MARK GARLICK / SCIENCE PHOTO LIBRARY MARK GARLICK / SCIENCE PHOTO LIBRARY
Internal structure of Mercury, cutaway artwork. The inset box at lower right shows Earth and Mercury at the same scale. Mercury is a rocky planet, and the closest of the major planets to the Sun. Its interior comprises a large iron core (yellow). This underlies a thick silicate crust (orange). Mercury has a very thin atmosphere, which does not trap much heat. Because of this, and its proximity to the Sun, it undergoes drastic surface temperature changes, from around -200 degrees Celsius at night to over 400 degrees Celsius in the day.
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