The change of eruption styles of Martian volcanoes and estimates of the water content of the Martian mantle

Estimated water contents in the Martian mantle range from 36 ppm to more than 1%. These values are based on the chemical analyses such as hydrous minerals in SNC meteorites and formation models of Mars. This study evaluates the water content of the Martian mantle using the change with time of volcanic eruption style on Mars as an observational constraint. Styles of volcanic activity depend on the volatile content of the magma and the atmospheric pressure. Because a low atmospheric pressure leads to a more explosive volcanic eruption, it has been believed that the volcanism on the current Martian environment would be very explosive. Our calculations, however, show that, under the current Martian atmospheric conditions, erupted magma cannot entrain the ambient air effectively, so the decrease in temperature of the magma during ascent is small. Consequently, the erupted magma may form a lava-like deposit when it falls back on the ground. This effusive-like style of eruption is a counterpart of clastogenic lava on Mars. On the other hand, numerical calculations under a thick CO2 atmosphere, which may correspond to an ancient Martian atmosphere, reveal a rather explosive eruption style. Geological features of earlier stages of Martian history in the Noachian and Hesperian eras suggest that the volcanic eruptions on Mars were explosive then. Effusive eruptions, however, became dominant in more recent times. It has been widely accepted that Mars experienced a major climate change. in addition, the release factor of volatiles on Mars has been suggested to be as small as 0.017-0.112. This may imply that the volatile content has been almost constant throughout Martian history. Consequently, we assume that this change in eruption style was caused by the change in atmospheric pressure. For a given water content of magma, a major climatic change may lead to a transition in eruption style. If we know the atmospheric pressure at the time of this transition, we can calculate the possible range of the volatile content of the mantle using our numerical simulations. If the atmospheric pressure on Mars around late Hesperian era is about 1 bar, the estimated values for a typical Martian magma are 0.05-0.25 wt.%, which is within the range of the water content of typical terrestrial basaltic magmas, (C) 2000 Elsevier Science B,V. All rights reserved.