The planet Mercury is the closest planet to the sun. Because of its nearness to the sun, it is seven times hotter than on Earth. However, it is very cold at night because the atmosphere of the planet cannot hold in the heat because it is too thin. Mercury is usually hidden from our sight by the brightness of the sun. In spring and fall, its orbit brings it into view on either side of the sun.
The planet is named after the Roman messenger of the gods. It goes around the sun faster than any other planet. A complete orbit takes just eighty-eight days. The planet spins very slowly on its axis, however. It rotates just once in fifty-nine days. A day is a time from one sunrise to the next. Mercury's day is much longer than Earth's day. It lasts 176 of Earth's days.
The planet's surface is very rocky with many craters or deep indentations probably made when asteroids smashed into the planet. Volcanic lava flowed over the surface of the planet and made smooth plains. Ridges and cliffs appeared after the lava cooled. It has no moons, unlike some of the other planets.
The largest crater is called the Caloris Basin and is 960 miles wide. The Caloris Basin was made when a huge asteroid crashed into the surface of Mercury. The force of the asteroid even affected the other side of the planet. Many hills were formed on the other side.
The spacecraft Mariner 10 made three flybys in 1974-1975 and took pictures of almost half the planet. Messenger had mapped the entire planet by 2015. Bepicolombo will launch in 2018 and eventually orbit Mercury. One orbiter will study the planet's surface. Another will study its magnetosphere, the part of the planet where magnetism is strongest.
Mercury is not a gas giant. Its surface is solid, made of metal and rock, and is called terrestrial. The four planets closest to the sun, Mercury, Venus, Earth, and Mars are smaller than the four gas giants which are much farther from the sun. These are Jupiter, Saturn, Uranus, and Neptune. These four larger ones are not solid and are made of gases.
Mercury is only 3,030 miles across. This is the distance across the United States from one ocean to the other. Its orbit around the sun is an oval. Therefore, the distance from Mercury to the sun changes constantly. The farthest point from the sun is called the aphelion. The closest point is called the perihelion. Mercury's aphelion is 43,000,000 miles. Its perihelion is 29,000,000 miles.
Mercury is not visible to Earth during the day because of the brightness of the sun. It is always closer to the sun than to Earth. The planet has no seasons. Earth has seasons because it is tilted on its axis. When Earth's North Pole is tilted toward the sun, the Northern hemisphere has summer because the weather is warm from the nearness of the sun. When the Earth's North Pole has tilted away from the sun, the weather is colder due to lack of direct heat. Mercury has just about no tilt so does not experience seasons.
Mercury's core is probably part molten iron and part solid. Thick solid rock covers this core. The surface is a thin crust. It has just about no atmosphere because its surface gravity is not strong enough to hold gases near the planet. The planet is also hit constantly by strong solar winds which would blow away any atmosphere. Mercury has no clouds, rain or fog because it has no atmosphere.
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