The Solar System
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 th[...]DOWNLOAD REFERAT
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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 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. « mai multe referate din Astronomie