Mercury Flyby 2
Today the MESSENGER spacecraft is due to make its second flyby of Mercury. In 2011 the probe will finally settle into orbit. The innermost planet has been known to have a magnetic field since the Mariner 10 flyby of 1977. MESSENGER's first flyby confirmed the prediction of vulcanism, indicating an internal heat source. To explain the magnetic field, scientists have inferred an iron-rich core, as with Earth. Though the planet's volcanic surface is full of materiel erupted from within, none of the volcanic plains show traces of iron. An iron core is solely an inferrence, with no direct evidence. Something else may be causing internal heat and a magnetic field. Mercury's centre would be a good place to seek a Black Hole.