The dynamics of pyroclastic density currents on Mars

The products of explosive volcanism have long been observed on the surface of Mars, and their corresponding dynamics, associated with phenomenon such as dike propagation, magma fragmentation, and eruption columns under Martian conditions, have been modeled with significant success (e.g., Wilson, L., J. W. Head (1994), Mars- Review and analysis of volcanic eruption theory and relationships to observed landforms, Rev. Geophy, 32, 221-263). However, the dynamics of pyroclastic density currents (PDCs) under Martian conditions is still poorly constrained. Our increasing capability to image the surface at high resolution, both from orbit and from rovers, presents an opportunity for more rigorous deposit observations and descriptions. For example, observations and measurements from Orbiters identify what have been interpreted as extensive aprons of volcanic ash deposits in several volcanic regions, namely those surrounding several southern highland patera, which have been interpreted as the deposits of PDCs. In addition the bedded deposits identified by the Spirit rover at “Home Plate,” an outcrop within the Columbia Hills in Gusev Crater, have been interpreted by many as the deposits of dilute pyroclastic density currents. This demonstrates that the need to understand the role of the Martian atmosphere on flow dynamics and depositional processes is much more important and relevant than it has been in the past. We have developed a quantitative, axi-symmetric model for flow of and sedimentation from a steady-state, vertically uniform dilute density current for application to PDCs on Earth and Mars (following Bursik, M. I., A. W. Woods, 1996, The dynamics and thermodynamics of large ash flows, Bull Volcan, 58, 175-193). The conservation of mass, momentum, and energy are solved simultaneously, and include the effects of atmospheric entrainment, particle sedimentation, basal friction, temperature changes, and variations in current thickness and density. For a given set of identical initial conditions, our models show that PDCs on Mars will out distance those on Earth by approximately 33%, primarily due to 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. In addition, we find that when sedimentation of particles and entrainment of atmosphere are included, the runout distance becomes six times shorter than previous model results suggest (e.g., Crown, D. A., R. Greeley (1993), Volcanic geology of Hadriaca Patera and the eastern Hellas region of Mars, J. Geophys. Res., 98, 3431-3451). Additionally, we calculate the Rouse number and Brunt-Väisäla frequency to estimate the wavelength of internal gravity waves in the density stratified currents, which are thought be the primary control on deposit bedform wavelength and amplitude (Valentine, 1987). The model predicts realistic wavelengths on Earth (dunes from 20-200 m), whereas longer wavelengths are predicted on Mars. This difference likely reflects the fact that lower particle settling velocities on Mars result in density stratification over a greater vertical extent, and thus longer-wavelength standing waves.