Olivine and pyroxene tell the story of volcanism, with differences in the chemical composition of the solidified lavas representing the evolution of the temperature and pressure inside the planet. Some of the most pristine olivines are found around impact craters that have excavated very ancient samples of the mantle from below the crust.
Most of the olivine-rich surfaces are associated with a global lava-flooding event around 3.7 billion years ago when magma erupted onto the surface through fractures in the floors of impact craters, forming lava ‘seas’ similar to the lunar mare.
The colour scale indicates a transition in chemical composition of the olivine lavas from magnesium-rich (blue) to iron-rich (red). The signature of olivine is also related to its grain size and abundance, such that the colour scale from blue to red also describes a small (<100 microns) to large (a few mm) grain size, and/or a low (10%) to high (40%) abundance in comparison to the mean martian surface. For example, detections mapped in red (such as the prominent spot indicating Nili Fossae) correspond to olivine with high iron content, and/or a large grain size and/or a high abundance.
This map was released on the occasion of the tenth anniversary of the launch of Mars Express, along with global maps of hydrated minerals, pyroxene, ferric oxide and dust. Click to access the required image.