THE DENSITY OF THE MEDUSAE FOSSAE FORMATION: IMPLICATIONS FOR ITS COMPOSITION, ORIGIN, AND IMPORTANCE IN MARTIAN HISTORY

The Medusae Fossae Formation (MFF) is one of the largest sedimentary deposits on Mars. The origin of the MFF is unknown though several processes including volcanic, aeolian and ice-related mechanisms have been proposed in its formation. Here, we localize the gravity and topography signature of the MFF and place the first direct constraint on its density (1500-1800 kg m^-3). When combined with previous radar measurements, our density constraint rules out the presence of ice as the cause of the formation’s unusual radar permittivity. Based on the enrichment of volatile elements (Cl and S) from new global elemental maps, along with its relatively low density, we propose that the MFF was deposited by pyroclastic eruptions. The mass of the MFF is found to be two orders of magnitude greater than the largest terrestrial pyroclastic deposit, making it the largest known pyroclastic deposit in the solar system. Outgassing of volatiles such as CO₂ and H₂O from the MFF would have substantially contributed to the Martian atmosphere and hydrosphere. Additionally, based on the S/Cl ratio and estimated mass of eroded material, we suggest the MFF has been a primary contributor to the global dust inventory of Mars.