The Moon - A Natural Satellite Figure 1: Saturn-bound Cassini spacecraft looking back to take a snapshot of the Earth and moon on August 18, 1999.To an astronomer on Mars looking at the part of the solar system between himself and the sun, one of the most interesting features would be its only double planet-a very bright, bluish-white globe with a smaller white companion a short distance away. This double planet system is, of course, our Earth and its moon. Let's look through the Martian astronomer's eyes and see how he would compare these two bodies. Seen from space, the Earth is a sparkling globe of blue, white, brown, and yellow, wreathed in spiral cloud patterns and largely covered with blue water. The continental areas, as you have seen, are in many places wrinkled by mountains, dissected by rivers, and studded with volcanoes. If our Martian astronomer watches the earth for a few days, he will be impressed by the ever-changing cloud patterns, the blinding reflection of the sun shimmering on the oceans, and perhaps even a volcanic eruption. The moon presents a strikingly different appearance. The Martian astronomer sees a bewildering maze of craters, ranging in size from hundreds of kilometers in diameter to a few meters across. Parts of the moon are covered with smooth dark plains. There are no oceans, no great river valleys, and none of the wrinkled mountain belts so prominent on the Earth. The Martian astronomer could watch the moon for days, months, and perhaps years before noticing any change in its landscape. The sun lights the same features over and over as the moon turns slowly on its axis. Why are the earth and its companion so different? Is the moon merely a small, dead planet still scarred by the events of its birth? Or is the moon, like the Earth, still evolving?
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