The thermal state and interior structure of Mars

©2018. American Geophysical UnionThe present‐day thermal state, interior structure, composition, and rheology of Mars can be constrained by comparing the results of thermal history calculations with geophysical, petrological, and geological observations. Using the largest‐to‐date set of 3‐D thermal evolution models, we find that a limited set of models can satisfy all available constraints simultaneously. These models require a core radius strictly larger than 1,800 km, a crust with an average thickness between 48.8 and 87.1 km containing more than half of the planet's bulk abundance of heat producing elements, and a dry mantle rheology. A strong pressure dependence of the viscosity leads to the formation of prominent mantle plumes producing melt underneath Tharsis up to the present time. Heat flow and core size estimates derived from the InSight (Interior Exploration using Seismic Investigations, Geodesy and Heat Transport) mission will increase the set of constraining data and help to confine the range of admissible models.DFG, 280637173, FOR 2440: Materie im Inneren von Planeten - Hochdruck-, Planeten- und Plasmaphysi

Plume formation in strongly temperature-dependent viscosity fluids: Application to early Mars

One of the most prominent features of Mars is the hemispherical dichotomy. The Martian surface consists of a heavily cratered elevated southern hemisphere and a resurfaced depressed northern hemisphere. The dichotomy seems to have formed very early in the history of the planet. Another interesting feature is a remnant magnetization of the crust, which suggests that early Mars had a magnetic field. Investigation of the origin of these features provides insights into the early history of Mars as well as other terrestrial planets including Earth. We develop a hypothesis that the dichotomy is caused by an early transient superplume produced by a hot Martian core. At first glance, the superplume hypothesis seems unlikely because the number of plumes in typical fluids heated from below is very large and the plumes are relatively small. However, solid rocks are rather unusual fluids whose viscosity varies with temperature by many orders of magnitude. Plume formation in such fluids is a complex and poorly understood phenomena. Thus, we begin with a systematic two-dimensional numerical and theoretical investigation of plume formation in strongly temperature-dependent viscosity fluids. Then we extend both the numerical calculations and the theory to fully three- dimensional geometry. We find the conditions under which a single transient superplume forms. One of the most important conditions is the requirement that the core was at least several hundred degrees Kelvin hotter than the mantle. Geophysical data and theoretical models of core formation suggest that this is likely to be the case. We find that the superplume can easily satisfy the timing constraints on the formation of the dichotomy. In the last part we consider the coupled core-mantle thermal evolution and investigate the cooling of the initially superheated core and the generation of the magnetic field on early Mars. We show that the core cooling is sufficiently rapid to induce convection inside the core and allow the operation of the magnetic dynamo. In our models, the magnetic field exists for millions to hundreds of millions of years after planetary formation, which is consistent with observations

Coupling SPH and thermochemical models of planets: Methodology and example of a Mars-sized body

Giant impacts have been suggested to explain various characteristics of terrestrial planets and their moons. However, so far in most models only the immediate effects of the collisions have been considered, while the long-term interior evolution of the impacted planets was not studied. Here we present a new approach, combining 3-D shock physics collision calculations with 3-D thermochemical interior evolution models. We apply the combined methods to a demonstration example of a giant impact on a Mars-sized body, using typical collisional parameters from previous studies. While the material parameters (equation of state, rheology model) used in the impact simulations can have some effect on the long-term evolution, we find that the impact angle is the most crucial parameter for the resulting spatial distribution of the newly formed crust. The results indicate that a dichotomous crustal pattern can form after a head-on collision, while this is not the case when considering a more likely grazing collision. Our results underline that end-to-end 3-D calculations of the entire process are required to study in the future the effects of large-scale impacts on the evolution of planetary interiors.Comment: 29 pages, 10 figures, accepted for publication in Icaru

Pre-mission InSights on the Interior of Mars

Abstract The Interior exploration using Seismic Investigations, Geodesy, and Heat Trans- port (InSight) Mission will focus on Mars’ interior structure and evolution. The basic structure of crust, mantle, and core form soon after accretion. Understanding the early differentiation process on Mars and how it relates to bulk composition is key to improving our understanding of this process on rocky bodies in our solar system, as well as in other solar systems. Current knowledge of differentiation derives largely from the layers observed via seismology on the Moon. However, the Moon’s much smaller diameter make it a poor analog with respect to interior pressure and phase changes. In this paper we review the current knowledge of the thickness of the crust, the diameter and state of the core, seismic attenuation, heat flow, and interior composition. InSight will conduct the first seismic and heat flow measurements of Mars, as well as more precise geodesy. These data reduce uncertainty in crustal thickness, core size and state, heat flow, seismic activity and meteorite impact rates by a factor of 3–10× relative to previous estimates. Based on modeling of seismic wave propagation, we can further constrain interior temperature, composition, and the location of phase changes. By combining heat flow and a well constrained value of crustal thickness, we can estimate the distribution of heat producing elements between the crust and mantle. All of these quantities are key inputs to models of interior convection and thermal evolution that predict the processes that control subsurface temperature, rates of volcanism, plume distribution and stability, and convective state. Collectively these factors offer strong controls on the overall evolution of the geology and habitability of Mars

On the relative importance of thermal and chemical buoyancy in regular and impact-induced melting in a Mars-like planet

