July 31,2014
The
moon may look like a beautiful circle in the night sky, but scientists
have long known that it’s actually shaped more like a lemon, with a bit
of a bulge around its middle. Now, new research has solved the mystery
of the bulge, finding that it formed from tidal effects early in the
moon’s development.
Usually, this sort of bulge
is an effect of rotation — like “a water balloon flattening out as you
spin it,” Ian Garrick-Bethell, an author of the new study, told The New York Times.
The Earth itself has just such a bulge. But the moon barely spins,
certainly not enough to fully account for its shape. And at a distance
of 238,000 miles, the gravitational pull of the Earth isn’t strong
enough to explain it either.
The new research incorporated both of
these effects, and looked at more subtle factors, including the moon’s
gravity field and the huge craters leftover from meteor impacts,
according to a release. But the big culprit is a phenomenon called tidal heating.
When
the moon first formed more than 4 billion years ago, it started as a
ball of molten rock with a thin crust floating on top. Tidal heating
would have caused the crust at the poles to be thinner than the crust in
regions aligned with the Earth, over time settling into the bulge shape
we see now.
“Tides start to flex and pull on it,” Garrick-Bethell told the BBC. “And that’s easy to do because the crust is just floating there. It’s not attached to the rest of the moon.”
Although
the lemon shape of the moon has been known for a long time — the
“fossil bulge hypothesis” explaining its shape was first posited in
1898, according to the release — the moon’s history of being slammed
with meteors and other objects have complicated efforts to map its
topography, making the answer elusive until now.
“The craters are
like gaps in the data," Garrick-Bethell said in the release. "We did a
lot of work to estimate the uncertainties in the analysis that result
from those gaps."
The research is published in the journal Nature.
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