'Big splat' may explain the moon's mountainous far side

The mountainous region on the far side of the moon, known as the lunar farside highlands, may be the solid remains of a collision with a smaller companion moon, according to a new study by planetary scientists at the University of California, Santa Cruz.

The striking differences between the near and far sides of the moon have been a longstanding puzzle. The near side is relatively low and flat, while the topography of the far side is high and mountainous, with a much thicker crust. The new study, published in the August 4 issue of Nature, builds on the "giant impact" model for the origin of the moon, in which a Mars-sized object collided with Earth early in the history of the solar system and ejected debris that coalesced to form the moon. The study suggests that this giant impact also created another, smaller body, initially sharing an orbit with the moon, that eventually fell back onto the moon and coated one side with an extra layer of solid crust tens of kilometers thick.

"Our model works well with models of the moon-forming giant impact, which predict there should be massive debris left in orbit about the Earth, besides the moon itself. It agrees with what is known about the dynamical stability of such a system, the timing of the cooling of the moon, and the ages of ," said Erik Asphaug, professor of Earth and planetary sciences at UC Santa Cruz.

Asphaug, who coauthored the paper with UCSC postdoctoral researcher Martin Jutzi, has previously done computer simulations of the moon-forming giant impact. He said companion moons are a common outcome of such simulations.

In the new study, he and Jutzi used computer simulations of an impact between the moon and a smaller companion (about one-thirtieth the mass of the moon) to study the dynamics of the collision and track the evolution and distribution of lunar material in its aftermath. In such a low-velocity collision, the impact does not form a crater and does not cause much melting. Instead, most of the colliding material is piled onto the impacted hemisphere as a thick new layer of solid crust, forming a mountainous region comparable in extent to the lunar farside highlands.

"Of course, impact modelers try to explain everything with collisions. In this case, it requires an odd collision: being slow, it does not form a crater, but splats material onto one side," Asphaug said. "It is something new to think about."

He and Jutzi hypothesize that the companion moon was initially trapped at one of the gravitationally stable "Trojan points" sharing the moon's orbit, and became destabilized after the moon's orbit had expanded far from Earth. "The collision could have happened anywhere on the moon," Jutzi said. "The final body is lopsided and would reorient so that one side faces Earth."

The model may also explain variations in the composition of the moon's crust, which is dominated on the near side by terrain comparatively rich in potassium, rare-earth elements, and phosphorus (KREEP). These elements, as well as uranium and thorium, are believed to have been concentrated in the magma ocean that remained as molten rock solidified under the moon's thickening crust. In the simulations, the collision squishes this KREEP-rich layer onto the opposite hemisphere, setting the stage for the geology now seen on the near side of the moon.

Other models have been proposed to explain the formation of the highlands, including one published last year in Science by Jutzi and Asphaug's colleagues at UC Santa Cruz, Ian Garrick-Bethell and Francis Nimmo. Their analysis suggested that tidal forces, rather than an impact, were responsible for shaping the thickness of the moon's crust.

"The fact that the near side of the moon looks so different to the far side has been a puzzle since the dawn of the space age, perhaps second only to the origin of the moon itself," said Nimmo, a professor of Earth and planetary sciences. "One of the elegant aspects of Erik's article is that it links these two puzzles together: perhaps the giant collision that formed the moon also spalled off some smaller bodies, one of which later fell back to the to cause the dichotomy that we see today."

For now, he said, there is not enough data to say which of the alternative models offers the best explanation for the lunar dichotomy. "As further spacecraft data (and, hopefully, lunar samples) are obtained, which of these two hypotheses is more nearly correct will become clear," Nimmo said.

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Aug 03, 2011
Is it trendy to label things " Big " ? Wouldn't this have been more of a small splat ?

Aug 03, 2011
"Wouldn't this have been more of a small splat ?"

Relative to the average size of a human being this was likely an enormous splat... at relatively low speed. based upon the computer simulation the smaller moon would have had a mass of ~2.45E22 kg. I would like to have witnessed the collision and see the resulting debris! Oh wells. In ~3,999,999,999 years we'll be able to Andromeda in the night sky... i suppose i'll just have to wait until then for the next astronomically spectacular event. LIGHT & GRAVITY- WHY YOU MOVE SO SLOW?

Aug 03, 2011
This newest in a series of various lunar collision theories is no great shakes, but the the fact that large amounts of phosphorus are on the moon is great news to an earth load of 7 billion people on the way to 9 billion - with only enough phosphates to last for about 50-100 years with current usage levels. Phosphate - peak phosphate (30 years) is absolutely critical to 95% of the global food production. If we want to be around more than another century we damn well better be finding other sources. Then again peak phosphate will solve our obesity problems.

Aug 04, 2011
I might add that for many years the original photos of the far side of the moon were classified. To this day I do not know if the pix that we have have not been 'sanitized' It was alleged that there was an extraterrestrial habitation of some sort visible on the far side. Would have been absurdly easy to 'fotoshop' it out and then release the picture. Would take some cooperation from all spacefaring powers' governments to carry it off. Probably in the name of safeguarding various religious dogmas here planetside.

Aug 04, 2011
Should be easy to test whether we had a big splat or not. The chemical makeup around the site of such a huge impact should be markedly different than on the near side of the moon (assuming that the impacting object wasn't another moon of Earth's but formed somewhere else)

Aug 04, 2011
Where's the massive debris left by a moon-forming giant impact? How about within our very own asteroid belt, which is right now being viewed and analyzed by the Dawn spacecraft, while currently orbiting the massive asteroid Vesta! There's a famous mathematical calculation, I forget by whom, which determines that a planet should absolutely exist between Mars and jupiter. The Sumerians wrote that this planet between Mars and Jupiter was named Tiamet and did in fact exist, While still forming and cooling approximately four billion years ago it was hit by one of the orbiting moons of a passing planet, the tenth planet Niburu, which 1) created the massive asteroid belt where Tiamet's orbit had been, 2) forced a new orbit upon the newly formed "Earth" into its current and closer orbit of today and 3) also created the Earth's single moon, which may very well have absorbed other debris bodies as suggested in this article. The solar system's asteroid belt does exist where a planet ought to be!

Aug 04, 2011
Where's the massive debris left by a moon-forming giant impact? How about within our very own asteroid belt, which is right now being viewed and analyzed by the Dawn spacecraft, while currently orbiting the massive asteroid Vesta!

How about: No?

The asteroid belt is a bit far away for any 'splash debris' to accrete there. Very, very, VERY far away.

Any debris that managed to escape the Moon's gravity well would probably either get eventually caught by Earth's gravity well or stay in roughly the same orbit as Earth and get sucked up over time (possibly some could still be found in the stable Lagrange points of the Earth-Mooon system - but only those parts that had juuuust the right velocity and direction after impact)

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