Men have always wondered about the earth they live on and about the ① ______ above them.

When early men looked at the night sky, they asked themselves a lot of questions. They looked at the moon, and wondered what this strange bright object in the sky was. They did not understand why it changed its shape from night to night. They looked at the stars and planets. They also wondered why some of them changed their positions, while others did not. ② 他们也思考地球的形状。Most early people believed the earth was flat.

It was not until the sixteenth and seventeenth centuries that scientists found the answers to some important questions. ④ With the help of Galileo’s telescope, people could see the sky more clearly, and find the positions of the stars and planets. Finally in 1665,Isaac Newton explained to the world what gravity (重力) was and people at last understood why they did not fall off the earth.

Hey! The people of the earth. Can you hear this radio message? Do not be afraid! We come in peace from another planet— and why not? We are your cousins. We are men, as you are. We have been here before.

You will see a huge spaceship, a few hours from now. We are flying towards the solar system (太阳系) almost as fast as this radio message. Your sun is already in sight. Our ancestors (祖先) and yours shared the sun ten million years ago. I’m sure you have forgotten your history while we have remembered ours.

Our ancestors lived on the Earth when dinosaurs were dying. Sadly, they could hardly save the beasts. Your planet was very warm then. Our ancestors believed it would make a nice home for them.

They were wrong. Though we were masters of space, we knew so little about the Earth.

For millions of summers — there were no winters in those ancient days — the Earth had rapid (飞速的) development. It took about thirty years to fly from one planet to another, so our ancestors seldom travelled in space. But they never felt lonely. They kept in touch with their relatives and friends on other planets. Three or four times every century, starships would call and bring them space news.

But two million years ago, the Earth began to change. The temperatures fell quickly. The ground was covered with snow and ice. At that time, however, our ancestors knew little about that. Those who had called the Earth their home believed there was just a strange disease spreading on the earth. The disease wouldn’t cause death to them, but it would bring much pain and do harm to men’s health.

Mars is full of surprises. Not long ago, something strange showed up in a picture of this planet. It is a pattern (图案) on the ground that looks like a bear face.

The picture was taken from high above Mars. Scientists took a closer look at the bear. Different marks on the ground make up the face of the bear. Its two eyes are big holes known as craters (坑). Rocks from space hit the surface of Mars long ago and then made the craters. The nose of the bear is a hill. Part of the hill has broken off in the shape of a V. The circle that forms the head is a crack (裂缝) in the ground. It might be the edge (边缘) of a crater that was much larger. Later, something like lava (岩浆) filled it. The “lava” might have come out of the hill that formed the nose.

Seeing a bear face on Mars is not as unusual as it might sound. The human mind is finding patterns in the world all the time. This is why you might see a cloud and think that its shape is like an animal or object.

People watching the surface of Mars have had experiences like this. In 1976, people were excited to find something like a human face in a picture of Mars. But it was just a huge rock.

These days, people take pictures of all kinds of things on Mars. Some have strange shapes. They might look like Martians made them, but it is probably not true. So far, Mars seems to be lifeless. The strange shapes people see are just patterns in the land.