Has the Moon Changed Its Face?

by Nancy Atkinson on January 22, 2009

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The far side of the moon, as seen by the Apollo 16 astronauts.  Credit: NASA

The far side of the moon, as seen by the Apollo 16 astronauts. Credit: NASA


The moon’s “near side” always faces Earth, because the moon spins once on its axis in precisely the same amount of time it takes to revolve around the Earth. But things could have been different billions of years ago. A computer analysis of the amount of craters on the different hemispheres of the Moon shows that the far side may have once been facing Earth. A large asteroid impact may have caused the moon to change the way its faces Earth.

One consequence of the Moon being locked in a spin-orbit resonance of synchronous rotation is that more impacts should occur on the Moon’s western hemisphere than the eastern, as that side would be facing into orbit, making it more likely to be hit by debris.

But Mark Wieczorek and Matthieu Le Feuvre at the Paris Institute of Earth Physics in France compared the relative ages of the craters, using data about the sequence in which ejected material was deposited on the surface, and they found the opposite to be true. Although the youngest impact basins were concentrated in the western hemisphere, as expected, the older craters were mostly congregated in the east. This suggests that the eastern face had once been bombarded more than the western face.

This could have happened if a large asteroid impact caused the moon to do an about face. The researchers estimate after the impact, the moon would have appeared to turn slowly as viewed from Earth, and slowly would have come into its current position.

In looking at several of the largest lunar impact basins, there are several suspects for impacts that could have temporarily unlocked the Moon from synchronous rotation.

“We show that there is less than a 2% probability that the oldest lunar impact basins are randomly distributed across the lunar surface,” the researchers say. “Furthermore, these basins are preferentially located near the Moon’s antapex of motion, and this configuration has less than a 0.3% probability of occurring by chance.”

The team studied the relative age and distribution of 46 known craters. Wieczorek says the Chandrayaan-1 or Kaguya orbiters could provide information on more craters that would help in further research in this area.

For more information, see the abstract.

Sources: New Scientist

About 

Nancy Atkinson is Universe Today's Senior Editor. She also is the host of the NASA Lunar Science Institute podcast and works with Astronomy Cast. Nancy is also a NASA/JPL Solar System Ambassador.

dollhopf January 22, 2009 at 5:35 PM

This is the fairytale:

Early in the Moon’s history, its rotation slowed and became locked in this configuration as a result of frictional effects associated with tidal deformations caused by the Earth.

It is challenged.

trux January 22, 2009 at 6:28 PM

“The team studied the relative age and distribution of 46 known craters.”
Not that I have any doubts about the possibility of the Moon not facing the Earth always with the same side, but doing such a conclusion after reviewing just 46 craters does not seem to be very credible.

Marco January 22, 2009 at 7:03 PM

The moon is a fairly substantial chuck of rock. In order for an impact to knock it 180 degrees around, I would imagine that it would have to be a pretty big impactor. That would result in a substantial scar, pprobably something on the order of the northern hemishpere deformation on Mars. There are some large potholes on the Moon, but nothing that looks big enough. The Moon’s orbit-spin resonance is stable now, but this may not have always been so. If it did rotate faster, that implies that it slowed down. At some point the rotation would have been excruciatingly slow. If that period coincided with the Late Heavy Bombardment, then the east face could have been the west face at the time and no impactor is needed to explain the craters’ age and placement.

dollhopf January 23, 2009 at 2:17 AM

The Earth became somewhat flat due to rotation. So the center-equator radius is bigger than the center-pole radius. And there is also this equatorial bulge due to rotation.

The Moon is also flatened at the poles (1738.1 km to 1736.0 km according to the NASA Moon Fact Sheet).

But the puzzling thing is, that the bulge is bigger on the opposite side to the Earth. That should not be, right?

marcellus January 23, 2009 at 9:59 AM

Does anyone know when the next favorable libration to view Mare Oriental will be?

Excalibur January 23, 2009 at 2:59 AM

dollhopf :

What do u mean with bigger? Higher up from surrounding terrain, higher up from geometric center, more mass ?

dollhopf January 23, 2009 at 3:15 AM

#2 : higher up from geometric center

dollhopf January 23, 2009 at 3:49 AM

A large asteroid impact may caused the moon to change the way its faces Earth.

Of course, that must have happened after the moon rotation has already been locked. So the forces that worked to lock the moon (internal friction) had already ceased.

But then, which forces had stopped the spin caused by the impulse given by the impact? A second impact?

Essel January 23, 2009 at 6:47 AM

“One consequence of the Moon being locked in a spin-orbit resonance of synchronous rotation is that more impacts should occur on the Moon’s western hemisphere than the eastern, as that side would be facing into orbit, making it more likely to be hit by debris.”

The author appears to be bit confused, apparently forgetting that earth and moon as a system go round the sun and in the interplanetary space the western hemisphere of the moon is not always the leading edge in its orbit around the sun.

Essel January 23, 2009 at 6:56 AM

Just to add to my above comments pls note that the earth moon system orbital velocity is about 30 km/sec whereas moons orbital velocity around earth is just 1 km/sec.

The dominating factor for striking with a heavenly body would be the 30 km/sec component and not 1 km/sec component.

middenrat January 23, 2009 at 9:20 AM

excellent thought-provoking debate.
dolhopf: while internal friction of molten core is a good mechanism for slowing rotation, Earth’s gravity will also slow down the rotation I think by acting unequally on near/farside of the satellite. I think there is also a conservation of momentum effect as the Moon increases its orbital radius.
Thus I reveal my deep ignorance of planetary dynamics, but I’m posting it up here so someone will enlighten me :)

Dark Gnat January 23, 2009 at 10:10 AM

There’s also the possibility that Earth’s plate tetonics have played a role. As the continents drift, the gravity distribution would change as well.

dollhopf January 23, 2009 at 4:13 PM

Essel said: The dominating factor for striking with a heavenly body would be the 30 km/sec component and not 1 km/sec component.

No, that is not the dominating factor at all.

Because as seen from the Northpole of the ecliptic the Moon goes the same direction around the Earth as does the Earth around the Sun: anti-clockwise.

Thus, the Moon’s western hemisphere (0° N, 90° W) goes with 31 km/sec, while its eastern hemisphere (0° N, 90° E) goes with 29 km/sec. And therefore more impacts should occur on the western hemisphere. Indeed, the 2 km/sec difference dominates the situation, because “the youngest impact basins were concentrated in the western hemisphere, as expected.

But by the way, you claimed that “the author appears to be bit confused“, so I really ask myself what you actually want.

sincerusone January 24, 2009 at 4:31 PM

What a great picture of the back side of the Moon. You said: “This could have happened if a large asteroid impact caused the moon to do an about face. ”

Well sure, and considering the largest known impact crater in the solar system is the Aiken Crater(Basin?) which has other notable anomalies showcasing the immensity of the impact.

How unfortunate this picture didn’t include it in the shot by clipping the bottom of the Moon from the image.

dollhopf January 24, 2009 at 5:01 PM

“the largest known impact crater in the solar system … how unfortunate this picture didn’t include it”

Dear Syncerus One, what do you mean by that?

Yesterday, I talked about the about-face story to my workmates. And one of them, she said: it’s very clear, the Moon at that must have traversed the Earth orbit in the opposite direction. I can’t help it but I found it sooooouuuh cute.

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