What’s wrong with this picture? I pulled a switcheroo on you to illustrate how different the full moon would appear if instead of its familiar front side we could see the hidden far side. The difference is obvious – a near total lack of dark spots or lunar ‘seas’ that make up the face of the ‘man in the moon’.
The front side of the moon has numerous seas, better known as ‘maria‘ (MAH-ree-uh) from the Latin.Â Neil Armstrong and Buzz Aldrin, the first humans to set foot on the moon in 1969 astronauts did so in Mare Tranquillitatis or Sea of Tranquillity. Of the 23 or so named maria, only two are found on the far side – Mare Ingenii (Sea of Cleverness) and Mare Moscoviense (Sea of Moscovey, a reference to the Moscow region) – though several maria around the moon’s edges are partially visible on both sides. All maria are formed of dark, solidified lavas that long ago oozed into large craters and basins from the lunar mantle through cracks in the moon’s crust.
The first pictures of the lunar far side were taken by the Soviet Luna 3 probe in October 1959. They were of low resolution but revealed the stark difference between the two hemispheres for the first time.
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.
The only people who’ve seen the hidden hemisphere are the Apollo astronauts. Apollo 8 astronaut William Anders described the view from lunar orbit in 1968: “The backside looks like a sand pile my kids have played in for some time. It’s all beat up, no definition, just a lot of bumps and holes.”
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.Â The extra thickness is the most likely reason for its dearth of seas;Â 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.
Currently the Lunar Reconnaissance Orbiter (LROC) has been mapping the moon at a resolution of 1-meter or about a yard. I encourage you to make a visit to the far side by clicking on the photo above assembled from thousands of LROC images. And if you’d like to get familiar with the names of features on the hidden side, check out this large, annotated lunar map. There you’ll discover the enormous crater Apollo, the unique lava-flooded floor of Tsiolkovsky crater (named after Soviet rocket scientist Konstantin Tsiolkovsky. Don’t miss this video flyover!) and the South Pole-Aitken Basin, one of the biggest, deepest impact scars in the solar system.
When you look up at tonight’s full moon, use your new ‘X-ray’ vision to see beyond to its rugged other half.