This is geology B with Oliver stromwell.
Following the Apollo missions to the moon in the late 1960s and
early 1970s, it became widely accepted that the moon formed as
a result of a cataclysmic Collision.
When the Earth was forming about four and a half billion years
ago. But a puzzle has remained as to
just what sort of collision must have occurred in order to result
in the earth-moon system. We see today.
How big was the impacting body? What was its composition and
what happened in the aftermath of the Collision?
Sarah Stewart is a professor in the department of Earth and
planetary Sciences at the University of California Davis.
She conducts, lab experiments in conjunction with numerical
modeling and out-of-the-box thinking to develop new
scenarios for a moon-forming impact.
And the subsequent dynamical Evolution, that appears to
resolve many of the outstanding difficulties associated with
previous Solutions. Sarah Stewart.
Welcome to geology B. You Oliver for inviting me.
Why do planetary scientists believe it was a giant impact
that created the moon. It's the best idea to explain
the present day rotation of the earth, and the orbit of the moon
along with the composition of the moon.
But it wasn't until after the Apollo missions that scientists
realized how common giant impacts would have been during
Planet formation. And right now, we know that
giant impacts were very likely events as planets were growing.
So what was the traditional Moon formation scenario?
That prevailed before the Apollo missions to the Moon before the
Apollo missions, we didn't have a single best idea for how the
moon formed there were actually multiple competing ideas.
The moon could have been captured or a created alongside
the Earth the whole time or there was even the idea that the
moon spun out of the Equator of the Earth in a process called
fission, but the analysis of the Apollo lunar samples changed our
ideas, the data from Apollo ruled out, all of the ideas that
scientists had for the origin of the Moon.
The moon's core was too small to explain the moon.
If it had been captured from somewhere else in the inner
solar system, the moon's composition is similar to
Earth's mantle but different enough that it doesn't explain
the idea that the moon just grew along The Earth together and
fission had so many problems that people really didn't take
it. Seriously.
It didn't explain today's length of day or the total angular
momentum of the Earth, Moon system.
So the Apollo Mission sent everyone back to the drawing
board on the question of how did the moon form the giant impact
idea only emerged in this aftermath where we were looking
for new ideas and it was at that time, that the Studies on how
all the planets grew together around the Sun realized that,
there would have been these giant hugely energetic
collisions, and before that people had really been thinking
that the energy of Planet accretion involve much smaller
bodies colliding. So how does the giant impact
scenario result in a Moon Earth system that we see today?
So, the way the giant impact was proposed is that the energy of
the event would Part of Earth's mantle and send it into orbit in
a disk around the earth and the moon would grow out of that disc
of hot debris and in the conceptual phase, this could
explain why the Moon looked similar to the Earth.
But not exactly like the Earth because it was a hot disc and
the moon didn't grow with any water or what we call moderately
volatile elements elements, like, potassium and sodium that
vapor. Her eyes at intermediate
temperatures, like, 1,000 degrees and so these
distinctions could be explained by a giant impact.
In addition to the large, massive, our moon, our moon is
1.2 percent, the mass of the Earth, and that's much larger by
fraction compared to the moon's that we see in the outer solar
system. And finally, the giant impact
gave us a way to explain why we have a 24-hour day today along
with where the Moon is in its orbit.
So there are a lot of great features in the idea of the
giant impact from our earlier conversation.
It seems, as if there are three main characteristics of today's
earth-moon system, all of which a successful model has to be
able to replicate, these are the compositional similarity of the
Moon and the Earth that you already referred to.
The tilt of the earth spin with respect to the ecliptic, which
is the plane of the Earth's orbit around the sun and the
inclination of the moon's orbit. It's been difficult to develop a
single model that can reproduce all these characteristics at
once. Why is that there are so many
pieces to the puzzle on the origin of the Moon, and they
involve both understanding the Eric's of collisions, the
chemistry of what occurs after those collisions and the
dynamical evolution of the lunar orbit and the giant impact
hypothesis. Gave us the bulk
characteristics, like the mass of the Moon.
The fact that its core was so small because most of the debris
was formed from the rocky mantels of the colliding bodies,
but the way the original hypothesis worked out In terms
of, where did the material come from that went into the moon
forming disc. Most of that material came from
the body that struck the young Earth, and we gave that body a
name. Its name is Thea.
So the moon would look compositionally a lot like this
other body. And we know, from studying
meteorites that no two planets in the solar system are the
same, no, two asteroids, you're identical.
And so we would expect there to be different In its chemistry
and in the Isotopes of the different elements in the body.
What we learned from the Apollo samples is that the moon was
identical to the Earth in many ways and that really called into
question, the giant impact hypothesis, were the moon form
mostly from this other body. We tried to come up with an
explanation of how to make the Moon look more like the Earth.
And the solution was to mix the two bodies to a greater degree
than in the original giant impact.
