English Across the Curriculum
The Solar System
Mars
Mars is the fourth planet in our solar system. It has two moons - Phobos and Deimos. Some astronomers think that the moons are like asteroids that were captured by Mars very early in its history.
If you look at Mars without a telescope you can see a reddish object that is sometimes very bright. When it is closest to the earth - at about 55 million km - Mars is, after Venus, the brightest object in the sky. Through a telescope, Mars has bright orange and darker areas. Some of the colors change with Martian seasons.
The Martian atmosphere is mostly made up of carbon dioxide with small amounts of nitrogen, oxygen and water. It is very thin and high yellowish clouds can often be seen. Scientists think they are made up of dust that is carried around by high winds on the planet.
The temperatures on Mars depend on the season and the time of day. But most of the time it is very cold. Summer temperatures may get as high as 17° C - but on average they don't get much higher than -30° C. The coldest parts of the planet are down to - 125 °. That's why the carbon dioxide freezes , turns white and forms the polar ice caps . In the winter they cover almost half of the planet and in the summer they get smaller as the sunlight melts them. They are about 300 km wide at the South Pole and more than 1,000 km wide at the North Pole. They are probably made up of frozen water and gases and may be up to 2.5 km thick.
Some of the surface of Mars consists of the biggest volcanoes in the solar system. One of them , Olympus Mons , is 25 km high and 600 km wide. There is no sign that volcanoes are active today. There are also many craters because in its early days Mars was hit very hard by asteroids and comets. Canyons run for over thousands of kilometers across the surface .
Most of the knowledge we have about Mars has come from spaceships that have been sent to visit the planet since 1964. The first views of Mars came from spacecraft that flew by the planet at a larger distance. The first spacecraft to orbit the planet was Mariner 9 . It studied the atmosphere and the surface for almost a year and gave scientists the first pictures of its two moons. In 1976, two Viking spacecraft landed on Mars and took a look at the surface of the planet. In 1997, Mars Pathfinder, set down on the surface and a 10kg heavy vehicle, called Sojourner, moved around the planet and examined rocks.
The idea that life could, or even does, exist on Mars has a long history. In 1877 the Italian astronomer Schiaparelli said he saw a system of canals all over Mars. But close-up pictures from Mars have shown that no such canals exist. The early Viking spaceships that landed on Mars in 1976 proved that there is no organic material on the planet.
