Mars has long been described as a stagnant lid planet: its surface is not broken into mobile tectonic plates, so scientists assumed it lacked the machinery that drives complex crust formation on Earth. Research from Oxford's Departments of Earth Sciences and Statistics, published in Nature Astronomy, challenges that picture, using seismic recordings of marsquakes and meteoroid impacts captured by NASA's InSight mission to probe a mysterious boundary about 24 kilometres below the surface.
The team compared hundreds of possible rock compositions against the seismic data using thermodynamic modelling and statistical techniques. Only ultramafic rocks, rich in iron and magnesium but low in silica, consistently matched the material beneath the boundary, while the rocks above matched more silica-rich mafic compositions. The researchers interpret the buried layer as the residue of molten rock that pooled deep underground and gradually separated, with dense crystals settling while lighter, more evolved melts rose upwards.
On Earth, comparable processes operate beneath volcanic arcs and are linked to the building of continents. Their presence on Mars suggests the planet sustained massive, long-lived magmatic systems capable of reprocessing molten rock throughout the crust, with implications both for how rocky planets become habitable and for the distribution of metal deposits that future crewed missions might one day exploit.