Water in the Martian interior—The geodynamical perspective

Abstract

Petrological analysis of the Martian meteorites suggests that rheologically significant amounts of water are present in the Martian mantle. A bulk mantle water content of at least a few tens of ppm is thus expected to be present despite the potentially efficient degassing during accretion, magma ocean solidification, and subsequent volcanism. We examine the dynamical consequences of different thermochemical evolution scenarios testing whether they can lead to the formation and preservation of mantle reservoirs, and compare model predictions with available data. First, the simplest scenario of a homogenous mantle that emerges when ignoring density changes caused by the extraction of partial melt is found to be inconsistent with the isotopic evidence for distinct reservoirs provided by the analysis of the Martian meteorites. In a second scenario, reservoirs can form as a result of partial melting that induces a density change in the depleted mantle with respect to its primordial composition. However, efficient mantle mixing prevents these reservoirs from being preserved until present unless they are located in the stagnant lid. Finally, reservoirs could be formed during fractional crystallization of a magma ocean. In this case, however, the mantle would likely end up being stably stratified as a result of the global overturn expected to accompany the fractional crystallization. Depending on the assumed density contrast, little secondary crust would be produced and the lithosphere would be extremely cool and dry, in contrast to observations. In summary, it is very challenging to obtain a self-consistent evolution scenario that satisfies all available constraints. 1960D. Breuer et al.

Hemispheric Dichotomy in Lithosphere Thickness on Mars Caused by Differences in Crustal Structure and Composition

“…Considering a dry mantle and dry crustal rheology also provides a good fit to the three T e constraints. By using crustal models similar to our UCM case, Breuer et al (2016) and Grott and Breuer (2008) found that a dry mantle and dry crustal rheology is not in agreement with the Noachian low T e . Indeed, a dry crust is stronger and the thinner incompetent bottom crustal layer disappears earlier both in the north and in the south in Mars evolution.…”

The Gravitational Signature of Martian Volcanoes

“…This can be regarded as an addition of dense materials, such as magmatic intrusions in the crust. For L <0, a negative density contrast is defined to reside in the upper mantle at 150‐km depth, where the largest amount of melting is expected (Breuer et al, ). This can represent either a positive temperature anomaly or a depleted layer resulting from the prior extraction of magma in the upper mantle.…”

Hydrogenation of the Martian Core by Hydrated Mantle Minerals With Implications for the Early Dynamo

“…Modelers often assume equilibrium crystallization of the magma ocean after accretion so whole-mantle convection begins immediately after solidification (Breuer et al, 2016). In contrast, fractional crystallization of the magma ocean would produce a gravitationally unstable configuration that overturns in <100 Myr via Rayleigh-Taylor instability (Elkins-Tanton, 2008;Elkins-Tanton et al, 2003, 2005.…”

“…Hydrous melting promotes efficient degassing of water from the upper mantle as proto-Mars grows, but the lower mantle may remain unmelted (Médard & Grove, 2006;Pommier et al, 2012). The water content of the upper mantle is~200 ppm according to measurements of water in apatite inclusions from basaltic shergottites (Breuer et al, 2016;Taylor, 2013). The saturation limit of olivine is only a few times higher at~1,000 ppm.…”