Learning from the Lunar Landings ~by Curt Nason
A lot of media attention, and telescopes, was focused on the Moon for the fiftieth anniversary of the Apollo 11 landing. Baseball and girls occupied my mind more than the landing did in that Summer of 69; and now I am less interested in the anniversary than in what the Apollo program had revealed to us about that leading light of our night sky.
Fifty years ago astronomers pondered theories of how the moon formed, and by studying craters they had only a general idea of their relative ages and formation. For example, a crater that formed inside another must be the younger of the two, but just how long ago they formed remained uncertain. The Apollo 11 crew returned lunar rock samples for chemical analysis and dating. Most of the samples were breccias, rocks that had been broken up by asteroid impacts, and basaltic lavas. Smaller amounts were of glassy rocks, formed by the heating and rapid cooling of the surface caused by impacts, and lighter rocks of once-molten calcium and aluminum silicates called anorthosites.
If you look at the full moon, especially with binoculars, you see dark and light areas which appear mainly in the northern and southern halves, respectively. You also see bright rays stretching great distances in all directions from some of the craters. The dark areas are ancient lava flows we call maria or seas, and Apollo 11 landed in the Sea of Tranquility. Radiostrontium dating of the basaltic samples showed that they were 3.6 billion years old. The anorthositic samples had been ejected into the area from ancient impacts in the lighter highland regions.
Analyses of these samples and others returned by subsequent Apollo and robotic Soviet Lunik missions allowed scientists to determine that the moon formed from the debris of a collision between the young earth and a smaller planetoid 4.5 billion years ago. Molten from the tremendous heat released in the collision, heavier material gradually sank to the inner part of the moon while the lighter anorthosites floated and later cooled on top. Most of the craters we see were formed about four billion years ago when the inner solar system was bombarded with asteroids tossed inward by the gravity of the larger planets. This was followed by half a billion years of lava flows which filled in the larger impact basins and gradually cooled.
The Apollo and, to a lesser extent, Lunik missions placed retroreflectors on the moon, which bounce laser signals back to their origin. Accurate measurement of the time taken for the return signal gives a precise distance to the moon. We know it is receding at a rate of about 3.8 cm annually, a result of mutual tidal effects slowing the rotation rates of both the earth and moon. Other Apollo experiments included seismology to determine the lunar composition at lower levels, and measurements of the solar wind and of heat flow from the lunar surface.
When I show the moon through my telescopes, some people half-jokingly ask if we can see the Apollo 11 flag. Even the largest telescopes cannot show anything that we have sent to the moon. However, NASA’s Lunar Reconnaissance Orbiter has been taking high resolution images of the lunar surface for the past decade. Their website has pictures showing lunar landers, tracks of the rovers, and even trails of astronauts’ footprints.
Having recently celebrated our nation’s 152nd birthday, it is interesting to note that the legs for the landing gear of the Apollo missions were made in Quebec. We can say with pride that Canada has given an arm and a leg to space exploration.
Image photo: Apollo Landing Sites (Image Credit: Michael Watson)