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The Mariner 10 mission sent back more than just pictures of Mercury's surface. On-board instruments detected a weak magnetic field - about 100 times weaker than Earth's. This came as a surprise, since the photographs from this mission showed a Moon-like surface, and the Moon's magnetic field is about 100 times weaker yet. A planet's magnetic field shields it from constant bombardment of charged particles streaming outward from the Sun - the solar wind.

Mercury’s magnetic field and high density both indicate that the planet has a large, iron-rich core, probably about 3600 kilometers in diameter. This core takes up a large fraction of the planet's volume, since Mercury's total diameter is only 4880 kilometers. Mercury’s ratio of core size to planet size is the largest of any planet in the solar system. Mercury prbably has only a thin lunar-like silicate mantle.

Learn about the Surface of Mercury

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Inside and Out
Mercury's iron core is thought to extend from the planet's center to nearly three quarters of the way to the surface, making it about the size of our Moon.