Mercury is the closest planet to the Sun and it is the smallest planet in the Solar System, just 4,879 Kilometres across its equator, compared with 12,742 Kilometres for the Earth.

After Venus, Mercury is  the second hottest planet, experiencing temperatures of up to 427°C on the side which faces the sun, whilst on the alternate side it can be as low as -173°C.

Mercury has no moons and no rings.

mercury 3
Messenger spacecraft image: Credit: NASA/John Hopkins University

Mercury is very dense.  This is largely due to  being composed mainly of heavy metals and rock.
As the iron core of the planet cooled and co
ntracted, the surface of the planet became wrinkled. Scientist have named these ‘wrinkles’, Lobate Scarps. They can be up to a mile high and hundreds of miles long.

One solar day (the time from noon to noon on the planet’s surface) lasts the equivalent of 176 Earth days while the sidereal day (the time for 1 rotation in relation to a fixed point) lasts 59 Earth days.

One year on Mercury is just 88 days long. For every two orbits of the Sun, Mercury completes three rotations on its axis.

Due to Mercury’s proximity to the Sun,  it is not easily seen except during twilight.

Only two spacecraft have visited this rocky planet: Mariner 10 in 1974-5 and MESSENGER, which flew past Mercury three times before going into orbit around Mercury in 2011.

“Thirteen times a century Mercury can be observed from the Earth passing across the face of the Sun in an event called a transit, the next will occur on the 9th May 2016.”

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