We’d like to thank Speakly for supporting
PBS. Einstein once asked if “the moon exists
only when I look at it?". It was rhetorical objection to the idea that
measurement in quantum mechanics causes reality to become real. But there was a time when the moon didn’t
exist, and then hours later suddenly did. At least, according to the latest simulations
of its formation. No one was there to see the Moon form, but
let’s side with Einstein and assume that this was a real event that actually happened. Without direct observation, we have to
get clever to figure out what went down. And there’s good reason to be curious. Earth’s moon is very, very unusual. For one thing, it’s gigantic - by far the
largest relative to its planet in the whole solar system. Excluding Pluto, which isn’t a planet anymore
so whatever. The moon's composition is also unusual - it
has a relatively tiny iron core for its size. On the other hand, the elements that it does
have look like they could have been scraped off the surface of the Earth, which you would
absolutely not expect for a celestial body that wasn’t scraped off the surface of the
Earth. Before we get into how such a body could have
formed, let’s talk about how we even know all of this, and why these properties of the
moon are so unusual. There was this magical time half a century
ago - it wasn’t very long, just a few years - when humans actually visited the moon. The Apollo program may have had a political
side, but it also provided a trove of scientific data about our celestial little sibling. For example, seismometers were set up at every
landing site, which allowed geologists to tune into the moon’s vibrations. Impacts from asteroids or even the spent fuel
stage of the Apollo 17 Saturn V rocket cause seismic waves - moonquakes. They resonated through
the lunar interior. From the nature of the waves that reach the
seismometer we can reconstruct the moon’s interior. And it turns out the insides
of the moon are weird. Most notably, its inner core of iron is really small, only about 20% of the
moon's diameter - surrounded by a long-since frozen mantle. By comparison, Earth’s core is closer to
50% of its diameter. Apollo astronauts brought back a half-ton
of material from the Moon’s surface, which adds the second layer to the mystery. Each element on the periodic table can have
different numbers of neutrons - they come in different isotopes. The ratio of isotopes for a given element
varied across the protoplanetary disk that our solar system formed from. Each world in our system has a signature set
of isotopic ratios. Mars is different to Earth is different to
some random asteroid. But in the Apollo samples, isotopes of oxygen
in the lunar silicate rocks are identical to those on Earth to a few parts per million. OK. So the moon has an isotopic composition
identical to Earth’s crust, but it has an abundance of heavy elements that’s very
different, evidenced by its tiny iron core. We’re going to need more data. And we can get that by just looking at the
moon. The types of rock on the lunar surface also
speak to its history. The moon clearly has darker stuff and lighter
stuff. The dark regions are basaltic flows, where
volcanic magma oozed onto the surface and froze, similar to the dark rocks of Hawaii
and Iceland. These so-called ‘mare,’ Latin for ‘seas’,
fill many relatively flat impact craters in lowlands, mostly on the near side of the moon. Meanwhile, the light spots are anorthositic
rocks. This is also an igneous rock like basalt,
but it forms differently. Basalt forms ‘extrusively’, solidifying on the surface, while
anorthosites form ‘intrusively’ inside the magna. They are made of light elements like calcium,
silicon, and oxygen and are less dense than the magma that makes them. Because of this, when bits of anorthosite
start to freeze within the still-molten magma they tend to float to the top. Together, these rocks tell us that the moon
was once covered by a layer of liquid magma - and that would have been for tens to hundreds
of millions of years. Over that time it would have formed a light
anorthositic crust that was later splotched by darker basalt as eruptions pushed some
magma back up on top. And then to finish it off, add a couple billion
years of impact craters. So then, how do we make a gigantic moon from
Earth-like material with a really dinky core and completely covered with a magma ocean? Let’s talk about some of the ways to make a
moon, and see if we can find one that we like. Maybe the most obvious way is to do it like
we make planets. Planets pull themselves together gravitationally
out of the protoplanetary disk leftover from the Sun’s formation. But in that process they have
their own ‘circumplanetary disks’. In this environment, planets and moons grow
simultaneously until the disk is exhausted, and this is the kind of
