Mars's dense layer reveals the fate of rocky planets express
If you don’t stir a layered cocktail, the heavy syrup sinks to the bottom. A young planet is much the same — a seething ocean of molten rock. As it cools, minerals crystallize: some, like iron, get heavy and sink, while others float. You’d think a dense layer would pile up at the core.
But from below, out of the solidifying depths, melts rise: light droplets, like oil in water, float up and churn the ocean. The bigger the planet and the hotter the 'stirring,' the harder it is for iron to settle.
The Martian ocean was shallower, the heat-driven motion weaker, and the iron quietly flowed to the bottom. This layer was recently detected through the echoes of marsquakes: it acts like a sponge, damping seismic waves.
The discovery explains the inner structure of many rocky exoplanets. In essence, we’ve read the recipe nature uses to cook up planetary interiors: some it stirs, others it lets separate into layers.
Original abstract
as written by the authors arXiv:2605.04840
Magma ocean crystallisation sets up the early structure and long-term evolution of terrestrial planets. Recent seismic evidence signals the presence of a silicate layer at the base of Mars' mantle. Magma-ocean crystallisation and subsequent overturn has been invoked as a hypothesis for this layer's origin. However, while a magma ocean existed in both Earth and Mars, there is no seismic evidence for a basal layer in present-day Earth. In this study, we apply a parameterized-convection model to study whether the effect of partial melting in the growing mantle on overlying magma ocean composition can explain this discrepancy. Melts from the mantle buffer the crystallising magma ocean, limiting progressive differentiation, iron enrichment and the density anomaly of the overturned layer. This buffering is more efficient for larger planets with more vigorous mantle convection and for planets that are originally less enriched in iron. Consequently, a shallow magma ocean is more iron enriched and denser on Mars than on Earth, providing an explanation for the Mars-Earth difference in present-day structure of the mantle. We also predict a dichotomy in terrestrial-exoplanet interior structures, with a population with small, stratified mantles and another with large, mostly-homogeneous mantles.
If Earth's interior had been as calm as Mars's, we would have formed a hidden dense layer too, and that would have completely changed the planet's look: continents would have moved differently, volcanoes would have erupted differently.