Mercury's interior has a complex differentiation consisting of various alloys. Beneath the thin rocky crust lies a mantle of solid iron sulfide. This sits on top of a liquid outer shell. Beneath that is an iron silicate layer wrapped around an iron core. These multiple layers don't mix, giving Mercury a much more complex and stratified interior than the moon is believed to have.
The retention of such an interior with such a mix of elements rules out formation theories that have Mercury born under extremely hot conditions in the protoplanetary disk that encircled the sun 4.5 billion years ago.
A popular idea also was that Mercury had a much thicker rocky mantle that was blown away in a tremendous asteroid collision.
In reality, Mercury may have just formed under cooler conditions, trapping materials that would normally have been driven off by intense heating. This idea is bolstered by emerging models of circumstellar disks that show they could have a comparatively cool, dynamically "dead zone" near a star where even volatile elements could survive before the disk dissipates.
In the 1990s, scientists recorded bright radar reflections from Mercury's polar regions. The conventional wisdom is that water ice - perhaps deposited on Mercury by comets - was preserved in permanently frozen shadowed polar craters.
Like our moon, Mercury's spin axis is nearly perpendicular to its orbit. If you stood at the bottom of craters at the poles you would never see sunlight coming over the rims.
MESSENGER's mapping shows that these bright radar regions lay precisely in the shadows at the bottom of the polar craters. This doesn't automatically prove water is present, but there certainly is some kind of radar bright material that was deposited only at the bottom of the permanently shadowed craters. Water is probably the best answer.
The MESENGER mission has transformed our view of Mercury from a moonlike world to a surprisingly complex and dynamic place. It underscores how little we can estimate conditions on extrasolar planets when one of our nearest neighbors in space yields so many mysteries.
Image credit: NASA