regolith patterns. By using multiple numerical simulations, we found that explosive volcanism likely contributed to certain patterns of hydration in the regolith along with other geologic processes.