Humans live on a small planet in a tiny part of a vast universe. This part of the universe is called the solar system, and is dominated by a single brilliant star-the sun. The solar system is the earth’s neighbourhood and the planets Mercury, Venus, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto are the Earth’s neighbours. They all have the same stars in the sky and orbit the same sun.
Scientists believe the solar system began about 5 billion years ago, perhaps when a nearby star exploded and caused a large cloud of dust and gas to collapse in on itself. The hot, central part of the cloud became the sun, while some smaller pieces formed around it and became the planets. Other ...view middle of the document...
Elsewhere, the Moon still bears the scars of every rock that has hit it from space, both large and small. The footprints of the visiting astronauts will survive for millions of years.
Crash and splash
Astronomers now believe the Moon is the remains of a giant cosmic traffic accident. In its very early days, the Earth was hit by a runaway planet the size of Mars. White-hot molten rock were splashed into space, and solidified into a ring of rocks around the Earth. These then came together to make up the moon.
Ice on the moon
Circling over the moon’s poles in 1998, the American Lunar Prospector spacecraft discovered patches of ice within some deep craters, where it is always shaded from the Sun, so the ice never melts or evaporates. Future manned expeditions could melt the ice into water, for drinking, washing and turning into rocket-fuel: there is enough ice to make a lake 10km across and 10m deep.
If you approached the Solar System from space, one planet would stand out as very odd. The third world from the sun is brightly coloured, in shades of mainly blue, with patches of red and green, and constantly shifting patterns of white cloud. And it has an unusually large moon; which-by contrast- is dull and uniformly brown.
The perfect planet
Come closer, and you find blue is liquid water. This is the only rocky planet with water. Test the atmosphere, and again this planet is unique: the air contains a lot of reactive gas oxygen. And finally, take a closer look at the green areas. Here there is ample vegetation and animal life. This is the only planet in the solar system obviously inhabited life forms.
The earth is special mainly because it is located at the perfect distance from the sun. The temperature here is just right for water to exist as a liquid: get in as close Venus and the water just boils away as steam, while as far out as the orbit of Mars water is frozen to ice.
Soon after the birth of earth, it was a very different place from the one we know today. Its “air” was made of the unbreathable and poisonous gases we still find on other planets, such as carbon dioxide and hydrogen cyanide. Comets fell from the sky as giant fireballs, and lethal ultraviolet light rays from the Sun irradiated the surface.
In all this mayhem, simple chemicals from the air and the infalling comets dissolved in the oceans. They gradually joined together in a “primordial soup” to make the first living cells
Life on Earth
Life became an important force in shaping the environment. The first plant cells in the sea took in carbon dioxide from the air, and gave out oxygen by the process of photosynthesis. Animals emerged which could breathe the oxygen. And some of the oxygen turned into an ozone layer high in the atmosphere that shielded the surface from the harmful ultraviolet rays of the sun. Lifeforms emerged on to dry land, evolving into the familiar variety of plants and animals we know today.
Mercury is the planet nearest...