The Density of the Medusae Fossae Formation: Implications for its Composition, Origin, and Importance in Martian History

Abstract

The Medusae Fossae Formation (MFF) is one of the largest sedimentary deposits on Mars. The origin of the MFF is uncertain, though several processes including volcanic, eolian, 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. We find that the MFF is a relatively porous unit with a bulk density of 1,765 ± 105 kg m−3. When combined with previous radar measurements, our density constraint rules out the presence of ice as the cause of unusual radar permittivity. Rather, we find the joint radar and gravity constraints imply a dry and highly porous rock unit. Based on the relatively low density, lack of ice, and the previously known enrichment of volatile elements associated with volcanic emissions (Cl and S), we propose that the MFF was deposited by pyroclastic eruptions. Using our density estimate, the mass of the MFF is found to be 2 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.