Volatile Losses from Lava Flows Revisited: Implications for Large Sheet Flows on Mars

Introduction: For many years, the nature of emplacement of large ‘flat-topped’ sheet flows on Mars has puzzled planetary scientists. Large terrestrial basaltic a’a flows may extend for tens of km and tend to exhibit significant viscosity increases (up to several orders of magnitude) along the path of the flow due to incipient cooling and crystallization. The change in viscosity usually causes a thickening of the flow with distance and this relationship often serves as a basis for developing inferences about Mars flow emplacement and comparisons with terrestrial analogs. There are many flows on Mars, particularly on the plains, that extend for hundreds of km with modest or non-existent thickening with distance in contrast to highly characterized terrestrial flows like those at Mauna Loa, Hawaii, or Mt Etna, Italy. One possible explanation for the flat-topped sheet flows on Mars is a style of emplacement we do not see on the Earth. Specifically, a balance exists between the formation of the cooler crust and the transfer of the cooler component to embanking levees, stationary margins, and overspills [1, 2]. Another possibility that deserves consideration is a loss of volatiles that masks an increase in viscosity. The importance of volatile losses in determining the final dimensions of a lava flow has been recognized for many years [3, 4, 5]. In recent years, there have been field studies documenting how the loss of volatiles affects the density of the lava between the source of the flow and its terminus [e.g., 6, 7]. The data available suggests that density may change by a factor of 2 or greater. Recently, we proposed an elementary quantitative model to investigate how longitudinal thickness profiles would be altered for different types of volumetric flow rates, (i.e., Newtonian and basal glide) and different types of viscosity changes with distance [8]. The intent of the model was simply to assess the importance of degassing on the dimensions, morphology, and advance rate of lava flows. We noted that actual degassing rates are unknown and used two different ad hoc forms for the loss rate from the surface of the flow during emplacement. Because this process could be an important factor in explaining the dimensions of the sheet flows ubiquitous on the Mars plains, we have begun to develop a more rigorous formalism for the influence of volatile losses on the thickness and density profiles. Governing Equations: We consider a vertical columnar control volume in the advancing lava flow to consist of lava and a volatile component that migrates to the surface and escapes. We assume that the lava component of the flow is conserved during transit. That is, there are no losses to stationary margins or levees and there is no significant assimilation of preexisting solids along the path of the flow. Here we also take the width of the flow and the underlying slope to be constant. Thus the generic local governing equation for the lava is