Whenwe look at Earth from space, we see a blue, watery world. It is no coinciden-组卷网

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When we look at Earth from space, we see a blue, watery world. It is no coincidence that three quarters of the surface is covered by oceans. Water is very common in the universe and is essential for lives. But life needs water in the form of liquid, not ice, and not vapor and that is a little bit less common.

For a planet to have liquid water at its surface, three things are important. First, the planet needs to be large enough that the force of gravity keeps the water molecules （分子）from flying off into space. For example, Mars is smaller than Earth, so it has less gravity, and that is one important reason that Mars has a very thin atmosphere. Second, the planet needs to have an atmosphere. Without an atmosphere, the planet is in a vacuum （真空）,and liquid water isn't stable in a vacuum. For example, our moon has no atmosphere, so if you spill some water on the moon, it will either boil away as vapor, or freeze solid to make ice. Third, the planet needs to be at the right distance from its star.

If you were an alien astronomer, you'd think Venus is a pretty good bet for life. It is the right size. It has an atmosphere and it is in the habitable zone of our sun. But Venus is not habitable, at least not at its surface. It is too hot, because Venus' atmosphere is full of carbon dioxide, an important greenhouse gas. In fact, its atmosphere is almost entirely carbon dioxide, and almost 100 times thicker than our own. As a result, the temperature on Venus is hot enough to melt lead, and the planet is dry as a bone.

So finding a suitable planet for humans to live on is only a beginning. Don't expect much.

【小题1】What do we know about water from the text?

A．Water has two forms.

B．Water is a necessity for humans.

C．It is common to see liquid water on the moon.

D．Water flies into space easily with the help of gravity.

【小题2】Venus is not suitable for humans to live on because .