It's difficult to examine Mercury from Earth because the sun is so bright. It's also difficult to send spacecraft to the planet because of the sun's enormous gravity. The European Space Agency has never sent craft so close to the sun and there have only ever been two missions to Mercury, both from NASA. The first was Mariner 10, which made flyovers in 1974 and 1975. The second was Messenger, which is still in orbit there. From those missions, NASA learned the strength and shape of Mercury's magnetic field and that the planet has an iron-rich core and no atmosphere. Messenger found ice on Mercury's north pole. There could be as much as 1.1 trillion tons of water in the form of ice on Mercury, astronomers estimate.