[music] Tom Watters: Two years ago, we reported
evidence that the moon is shrinking. Now, we've found
evidence that the moon is actually being pulled apart--forming
features called graben. So, the "shrinking moon"
--it turns out, is not shrinking everywhere.
Some places, the moon is actually expanding by
a little bit. So, finding these young graben was a real surprise
because we thought, well, all these lobate scarps are telling us
the moon is shrinking, so what are these little, small
graben that are telling us the moon is pulling apart--doing
in this picture? How does this all fit together? All that's related
to how the moon has evolved--how the moon has lost heat.
over its four-and-a-half billion year history. Most of the
terrestrial planets, when they formed, were very hot, and they got so hot
that they actually completely melted. When that happens,
they will be in a general state of contraction because they're still hot
on the inside and cooling down, and as they cool, they want to shrink.
Only the outer part of the moon melted, forming what is called a magma ocean,
and in that model, the balance of stresses or forces
that are acting on the moon would allow us to form both
these small lobate scarps that show contraction, as well as
these small graben that show the moon being pulled apart.
One of the really, really exciting returns
of the Lunar Reconnaissance Orbiter mission is that we've seen this
now growing evidence of very young geoloic activity on the
moon. Many, many people have felt that the moon
is pretty much geologically dead, and what we're finding is that that's totally wrong,
that the moon appears to be geologically active now.
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