We're all familiar with the face of the Moon — it's blobby shapes and colors are part of the unconscious map of our environment we all share. And we all know that the Moon doesn't rotate as seen from Earth. But now, research suggests that once it was the rear face of the Moon that pointed at our planet.
The study was carried out at the Paris Institute of Earth Physics, where scientists were examining the complex crater patterns on the Moon. By carefully observing which debris sprays, rays, and crater formations overlap others, and combining that data with geological evidence gathered from the precious few moon rock samples, it's possible to determine an approximate timeline for the events that formed the lunar surface.
Modeling the Moon's orbital path around the Earth and comparing projected results with the crater history can tell us about the Moon's history. Simulations show that the Western hemisphere of the Moon (as seen from Earth) should have about 30% more craters than the Eastern half — simply as a result that the Western face is the one that points along the Moon's orbit, and thus should "sweep up" more asteroids than the trailing Eastern face.
But the French team found that data from the 46 characteristic craters they studied didn't support that theory all the way through the lunar history. In fact, they found that the Eastern hemisphere had the highest density of oldest craters. And that suggests the Eastern hemisphere once pointed forward, and what we now know as the "dark side" of the Moon once faced us.
They conclude that a massive asteroid impact at some point around 3.9 billion years ago set the Moon spinning on its axis. When gravitational tidal forces eventually robbed the satellite of its spin, it settled into its current orbit with its current alignment of hemispheres.
But there's an important lesson for us hidden in this data — one that's been repeatedly underlined as scientists make discoveries.
We know the Earth has been hit by giant asteroids in the past, but the evidence is generally concealed. The fact that our next door neighbor encountered an asteroid big enough to change its orbital behavior should be a new spur for our near-Earth asteroid "spaceguard" programs so that we might avoid something equally disastrous happening over here.