And the big idea that we had at the time was that the
moon-forming impact did not lead to our 24 hour a day, but
actually made the earth spin much faster and in giant impacts
were the Earth spins faster. The two bodies mix to a greater
degree and you could make the disc out of an equal portion
mixture as what went To the Earth.
And this was one way to explain how the chemistry and the
Isotopes of each element were more similar between the Earth
and the moon than the original giant impact model.
The catch though, is that you still have to get to today's
24-hour day and you must do that during the lunar tidal evolution
by transferring angular momentum away.
Probably through a resonance with the Sun that slows down the
spin of the Earth and Leads to the path that gets to our 24
hour day. So if so, to speak aloud to have
a high angular momentum impact in which the Earth starts out
spinning very fast. How does that help us with?
The question of the same composition in the earth and
moon and ending up with the right orbital characteristics
today compared to the original giant impact model these high
angular momentum Giant Facts were also more energetic and led
to more vaporization and more complete mixing between the two
bodies. However, when they were first
proposed in 2012, there were only a few very specific impact
scenarios that could lead to the exactly the same mixture in the
moon, as in the earth and the response from the scientific
Community was to have some skepticism.
Mm, you know, is that really correct that you would need?
Perhaps a special case giant impact to explain the Earth and
the moon. And so there was some pushback
and it seemed like we were still missing something.
Okay, so what did you do? Next?
It took several years of working on these high energy, high angle
momentum giant impacts to try and understand them and they
were just very different from what anyone had studied before
because Previously, the entire Community was focused on
explaining the 24-hour day that we have today, but once we were
free to explore a larger parameter space, we made giant
impacts that had outcomes that we hadn't seen before.
And it turns out that giant impacts can transform a planet
into a completely different type of astronomical object.
And we we figured this out around 2016 by looking at the
computer calculations of these giant impacts and realizing that
the object in the middle. Just didn't look like a planet
anymore. So there's a very specific
definition for a planet that the international astronomical Union
wrote down when we tried to decide, if Pluto was still a
planet planet has to orbit the sun, it has to clear its Zone
around. In its orbit around the sun and
it needs to have a spherical shape that represents what we
call hydrostatic equilibrium. And so, any rotating body will
have a slightly oblate spheroidal shape.
That's a balance between gravitational and pressure
forces within the body. And what happens in a giant
impact is that the planet becomes so hot that it partially
vaporizes and puffs up and it's radius.
Has become so big that it can't rotate all together anymore and
maintain that spheroidal shape. And at that time it transitions
to an object that forms a disk that's constantly changing and
shape. So it's no longer hydrostatic
and it no longer satisfies the definition of a planet.
And so we had discovered in our computers, this entirely new
object and so we had to give it a name we call these objects.
It's siness Diaz and we derive the word from the Goddess.
Hestia is the goddess of the Hearth and Home.
We were studying the origin of the earth.
It was hot. And we attach the prefix in to
emphasize that the disc that the Moon is forming in was actually
part of the earth. And we proposed that the moon
formed basically inside the Earth, when the Earth had been
transformed to this huge object that we call a synesthesia Can
you describe what exactly a semester is?
When you think of a planet, you think of a spherical body that
rotates around altogether and the shape of that body is
spherical by a balance of forces that we call hydrostatic
equilibrium. But if you make the body larger
by heating it up and vaporizing, the outer layers, and making it
spin a little faster, increases, the size of the Equator of that
object, And there's a certain distance from the center where
the body can no longer rotate altogether.
And any material outside of that special distance, rotates more
slowly in a disk, like region around the Central Mass that
transition, we call the co rotation limit for a planet and
it depends on what the It is made out of and how massive it
is exactly what that critical distance is.
But once it's exceeded the shape of the object is no longer a
nice spheroid but instead makes a fin disk that's connected to
the interior mass and that object is called a Celestia but
is the Sonesta just the bit beyond the rigid part When a
body exceeds the co rotation limit, There's no distinction
between what is on the inside of that distance and on the
outside, all of the material is connected and interacting in the
same fluid system. And so the name Sonesta
represents the entire system. All of the mass because there is
no distinction between a planet in a disk anymore.
It's all one body so that'd be no sir.
Suffice to stand on when a rocky planet.
Like the Earth has turned into a synesthesia by a giant impact.
It's so hot that the gas and the outer layer becomes a high
pressure gas and a supercritical fluid as you go deeper and
deeper at higher and higher pressures.
And there's no liquid surface layer present at these extreme
energy conditions. So that's right.
There would be nowhere to float. About.
I can see that this would give you a framework where you would
get an awful lot of mixing which would solve the problem of the
Moon having the same composition as the Earth.
But does it result in the orbital characteristics that we
see in the earth-moon system today?
There are still big requirements on what happens after the Moon.
Accretes from that debris before we can get to the present day.
