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Moon Meets Moon

Early Earth may have had two moons! (Not lunacy, its a new theory!)

Collision with lost second satellite would explain Moon’s asymmetry.

A previous collision with a smaller companion could explain why the Moon's two sides look so different.

A previous collision with a smaller companion could explain why the Moon's two sides look so different.

Earth once had two moons, which merged in a slow-motion collision that took several hours to complete, researchers propose.

Both satellites would have formed from debris that was ejected when a Mars-size protoplanet smacked into Earth late in its formation period. Whereas traditional theory states that the infant Moon rapidly swept up any rivals or gravitationally ejected them into interstellar space, the new theory suggests that one body survived, parked in a gravitationally stable point in the Earth–Moon system.

Several such ‘Lagrangian’ points exist, but the two most stable are in the Moon’s orbit, 60° in front or 60° behind. Traces of this ‘other’ moon linger in a mysterious dichotomy between the Moon’s visible side and its remote farside.

The Moon’s visible side is dominated by low-lying lava plains, whereas its farside is composed of highlands. But the contrast is more than skin deep. The crust on the farside is 50 kilometres thicker than that on the nearside. The nearside is also richer in potassium (K), rare-earth elements (REE) and phosphorus (P) — components collectively known as KREEP. Crust-forming models show that these would have been concentrated in the last remnants of subsurface magma to crystallize as the Moon cooled.

What this suggests is that something ‘squished’ the late-solidifying KREEP layer to one side of the Moon, well after the rest of the crust had solidified. An impact, he believes, is the most likely explanation.

“By definition, a big collision occurs only on one side,” he says, “and unless it globally melts the planet, it creates an asymmetry.”

Scientists have created a computer model showing that the Moon’s current state can be explained by a collision with a sister moon about one-thirtieth the Moon’s mass, or around 1,000 kilometres in diameter.

“Why the nearside of the Moon looks so different to the farside has been a puzzle since the dawn of the space age.”

 

Such a moon could have survived in a Lagrangian point long enough for its upper crust and that of the Moon to solidify, even as the Moon’s deeper KREEP layer remained liquid.

Meanwhile, tidal forces from Earth would have been causing both moons to migrate outward. When they reached about one-third of the Moon’s present distance (a process that would take tens of millions of years), the Sun’s gravity would have become a player in their orbital dynamics.

“The Lagrange points become unstable and anything trapped there is adrift,” Asphaug says. Soon after, the two moons collided. But because they were in the same orbit, the collision was at a relatively low speed.

“It’s not a typical cratering event, where you fire a ‘bullet’ and excavate a crater much larger than the bullet,” Asphaug says. “Here, you make a crater only about one-fifth the volume of the impactor, and the impactor just kind of splats into the cavity.”

Like a Pancake!

In the hours after the impact, gravity would have crushed the impactor to a relatively thin layer, pasted on top of the Moon’s existing crust. “You end up with a pancake,” scientists say. The impact would have pushed the still-liquid KREEP layer to the Moon’s opposite side.

This theory isn’t the only attempt to explain the lunar dichotomy. many scientists have invoked tidal effects from Earth’s gravity, or convective forces from cooling rocks in the Moon’s mantle.

“The fact that the nearside of the Moon looks so different to the farside has been a puzzle since the dawn of the space age,” says Francis Nimmo, one of the authors of a 2010 paper in Science proposing tidal forces as the cause.

But despite his competing model, Nimmo calls this new theory “elegant”.

And Peter Schultz of Brown University in Providence, Rhode Island, calls it “interesting” and “provocative”, despite his own theory involving a high-angle collision at the Moon’s south pole, which he believes would have pressed crustal material northward to form the farside highlands.

“All this is great fun and tells us that there are very fundamental questions that remain about the Moon,” he says.

NASA’s upcoming GRAIL mission, designed to probe the Moon’s interior using precise measurements of its gravity, may help figure out what happened billions of years ago. “But in the end,” Schultz says, “new lunar samples may be necessary.”

