Evening Mercury 
Mercury is unlike any other planet in the solar system. Its core of iron and nickel makes up about three-quarters of the planet’s diameter -- a far larger fraction than on Earth or any other planet. And for decades, most explanations have said that odd construction was the result of a lot of energy.
But observations by a spacecraft in orbit around Mercury suggest that those ideas are wrong. Instead, the planet may have formed in a quiet process that was similar to the one that gave birth to the building blocks of planets.
The earlier ideas said that intense heat -- either from the Sun or from a collision with a giant asteroid -- stripped Mercury of many of the lightweight elements found in Earth and the other planets. But the Messenger spacecraft has found higher concentrations of those elements than expected. The finding seems to rule out a major role for extreme heat in Mercury’s formation.
Instead, it suggests that Mercury formed from a slow accumulation of both light and heavy materials -- the same process that gave birth to the building blocks for all the planets. But that doesn’t explain why Mercury’s core is so large. That’s a mystery that will require more observations -- and a lot more thought about the birth of the Sun’s closest planet.
Look for that planet in the west shortly after sunset the next few evenings. It looks like a fairly bright star, but it’s so low in the sky that you need a clear western horizon to see it.
Script by Damond Benningfield, Copyright 2012