The craters of the Moon

It looks like something familiar. . .

When our astronomer pulls out his portable telescope to show objects in the sky to non-astronomers (which is most of humanity), he likes to end the show with a look at the Moon.  Not only is it brighter than almost anything else, so one’s night vision is gone after a glance; it is impressive, revealing detail and features that are almost overwhelming after faint galaxies or star clusters.  And it looks much more familiar: there are mountains and plains, shining or shadowed as may be.  And there are craters.

Not quite visible to the naked eye, even a small telescope shows some of these round rings of mountainous material, and a good piece of optics on a good night will show hundreds.  The obvious question since the time of Galileo has been: where do they come from?  What causes them?  And the answer, found in almost every astronomy book up until the Space Age, has been: they are volcanoes.  The only objects that look at all like them in human experience, round and hollow and mountainous, are volcanoes, active or inert.  Some nineteenth-century astronomical observing guides have illustrations of Hawaiian calderas, to emphasize the point.

It’s true that an object of some size hitting the Moon at high speed would create a hole, and without an atmosphere to slow or burn up meteors many more would reach our satellite than hit the Earth.  But the argument seems to have been that meteors would come in from any angle, so impact craters would be mostly elliptical, and all those on the Moon (accounting for foreshortening and other effects) are quite round.  This held the day until someone actually did high-velocity experiments on something resembling the Moon’s surface.  It turns out that a meteorite converts its kinetic energy into impact energy where it hits, and the effects travel out in all directions about equally; its momentum (which has a direction) isn’t terribly important.  (Our tutor is trying to work out how to make this into a clear and useful lesson in Physics.)  So impact craters are round also.

[There has been plenty of volcanic activity on the Moon in the past, but craters aren’t the result.]

More important, we think, is the point that lunar volcanoes were accepted because craters looked like those on Earth.  It wasn’t realized how weak this line of reasoning actually was.  It was only during the progress of the twentieth century that unfamiliar and extreme environments to be met with in space became more clear, and how unusual the Earth actually is, how much it’s not a good guide to what’s out there.  We suspect that most people still don’t quite understand this point.

A more theoretical lesson is: you may come up with an explanation; that doesn’t mean it’s necessarily the explanation.