GRAIL and the mystery of the missing moon

September 7, 2011 By Dauna Coulter
GRAIL and the mystery of the missing moon
The "Big Splat." Four snapshots from a computer simulation of a collision between the Moon and a smaller companion show how the splattered companion moon forms a mountainous region on one side of the Moon. Credit: M. Jutzi and E. Asphaug, Nature.

( -- As early as Sept. 8th, NASA's GRAIL mission will blast off to uncover some of the mysteries beneath the surface of the Moon. That cratered gray exterior hides some tantalizing things – even, perhaps, a long-lost companion.

If a paper published recently in the journal Nature* is right, two moons once graced our night skies. The proposition has not been proven, but has drawn widespread attention.

"It's an intriguing idea," says David Smith, GRAIL deputy principal investigator at the Goddard Space Flight Center. "And it would be a way to explain one of the great perplexities of the Earth-Moon system – the Moon's strangely asymmetrical nature. Its near and far sides are substantially different."

The Moon's near side, facing us, is dominated by vast smooth 'seas' of ancient hardened lava. In contrast, the far side is marked by mountainous highlands. Researchers have long struggled to account for the differences, and the "two moon" theory introduced by Martin Jutzi and Erik Asphaug of the University of California at Santa Cruz is the latest attempt.

Scientists agree that when a Mars-sized object crashed into our planet about 4 billion years ago, the resulting debris cloud coalesced to form the Moon. Jutzi and Asphaug posit that the debris cloud actually formed two moons. A second, smaller chunk of debris landed in just the right orbit to lead or follow the bigger Moon around Earth.
"Normally, such moons accrete into a single body shortly after formation," explains Smith. "But the new theory proposes that the second moon ended up at one of the Lagrange points in the Earth-Moon system."

Lagrange points are a bit like gravitational fly traps. They can hold an object for a long time--but not necessarily forever. The second moon eventually worked its way out and collided with its bigger sister. The collision occurred at such a low velocity that the impact did not form a crater. Instead, the smaller moon 'went splat,' forming the contemporary far side highlands.

In short, the lunar highlands are the lost moon's remains.

"By probing the Moon's gravity field, GRAIL will 'see' inside the Moon, illuminating the differences between the near and far sides."

GRAIL will fly twin spacecraft around the Moon for several months. All the while, a microwave ranging system will precisely measure the distance between the two spacecraft. By watching that distance expand and contract as the pair fly over the lunar surface, researchers can map the Moon's underlying gravity field.

"These measurements will tell us a lot about the distribution of material inside the Moon, and give us pretty definitive information about the differences in the two sides of the Moon's crust and mantle. If the density of crustal material on the lunar far side differs from that on the near side in a particular way, the finding will lend support to the 'two moon' theory."

Flying in formation around the Moon, NASA's twin GRAIL spacecraft will make precise measurements of the lunar gravitational field. Credit: NASA/JPL-Caltech

But this information is just one "piece of the jigsaw puzzle." To prove a sister ever existed, other pieces are needed. NASA's Lunar Reconnaissance Orbiter has already provided key information on the Moon's surface topography. Scientists can also refer to lunar surface chemistry data and look at old seismic information from Apollo for clues.

But what's really needed, says Smith, is a sample return mission to the far side to determine the ages of rocks there.

"The smaller moon, if there was one, was about 1/3 the size of our current . So upon collision it would have cooled down faster, and the rocks on the far side, where its remains are thought to have spread, would be older than the ones on the near side."

In any case, we have something new to think about. Shall we try singing "fly me to the moons" or "shine on harvest moons"?

"Don't go changing any song lyrics just yet," says Smith.

Explore further: 'Big splat' may explain the moon's mountainous far side

More information: * Jutzi, M. & Asphaug, E. Nature 476, 69-72 (2011).

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1 / 5 (17) Sep 07, 2011
Of even greater interest would be to explain why there's evidence of recent volcanic activity on that far side of the moon. How can such a small body still be volcanically active after about 4 billion years? Where does the energy come from?
Or would it be possible that the moon is perhaps not much older than 100k years?
5 / 5 (16) Sep 07, 2011
what on earth would a person with your personal beliefs even be doing on a website concerned with science?
5 / 5 (6) Sep 07, 2011
5 / 5 (10) Sep 07, 2011
what on earth would a person with your personal beliefs even be doing on a website concerned with science?
Trolling, of course. He's obviously not interested in debating and learning here.
5 / 5 (10) Sep 07, 2011
kevin - First, the impact that created the moon did not require just the right angle at just the right spot with just the right momentum - if these had been different, we would have had a different moon. And the same is true for the proposed two-moon impact - if it came in at a different speed or was a different size, the moon would now be different.
Second, just as the earth / big-moon L4 or L5 (the meta-stable Lagrange points) may have been a 'fly trap' for the second moon (if this recent hypothesis is correct), and would produce the right speed for that impact, it is very likely that the earth/sun L4 or L5 point served as a 'fly trap for the impactor that created the moon (or moons). An impactor freed from the earth/sun L4/L5 would tend to come in at the right speed for its impact to create a moon like our moon.
And third, scientists have tried lots of explanations, and thrown out those that don't fit the data. You should try that with creationist theories and see what's left...
5 / 5 (10) Sep 07, 2011
If you don't have any other explanation then it simply boils down to the majority vote so everyone can keep quiet and carry on with their lives.

Isn't that kind of like how Christianity came about?
4.6 / 5 (10) Sep 07, 2011
Please do not feed the trolls it only encourages them.
5 / 5 (2) Sep 08, 2011
Of even greater interest would be to explain why there's evidence of recent volcanic activity on that far side of the moon. How can such a small body still be volcanically active after about 4 billion years? Where does the energy come from?
Or would it be possible that the moon is perhaps not much older than 100k years?

"Just recent" must be considered in "GOD-time". With thoughts like that, you could be waiting a while for the second coming.
not rated yet Sep 08, 2011
If a second moon impacted the Moon it would introduce some extra spin to it. Being lopsided after the impact there would be a more stable position and as the spin reduced it would eventually have rocked backwards and forwards around the most stable position.

This would explain why the front and back are different as if it was west and east hemisphere that would have been a less stable position. The probes should be able to determine if the mass distribution reflects this.
5 / 5 (2) Sep 08, 2011
"The Moon's near side ... is dominated by vast smooth 'seas' of ancient hardened lava."

These maria are themselves large impact basins back-filled with lava. Presumably, these impact basins covered the whole of the moon's surface at one stage (i.e. before the "Big Splat" covered those on the far-side).

Radiometric dating of maria basalts suggests that the greatest volume of these erupted from 3.9 to 3.2 bya and these eruptions occurred about 0.5by after the cratering events.

Various reports I've read on the "Big Splat" theory mention that the secondary moon would have remained at whichever Lagrange point for a few 10's or 100's of millions of years before being de-stabilised and eventually impacting with the primary moon.

Could it be that the "Big Splat" impact event provided sufficient energy to trigger the eruptions of lava that filled the maria on the diametrically opposite near-side. If so, then we could pin down the timing of this event to the start of these eruptions.

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