At a NASA teleconference today, Arizona State University geologist and mission co-investigator Mark Robinson said new images of Mercury from the MESSENGER spacecraft reveal "mysterious dark blue material" that is likely result of an extensive volcanic history of the planet. [Credit, NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington]
Robinson said they aren't sure what the material is that fills some craters but it shows the planet is not as tectonically quiet as previously thought - there was active volcanism all over the planet.