Mercury's carbon-rich crust is surprisingly ancient

Before its planned crash into Mercury last year, NASA's MESSENGER spacecraft gave scientists a parting gift. In its final orbits, MESSENGER not only confirmed Mercury's dark hue is due to carbon, but also revealed the carbon was not deposited by impacting comets. Instead, scientists now believe they are seeing remnants of the planet's primordial crust, which likely formed when a global ocean of super-heated magma cooled, allowing minerals to solidify.