Home
Login
Register
Loading...
Practice Listening
Practice Listening
/
Video
/
SciShow
/
Where Did The Moon Come From?
Where Did The Moon Come From?
Select learning mode:
View subtitles
Pick word
Rewrite word
Highlight:
3000 Oxford Words
4000 IELTS Words
5000 Oxford Words
3000 Common Words
1000 TOEIC Words
5000 TOEFL Words
Subtitles (238)
0:00
Ahh, the moon.
0:01
Earth’s steadfast companion,
0:03
bringing light to the night and inspiring oodles of art and music.
0:07
It’s always been a central character in our cultural stories,
0:10
whether as a spiritual figure
0:12
or as a symbol for the race toward scientific progress.
0:15
For as long as we can remember, the moon has also inspired questions.
0:19
Early on we wondered,
0:20
“where does the moon’s light come from?”
0:22
And as we learned more,
0:23
we asked more complex questions,
0:25
like, “how does the moon affect the tides?”
0:27
But there’s one big, unanswered question that’s still bugging us to this day:
0:31
Where did the moon come from?
0:33
[intro]
0:36
The first thing you have to understand about our moon is that it’s weird.
0:40
Our moon is about 81 times less massive than Earth.
0:44
And that’s really strange because most moons in our solar system
0:47
are truly tiny compared to their planet.
0:51
Like most of these satellites orbit planets more than one thousand times their mass.
0:56
The only other oddball in our Solar System sits at the other extreme:
0:59
the dwarf planet Pluto and its so-called moon, Charon,
1:03
which is only about eight times smaller than Pluto.
1:06
They’re not really like a planet and a moon, where the moon orbits a planet.
1:10
Instead, they act as a binary system, where they both orbit a point in between them.
1:15
But besides them earth and our moon really stand out
1:19
We can’t glean any clues from the rest of our solar system regarding lunar origins,
1:24
because there are no other satellite-planet relationships quite like ours.
1:28
We need to get creative to come up with theories about how the Moon formed,
1:32
where it came from, and why its relationship with the Earth is so peculiar.
1:36
Now, if you’ve ever learned anything about the moon’s formation,
1:39
it was probably something about the Giant Impact Hypothesis.
1:42
That’s the most commonly accepted theory on the moon’s origin.
1:45
We've made several videos about the hypothesis,
1:47
including one that had a pin to go with it
1:49
As the story goes, some four and a half billion years ago,
1:52
ancient Earth was minding its business,
1:55
doing its gooey-lava-ancient Earth thing
1:58
when a huge chunk of space rock very rudely interrupted.
2:01
We call the intruder Theia,
2:02
and she is assumed to have been about the size of Mars.
2:05
When Theia collided with Earth,
2:07
the debris from this spectacular collision eventually coalesced
2:10
to form what we now call the Moon.
2:12
Sounds pretty reasonable, right?
2:14
The solar system certainly had a chaotic, youthful phase
2:17
before it mellowed out into the relatively quiet neighborhood we know today.
2:20
We’ve seen the aftermath of big crashes in other young star systems,
2:24
even in systems relatively close by, like in the Aries constellation,
2:29
just 300 light years away from us.
2:31
So it’s not too hard to imagine that an enormous interloper
2:34
may have impacted our own planet in its early days.
2:37
Besides the fact that it makes some intuitive sense,
2:40
the Giant Impact Hypothesis also successfully explains some weird Moon quirks
2:45
that other theories struggle with.
2:46
For example, recent lunar exploration
2:48
has shown that in its earliest days,
2:50
the moon was covered in magma.
2:53
Samples recovered from the near and far sides of the moon
2:56
have shown similar composition,
2:58
meaning it’s likely those samples
3:00
formed in one big, Moon-wide molten sea.
3:03
That’s puzzling because we typically think about planets and satellites
3:07
forming from the slow accumulation of tiny space dust into a big hunk of rock,
3:13
like rolling a massive snowball!
3:15
But that snowballing process can’t account for the amount of heat necessary
3:19
to create a layer of molten rock all over the Moon’s surface.
3:23
However, the Giant Impact could have created all that heat,
3:27
like striking flint against steel multiplied by a gazillion.
3:30
Plenty of energy to create a magma ocean on the moon’s surface.
3:34
There are also striking similarities between Earth rocks and Moon rocks.
