Iron Snow in the Martian Core?

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, x0. 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 x0~10% and snow zones that occupy approximately the top 100 km of the present-day Martian core.

💡 Research Summary

The paper revisits the long‑standing problem of why Mars lost its global magnetic field around 3.8–4.1 billion years ago, linking the cessation of the core dynamo to the thermochemical evolution of the planet’s core. While Earth’s dynamo is sustained by the growth of a solid inner core at the bottom of the liquid outer core, recent high‑pressure experiments on Fe‑S alloys show that the liquidus temperature of iron‑sulfur mixtures actually increases as pressure decreases under Martian core conditions. This counter‑intuitive behavior opens the possibility that Mars’ core could crystallize from the top down, forming what is termed an “iron snow” layer: pure iron precipitates at the top of the core, is denser than the surrounding Fe‑S liquid, and therefore sinks.

To assess this scenario, the authors develop a global energy and entropy framework for a two‑phase, two‑component (Fe and S) system. They assume the snow zone remains in phase equilibrium at all times and that any solid that forms immediately separates from the liquid, falls through the snow zone, and remelts at the base of the zone in each timestep. Under these assumptions, the growth of the snow zone releases gravitational potential energy (as dense iron descends) and latent heat of crystallization. The authors derive expressions for the total heat flow, the entropy production associated with secular cooling, ohmic dissipation, and the additional terms arising from snow formation.

A broad parameter sweep explores the influence of the initial sulfur concentration (x₀), core temperature profile, thermal conductivity of the core and core‑mantle boundary, and the rate of heat loss to the mantle. The key findings are:

1. Snow‑zone formation is highly sensitive to x₀. For initial sulfur contents between roughly 8 % and 12 % (by weight), the top of the core can become supersaturated in iron and begin to solidify. The most realistic value, constrained by geochemical and geodetic data, is x₀ ≈ 10 %.

2. Typical snow‑zone thickness in present‑day Mars is modest. Models that satisfy the observed magnetic field history and the present‑day moment of inertia predict a snow zone occupying only the upper ~100 km of the core. Such a layer can be stable for 1.5–2 Gyr, consistent with the timing of dynamo shut‑down.

3. Entropy budget does not favor dynamo re‑ignition. Although the sinking iron releases gravitational energy and the crystallization releases latent heat, the associated entropy production is insufficient to overcome the ohmic dissipation threshold required for a self‑sustaining dynamo, unless the snow zone is at least ~400 km thick—a thickness far larger than that allowed by current constraints.

4. Thermal stratification may delay snow formation. The model does not include a thermally stratified upper core, which would suppress convection and postpone the onset of iron snow. Incorporating this effect would likely shift the formation time to later epochs, but would not change the fundamental conclusion that the snow zone remains too thin to power a dynamo.

5. Implications for the present core state. The results suggest that Mars’ core is not entirely liquid today; rather, a thin, iron‑rich snow layer likely exists at the top. This configuration can explain the residual weak magnetic signatures detected by orbiters and is compatible with the measured Love numbers and moment of inertia.

The authors acknowledge several simplifications: instantaneous solid fall‑out, neglect of compositional gradients that could develop as iron sinks, and the omission of a possible solid inner core at the very center. They argue that future work should incorporate 3‑D convection, realistic solid‑particle dynamics, and the effect of a thermally stable layer to refine the estimates of snow‑zone thickness and longevity.

In summary, the study provides a robust thermodynamic argument that a top‑down crystallization (“iron snow”) regime is plausible for Mars, driven by the pressure‑dependent Fe‑S liquidus. While the snow zone can persist for billions of years, its modest thickness means that the released gravitational and latent‑heat energy cannot restart the planetary dynamo. Nonetheless, the presence of an iron‑snow layer offers a coherent explanation for the observed magnetic and geodetic constraints and reshapes our understanding of the present‑day physical state of the Martian core.