Mercury

The MESSENGER's global map of Mercury included the "most spectacular" volcanic vent feature yet seen on the planet, as well as evidence for geologic activity on Mercury as recently as a billion years ago. "After Mariner 10, it was thought that internal volcanic activity on Mercury ended earlier than on any other planet," MESSENGER imaging team member Brett Denevi said. "Now we're realizing that's not the case."

Mercury is known to have the largest metallic core, relative to its diameter, of any of the rocky planets. But, Lawrence said, "many space and Earth based measurements have shown Mercury's surface has a low concentration of iron within silicate minerals. Since silicate minerals typically dominate the bulk composition of planetary bodies, this observation has led to a commonly-held view that Mercury's surface has generally low abundances of iron as well as titanium. As a consequence, it has been a puzzle for studies of Mercury to explain how a planet can have such a large, iron-rich core but have a surface with such a very low iron content."

As MESSENGER flew past Mercury, it pointed its neutron

spectrometer at the surface, providing the first direct test of the exact abundance of iron and titanium in Mercury's rocks.

The high-resolution photos snapped by MESSENGER on approach to this flyby make it clear that the splash is a halo of material, probably volcanic cinders, tossed out of an enormous, steep-sided hole in the ground that is almost certainly a volcanic vent, Denevi said: "It doesn't have a raised rim, it is very steep, and it has this odd shape, all of which are characteristics of a volcanic vent. It's one of the best examples on the planet." At roughly 30 kilometers across, it is one of the largest such features yet spotted by MESSENGER.