Planetary magnetic fields shield planets, their surfaces and their atmospheres, from charged particles that are always coming off the sun.

 

The auroras that light up our polar skies are evidence of our protective shield\ at work. Without it, life would not exist here. Visit Mercury, and you will be visiting the only other planet in the inner solar system with a magnetic field. But its very existence defies explanation.

 

Now the mystery there is that in order to have a magnetic field, you need to have an interior to the planet that is at least partially molten. Mercury, one of the smallest planets, would have been expected to be really frozen all the way through. And yet, we have a relatively strong, very well defined magnetic field that appears to have a source that's located deep in the planet.

 

Whatever mechanism is driving Mercury's magnetic field, it's too weak to protect it from the full force of the sun. The solar wind buffets Mercury's thin atmosphere. And in the process, it puts on a light show.