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We’ve been looking up at the Moon for as long as there have been people to look.
And because the natural satellite is tidally locked to the Earth,
it only ever shows the same face to us. And for a long time,
we knew absolutely nothing about its so-called dark side.
But its secrets have gradually been revealed over the last fifty years,
and scientists probing the far side of the Moon have recently discovered
something new and unexpected.
There’s a chunk of metal buried deep beneath an even bigger crater.
And they hope that studying it will help solve some of
the outstanding mysteries around our solar system’s formation.
We first got a good look at the Moon's far side about 60 years ago,
when various US and Soviet probes returned the first images.
These pictures revealed a huge crater near the South Pole,
which is now known as the South Pole-Aitken basin.
At about 2,500 kilometers across and 12 kilometers deep,
it’s the biggest and oldest crater we’ve found in the entire solar system.
And scientists think that around 4 billion years ago,
an asteroid of about 170 kilometers wide impacted the Moon, creating this crater.
That impact would have been about 1000 times more explosive
than the asteroid that brought about the end of the dinosaurs.
But its incredible size isn’t the only remarkable thing about the South Pole-Aitken crater.
There seems to be something surprising hidden beneath the surface.
Modern surveys with new instruments and technologies
have revealed the structure of the Moon in unprecedented detail.
For instance, the Lunar Orbiter Laser Altimeter, or LOLA,
onboard the Lunar Reconnaissance Orbiter,
has given us precise measurements of the Moon’s topography.
And NASA’s Gravity Recovery and Interior Laboratory,
or GRAIL mission, has measured the strength of gravity all over the Moon.
Combining these two datasets revealed an incredibly high gravity reading
in the floor of the South Pole-Aitken basin.
And since extra gravity must come from extra mass,
that means there’s something huge and very dense hidden there under the surface.
This mass concentration seems to extend several hundred kilometers
beneath the lunar surface. And its density is closer to that of metal than rock.
And there’s a lot of it.
Scientists estimate that it’s about five times the size of the big island of Hawaiʻi,
amounting to about three and half million cubic kilometers of metal.
What’s more, the extra gravity is enough to pull down
the middle of the South Pole Aitken crater by another 800 meters!
Other than the dip, there’s nothing else on the crater bottom to suggest what it is,
but the lunar scientists from Baylor University in Texas have a couple of ideas.
They believe that the signal could be coming from the asteroid’s core
that collided with the Moon to make the South Pole Aitken crater.
The theory goes that a big enough asteroid would have been able to differentiate,
which is when things like planets, or in this case an asteroid
melt and the stuff in them separates from one another
forming layers
Just like what happened to Earth, back in the day.
So when the asteroid impacted the Moon,
it shattered into bits of rocks, but the metal core
would have plowed through and into the Moon’s mantle,
leaving a metallic core of iron and nickel surrounded by a rocky shell.
And because the mantle was already partly solidified, it just kind of…
stuck there, and stayed there to this day.
There is another interpretation for this extra mass,
which doesn’t call for interplanetary ballistics.
It’s possible that the extra density comes from a concentration of heavy metal oxides,
which formed near the end of the mantle solidification.
But these kinds of processes aren’t very well understood,
and the fact that the metal is right beneath
the biggest known impact crater in the solar system
is a big clue that the two are probably connected.
Ultimately, impact sites like the South Pole-Aitken basin
are important natural laboratories for understanding planetary formation processes.
By studying this crater and the metal underneath,
scientists hope to learn more about the cooling and solidification of the Moon,
the differentiation of asteroids,
and the timing of collisions in the early solar system.
For example, we still don’t know exactly when the South Pole-Aitken basin formed.
Getting a handle on that would help us date other events in the solar system,
for which we currently only have relative ages from things like overlapping craters.
So scientists are currently making a case for sample return missions
to the far side of the Moon.
They won’t be able to access the massive chunk of metal buried beneath the surface,
but physical samples of the surface material would tell us a lot.
Like, the state of the asteroid and the Moon during the South Pole-Aitken impact.
Scientists can also pinpoint which radioactive elements
got trapped within the melted rocks to potentially determine the precise age of the impact.
Combining these and other data will help researchers to figure out
the true nature of the extra buried mass, and decide whether it really is asteroid shrapnel,
or something else.
So even as we look further afield to uncover the secrets of the cosmos,
there are still plenty of mysteries still to be solved in our own planetary neighborhood!
Thanks for watching this episode of SciShow Space!
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