Initially, any giant impact makes a disc in the equator of
the new post-impact body. And so we expect the moon's
orbit to be in the equator of the young Earth but as the Moon
tidally migrates away from the Earth it transitions to having
its orbit be dominated by interactions with the sun and
the ecliptic plane of the solar system that the sun defines.
And in that transition, we expect the moon's inclination to
end up perfectly in that plane. And if the moon were in that
same plane, then we would have a solar eclipse every month.
The moon would line up perfectly with the Sun, but we don't, and
we don't, because the Moon is tilted by 5 degrees from the
ecliptic plane. And that's not what we expect,
if the moon formed in a giant impact.
So there's still another major puzzle to be solved.
So how did you go about resolving that?
We went back again to the idea of a high angular momentum giant
impact. It turns out that there are many
ways to make a siness TIA, most giant impacts have a lot of
angular momentum, and many different kinds of giant impacts
can make a synesthesia. And so, we now have more choices
for the kind of giant impact, but we proposed a path that
could lead to the five degree inclination.
That Involves a lot of angular momentum.
More than two, two and a half times what we have today and
that the giant impact tilted, the axis of the new Earth over
very far, maybe 65 70 degrees from the celestial North.
Today, we're at a 23-degree tilt and a giant impact has equal,
probability of the Earth's, spin axis pointing On the sky after
the giant impact. So it is plausible that the
Earth's spin axis was tilted over but then something has to
happen to change the tilt of the earth and that's part of the
title, migration of the Moon. If the Earth starts tilted, over
in the process of the Moon, migrating away from the earth,
there's an interaction with the Sun that leads to the Earth's
Tilt being restored to where it is today.
But leaves the Moon in a high, inclination orbit tilted away
from the plane of the solar system high enough so that
during its title Evolution, it doesn't reach zero today.
So this is a brand new idea that my colleagues and I have
published as a way of explaining this 5 degree tilt and it's just
been launched into the community to try and work on.
But currently it is at least a self-consistent way to make.
Our flavor Moon and its peculiar orbit.
I understand you figured this out by running some numerical
simulations on a computer. But do you have a qualitative
understanding of how this inclination becomes reduced,
while the Earth's Tilt gets back to what it is today?
And we settled down into this stable orbit that we have now
During the lunar tidal, migration outwards there are a
couple of dynamical resonances that occur and you can think of
these resonances being times where the orbit is a bit
unstable and very sensitive to large changes.
And so if people intuitively want to understand what is
happening with the moon's orbit and the Earth's Tilt, the first
major process has to do with that transition from the Moon.
In being an orbit around the earth's equator and dominated by
interaction with the tilt of the Earth, to the Moon, precessing
around the ecliptic plane and that transition is called the
LaPlace plain transition and when the earth begins tilted
over there's a big instability during that transition that
changes the tilt of the earth and changes the length of day of
the earth to get us on a path to today.
And leaves the moon and a high inclination orbit.
And then as the moon continues to migrate away from the earth,
it feels special Tides with the Earth that high inclination,
that lower the inclination, but not all the way to 0, which is
what we want. It doesn't seem intuitive at all
to me, but I'm sure the computer models are correct.
I can see that this is something that was surprising to the
community. Did you expect to get this
result? Out.
Now, I have been surprised at pretty much every stage of my
work on the origin of the Moon, and it's partly because we live
in a world where our textbooks have given us an explanation of
a mars-sized object called FEA. That struck the Earth and gave
us the angular momentum that leads to 24 hours, but the
details were missing and there were some serious observations
that, couldn't be explained by this specific giant impact
event. And and what has happened with
the angular momentum work? And these new title models is
there's actually a lot of new ideas, a lot of changes, the
dust has not settled at all. I'm trying to explain the lunar
orbit and the Isotopes. So it's a very exciting time in
trying to understand the origin of the earth.
And moon, was that mentioned in the introduction.
You also conduct lab experiments?
Can you tell us about those? The experiments are amazing
because we can now reach in the lab.
All of the pressures and temperatures that are generated
during Planet formation, even in these humongous giant impacts,
And what's hard to appreciate is?
How much power is in a giant impact.
You calculate, the total kinetic energy of the two colliding
bodies over the time scale of the event.
The power of a giant impact is like all of the energy coming
out of the Sun and so all of that energy going into a planet
leads to nearly complete melting and partial vaporization.
And we can recreate that thermodynamic path in the lab.
How on Earth could you do that? No pun intended.
We do what are called Shockwave experiments in the lab and we
can generate these huge pulses of pressure that lead to
temperatures. Tens of thousands of degrees by
using lasers or huge capacitors. That can launch metal plates up
to 20, 30, 40 kilometers per second and that's close to the
speed that the Earth is orbiting around the Sun.
So these facilities that can reach these extreme conditions
were built by the department of energy.
