Martian clays may have formed when the planet’s crust reacted with a dense steam or atmosphere that was released during the cooling of its magma ocean, suggests a study published in this week’s Nature.

It is thought that the basaltic crust on Mars reacted with liquid water to form hydrated clay materials. Previous studies of the origins of Martian clays focused on the Noachian period (4.1 to 3.7 billion years ago) because most of the rocks that contain these materials had been dated to that time. However, Kevin Cannon and colleagues propose an alternative theory of clay formation, earlier in the history of the planet, when its solid crust first formed and reacted with a dense steam or atmosphere of water and carbon dioxide that was released as the magma ocean cooled.

The authors demonstrate that clays form rapidly under conditions present at the base of such an atmosphere and deeper in the porous crust. Using a crustal evolution model, they find that impacts and volcanic activity contribute to the distribution of the clay exposures that can be observed by orbiting spacecraft.