formation we expect produced the big moons of Jupiter. This mode of formation would explain the matching
isotopic ratios of the Earth and moon, because they both would have formed from the same
general region of the protoplanetary disk. But if this were the whole story then we’d expect the moon to have
roughly the same proportion of different elements, not just the same isotope
ratios for a given element. And the moon’s tiny core tells us this is
not the case. Another piece of evidence against this formation
mechanism is that the rotation axis of the earth and the plane of the moon’s orbit
don’t line up. If they formed from the same
disk, they should line up. OK, so the moon probably did not form in a
circumplanetary disk at the same time as the Earth. So what else we got? If you don’t make your moon at home, the
other way to get one is to order out. Plenty of moons in the solar system show evidence
of forming elsewhere before being gravitationally captured by their planet But that’s harder
than it sounds. The biggest challenge is slowing down your
potential moon during a close flyby and stopping it from just whizzing right back off into
space. You can do that with atmospheric drag, and
the gas giants capture asteroids that way. It’s also a possible explanation for Mars’s
sad little potato moons. But this is pretty implausible for a moon
like ours, which completely dwarfs our atmosphere. Another way to capture a moon is to drop in
two at the same time. The complicated gravitational ballet of a
binary - or really trinary system - can leave one body in orbit and kick the other out with
even higher velocity than it entered with. This sort of ‘exchange’ was probably the
source Neptune’s moon Triton, with its huge tilt and backwards orbit. Capture was actually one of the most popular
explanations for our moon’s origin until, you guessed it, the Apollo missions killed
it. Those isotopic ratios really are a smoking
gun for the moon and earth formed from the same stuff. But how did it form from the same stuff if
it didn’t form in Earth’s circumplanetary disk? The last, most likely scenario, and by
far the most awesome scenario is the giant impact hypothesis. The basic idea is that two proto-planets were
forming in the same orbit and collided. There was a larger one that
we’ll call Proto-earth, and probably a Mars-ish sized body that we call Theia after the mother of the moon in
Greek mythology. Theia most likely formed at the L4 or L5 Lagrange
point, leading or trailing proto-earth by a sixth of an orbit. When the solar system was about 100 million
years old and the planets had already eaten up most of their raw materials, Theia’s orbit destabilized – probably
from the gravitational tugs of the other planets. It began to drift toward the earth. Let’s take a moment to imagine what that
would have looked like … A giant planet getting bigger and bigger and bigger on the
sky until… Well, for decency let’s zoom out and look
at a simulation of what may have happened. Theia smashes into proto-Earth, and the debris
forms a disk around the liquified planet. Much of that debris eventually rains back
onto the Earth, but some recedes outward and over months to years pulls itself together
into the moon with its own gravity. The giant impact hypothesis solves a lot of
our problems – the isotopic similarity of the earth and moon would come from mixing
during the impact. The moon’s iron core is small because the
lighter stuff from both planets got sprayed into space, while Theia’s iron core was
absorbed by the Earth. And a collision like this would have easily
liquefied rock, giving us our lunar magma ocean. Besides explaining the anomalies of the moon,
this hypothesis gives us some bonus answers about the Earth’s own weirdnesses. Earth has a particularly robust iron core,
which is responsible for our planet’s strong protective magnetic field. Absorbing Theia’s core may have really helped
out there. Finally, Earth’s own rotational axis should
have started out the same as its orbital axis, but now it’s tilted. This collision would have been more than enough
to knock our tilt off axis, depending on the angle of impact. This story seems to fit a lot of the data,
but there are lots of uncertainties. What were the real masses of the bodies? Their relative speeds? Was it a glancing hit, or a head-on collision? And how do you even test something like this? While we can’t go and smash proto-planets
into each other to test these different scenarios - at least not in the real universe … yet
- we can obliterate worlds to our hearts content in computer simulations. Using massive hydrodynamic simulations run
on supercomputers, scientists explore the wide range of outcomes for different giant
impact scenarios. If a given setup for a virtual collision between
