SEE ALSO: BEYOND THE SOLAR SYSTEM -- HOW BIG IS THE MILKY WAY?
Mercury from Mariner 10
Mercury is the closest planet to the Sun. It is a terrestrial planet. That means a planet made from rock.
Only one spacecraft, Mariner 10, has visited Mercury. Much of what we know about this planet came from this mission.
How big is this planet?
is 4879 kilometres (km) across. That makes Mercury just less than half the size
of the Earth. It is the second smallest planet in the Solar System. Only Pluto
is smaller. Mercury's small size and its closeness to the Sun sometimes make it
difficult to see without a telescope or binoculars.
What is its surface like?
Mercury has craters like those on the Earth's moon. The largest crater on Mercury is the Caloris Basin. It is about 1300 km wide. It was created by a huge asteroid hitting Mercury. The asteroid was probably 100 km wide.
The surface also has big cliffs called scarps. They were made long ago when Mercury cooled down. It shrank, causing the surface to get wrinkled in some places. This wrinkling created the scarps.
There may also be ice on the top and bottom of Mercury. Like the Earth, these areas (called poles) don't get much warmth from the Sun. Any ice there won't melt.
It gets very hot during the day and very cold at night on Mercury. It is very hot during the day (over 400°C) because Mercury is so close to the Sun. At night, however, Mercury loses almost all its heat because there is almost no atmosphere to keep it there. The temperature can fall to almost -200°C.
How long is a day on this planet?
Mercury rotates (spins around) much more slowly than the Earth, so one day on Mercury lasts as long as 58 days on Earth! So there is plenty of time for the temperature to rise so high or fall so low.
How long is a year on this planet?
Mercury has the shortest year in the Solar System. It is about 88 Earth days long. That means there are only 1Â½ days in each Mercury year!
What is it made of?
As far as we know, the center of Mercury is made of iron. It contains more iron for its size than any other planet in the Solar System. The rest of Mercury is made of rock.
How much would Mercury's gravity pull on me?
If you were on Mercury, it would pull you down less than half as much as the Earth.
Who is it named after?
In Roman mythology, Mercury was the messenger of the gods. He wore a hat and sandals with wings on them, allowing him to travel around the world very quickly. The planet Mercury was named after him because it moves around the Sun faster than any other planet in the Solar System. It moves nearly 48 km every second!
is a terrestrial planet."  
TEST YOUR KNOWLEDGE OF THE MATERIAL
1) Mercury has the shortest year. A year on Mercury is
Please enter your answer in the space provided:
2) What is the diameter of Mercury?
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