Onset of solid-state mantle convection and mixing during magma ocean solidification

©2017. American Geophysical UnionThe energy sources involved in the early stages of the formation of terrestrial bodies can induce partial or even complete melting of the mantle, leading to the emergence of magma oceans. The fractional crystallization of a magma ocean can cause the formation of a compositional layering that can play a fundamental role for the subsequent long‐term dynamics of the interior and for the evolution of geochemical reservoirs. In order to assess to what extent primordial compositional heterogeneities generated by magma ocean solidification can be preserved, we investigate the solidification of a whole‐mantle Martian magma ocean, and in particular the conditions that allow solid‐state convection to start mixing the mantle before solidification is completed. To this end, we performed 2‐D numerical simulations in a cylindrical geometry. We treat the liquid magma ocean in a parameterized way while we self‐consistently solve the conservation equations of thermochemical convection in the growing solid cumulates accounting for pressure‐, temperature‐, and, where it applies, melt‐dependent viscosity. By testing the effects of different cooling rates and convective vigor, we show that for a lifetime of the liquid magma ocean of 1 Myr or longer, the onset of solid‐state convection prior to complete mantle crystallization is likely and that a significant part of the compositional heterogeneities generated by fractionation can be erased by efficient mantle mixing. We discuss the consequences of our findings in relation to the formation and evolution of compositional reservoirs on Mars and on the other terrestrial bodies of the solar system.DFG, 276817549, Kristallisation des irdischen Magmaozeans: Thermo- und Geodynami

Delta Deposits on Mars: A Global Perspective

Deltas have been long considered a constraining element to reconstruct the water level of an ancient ocean that may have once occupied the northern lowlands of Mars, and just recently this hypothesis started to be challenged. We investigate this hypothesis and present a global inventory of fan shaped features showing typical deltaic traits across the entire Martian surface. For each element we provide descriptive details and classifications based on morphology, location, and relation with characterizing environmental features. In this catalogue of 162 deltas, we identified only six having high potential to constrain an oceanic paleo-shoreline. Nonetheless, age and location of these candidates display discrepancies with what was previously suggested from independent datasets about shoreline age and locations. Our analyses hence indicate that deltas alone are insufficient to delineate a globally consistent ancient oceanic shoreline, but they have the potential to locally constrain the water level both in space and time

The two parameterisations of the Andrade rheological model in planetary science: a comparative study

We discuss two parameterisations of the popular Andrade rheological model that appear in planetological literature and illustrate how different assumptions affect the estimates of tidal dissipation and Love numbers

The Temperature and Composition of the Mantle Sources of Martian Basalts

The composition of basaltic melts in equilibrium with the mantle can be determined for several Martian meteorites and in-situ rover analyses. We use the melting model MAGMARS to reproduce these primary melts and estimate the bulk composition and temperature of the mantle regions from which they originated. We find that most mantle sources are depleted in CaO and Al2O3 relative to models of the bulk silicate Mars and likely represent melting residues or magma ocean cumulates. The concentrations of Na2O, K2O, P2O5, and TiO2 are variable and often less depleted, pointing to the re-fertilization of the sources by fluids and low-degree melts, or the incorporation of residual trapped melts during the crystallization of the magma ocean. The mantle potential temperatures of the sources are 1400–1500°C, regardless of the time at which they melted and within the range of the most recent predictions from thermochemical evolution models

Estimation of the Seismic Moment Release Rate of Mars from InSight Seismic Data

Seismicity models for Mars usually estimate the long-term average annual seismic moment rate, and also the average annual event rate. This holds for estimations based on geological evidence (Golombek et al., 1992, Golombek, 2002, Taylor et al., 2013) as well as for models based on thermal evolution and cooling of the Martian interior (Phillips, 1991, Knapmeyer et al., 2006, Plesa et al., 2018). All studies are compatible with the conclusion based on the non-observation of any unambiguous event by Viking (Anderson et al., 1977, Goins & Lazarewicz, 1979) that Martian seismicity lies somewhere between that of the Moon and that of the Earth. We developed tools to derive reasonable estimations of the annual seismic moment rate from a number of events as small as one, provided that the observed events are beyond the global completeness threshold for observable events. Numerical tests as well as evaluation of terrestrial data shows the feasibility of the approach

The long-term evolution of the atmosphere of Venus: processes and feedback mechanisms

In this chapter, we focus on the long-term evolution of the atmosphere of Venus, and how it has been affected by interior/exterior cycles. The formation and evolution of Venus's atmosphere, leading to the present-day surface conditions, remain hotly debated and involve questions that tie into many disciplines. Here, we explore the mechanisms that shaped the evolution of the atmosphere, starting with the volatile sources and sinks. Going from the deep interior to the top of the atmosphere, we describe fundamental processes such as volcanic outgassing, surface-atmosphere interactions, and atmosphere escape. Furthermore, we address more complex aspects of the history of Venus, including the role of meteoritic impacts, how magnetic field generation is tied into long-term evolution, and the implications of feedback cycles for atmospheric evolution. Finally, we highlight three plausible end-member evolutionary pathways that Venus might have followed, from the accretion to its present-day state, based on current modeling and observations. In a first scenario, the planet was desiccated early-on, during the magma ocean phase, by atmospheric escape. In a second scenario, Venus could have harbored surface liquid water for long periods of time, until its temperate climate was destabilized and it entered a runaway greenhouse phase. In a third scenario, Venus's inefficient outgassing could have kept water inside the planet, where hydrogen was trapped in the core and the mantle was oxidized. We discuss existing evidence and future observations/missions needed to refine our understanding of the planet's history and of the complex feedback cycles between the interior, surface, and atmosphere that operate in the past, present or future of Venus

The Long-Term Evolution of the Atmosphere of Venus: Processes and Feedback Mechanisms: Interior-Exterior Exchanges

This work reviews the long-term evolution of the atmosphere of Venus, and modulation of its composition by interior/exterior cycling. The formation and evolution of Venus’s atmosphere, leading to contemporary surface conditions, remain hotly debated topics, and involve questions that tie into many disciplines. We explore these various inter-related mechanisms which shaped the evolution of the atmosphere, starting with the volatile sources and sinks. Going from the deep interior to the top of the atmosphere, we describe volcanic outgassing, surface-atmosphere interactions, and atmosphere escape. Furthermore, we address more complex aspects of the history of Venus, including the role of Late Accretion impacts, how magnetic field generation is tied into long-term evolution, and the implications of geochemical and geodynamical feedback cycles for atmospheric evolution. We highlight plausible end-member evolutionary pathways that Venus could have followed, from accretion to its present-day state, based on modeling and observations. In a first scenario, the planet was desiccated by atmospheric escape during the magma ocean phase. In a second scenario, Venus could have harbored surface liquid water for long periods of time, until its temperate climate was destabilized and it entered a runaway greenhouse phase. In a third scenario, Venus’s inefficient outgassing could have kept water inside the planet, where hydrogen was trapped in the core and the mantle was oxidized. We discuss existing evidence and future observations/missions required to refine our understanding of the planet’s history and of the complex feedback cycles between the interior, surface, and atmosphere that have been operating in the past, present or future of Venus