Volcanoes reveal the cooling of Mars

The history of water and of climate evolution on Mars has received considerable attention over the past few decades. However, the evolution of a planet needs to be considered in its entirety. This requires an understanding of the thermal and dynamic evolution of the planetary interior in relation to volcanic or tectonic activity. The formation of volcanoes results from the partial melting of mantle rocks*, the upwelling of these magmatic liquids, and their eruption at the surface. The composition of magmatic liquids is controlled by the depth (pressure) and temperature at which melting takes place. For instance, terrestrial rocks of Archaean age (around 3 billion years ago) suggest that Earth's mantle was hotter at that time than it is today. This link between magma chemistry and melting conditions was utilized for Mars by researchers at the Institut de Recherche en Astrophysique et Planétologie (CNRS/Université Paul Sabatier), at the Observatoire Midi-Pyrénées.

The Gamma Ray Spectrometer on board the US Mars Odyssey mission has produced maps of the abundance of several chemical elements on the surface of Mars. The researchers focused mainly on silicon, iron and thorium, which are especially sensitive to melting conditions. The abundance of these three elements in twelve Martian volcanic provinces makes up a remarkable record of melting processes within the mantle over the past four billion years.

The researchers showed that variations in iron, silicon and thorium in volcanic rocks are evidence of a fall in the temperature of the mantle over time, of thickening of the lithosphere at the base of which melting takes place, and of magma being produced at increasingly great depth.

These results make it possible to precisely reconstruct the thermal evolution of Mars, which appears to have cooled down more slowly (30-40°C per billion years) than Earth (70-100°C per billion years). The reason for this probably lies in the absence of plate tectonics on Mars. This study also sheds light on the diversity of scenarios of planetary evolution and makes it possible to understand why a planet's volcanic activity eventually ceases (when the magma is no longer able to pass through the lithosphere that has become too thick). These results provide a new framework within which to tackle numerous questions such as the reasons for the shutdown of Mars' internal magnetic field 4 billion years ago, the origin of crust of over 4 billion years, and the connections between volcanism and the evolution of the physical and chemical parameters of the Martian atmosphere.

Note: *Partial melting takes place inside the mantle, at a depth where the right temperature and pressure conditions for melting are met. This depth corresponds to the base of the lithosphere (which consists of the crust and the upper mantle).