The inner planets – Mercury, Venus, Earth and Mars – are all made of rock. They are quite small compared with some of the other planets. Mercury is closest to the Sun. It is extremely hot during the day but the temperature falls to -175 degree C at night. Mercury is only a little bigger than our Moon.
Although Venus is further from the Sun than Mercury, it is the hottest planet in the Solar System. Venus spins in the opposite direction to all the other planets. It also spins so slowly that its ‘day’ is longer than its ‘year’!
Earth has water, oxygen and moderate temperatures. Because of this it can support life. Mars is about half the diameter of Earth. Temperatures here are never above zero degree C and the only water is ice at the frozen poles.
Venus
On the surface of Venus, below, there are deep cracks and dead volcanoes. The atmosphere is thick carbon dioxide gas and the planet is surrounded by clouds of sulphuric acid.
Mercury
Like the Moon, the surface of Mercury is pitted with crates and covered in dust and stones. There is no air or water. It is bathed in dangerous radiation from the Sun.
Earth
About three-quarters of the Earth’s surface is covered by water. The atmosphere contains the gases oxygen, nitrogen and a small amount of carbon dioxide.
Mars
Mars is made of red rocks. It has craters and dead volcanoes on its surface. The atmosphere is dusty and made of carbon dioxide. There are strong winds which blow up dust storms.