Land of Confusion
When the Messenger spacecraft reaches Mercury in 2011, it will be the planet's first Earthly visitor in more than 36 years. The Mariner 10 spacecraft was Mercury's original and only guest back in 1974 and 1975. However, we haven't visited since and still don't know very much about the planet.
Much of what we do know is quite puzzling.
Mercury has an odd sense of time. The planet streaks around the Sun once every 88 Earth-days, making its year just a quarter as long as a year on Earth. With that sort of speed, it would be easy to think that a Mercury day would pass in the blink of an eye. But with the planet's unusually slow rotation, a Mercury day sluggishly passes once every six Earth months. Mercury's orbital habits are strange and interesting, indeed.
Image to right: Mercury looms in the darkness of space as Mariner 10 approaches the planet. Credit: NASA
On the surface, we've only managed to map half of Mercury's terrain. When Mariner 10 flew by the planet, the spacecraft photographed a landscape of intermixed craters and plains scored with ridges - some more than a mile high and hundreds of miles long. Unfortunately, due to the limitations of early 1970s technology, that's about all we could see. As incredible as Mariner 10's pictures are, they don't show enough detail to give scientists a good idea of how Mercury's surface was formed.
Below Mercury's landscape, something peculiar is going on. Mercury's core is a mass of iron surrounded by a mantle and crust of magnesium and iron silicates. For rocky, "terrestrial" planets like Earth, Mars and Venus, this geological structure is business as usual. What has grabbed scientist's attention are indications that Mercury appears to be made largely of iron-core - possibly as much as 65 percent. That's twice the percentage of our own Earth and a real mystery to scientists.
Earth and Mercury are the only two rocky planets with a global magnetic field. Earth's magnetic field is believed to be produced by the liquid magma swimming around the outside of the solid planet core. However, scientists continue to question the origins of Mercury's field.
Image to right: A view of Mercury’s south pole taken as Mariner 10 cruised beneath it. Credit: NASA
Like Earth, Mercury could have flowing magma generating a magnetic field. But that possibility raises a question of how the core of such a small planet could still be warm after all this time. Another prospect is that the present field is a weaker remnant from the time when magma was flowing and powering a stronger field.
Fast and slow, magnetic from pole to pole, Mercury is a confusing planet that's time for exploration has come. When the MESSENGER spacecraft arrives in 2008, modern technology and science will set to work untangling the knotted logic that governs Mercury. Not only will the adventurous mission shed light on this perplexing planet, but it could also change what we know about our own.
NASA's John F. Kennedy Space Center