(c) Copyright, 2011 IYSN. All rights reserved.

References

  1. Jutzi, M. & Asphaug, E. Nature 476, 69-72 (2011).
  2. Garrick-Bethell, I., Nimmo, F. & Wieczorek, M. A. Science 330,949-951 (2010).
  3. Schultz, P. H. & Crawford, D. A. Geol. Soc. Am. Spec. Pap. 477,141-159 (2011).
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Discussion

9 thoughts on “Moon Meets Moon

  1. Surely the difference in appearance and composition of the two sides of the Moon is mainly due is to the fact that it is rotating in a captured orbit around the Earth.

    This means that the far side from Earth is always on the outside of the orbit. Therefore, any objects coming in from the outer reaches of the Solar System have a higher chance of striking the far side of the Moon than striking the near side which is always facing the inside of the orbit towards the Earth.

    I would have thought that this fact alone explains why the near side of the Moon is a lot smoother and has less craters than the far side.

    Posted by John Horsell | August 5, 2011, 12:11 AM
    • I have an alternative theory that makes a lot more sense. Since the moon is in a fixed rotation, the centrifugal force on the back side of the moon is continually in the same area. After billions of years revolving around the earth in this fixed position, it would make sense that the near side is flat and the far side has protrusions. No need for a second moon to make this happen. Seems like a pretty logical and reasonable theory. I am not sure why it would be a mystery?

      Posted by Jon Holtzmann | August 5, 2011, 12:13 AM
    • The theory that the far side of the moon gets hit more often with space junk does not make a lot of sense, as this can come from any angle in space. The earth would block very little of it.

      However, what makes sense to me is just pure centrifugal force. Since the moon is in a fixed orbit, the centrifugal force of billions of years has been on the same portion of the moon. It would surely result in one flat side and one with protrusions. Not sure why it’s a mystery?

      Posted by Jon Holtzmann | August 5, 2011, 12:17 AM
  2. Assumed the theory is true, this slo-mo but nevertheless dramatic event must have caused some awesome “collateral damage” on Earth in my view. Hence would we be able to find witnesses of that event here at home if we’d know were (or rather: when) too look.

    Anyway, exciting idea, it would have been the celestial show of the millenium for those who had been able to witness.

    Posted by Ole Brum | August 5, 2011, 12:18 AM
  3. If the theory that the Moon was the result of a glancing blow between the Earth and a rogue, Mars-sized planet early in its history is correct, then at some point in Earth’s past there would have been two moons, and before that, three, and so on, ad infinitum, back to the original crash event. So this would just be one of the final coalescence events. Thus, it’s not all that surprising.

    Posted by Stephen Savage | August 5, 2011, 12:19 AM
  4. This idea is a logical consequence of the theory that the Moon was the result of a glancing blow between the Earth and a rogue, Mars-sized planet in its early history. If that theory is correct, then at some point in Earth’s past there would have been two moons, and before that, three, and so on, ad infinitum, back to the original collision that blew part of the Earth’s mantle into space. So this would just be one of the final coalescence events.

    Posted by Stephen Savage | August 5, 2011, 12:20 AM
  5. Based on this discussion, it appears that many agree, or tentatively agree with this theory.. what have our observations revealed of other planets with multiple moons? Also, if the Earth did at one point have more than one moon, what is the origin is this smaller satellite?

    Posted by W. Alex Edmunds | August 5, 2011, 12:21 AM
  6. perhaps we could check this theory by observing moons of other planets

    if the fact that earth shielded the moon explains the asymmetry then perhaps all other moons of other planets would be asymmetric as well , but if not all other moons are asymmetric then perhaps the asymmetry of earths moon might be due to chance phenomena such as collision with another moon

    what do you people think

    Posted by devang1994 | September 26, 2011, 1:06 PM

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