Mercury

Mercury is the closest planet to the sun. It is a terrestrial planet and the second smallest planet after Pluto. Only one spacecraft has flown by Mercury, Mariner 10, which was launched by the U.S. on November 3, 1973. Much of what we know about this planet came from this mission.

How big is Mercury?

Mercury is about 4879 km in diameter, which makes it a little over a third as big as the Earth is across. It's small enough that eighteen balls of modeling clay the size of Mercury could be rolled together to make one the size of Earth. In fact, the diameter of Mercury is only about one and a half times the diameter of our moon. It is the second smallest planet in the Solar System, just larger than Pluto. The small size and how close it is to the Sun sometimes make it difficult to observe Mercury in the sky, especially without a telescope or binoculars.

What is its surface like?

Mercury has craters like those on the Earth's moon. They were made when asteroids or comets crashed into the surface. The largest crater we've seen on Mercury is the Caloris Basin. It is about 1350 km in diameter and was caused when a huge asteroid, probably about 100 km wide, hit Mercury about 4 billion years ago.

The surface also has big cliffs called scarps. They were made long ago when Mercury cooled down and shrank. This shrinking caused the surface to get wrinkled in some places, which created the scarps.

There are also plains. Some of them may have been made by lava flows long ago. There may also be water ice on both of Mercury's poles. The poles, like Earth's, get very little warmth from the Sun, so the ice doesn't melt or evaporate.

It gets very hot during the day and very cold at night on Mercury. It's so hot during the day (430°C, hot enough to melt tin!) because Mercury is so close to the sun. At night, however, it gets down to -180°C, much colder than Antarctica in winter. It loses almost all of its heat during the night because Mercury has very little atmosphere, which would normally help to keep in the warmth.

What is its moon like?

Mercury does not have a moon. Mercury's rotation is so slow that if Mercury had a moon, it would crash into Mercury or get broken up. This would happen because the moon's gravity would cause tidal effects on Mercury. There would be two bulges called tidal bulges on Mercury. One would bulge toward the moon, with the other bulge being on the opposite side of Mercury. The moon's motion in its orbit would be faster than Mercury's rotation because Mercury's rotation is very slow. That would cause the moon to be ahead of the tidal bulge all the time. The gravity from the bulge would pull back on the moon. This would cause the moon to become closer to Mercury and Mercury's rotation to speed up. This would continue to happen over millions of years until the moon got broken up by Mercury's gravity or crashed onto Mercury. Mercury had existed for billions of years, so if it had any moon, it is long gone.

How long is a day on this planet?

Mercury rotates much slower than Earth. Its day is 58.6 Earth days long.

How long is a year on this planet?

Mercury is the planet with the shortest year in the solar system. It is about 88 Earth days long, roughly one quarter of an Earth year. This means that an eight- year-old person would be 32 years old on Mercury. One interesting thing about Mercury is that there are three Mercury days for every two Mercury years.

What is planet Mercury made of?

Mercury has a large iron core for a planet of its size. The planet contains a larger portion of iron than any other planet in the solar system. The outer layer of the planet is made of silicates, which are ordinary rocks.

How much would Mercury's gravity pull on me?

If you were on Mercury, it would pull you down with a force about one third as strong as the force of Earth's gravity.

Who is planet Mercury named after?

In Roman mythology, Mercury was the messenger of the gods. He wore a hat and sandles with wings on them, allowing him to travel around the world extremely quickly. The planet Mercury was named after him because it orbits around the sun faster than any other planet in the Solar System, traveling nearly 50 km every second!

Mythology Not only was Mercury the messenger of the gods, he was also the god of buying and selling. His name probably comes from the Latin word mercator, which means "merchant." Mercury had his own day of the week, dies mercuri, or "Mercury's Day". In France it is mercredi. In English we named this day after a Norse god, Woden, making it "Woden's Day", or Wednesday