Early Earth May Have Had Two Moons

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

Richard Lovett
Published online 3 August 2011 | Nature |
doi:10.1038/news.2011.456
http://www.nature.com/news/2011/110803/full/news.2011.456.html

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

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, says Erik Asphaug, a planetary scientist at the
University of California, Santa Cruz, who co-authored
the study with Martin Jutzi, now of the University of
Berne[1].

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, Asphaug says, 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."

Asphaug and Jutzi 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.

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," Asphaug says. The impact would have pushed the
still-liquid KREEP layer to the Moon's opposite side.

Apshaug's theory isn't the only attempt to explain the
lunar dichotomy. Others 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[2].

But despite his competing model, Nimmo (a colleague of
Asphaug's at Santa Cruz, but not an author of the new
study) calls the 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[3].

"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."

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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