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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 that 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 and 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 are 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’s called a magma ocean and in that
model the balances of stresses or forces that are acting on the moon and 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
Lunar Reconnaissance Orbiter mission is that we’ve seen this now growing evidence of
very young geologic 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.