The Dispersal of Pyroclasts in the Martian Atmosphere

Introduction: The Medusae Fossae Formation (MFF) is an enigmatic and discontinuous formation located in the southern parts of Elysium Planitia and Amazonis Planitia (130°-230°E and 12°S-12°N), covering an area of approximately 2.1 x 10 km and having an estimated volume of 1.4 x 10 km [1]. It is thought to have been deposited during the Amazonian period [2,3]. However, much of the cratering record may have been erased as friable units were eroded and long-buried terrains exhumed [4-6]. The formation is characterized by large accumulations of fine-grained, friable deposits and evidence of large amounts of erosion. There are many theories regarding the emplacement of this formation; recently the literature has focused on three possibilities: ignimbrites [2], ash fall [2,7,8] (perhaps from Tharsis Montes [7]), and aeolian dust [2,8]. If the formation is made of ash, its volume would suggest a large amount of pyroclastic activity in the past, which would have a significant effect on the atmosphere of Mars. Volcanic ash particles can effectively scatter incoming solar radiation and reduce the overall temperature of the planet. However, volcanoes which emit large amounts of greenhouse gases may warm the planet with time. Examining large deposits of possible explosive volcanic material may lead to better constraints on how volcanoes interacted with the atmosphere over time. In order to test the hypothesis that the MFF is made of ash, we combined a Mars Global Circulation Model (GCM) [9] with a semi-analytical explosive eruption model for Mars [10]. The explosive model determines the rise-height of the eruption column and the release heights for volcanic clasts of various sizes while the GCM provides time-dependent wind profiles for calculating the range of each particle. The medium-sized Hesperian-aged volcano Apollinaris Patera (-8oS, 174oE) is located in a unique position close to the global dichotomy boundary, as well as near the Mars Exploration Rover (MER) Spirit landing site in Gusev Crater, where tephra deposits may be exposed in the Columbia Hills [11]. Given its proximity to the Medusae Fossae Formation, and the unique opportunity to compare data taken from orbit with information from the surface, we decided to test the likelihood that this volcano was the source for the deposit.