MODELING THE FORMATION OF HYDROVOLCANIC CONES ON MARS

New data from the Mars confirms the existence of hydrovolcanic constructs atop some young lava flows in the equatorial lowlands. Quantitative modeling of the formation of these features could fundamentally improve our understanding of the history of water on Mars. While many of these features are similar to terrestrial rootless cones in morphology and spatial distributions [1,2], they also exhibit some alien characteristics. These include moats, wakes, chains, smaller mounds, and larger low-relief rings. The chains, wakes, and moats can all be attributed to the Martian features forming atop a thin, translating crust on a voluminous sheet flow [3]. Earlier modeling efforts have shown that if steam explodes through a lava flow, it will throw material the appropriate distance to form the observed structures [1]. We focus on understanding the conditions that allow sufficient stream pressure to accumulate to cause an explosion. On Earth, hydrovolcanic explosions require the intimate mixing of liquid lava and water, forming "fuel-coolant interactions" that generate steam in a very rapid "run-away" process. Such a process is challenging on Mars because neither liquid water nor ice are currently stable at the surface at these latitudes. It is speculated that the lower atmospheric pressure and gravity on Mars allows less vigorous steam generation to produce explosions - and thus the water may have been frozen and separated from the hot lava by a substantial zone of desiccated substrate. We are using numerical models [4,5] to test this possibility and other hypotheses born of the new observations. Modeling hydrovolcanic processes is very challenging due to the extreme range of temperatures, the range of phases (and phase changes), the interaction with the atmosphere, the importance of both conductive and advective heat transfer, the geometry of the lava-ground-water interface, and the changing overburden pressure as the sheet flow deflated.

Greeley, R. and S. Fagents (2001), JGR 106: 20527-20546; Bruno, B. et al. (2006) JGR 108: 2004JE002273; Jaeger, W. et al. (2007) Science, in press; Keszthelyi, L. and R. Denlinger (1996) Bull. Volcanol., 58: 5-18; Ingebritsen, S. and D. Hayba (1994) USGS WRIR 94-4045.