3:38
Scientists have analyzed lunar samples brought back by Apollo missions,
3:42
and found they’re almost identical in composition to Earth-side samples.
3:46
But if the Giant Impact caused pieces of ancient Earth
3:49
to get mixed up in the newly-formed moon,
3:52
then it’s certainly plausible that their rocks would be similar,
3:55
even billions of years later.
3:57
At this point you might be thinking
3:58
the Giant Impact Hypothesis is a home run.
4:01
We know where the Moon came from, score one for science!
4:04
But hold your horses, I’m gonna poke some holes.
4:06
Because while the Giant Impact model
4:08
has been the prevailing theory for over forty years now,
4:11
it has some major weak spots.
4:13
And I’ll tell you all about those weak spots right after this short break.
4:17
Thanks to JMP for supporting this SciShow video!
4:21
JMP was designed with scientists and engineers in mind,
4:25
but it’s great for anyone solving problems with data.
4:28
The software is jam-packed with tools for data preparation, analysis, graphing, and so much more,
4:35
making it easy to go from raw data to meaningful analysis in just a few clicks, no coding required.
4:42
Plus, JMP integrates R, Python, and SQL support for smooth analytic workflows.
4:48
It basically doesn’t matter what kind of file your data is coming from
4:52
or what coding language you’re working with.
4:55
JMP works with the systems you already have in place.
4:58
To reap the benefits of visual statistics for yourself and get a 30-day free trial,
5:04
visit jmp.com/scishow.
5:09
The truth is, we know surprisingly little
5:11
about the reality of the Giant Impact and its supposed instigator, Theia.
5:15
The Giant Impact theory assumes that Theia
5:18
would have been about the size of Mars,
5:20
or about one-tenth the mass of the Earth.
5:22
That’s the most popular version of this theory,
5:25
but it’s not the only possibility.
5:27
Researchers have run computer simulations over a range of masses for Theia,
5:32
from Mars’ size to over double that
5:35
And they’ve found that despite the range of mass inputs,
5:38
many of these scenarios could have created similar impact outputs.
5:42
We also don’t know where Theia actually came from.
5:45
Recent research suggests that Theia formed in the inner solar system,
5:49
based on the chemical composition of samples
5:51
taken from the Earth, the moon, and meteorites.
5:54
Theia may have even formed closer to the sun than the Earth.
5:58
But if they formed so close to each other,
6:01
why didn’t they collide sooner,
6:02
before they were both so big?
6:04
Some scientists have proposed that Theia got caught
6:07
between a space rock and a hard place.
6:09
Theia could have been pinned
6:10
between the gravitational forces of the Earth and the sun,
6:13
at a point of gravitational equilibrium called a Lagrange point.
6:17
It’s possible that it hung out there,
6:19
gradually accumulating dust and space junk,
6:22
until it got massive enough to escape
6:24
and set a collision course with Earth.
6:26
This theory is possible,
6:28
but other studies suggest perhaps Theia formed
6:30
in the outer solar system, after all.
6:33
The jury’s still out.
6:34
Partly because the biggest remaining question
6:36
is just a major problem with the Giant Impact theory in general:
6:41
What was Theia made of?
6:42
This problem is called “the isotope crisis”
6:45
because, if you’ll recall,
6:46
we still don’t know why Earth and Moon rocks
6:48
share so much material with each other.
6:50
That’s not so much of a crisis if the Moon
6:53
was made mostly of Earth stuff that got blasted apart by Theia.
6:57
But the most widely accepted computer model of the Giant Impact Hypothesis,
7:01
which was published in 2001,
7:03
predicts that the proto-Moon debris
7:06
was actually made mostly of Theia, not Earth.
7:10
So if the Moon is made mostly of Theia,
7:12
it looks pretty suspicious and crisis-y that Theia and Earth
7:16
would have had such similar compositions.
7:19
For a lot of researchers,
7:20
that level of coincidence is just too outrageous.
7:23
So experts have scrambled to propose solutions
7:26
to the “isotope crisis”.
7:28
One potential scenario is that the Earth-Theia collision
7:30
was actually a “hit-and-run”.
7:33
The canonical Giant Impact model depicts the collision as a “slow graze”.
7:38
But “hit-and-run” proponents say it’s possible
7:40
that Theia hit Earth fast and at a steep angle.
