Numerical modelling of ejecta dispersal from transient volcanic explosions on Mars.

Fagents, Sarah A. and Wilson, Lionel (1996) Numerical modelling of ejecta dispersal from transient volcanic explosions on Mars. Icarus, 123 (2). pp. 284-295. ISSN 0019-1035

Abstract

The dynamics of ejecta dispersal in transient volcanic eruptions on Mars are distinct from those on Earth and Venus because of the low atmospheric pressure and gravitational acceleration. Numerical modeling of the physical mechanisms of such activity, accounting for the different martian environmental conditions, can help constrain the style of emplacement of the eruptive products. The scenario envisaged is one of pressurized gas, contributed from either a magmatic or meteoric source, accumulating in the near-surface crust beneath a retaining medium. On failure of the confining material, the gas expands rapidly out of the vent, displacing both the “caprock” and a mass of atmospheric gas overlying the explosion site, in a discrete, transient event. Trajectories of large blocks of ejecta are computed subject to the complex aerodynamic interactions of atmospheric and volcanic gases which are set in motion by the initiation of the explosion. Reservoirs of crustal and surface water and carbon dioxide may have increased the chances of occurrence of transient explosive events on Mars in two ways: by supplying a source of volatiles for vaporization by the magma and by acting to slow the ascent of the magma by chilling it, providing conditions favorable for gas accumulation. Results of the modeling indicate that ejection velocities ranging up to 580 m sec−1were possible in martian H2O-driven explosions, with CO2-driven velocities typically a factor of 1.5 smaller. Travel distances of large blocks of ejecta lie within the range of a few kilometers to the order of 100 km from the vent. The low martian atmospheric pressure and gravity would thus have conspired to produce more vigorous explosions and more widely dispersed deposits than are associated with analogous events on Earth or Venus. Other phenomena likely to be associated with transient explosions include ashfall deposits from associated convecting clouds of fine material, pyroclastic flows, and ejecta impact crater fields. It is anticipated that the martian environment would have caused such features to be greater in size than would be the case in the terrestrial environment. Ash clouds associated with discrete explosions are expected to have risen to a maximum of 25 km on Mars, producing deposits having similar widths. Another indication of a volcanic explosion site might be found in areas of high regolith ice content, such as fretted terrains, where ice removal and mass-wasting may have modified the vent's initial morphology. The modeling results highlight the implications of the occurrence of transient explosive eruptions for the global crustal volatile distribution and provide some predictions of the likely manifestation of such activity for testing by upcoming spacecraft missions to Mars.