Mysteries of the moon’s far side

If only we could. This is how the far side of the moon would appear at full moon. The foreground's mine with the original full moon replaced by a toned version of the moon's hidden side taken by NASA's Clementine probe. The only obvious dark lunar sea is Mare Moscoviense at upper left. The large shaded area near the bottom is the South Pole-Aitken Basin, one the largest impact basins known at 1550 miles across and 8 miles deep!

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 two hemispheres of the moon compared. Credit: NASA

The front side of the moon has numerous seas, better known as ‘maria‘ (MAH-ree-uh) from the Latin.  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.

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. Illustration: Bob King

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.

One of the first lunar far side photos taken by the Soviet Luna 3.

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.  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.

A composite of over 15,000 images taken by the Lunar Reconnaissance Orbiter currently in orbit about the moon. The detail visible is spectacular! See what I mean by clicking the photo for an enlarged view. Credit: NASA

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.

This series of photos taken by LROC show the moon through nearly a full rotation so you can see how the near side transitions to the far. Click to enlarge. Credit: NASA/Goddard/Arizona State Universit

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.

10 Responses

  1. Mike

    Good morning! I love the dewdrop photo! “Dew” tell how you got it! Camera, lens, exp, etc. VERY nice. Can you see what is in the reflections within the drops?

    1. Profile photo of astrobob

      Hi Daniel,
      Good questions. We’ve known for at least the past few decades though I don’t know which was the first satellite orbiting the moon that gave us the data to determine lunar crust thickness. The Clementine mission in the early 90s used topography and gravity measurements taken from lunar orbit to create a global map of crustal thickness.

  2. Feroz Siddiqui

    hi, i want to share what i think about why moon faces earth; my perception is “the center point of gravitation of moon is little tilt toward near side” and the result is what we see today.

    1. Profile photo of astrobob

      Hi Feroz,
      The moon’s center of mass is offset about 1.2 miles in the direction of Earth however this is not the reason the same side of the moon faces the Earth. The moon used to rotate much faster many millions of years ago, but the more powerful gravitational pull of the Earth gradually slowed its rotation so that today it rotates at the same rate as it revolves around the Earth. That’s why we see only one face.

      1. Feroz

        hi Bob,
        Ok in case I agree with you:-
        1- Earth’s gravitation is slowing down the rotation of Moon. That means still the time remain when it will stop the rotation completely, and in coming some lacks years we will able to see its far side too.
        2- And in case you say Moon has stop rotating; why it stop at facing this side? Because of tilt of center of gravitation.

        1. Profile photo of astrobob

          Earth’s gravitation has slowed down the moon’s rotation so that it’s now tidally locked with just one hemisphere facing Earth. It won’t continue to slow it down so that we’ll see the farside someday because Earth’s rotation is also slowing, maintaining the “lock”. As energy from our planet continues to be transferred to the moon via tidal friction, the Earth will spin more slowly and the moon will continue to recede. Some 50 billion years in the future, Earth’s rotation will equal the moon’s period of revolution and there will be a double tidal lock – only one face of the Earth will always face the same side of the moon. Neither the Earth nor the moon will stop rotating. Their rotation periods will simply be much longer in the distant future than they are now.

    1. Profile photo of astrobob

      The nearside crust is thinner than the farside, which allowed lavas from beneath the crust to more easily reach and flood the nearside impact basins, creating the lunar seas.

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