The Earth and other rocky planets

At the centre of the Earth is the core. This is surrounded by a thick coating of hot rock called the mantle. Above the mantle is the Earth's rocky skin called the crust. The other rocky planets, except Pluto, show a similar structure of core, mantle and crust.


Mercury and Venus


Mercury and Venus are two rocky planets closer to the Sun than the Earth.

Have any space craft been to Mercury?
Yes. The Mariner 10 space probe flew past the planet three times in the mid 1970s. It took a large number of photographs. These were used to make maps of the planet's surface.

Is Mercury as big as the Earth?
No, it is much smaller. It is just a little larger than our Moon.

Does Mercury have grey patches like the ones we can see on the Moon?
No, it is covered in craters and mountain ridges. The largest crater is called the Carolis Basin and is 1,300 kilometres in diameter. It formed when a huge piece of rock hit the planet. When the crash happened, the heat of the impact made the planet's crust melt and ripple like the surface of a pond. As the crust cooled, the ripples formed rings of mountains round the basin and made it look like a bull's eye.

Have any spacecraft been to Venus?
There have been many spacecraft sent to Venus. The first to fly-by was Mariner 2 in 1962. Since then 15 probes have landed on the planet but some were destroyed straight away by the conditions on the surface. More information about the planet's surface has been found by scanning the planet with radar equipment. The radar waves can pass through the clouds, reflect off the surface and form pictures of the hills, mountains and plains on the planet. The Soviet Venera 13 and 14 spacecraft took several pictures of the surface of Venus.


Mars


The surface of Mars has many features such as extinct volcanoes, deep valleys, hills, plains, craters and empty river beds. As Mars goes round the Sun these features cast different shadows. The shadows can be used to tell more about the features and the planet.
When astronomers look at photographs of the surface of Mars or a moon they look at the sizes and shapes of the shadows to help them identify the features on the surface.

The face on Mars

In 1976 a space probe to Mars, called the Viking 1 orbiter, took a photograph of the planet's surface and showed rocks that seemed to be carved into a human face. Some people became excited because they thought that the face was proof that there had been some intelligent life on Mars but other pictures taken when the light shone on the rocks from a different direction showed the 'face' to be just an ordinary mountain range.

What do the martian moons look like?

The two moons of Mars are called Deimos and Phobos. They are probably both asteroids that have been captured by Mars.
Deimos is shaped like a brick and is only 28 kilometres long, 22 kilometres deep and 27 kilometres wide. It is 24,000 kilometres above the martian surface and would appear about half the size of our Moon, but would be dimmer because the surface is made of very dark rock.
Phobos looks like a giant potato 27 kilometres across. It is in orbit 9,300 kilometres from the planet and also has a dark surface.
Phobos is slowly spiralling into the planet. Each year it gets a little closer and may hit the planet in less than a billion years.

Is there life on Mars?
So far no one is sure. In 1976 the Viking lander made some tests on the soil. In one test a gas was made which seemed to suggest life was present in the soil. A further check showed that the gas could be made by the rock. A lump of martian rock which landed on the Earth has tiny markings in it which a few scientists believed to be simple forms of life. Now most scientists believe that the markings formed naturally in the rock. The landscape of Mars shows that at one time rivers of water flowed across its surface. Perhaps life developed in this water. Some remains may yet be found below the martian soil.

How did martian rock get to Earth?
In the distant past lumps of space rock have hit Mars. During the impacts, chunks of martian rock have been thrown into the sky. As the pull of gravity of Mars is only about a third of the Earth's gravity, some of the rocks reached space and eventually travelled to the Earth. Many martian rocks landed as meteoroids and have been found in Antarctica, where they stand out against the snow and ice.