The moon is remarkable for two asymmetries
that are surely linked in some manner.
The obvious one is that it has a front and back because it is locked
into showing the same face to Earth. The
second is that the geological history is so different.
Here we attempt to explain it
away as fairly slow merging of two planetoids.
A more convincing explanation
would be for the molten moon while still spinning to have its lighter crustal
material to concentrate as a large continent while stripping the remaining
surface leave a smooth lava ocean. Once
cooled sufficiently, all locked down and after that the moon eventually settled
into its present configuration with the smooth side facing Earth. The combined tidal effects of Earth and Sun
would drive the whole process.
The GRAIL mission will provide us
with ample data and superior resolution of the structure of the moon and will
be welcome.
GRAIL and the Mystery of the Missing Moon
Sept. 7, 2011: 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.
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.
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
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
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.
Flying in formation around the Moon, NASA's twin GRAIL spacecraft will
make precise measurements of the lunar gravitational field.
"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."
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 Moon. 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.
Gravity
Recovery and Interior Laboratory -- GRAIL home page at nasa.gov
Footnotes:
* Jutzi, M. & Asphaug, E. Nature 476, 69-72 (2011).
** By very precisely measuring the tiny gravitational perturbations of
the two satellites at various locations, and then putting all those
measurements together for the whole Moon, you get a gravity map. In making all
their calculations, the GRAIL team will have to correct for factors such as
gravitational pull of the Sun, Earth, and other planets, and general relativity,
just to name a few.
GRAIL's launch period opens Sept. 8 and extends through Oct. 19. On
each day, there are two separate instantaneous launch opportunities separated
in time by approximately 39 minutes. On Sept. 8, the first launch opportunity
is at 8:37 a.m. EDT (5:37 a.m. PDT). The second launch opportunity is 9:16 a.m.
EDT (6:16 a.m. PDT).
Credits: NASA's Jet Propulsion Laboratory, Pasadena , Calif. Cambridge Marshall
Space Flight
Center in Huntsville ,
Ala. Lockheed Martin Space Systems, Denver Kennedy
Space Center
in Florida Pasadena
Interviewee David Smith is also from the Massachusetts Institute of
Technology.
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