a proto-Earth and Theia leads to something like the Earth-moon system, then it’s a
sign we’re on the right track. Up until now, typical simulations have played
out like I described - Theia crashes into the moon, spraying up a big tail of debris
that splashes around the earth and and also forms a big disk in orbit. Over months to years that forms the moon. While we can test pretty much any scenario
this way, there’s still a big limitation - and that's the resolution of the simulation. We can’t simulate every atom, or even every
pebble, but we can approximate the system as say hundreds of thousands of particles
all interacting gravitationally, and-or fluid cells interacting hydrodynamically. You can have confidence in the result of these
approximations if increasing the resolution doesn’t change the outcome too much. But apparently, that’s not the case yet,
because a very new simulation at a vastly higher resolution has found something completely
different to anything we’ve seen before. Earlier this year, Dr. Jacob Kegerreis at
NASA Ames and Durham University in the UK and his collaborators ran simulations with
as many as a hundred million little matter particles, a thousand times more detailed
than standard simulations. Let’s take a look at their best simulation
to see what they discovered. We starts with a pretty typical scenario-
a very nearly earth-sized proto-earth, and a roughly Mars-sized Theia. The collision happens at about a 45 degree
angle at very nearly the escape velocity- not a head on collision, but still a major
collision. While the proto-earth survives the collision,
Theia is basically obliterated. The debris from Theia shears an enormous amount
of the early earth’s mantle off with it, creating a plume towering thousands of kilometers
into space. But now, we see something no one has ever
seen before. For the briefest period, no more than a few
hours, there are two moons. Only in these most detailed simulations does
this happen. While the largest falls back into the earth,
its gravity plays an important role in helping raise the smaller one into a wider, stabler
orbit. These simulations show that in just under
two days’ time, Theia can hit the earth, make two satellites, and then destroy one
while leaving the other to become our moon. That’s very different to the previous picture
of a ring of material pulling together over many months. The new simulation gives us something very
like our own system - a moon that’s around 1% of the Earth’s mass, with an outer layer
heated to 4000K - plenty to give us our magma ocean. The surface of this virtual moon contains
a lot more material from proto-earth than other models, while the interior is almost
entirely from Theia. However the iron content is low, just like
in the real moon. So, like I said. One minute there’s no moon - just a giant
planet hurtling towards you - and a few hours later the moon is in place - according to
this simulation, with all the properties of the actual moon. Of course, there’s still a lot of uncertainty. Just because a few simulations give us something
that looks like the moon doesn’t mean this is exactly what happened. But given the quality of the simulation, we
could argue that this is a more plausible scenario than the earlier ideas. Imagine what we could learn if we threw even
more resources at the problem. For example, we could add magnetic fields,
which both bodies should have had prior to collision. We’ll never have a perfect description of
the moon’s formation, but as evidence mounts and simulations get better we can continue
to narrow down the range of possibilities for the moon’s formation until we’re left
with a general picture of what likely happened. And if these simulations lead to predictions
- like, for example, the nature of the moon’s interior - then we can test our theory. Perhaps soon we’ll be able to confirm that
cosmic cataclysm led to the weirdest moon in our local patch of space time. We’d like to thank Speakly for supporting
PBS. If time travel were invented and you thought
“Great! Now I can go back to the 1920’s and debate
interpretations of quantum mechanics with Einstein and Schrödinger!” – well, then you’d need to learn German. Unless you already know it… but even in
this current time period, if you’ve wanted to learn another language, then I’d like
to introduce you to Speakly. Offering 8 different languages to choose from,
Speakly was created by 2 polyglots who both speak 7 languages. They researched thousands of language learners
and created a unique method that teaches words and sentences based on their relevance in
real-life situations. This means that you don’t learn anything
that you actually can’t use to speak the language. Available on both web and mobile platforms,
you’ll learn new vocabulary, with speaking exercises, writing exercises,
