Iron Snow in the Martian Core?

Iron snow in the Martian Core? Christopher J. Davies1,2,* and Anne Pommier2 1School of Earth & Environment, University of Leeds, Leeds LS2 9JT, UK. 2Institute of Geophysics and Planetary Physics, Scripps Institution of Oceanography, University of California at San Diego, 9500 Gilman Drive, La Jolla, CA 92093-0225, USA. *Correspondence to: c.davies@leeds.ac.uk. Abstract The decline of Mars’ global magnetic field some 3.8-4.1 billion years ago is thought to reflect the demise of the dynamo that operated in its liquid core. The dynamo was probably powered by planetary cooling and so its termination is intimately tied to the thermochemical evolution and present-day physical state of the Martian core. Bottom-up growth of a solid inner core, the crystallization regime for Earth’s core, has been found to produce a long-lived dynamo leading to the suggestion that the Martian core remains entirely liquid to this day. Motivated by the experimentally-determined increase in the Fe-S liquidus temperature with decreasing pressure at Martian core conditions, we investigate whether Mars’ core could crystallize from the top down. We focus on the “iron snow” regime, where newly-formed solid consists of pure Fe and is therefore heavier than the liquid. We derive global energy and entropy equations that describe the long-timescale thermal and magnetic history of the core from a general theory for two-phase, two-component liquid mixtures, assuming that the snow zone is in phase equilibrium and that all solid falls out of the layer and remelts at each timestep. Formation of snow zones occurs for a wide range of interior and thermal properties and depends critically on the initial sulfur concentration, 𝜉0. Release of gravitational energy and latent heat during growth of the snow zone do not generate sufficient entropy to restart the dynamo unless the snow zone occupies at least 400 km of the core. Snow zones can be 1.5 −2 Gyrs old, though thermal stratification of the uppermost core, not included in our model, likely delays onset. Models that match the available magnetic and geodetic constraints have 𝜉0 ≈10% and snow zones that occupy approximately the top 100 km of the present-day Martian core. Keywords: Mars core, dynamo action, iron snow 1. Introduction Low-altitude vector magnetometer measurements from Mars Global Surveyor show that Mars presently lacks a global dipole field, but reveal large regions of strongly magnetized crust located mainly in the southern highlands (Acuña et al. 1998). The prevailing view is that this magnetization was acquired as the rock cooled in the presence of a global magnetic field (Stevenson, 2001; Breuer and Moore, 2015). The global field was likely produced in the liquid core by a dynamo process in which thermal (and possibly chemical) buoyancy forces drive convective motion (Stevenson, 2001). Inferences based on the age of impact craters (Acuña et al., 1998; Langlais et al., 2012) and Martian meteorites (Weiss et al., 2002) suggest that the global field decayed around 3.8-4.1 Ga. This event marks the demise of the Martian dynamo and may have been contemporaneous with changes in the planets’ heat loss (Ruiz, 2014) and oxidation state (Tuff et al., 2013). Explanations of Mars’ magnetic history are intimately linked to the thermal evolution and crystallization regime of its metallic core. A thermal dynamo can operate in an entirely liquid core, provided that the ancient core-mantle boundary (CMB) heat flow 𝑄𝑐𝑚𝑏 exceeded the heat 𝑄𝑎 lost by conduction down the adiabatic temperature gradient (assuming no radiogenic heating). In this scenario the core cooled, perhaps from an initially superheated state compared to the mantle (Williams and Nimmo 2004) or modulated by an early episode of plate tectonics (Nimmo and Stevenson, 2000), until 𝑄𝑐𝑚𝑏 fell below 𝑄𝑎. Impact-induced thermal insulation of the core (Monteux et al., 2013; Arkani-Hamed and Olson, 2010) would produce a similar outcome. On the other hand, a thermochemical dynamo can operate with 𝑄𝑐𝑚𝑏< 𝑄𝑎. It has been suggested that rapid growth of an inner core early in Mars’ history led to dynamo termination when the size of the liquid region fell below a critical threshold (Stevenson, 2001). This scenario has not been favored because inner core growth provides additional power sources that lead to a long-lived dynamo (Williams and Nimmo, 2004). In this paper we investigate a third scenario: the top- down crystallization of the Martian core. A necessary condition for top-down core freezing is 𝜕𝑇𝑙/𝜕𝑃< 𝜕𝑇/𝜕𝑃 for all pressures 𝑃, where 𝑇𝑙 is the liquidus temperature of the core alloy and 𝑇 is the ambient temperature. 𝜕𝑇/𝜕𝑃 is positive and for an adiabatic core 𝜕𝑇𝑎/𝜕𝑃∝𝑇𝑎∝𝑇𝑐𝑚𝑏, where 𝑇𝑐𝑚𝑏 is the CMB temperature. Melting curves for iron-sulfur systems, the mixture used throughout this work, have been extensively studied (Kamada et al., (2012); Morard et al., (2011); Figure 2d). Of particular in

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