Marie Schulenburg, age 11, of St. Paul, Minnesota, for her question:
What is the planet Mercury like?
Mercury is the smallest of the nine planets and we would expect the little fellow to be closest to Father Sun, the starry parent of our Solar System. In fact, the outstanding features of Mercury are caused by its closeness to the sun. No other planet is so dominated by our mighty sun.
The average distance between Mercury and the sun is 36 million miles and our average distance is 93 million miles. The little planet, then, gets almost three times more of the sun's seething heat and dazzling light. It also feels three times more of the sun's mighty pull of gravity. You would expect it to suffer. blistering, daytime temperatures. And so it does. But daylight falls on only half, the same half of the small planet. The other side faces away from the sun and knows only the cold and the mysterious darkness of eternal night.
Mercury has the hottest spot of any planet in our Solar System. Conditions on the day side and the night side are vastly different, but spacemen from earth would find both sides extremely unfriendly. Between the dark and the daylight, however, there is a band of twilight where the sun rises and sets and conditions are less extreme. If spacemen ever land on Mercury, they will set down their spacecraft somewhere in this twilight zone between the eternal day and the eternal night.
The first experience of spacemen on the little planet would be a strange one. Its surface gravity is only a quarter of the earth's. The visitors could leap four times higher and stride four times farther than they could on earth. A load of 100 earth pounds would weigh only 25 pounds. This is lucky, because the visitors could not move around without loads of oxygen, water and other equipment. Astronomers suspect that there is a scanty atmosphere above parts of the surface but certainly it is not breathable. It is 300 times thinner than our air and most likely composed of fine particles of drifting dust.
The center of the sunny side is almost 400 degrees Centigrade. This is hot enough to melt lead and far too hot to permit water or water vapor. Noon in the twilight zone may be somewhat cooler, though hot enough to melt tin. Spacemen could not walk on the blistering ground without special cooling equipment. And they would have to watch out for puddles of molten tin and several other substances that are frozen solid at normal earth temperature.
The Mariner 10 Spacecraft photographed the surface of Mercury while it orbited around the planet. The photographs revealed an ancient surface heaving cratered, perhaps bombarded by meteorites.
Mercury appears only now and then, low in our skies beside the rising or the setting sun. The little planet takes 88 earth days to complete its yearly orbit and 88 days to rotate once around on its axis. This is why the same side always faces the sun. However, as both planets orbit the sun, Mercury turns first its day side and then its night side to face the earth. Every 116 days, Mercury goes through phases like those of the moon.