We ran several series of two-dimensional numerical mantle convection simulations representing in idealized form the thermochemical evolution of a Mars-like planet. In order to study the importance of compositional buoyancy of melting mantle, the models were set up in pairs of one including all thermal and compositional contributions to buoyancy and one accounting only for the thermal contributions. In several of the model pairs, single large impacts were introduced as causes of additional strong local anomalies, and their evolution in the framework of the convecting mantle was tracked. The models confirm that the additional buoyancy provided by the depletion of the mantle by regular melting can establish a global stable stratification of the convecting mantle and throttle crust production. Furthermore, the compositional buoyancy is essential in the stabilization and preservation of local compositional anomalies directly beneath the lithosphere and offers a possible explanation for the existence of distinct, long-lived reservoirs in the martian mantle. The detection of such anomalies by geophysical means is probably difficult, however; they are expected to be detected by gravimetry rather than by seismic or heat flow measurements. The results further suggest that the crustal thickness can be locally overestimated by up to ~20 km if impact-induced density anomalies in the mantle are neglected.Comment: 29 pages, 10 figure

MEVTV Workshop on Early Tectonic and Volcanic Evolution of Mars

Although not ignored, the problems of the early tectonic and volcanic evolution of Mars have generally received less attention than those later in the evolution of the planet. Specifically, much attention was devoted to the evolution of the Tharsis region of Mars and to the planet itself at the time following the establishment of this major tectonic and volcanic province. By contrast, little attention was directed at fundamental questions, such as the conditions that led to the development of Tharsis and the cause of the basic fundamental dichotomy of the Martian crust. It was to address these and related questions of the earliest evolution of Mars that a workshop was organized under the auspices of the Mars: Evolution of Volcanism, Tectonism, and Volatiles (MEVTV) Program. Four sessions were held: crustal dichotomy; crustal differentiation/volcanism; Tharsis, Elysium, and Valles Marineris; and ridges and fault tectonics

Thermal perturbations caused by large impacts and consequences for mantle convection

We examine the effects of thermal perturbations on a convecting layer of incompressible fluid with uniform viscosity in the limit of infinite Prandtl number, for two upper boundary conditions (free- and no-slip) and heat sources (100% volumetric heating and 100% bottom heating) in 2-D Cartesian finite element simulations. Small, low-temperature perturbations are swept into nearby downflows and have almost no effect on the ambient flow field. Large, high-temperature perturbations are rapidly buoyed and flattened, and spread along the layer\u27s upper boundary as a viscous gravity current. The spreading flow severs and displaces downwellings in its path, and also thins and stabilizes the upper thermal boundary layer (TBL), preventing new instabilities from growing until the spreading motion stops. A return flow driven by the spreading current displaces the roots of plumes toward the center of the spreading region and inhibits nascent plumes in the basal TBL. When spreading halts, the flow field is reorganized as convection reinitiates. We obtain an expression for the spreading time scale, ts, in terms of the Rayleigh number and a dimensionless perturbation temperature (Θ), as well as a size (Λ), and a condition that indicates when convection is slowed at a system-wide scale. We also describe a method for calculating the heat deposited by shock waves at the increased temperatures and pressures of terrestrial mantles, and supply estimates for projectile radii in the range 200 to 900 km and vertical incident velocities in the range 7 to 20 km s−1. We also consider potential applications of this work for understanding the history of early Mars

Workshop on Hemispheres Apart

The purpose of this two-day workshop is to discuss our current understanding of the martian hemispheric dichotomy in light of new data returned by Mars Global Surveyor and Mars Odyssey and upcoming data from Mars Express.Sponsored by: Lunar and Planetary Institute, National Aeronautics and Space Administration.Conveners: Thomas R. Watters, Smithsonian Institution, Patrick J. McGovern, Lunar and Planetary Institute.PARTIAL CONTENTS: Gravity Modeling of the Isidis/Syrtis Major Region of Mars: Implications for Lithospheric Properties and for the Origin and Evolution of the Dichotomy Boundary / W. S. Kiefer--The Crustal Dichotomy as a Trigger for Edge Driven Convection: A Possible Mechanism for Tharsis Rise Volcanism? / S. D. King and H. L. Redmond--Mars and Earth: Two Dichotomies - One Cause / G. G. Kochemasov--Depth-dependent Rheology and the Wavelength of Mantle Convection with Application to Mars / A. Lenardic, M. A. Richards, F. H. Busse, and S. J. S. Morris--The Martian Reliefs Dichotomy and Planetary Axial Structural Symmetry / G. F. Malmrenko--Crustal Evolution of the Protonilus Mensae Area, Mars / G. E. McGill, S. E. Smrelmr, A. M. Dimitriou, and C. A. Raymond--Loading-induced Stresses and Topography Near the Martian Hemispheric Dichotomy Boundary / P. J. McGovern and T. R. Watters--Topographic Change of the Dichotomy Boundary Suggested by Crustal Inversion / G. A. Neumann--Tectonic Consequences of Dichotomy Modification by Lower Crustal Flow and Erosion / F. Nimmo--Glacial Modification of the Martian Crust in Aeolis Region, Mars / J. Nussbaumer

Early tectonic and volcanic evolution of Mars

Four sessions were held: crustal dichotomy; crustal differentiation/volcanism; Tharsis, Elysium, and Valles Marineris; and ridges and fault tectonics.sponsored by Lunar and Planetary Institute, NASA/MEVTV Study Projectedited by H. Frey ; sponsored by Lunar and Planetary Institute, NASA/MEVTV Study Project.Conditions on Early Mars: Constraints from the Cratering Record / Barlow, N.G. -- Origin of Fluvial Valleys and Early Geological History, Aeolis Quadrangle / Brakenridge, G.R. -- Towards a Chronology of Compressive Tectonics on Mars / Chicarro, A.F. -- Ejecta Deposits of Large Martian Impact Basins: A Useful Geologic Tool and Window to Early Martian History? / Edgett, K.S. -- Early Volcanism on Mars: An Overview / Greeley,