COMPARING THE DYNAMICS OF DILUTE PYROCLASTIC DENSITY CURRENTS ON EARTH AND MARS

While the products of explosive volcanism have long been identified on the surface of Mars, the dynamics of Martian pyroclastic density currents (PDCs) remain poorly understood. Recent numerical models show that PDCs formed by Martian eruptions are capable of producing pyroclastic flows that reach up to 600 km from source, consistent with observations of ash deposits around many highland Patera (e.g., Greeley and Crown, 1990, J Geophys Res 95 7133-7149). However, these models neglect processes such as entrainment, sedimentation and thermal evolution of the currents, which contribute to changes in bulk density. We’ve developed an axi-symmetric model for flow of and sedimentation from a steady-state, vertically uniform density current for application to PDCs on Earth and Mars (following Bursik and Woods, 1996, Bull Volcan 58 175-193). The conservation of mass, momentum, and energy are solved simultaneously, and the effects of atmospheric entrainment, particle sedimentation, basal friction, temperature changes, and variations in current thickness and density are explored. Employing the Mars conditions proposed in Crown and Greeley (1993), and neglecting entrainment and sedimentation, we too obtain run out distances up to 600 km. However, introducing sedimentation of 0.1 mm particles slows the current more quickly, and adding the effects of entrainment further slows the current, resulting in much shorter run out distances (<100 km). For a given set of identical initial conditions, our models show that PDCs on Mars will out distance Earth PDCs by approximately 33%, reflecting slower sedimentation rates. Although this general conclusion is consistent with previous studies, the difference between the Earth and Mars cases is much less than previously published. Additionally, we incorporate the Rouse number and Brunt-Väisäla frequency to estimate the wavelength of internal gravity waves in a density stratified current, which are thought to be the primary control on bedform characteristics (Valentine, 1987, Bull Volc 49 616-630). The model predicts realistic wavelengths on Earth (dunes from 20 - 200 m), whereas longer wavelengths are predicted on Mars (50 - 300 m). This difference probably reflects the fact that lower particle settling velocities on Mars result in density stratification over a greater vertical extent.