Why We Never See the Far Side of the Moon

The moon's near side (left) is covered with dark splotches of lunar maria that look like a man's face when seen from Earth. The moon's far side (right), with its many craters and elevated topography, looks quite different. (Photos: NASA)

Do you ever wonder why we never get to see the far side of the Moon?

New research suggests it’s because of the rate at which the moon’s rotation slowed before locking into its current orientation with Earth.

Artist's drawing of body hitting young earth to form our moon (Artwork: Joe Tucciarone)

Scientists now think the moon was formed from a very large impact, according to Dr. Oded Aharonson, professor of planetary science at the California Institute of Technology (Caltech).

Something hit our planet long ago, ripping a big chunk of Earth out. Some of that original impact material collected together under its own gravity and orbit to form the moon.

When that happened, according to Aharonson, the moon was closer to Earth and it spun a lot faster.

If anyone was on Earth at the time, they’d have looked up and seen different sides of the moon on different nights, rather than the same static view we see today.

So what caused the moon’s rotation to slow down?

Aharonson points to a process called “tidal dissipation.”  The moon, as it orbits our planet, raises tides on Earth.

A closer look at the South pole of the far side of the moon as seen from the GRAIL mission’s Ebb spacecraft. (Photo: NASA/JPL-Caltech)

At the same time, Earth also raised tides on the moon, causing it to bulge and change its shape slightly.  This also caused the moon to dissipate energy in the form of heat.   This process took energy from the rotation of the moon which caused its spin to slow down.

Over the years, the moon has fallen into a synchronous orbit with Earth.

“Like a ballroom dancer, the moon spins around the Earth at exactly the same rate as it spins around itself,” Aharonson says, “so it always maintains the same face towards its companion, Earth.”

Although the moon’s rotation speed has stabilized and is no longer slowing down, its orbit is evolving outward from the Earth, slowly expanding its circuit around our planet.

Right now, it takes the moon around 30 days to orbit the Earth, but in the future, says Aharonson, both the moon and Earth will eventually synchronize to each other.  The consequences of this synchronization, according to Aharonson, are that Earth would no longer experience changing tides and we would no longer observe the moon go through its monthly phases.

That’s when we would really see the exact same thing night after night when we look at the moon.

NASA’s GRAIL mission has beamed back its first video of the far side of the moon. The imagery was taken on Jan. 19 by the MoonKAM aboard the mission’s “Ebb” spacecraft.

The process of tidal dissipation has also affected the shape of the moon.  Looking at a big bright full moon, we see a perfectly round sphere. But it’s not, according to Aharonson, who says it is actually an ellipsoid, shaped a little like an American football.  The moon has three axes and each one of them is a little different.

Why we only see the near side of the moon was not due to random chance, but depended on the two sides of the moon being asymmetrical.

The side facing us is filled with large, dark, basaltic plains formed by ancient volcanic eruptions which are dense as well as low and closer to the moon’s center of mass.

And, with all due respect to the Pink Floyd album “Dark Side of the Moon,” the other side of the moon isn’t really necessarily dark.  When it’s illuminated by the sun, it can appear much brighter than the side we see.

Some new and unique views of the moon in this video “tour” from NASA

Its moonscape is quite different, too. It is filled with mountains rather than the basaltic maria and basins of the near side.

If the moon were flipped around, giving us a view of the far side, Aharonson says we’d notice a little bit more mass and, because of its high mountains, it appear to be closer to Earth.

Scientists still don’t know why the moon is so asymmetrical; why one side of the moon is so different than the other.

Dr. Oded Aharonson joins us this weekend on the radio edition of “Science World.”  Tune in (see right column for scheduled times) or check out the interview below.

Other stories we cover on the “Science World” radio program this week include:

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• Scientists turn small living things into electronic cyborgs
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• Hallucinogenic drugs used to treat PTSD, Sex Abuse Victims