Nothing stirs the soul of poets and lovers like the sight of a full Moon rising
on a warm autumn night. Nothing, that is, except the sight of two moons rising.
If a paper published recently in the journal Nature is right, two moons once
graced the night skies of Earth. The idea has not been proven, but it is drawing
the serious attention of researchers.
"It's an intriguing idea," says David Smith of the Massachusetts Institute of
Technology. "And it would be a way to explain one of the great mysteries of the
Earth-Moon system: the Moon is strangely asymmetrical."
The Moon's near side, facing us, is dominated by vast smooth 'seas' of ancient
hardened lava. The Moon's far side, on the other hand, is marked by mountainous
Researchers have long struggled to explain why the two sides of the Moon are so
different, and the "two moon" theory introduced by planetary scientists Martin
Jutzi and Erik Asphaug of the University of California at Santa Cruz is the
NASA's GRAIL mission, launched from the Kennedy Space Center on September 10th,
could help unravel the mystery.
Smith is the deputy principal investigator for GRAIL, which stands for Gravity
Recovery and Interior Laboratory.
GRAIL consists of two spacecraft that will orbit the Moon in tandem, precisely
mapping the Moon's gravitational field and thus revealing how the Moon's
interior is layered.
Among other things, this could test the two moon theory.
Most Scientists believe that when a Mars-sized object crashed into our planet
about four and a half billion years ago, the resulting debris cloud coalesced to
form the Moon.
Jutzi and Asphaug say that the debris cloud actually formed two moons. A second,
smaller chunk of debris landed in just the right orbit to pace the bigger Moon
"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.
When GRAIL examines the lunar farside, will it find evidence of a long-lost moon
splattered across the ancient gray landscape?
For answers to this and other soul-stirring questions as the GRAIL mission
unfolds, please visit science.nasa.gov.