They're housed, Lawrence Livermore, National Lab Sandia,
National Lab, they were built to try and Achieve nuclear fusion
by generating intense pressures. And so we use the same platforms
to study natural processes. Like giant impacts and
understand how planetary materials respond to very strong
shock waves. So you managed to get some time
on these giant. Laser Fusion machines, these
world-class facilities that were built by the department of
energy, all have basic science programs and so scientists from
all over the world can apply to conduct.
Experiments on these facilities and we compete with each other
for time. And so we have successfully
gotten precious days on these massive machines to make giant
impacts in the lab. So what do the results of your
experiments tell you? Because we could reach the exact
pressures obtained, during a giant impact.
Millions of times more than atmospheric pressure on the
earth, we could determine how hot geologic materials got
during an impact event. And we developed a special
technique to find the boiling points of rocks and metals that
are reached during giant impacts.
And so we could calculate the action of the earth and the
fraction of the bodies that collided with us, what fraction
became gas and what fraction melted into magma.
Oceans does this mean you exploring new parts of the phase
diagram of earth materials at high temperatures and pressures?
That's right, we can't reach these conditions in static
experiments when we think of phase diagrams.
When does a material melt and when does it turn into a gas for
many materials? Like water, we can reach all of
those conditions in the lab by controlling the temperature and
pressure. But for the main minerals in
planets, the temperatures are just too high for us to be able
to hold the material at those temperatures and study them.
So the only way to get to the boiling points of many of the
phases in the earth is to do it transiently during a shock wave
experiment. And then we We can generate a
little parcel of rock, that's boiling and study it and the
whole experiment could last tens of NS over, what sort of scale,
how big a piece can you experiment on?
And is it a piece of Olivine? You stick inside these machines?
We've been studying the major minerals in planets that
includes Olivine and Bridgeman night type compositions, which
is the main mineral in the lower mantle and our samples are tiny
because we can only take a small volume to these high pressures
in a controlled way so that we have time to study it.
So the sample thicknesses are only a few hundred microns
thick. So how do you use the results of
your Lab experiments to try and figure out what happened during
this Collision. We use our lab data to extract,
the thermodynamic properties of the major minerals in the earth.
And we construct a model for that material that is called an
equation of state and that model is fed into the computer codes
that calculate the giant impacts and so because of our lab data,
we could believe the temperatures being calculated in
the codes and Just that the amount of vapor that was being
calculated was real. And that's when we realized
that, these huge vaporous planets were as large as the
codes were calculating, that this was a, an accurate result
and that lent support to the idea that this new object called
hacen, esto is being created. So if we adopt your soon, Your
model and the results of your dynamical simulations for the
subsequent evolution of the Earth and Moon orbits.
Does that managed to replicate all the compositional and
orbital characteristics. We observe in the Earth and Moon
without positing a contrived low-probability scenario for the
nature of the moon-forming impact.
Everything about planet formation.
That we know, today tells us there were lots of giant impacts
and most of them are energetic and have a lot of angular
momentum. And many of them will make
Celestia's. And so I think the idea of a
Sonesta is not a low probability event.
There's a lot of work though, that still needs to be done in
order to fill in all of the puzzle pieces for what we see
today. And one of the biggest
challenges that we have is Linking.
Our physical model of a giant impact.
Where usually were only calculating the first 24 hours
for The First 48 hours. We now have to link that to data
that we have on the Moon from four and a half billion years
later and in between a lot has happened.
There's that core formation on the earth and the moon exactly
how did the moon form out of the disk, the title evolution of the
Moon, volcanism on the moon and all of Is affect the chemical
signatures that were using to guide us.
And so the challenge that we have is to make specific
chemical predictions from our physical model.
And to date, that's been really difficult.
We've only just gotten the temperatures right, all right?
And so now we're going to go another step to understand the
chemistry, the number and diversity of exoplanetary
systems has exploded of late. Does your work provides some new
predictions? About what we might expect to
see in these systems. All of our calculations to date.
Indicate that siness Diaz are a common outcome of giant impacts.
And what we know about planet formation is that every Star
forms with a disc that generates planets.
And we think that all of those systems had giant impacts.
So, the prediction at the moment is that almost every star in the
universe has had Celestia's one or more around it, but the catch
Is that they didn't last very long.
They cool off very quickly, because in most planetary
systems, it's not hot enough to sustain Rock Vapor, that would
only occur very close to the star.
So, most would cool off quickly and the gas would rain down onto
the planet to form a magma ocean.
And we would have to be very lucky to see that in action.
Now we've seen the debris from Giant packs around other stars
and so we can and astronomers are looking for other odd
features around young stars during the planet formation
stage and we have made predictions for what a
synesthesia should look like, around another star but we
haven't found one yet. Sarah Stewart.
Thank you very much. Thank you for having me for
illustrations that support this podcast.
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