We are used to tectonic activity on Earth, it being a daily occurrence somewhere on the
planet.
But what about on our closest neighbour, the Moon?
During the Apollo missions, seismometers were left on the moon to detect the presence of
“moonquakes”.
From these instruments, we now know that the moon does have quakes, and relatively frequently
too.
But unlike Earth, most can be attributed to meteors striking the moons surface, although
there is also evidence to suggest that a small percentage of these quakes could be due to
the moon shrinking and contracting.
Early in the solar system’s life, most of its celestial objects were hot, a result of
the process of how they were formed and followed up by a period of heavy bombardment.
Since then, impacts have become much less frequent, and these objects - including the
moon - have cooled down.
As something cools, it contracts.
Photos taken by the Apollo missions imaging the moon’s surface, and photos from the
current lunar mission, LRO, show that the moon is covered with these kilometres-long
ridges, or lobate scarps, which can rise up to 100 meters high.
Scientists believe they are connected to the moon cooling.
They predict that these scarps are caused by thrust faults, faults caused by the lunar
crust being pushed together until it gives, causing a distinctive scarp on the surface
which is steep on one side and shallow on the other.
It could be that some of these moonquakes detected by the old Apollo missions are caused
by these thrust faults even today!
The images that we have of lobate scarps on the moon show that they must be pretty young,
certainly formed within the last 100 million years.
We know this because they can cut right through older craters, craters which are big enough
that they would have likely formed billions of years ago, not recently.
Tidal forces from the Earth could also influence the location of these scarps as the Earth’s
gravity pulls on the Moon’s crust.
Here you can see the location of the scarps across the moon, and the arrows represent
the direction of pull caused by the Earth’s gravity.
Even with all these influences, however, it is thought that the Moon has only shrunk a
total of 250m since it was formed.
Mercury, on the other hand, has had a much more tumultuous past.
It is thought that early in its life, it was actually a molten world which has since cooled
and solidified.
Mercury also is special in that it has a massive neighbour exerting an incredible amount of
tidal forces on it, the Sun.
This means its contractions were much bigger than our moon’s.
Here, you can see one such lobate scarp which is about 2km high and hundreds of kilometres
long!
In comparison to the moon shrinking 250m, Mercury is thought to has shrunk by up to
4km!
So yes, the moon is shrinking, but we don’t need to worry about it disappearing anytime
soon.
Well thanks for watching!
Celestial objects are massively interesting and there’s still so much to learn about
them, so if you have some time, why not have a look through this playlist?
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