[Previous] | [Session 36] | [Next]
A. P. Zent (NASA Ames Research Center)
H2O was supplied to the Noachian atmosphere by eruptions, or in association with large impacts. Most water outgassed into an extremely cold atmosphere, and condensate deposits were inevitable. High heat flow could lead to subglacial melting only if ice thicknesses were greater than 500 -1000m., which is extremely unlikely. Subareal melting and flow is contingent upon temperatures periodically exceeding 273 K, and retarding evaporative loss of the flow.
In still air, evaporation into a dry atmosphere is in the free convection regime, and a stream with 2 m3s-1 discharge, flowing 1 m s-1 could persist for hundreds of days and cover distances greater than any valley reach. The zero-wind-shear condition is considered implausible however.
We investigate the possibility that evaporation rates were suppressed because the atmosphere was regionally charged with H2O as it moved over snow/ice fields. Our initial concern is precipitation from volcanic plumes. A Kilauea-style eruption on the martian surface would cover a 10km circular deposit with 10cm of H2O, if all H2O could be precipitated near the vent. The characteristics of the eruption at the vent, (vent size, temperature, H2O content, etc.) are independent of the environmental conditions. The subsequent behavior of the plume, including precipitation of ash and H2O condensate depends strongly on the environment. Hence, the proximal fate of volcanic H2O is amenable to treatment in a model.
A simple bulk thermodynamic model of the rise of an H2O plume through a stably stratified CO2 atmosphere, with only adiabatic cooling, produces runaway plume rise. A more complex treatment includes the effects of latent heat release, wind shear along the plume, divergence of ash and H2O, and will yield more realistic estimates of H2O transport in eruptive plumes. Results of these simulations will be presented.