7:44
That could have churned up more Earth debris,
7:46
leading to a Moon comprised more of Earth than of Theia.
7:50
Another option is the “merger” model,
7:52
which assumes proto-Earth and Theia
7:54
were actually two bodies of similar size,
7:57
instead of Theia being much lighter.
7:59
This would result in a more powerful collision
8:02
that thoroughly intermixed the material between those two bodies.
8:06
Then both the Earth and the Moon
8:08
would have formed from that mixture of the same basic “stuff.”
8:12
The final option is a “bigger bang”.
8:14
The Canonical Giant Impact Hypothesis
8:16
is based on a relatively tame collision.
8:19
But if Earth and Theia crashed into each other
8:21
with enough speed and angular momentum,
8:24
there would be enough energy in that collision to basically vaporize them both.
8:29
This cloud of Earth-and-Theia vapor
8:31
would then have formed a new kind of planetary structure called a “synestia.”
8:36
That name was coined specifically by the study authors
8:38
to describe this new shape they noticed
8:41
when running their computer models of the collision.
8:43
It’s a combination of the Greek prefix “syn”,
8:46
meaning together, like in “synthesis”,
8:48
and the name of the Greek goddess Hestia,
8:50
who represents home, hearth, and architecture.
8:53
So “synestia” means the assembly of our home!
8:56
That’s so sweet!
8:58
Anyway, this energetic vapor cloud would be spinning so fast
9:01
that the synestia would become a donut-y shape,
9:05
where the material is denser around the rim and emptier in the middle.
9:09
As the spinning donut cooled down,
9:11
it would have eventually settled into Earth and the Moon.
9:15
And, much like the merger model,
9:17
they would both be made from the same stuff,
9:19
explaining their similar compositions.
9:21
These are all essentially variations of the Giant Impact model,
9:25
tweaked to patch the original model’s leaks.
9:27
But some researchers are thinking outside the box.
9:30
In a paper published in 2025, one team is boldly asking:
9:34
What if the moon wasn’t formed by a single big collision at all?
9:37
Perhaps the moon formed over a longer stretch of time
9:40
due to multiple small intruders colliding with Earth.
9:43
That team’s model shows how each small collision
9:46
could have chiseled off a bit of proto-Earth’s material,
9:49
producing relatively small chunks that they’re calling “moonlets.”
9:53
another cute name.
9:55
We have some cute names in this episode!
9:57
These smaller impacts wouldn’t have generated enough momentum
9:59
for the moonlets to escape,
10:01
so a moonlet would have hung around Earth’s orbit for a couple million years
10:05
until the next impact generated a new one.
10:07
Eventually, gravity would pull all these chips off the old block into one big Moon:
10:12
the one we know and love today.
10:14
And because the moonlets are just chunks chipped off proto-Earth,
10:17
it would explain the “isotope crisis” pretty well.
10:20
Now this team isn’t the first to propose a “multiple impacts” scenario,
10:24
but it’s the most realistic one yet.
10:26
This model demonstrated that three moonlet-generating impacts
10:29
could have led to our Moon’s formation.
10:32
Whereas other simulations required
10:34
twenty or more impacts to make this work.
10:37
And that’s a lot of gravitational chaos
10:39
for all those poor untethered moonlets to withstand.
10:42
Basically, fewer impacts implies larger moonlets
10:45
that are less likely to accidentally escape Earth’s orbit
10:48
and be lost forever in the solar system.
10:50
After all, we would need them to have stayed close enough for gravity
10:54
to do its moon-making job.
10:55
Three impacts might just be the sweet spot theory
10:58
that accounts for the strange quirks in our moon data.
11:01
Without needing anything too outlandish to happen.
11:05
But of course, this still isn’t a definitive answer to the question
11:08
“Where’d the moon come from?”
11:10
As computer models get more advanced,
11:11
we’ll be able to run whatever new scenarios we concoct,
11:15
or reanalyze the chemical composition of old samples.
11:18
And it’s possible that bringing back more physical samples from the moon
11:22
might confirm or disprove our existing theories.
11:24
But it’s exciting to have a promising new explanation
11:27
after decades of the Giant Impact Hypothesis.
11:30
The three impacts hypothesis is a compelling alternate theory
11:33
for scientists to consider.
11:34
The moon continues to inspire us,
11:36
even if it remains kind of a mystery.
11:39
[ OUTRO ]
