What were the best results obtained from the MESSENGER spacecraft’s trip to Mercury?
The big surprise was that Mercury’s powder-rocky material, the part we can see on the surface, had been predicted for many decades to be deficient in elements and compounds that are easily removed at high temperatures.
The one thing we knew before any spacecraft ever went to Mercury is that it has got a very high fraction of metallic iron. The question is, how do you make a planet that is mostly iron, but is otherwise produced by the same properties that produced Earth, Mars and Venus? The prediction was that Mercury would be very deficient in elements that are easily removed at high temperatures, just like Earth’s Moon. These are what the chemists like to call ‘volatile elements’.
Once we started measuring what’s at Mercury’s surface, we realised that wasn’t the case. Mercury is high in sulphur. It has ten times the abundance of sulphur of the surface of Earth. Mercury has high abundances of sodium, potassium, alkaline metals that are easily removed at high temperatures and chlorine. Mercury has a higher chlorine abundance relative to several elements on Earth.