Lillian Goode, age 15, of Jarrah, Va., for her question:
HOW DID CRATERS GET ON THE MOON?
A few large lunar craters are visible from the earth, a distance of some 240,000 miles. But close up news from the Apollo missions was full of surprises. We now know that the lunar landscape is dotted and spotted with craters a la mode, including whoppers, mediums and an overall rash of minis.
Centuries ago, when the first maps of our side of the moon were made, some astronomers thought that the large round craters might have been formed by erupting volcanoes. After all, earth's volcanoes form large round shadowy craters. Others suspected that lunar craters might have been formed by the impact of fallen meteorites. After all, a few such meteors made craters also on the earth.
These two opinions were debated right up until the Apollo missions brought on the spot reports from the surface of the moon.
We now know that certain lunar lava flows most likely erupted from below. However, the craters, large and small, were caused by collisions with large and small space traveling meteors.
Most of the large ones crashed between 3 and 4 billion years ago, when no doubt other whoppers also fell upon the earth. However, the earth's eroding atmosphere has long since erased most of the evidence. The moon has no atmosphere, and the foot wide pits caused by even smallish meteorites remained for millions of years.
Hence, the lunar landscape has preserved a record of most of its meteorites dating back more than 4 billion years. There are craters cheek to cheek, craters in rows, overlapping craters and even craters within craters. The spotty evidence includes half a million bowls about a mile wide. There are an estimated 30,000 billion foot wide dents plus an uncountable number of pea size pits made by mini meteorites.
Many craters in the 5 to 10 mile class have flat floors and high, steep rims. Those of 15 miles or more tend to have bumpy floors, often with pointed peaks. Their rims are like rugged mountains with terraced slopes.
Sometimes a wise old male or female is the leader when the parade starts out. But flying in first place is extra hard work, and after a while the leader becomes weary. Then he or she drops back to an easier position in the parade. Then one of the other geese flies forward to take the role of leader for a while.