ranger moon

Astro Bob, diurnal, libration (an excerpt)   OUR MOON ROCKS

Since the moon completes a rotation in the same time it takes to revolve about Earth, an observer on Earth always see the same face. All parts of the moon receive sunlight during a lunar orbit – there is no lunar ‘dark’ side. Because the moon orbits the Earth in the same time it takes to spin on its axis, it always keeps the same hemisphere pointing at us. This is called synchronous rotation and is caused by the gravity of the Earth acting upon the moon to slow its rotation to a rate equal to its orbital period of 27 days. That’s why we’re stuck with seeing the same face for as long as humanity has gazed at the moon. Much farther back in time, the moon rotated faster. If early life forms cared or knew to look up, they would have seen all sides of the moon as it went through its phases.

Indeed the far side is nearly all ancient crust, like the white-colored near side, and saturated with impact craters from bombardment by asteroids and meteorites some 4 billion years ago. But check this out. The far side crust is 50 miles thick versus 37 miles for the near side. The extra thickness is the most likely reason for its dearth of seas; lavas that may have erupted from below couldn’t reach the surface to fill the basins carved by impacts.

Shortly after the moon formed, its solid, outer crust floated on an ocean of liquid rock. Did the difference in hemispheres have something to do with Earth’s gravity acting on this moveable crust on a faster-rotating moon? Another factor that may play into the frontside-backside difference has to do with heat produced by radioactive elements. NASA’s Lunar Prospector probe found more on the near side, where they may have encouraged the formation of hot magmas that eventually found their way to the surface.

The Earth plays apart in creating another difference between the far side and the familiar face of the moon. It acts like a shield for the near side, intercepting potential meteorites and (long ago) small asteroids that otherwise would have struck the moon. The far side has had no such protection once the moon’s rotation became locked to its orbit.