listening comprehension exercises, and even music recommendations in the language that you’re learning so
that you don’t get bored. There’s a link in the description to learn
more. Hello and happy new year! I’m excited to get into another year of
trying to understand this ridiculous universe with you. Another good place to try to understand the
universe is Curt Jaimungal’s wonderful Theories of Everything podcast. As it happens, he interviewed me recently
on topics ranging from consciousness and free will to godel incompleteness and dualities
in physics. I get quite a bit more opinionated than I
usually do on Space Time, so it’s at least colourful, and perhaps even interesting. If you want to, you could also
let Curt know in the comments that space time sent you. Link in the description. OK, today we’re doing comment responses
for the episode on supercritical fluids - that bizarre hybrid state of matter between liquid
and gas. mitchblahman13 asks if I can clarify what
liquid metallic hydrogen is - one of the states of hydrogen in the gas giants. Sure thing. Firstly, solid metallic hydrogen is a theoretical
state of hydrogen at very high pressures in which H2 molecules form a lattice with metallic
properties - namely the electrons can travel freely through the lattice. Liquid metallic hydrogen is when you have
the same electron-sharing properties due to the high density, but the temperature is high
enough to break the lattice bonds. It still has the high conductivity of a metal,
but can flow. ChaosPotato and Jason Bouvette ask similar
questions: could you swim or float in an open-top boat on an ocean of supercritical fluid. My intuition says no, unfortunately. Swimming requires substantial viscosity so
you have something to push against to move forward. Supercritical fluids have viscosities closer
to gases, so to swim you’d need really gigantic hands. Although some animals do have hands large
enough to swim in a gas. They’re called birds. An open-top boat probably wouldn’t work
because the boundary between the supercritical ocean and the gaseous atmosphere is not well
defined. The buoyancy of such a boat comes from the
fact that it’s filled with the medium of the atmosphere, which needs to be substantially
less dense that the medium of the ocean - enough of a difference to make up for the fact that
the material of the boat itself is more dense than both media. But in the blurry supercritical fluid-gas
boundary, the change in density between the base and rim of the boat is relatively small. So in order to float the boat has to be large
compared to the transition scale between media, which I think means gigantic. Or you could have a closed surface filled
with lower-density gas, in which case you could easily float at the boundary. This would be something between a zeppelin
and a submarine, which sounds super cool and steampunk. Roland Pihlakas requests that I please briefly
summarise again, how is supercritical fluid different from gas. Happy to, Roland. The main difference is density. Density is liquid-like, while viscosity and
absence of surface tension is gas-like. The high density means it interacts much more
readily with whatever its in contact with than a gas does due to the large number of
particles per unit volume, and can carry with it far more dissolved matter and heat. The low viscosity and surface tension means
it can get into tiny spaces much more easily than a liquid - allowing it to actually get
to the particles you want to dissolve. Together, these properties make supercritical
fluids amazing solvants, oxydizers, heat carriers, etc. as well as making them extremely corrosive. Lord Marcus asks if there’s a supercritical
phase where solids become indistinguishable from liquids. Gareth Dean correctly responds - not really,
because liquids and gases are both fluids - no rigid bonds between particles, which means an intermediate state between
liquids and gases is meaningful. Solids, however, do have rigid bonds which
means they can’t flow, and so there’s no true intermediate state with liquids. That said, there are cases where the solid
and liquid states are so mixed that the effect feels like a hybrid. Examples include gels - a crystal lattice
of fiber networks filled with fluid, or colloids - aka slimes - solid particulates suspended
in a liquid. By the way, that whole thing about glass really
being a liquid is false. It’s an amorphous solid. On the topic of solid-liquid hybrids, Vitaliy
Vuychych and Sascha Wust point out that cats are simultaneously solid and liquid. While it’s true that cats can be brought
into an equilibrium between solid and liquid, this means raising their temperature to the
cat melting point. While that has been shown to place the cat
in a state of being simultaneously solid and liquid, it’s no longer in a state of being
simultaneously alive and dead. It’s just one of those.
