We've looked at Earth there's another 
celestial body about which we know almost 
as much as we know about Earth. 
it shares with the Earth the amazing fact 
that it's been 
visited by humans. 
This is our moon. 
Twelve people, in total, have visited the 
moon. 
And this means that we have samples of 
lunar rocks and soil that were brought 
back. 
As we will see, this is not unique to the 
moon, in that sense. 
bits of other planets have been knocked 
off by impacts and have, some of them 
have eventually found their way to Earth, 
so that we have meteorites whose source 
can be traced back to Mars or other 
planets, but the samples brought back 
from the moon were brought back from 
precise positions. 
We know where they came from and this 
will become important. 
And there are experiments that the 
astronauts left upon the moon and so we 
should know quite a bit about the moon. 
It's a good place to test what we learned 
on Earth and apply it to another similar 
smaller object. 
And let's start by looking at the moon. 
The moon as we know has two sides. 
as it orbits the Earth one side always 
faces the Earth. 
Looking at these images it's pretty easy 
to recognize the familiar man in the moon 
face on the left, that's the near side. 
The far side is missing those dark 
splotches. 
They're called maria because originally 
they were assumed to be oceans. 
We now know that there is no water on the 
moon. 
We talked about there could be no water 
on the moon. 
These are not oceans, they're lava flows 
but they're named Maria styx. 
regions of the moon that are not maria 
are called highlands, and we see from 
this image that they're heavily cratered 
This is true. 
the far side is all highlands. 
And, the distinction, the dichotomy 
between the far side and the near side. 
the causes for why one side is different 
from the other are not, I think, clear in 
any sense that I can explain it. 
And so, we'll just leave the statement at 
that. 
It certainly has something to do with the 
presence of the earth on one side, but 
what exactly is far from obvious. 
let's take a closer look. 
There are beautiful 
Lunar maps, let's take a closer look at 
the surface of the Moon and see what we 
can see there. 
This is one of the benefits of having had 
so many spacecraft orbit the Moon, we 
have. 
Brilliant photographic maps of the moon, 
I'll post a link to this one. 
You can play with it if you would like. 
And here we are looking at the region of 
the Maria Crisium, one of those maria 
that I spoke about and we can see that 
while it's not an ocean it does share the 
properties of an ocean. 
It looks kind of like it, it's flat and 
straight and dark and whereas the 
highlands around it appear to be peppered 
with craters, and we see that this Maria 
appears to be 
a crater it's in it's own right, except a 
very large crater. 
Indeed when astronauts investigated, the 
Maria became clear that what these are, 
are old lava flows. 
This is are, these are traces of old 
volcanic activity on the moon. 
not in the form of large dome volcanoes, 
like on earth, though there are traces of 
that but mostly lava flows that 
essentially, flowed and filled a existing 
large crater. 
you will notice that the highlands 
surrounding the Maria are heavily scarred 
with craters of all sizes. 
But few as large as the aria. 
And, so we begin to be developing and 
understanding of time. 
And we see that there are craters. 
Inside the Maria but there are few and 
far between. 
what we take this to mean is that the 
terrain of the aria is more recent. 
The moon has been under constant 
bombardment and the mare only has craters 
reflecting those impacts that have 
occurred since the time it was resurfaced 
by the lava flow. 
This is why terrain in the Maria is by 
and large smooth, because it is younger 
terrain. 
Who. 
We've learned something about how to 
learn about the age of terrain 
older terrain will have more craters, the 
longer something is sitting there the 
more impacts it will absorb. 
This of course terminates at some point, 
you reach some saturation where new 
impacts destroys as many craters as they 
produce. 
you can get a little more refined sense 
of dating we focused here a little bit to 
the north of the Maria Crisium on this 
crater called Cleomedes. 
We've made a little bit of an 
enlargement, and what we can see is that 
Cleomedes itself are. 
Large, and rather old crater, and it's 
been degraded, its rim is not very sharp 
and not very high. 
And indeed, on its western rim, I'm 
using. 
Conventional east and west notations 
here, is this crater Tralles which 
clearly post dates Cleomedes. 
The impact that created Tralles post 
dates the impact that created Cleomedes 
because Tralles obliterated a part of the 
rim of Cleomedes. 
And similarly going up here to the north 
we see that Cleomedes E post dates 
Cleomedes and Cleomedes A, this nice 
sharp round one, post dates Cleomedes E. 
And so because we have craters upon 
craters upon craters, and if we magnify 
further we can see yet more craters upon 
them, what we can see is that we can 
begin to order things in terms of the 
order in which they occurred by not only 
the degree of cratering but also 
precisely ordering the craters one on top 
of the other. 
Enlarging still further, we see that when 
I say that the interior of Cleomedes was 
relatively unscarred, relatively was the 
operative word 
indeed this is a region that is less 
cratered than the surrounding. 
But, there have been impacts in 
Cleomedes. 
We also see another typical lunar 
feature. 
This Rima Cleomedes, these are these 
ridges, which are thought to have formed 
when the interior of the Moon cooled and 
contracted. 
The crust having already solidified. 
This is. 
The closest analog the moon has to plate 
tectonics, we do not find on the moon, 
aside from the impact craters, any 
indication of end of volcanism. 
But any indication of linear features 
that would indicate plate tectonics, the 
moon's crust is all one plate. 
The moon is geologically inactive now not 
completely. 
Its geologic activity such as it is, is 
that as the moon is shrinking. 
We get these 
linear features that are these ridges, 
which are basically the result of the 
surface cool shrinking as the interior 
cools. 
But the moon shows traces of past 
geologic activity, but certainly no 
current geologic activity. 
What we saw 
lunar craters are created by impacts, 
this is the mechanism of by which, the 
moon is weathered, a continuous barrage 
of impacts. 
No atmosphere to protect it, and so even 
small rocks make it all the way down to 
the lunar surface. 
The maria or lava plains, often they end 
up filling old large craters. 
We see that there are old craters and 
then it seems that with time the size of 
impactors has become smaller. Large 
impacts are always contained in, inside 
smaller, contained smaller ones that are 
more recent. 
So there's a progression in the size of 
the things that are crashing into the 
moon. 
We small these Rima Rilles and Graben 
that result from the shrinking of the 
interior. 
There's no volcanism, currently, on the 
Moon. 
This makes sense. 
The moon is a smaller object than the 
Earth. 
It will have cooled, the interior will 
have managed to radiate out its heat 
sooner. 
And, at some point, 
given enough time, the Earth too, will 
become volcanically inactive. 
On the Moon this has already happened so 
small planets we expect will cool faster, 
just because of scale. 
The moon has essentially no atmosphere, 
its gravity is not enough to bind an 
atmosphere. 
such molecules of gas as are found on the 
moon will be exposed in the absence of 
both a global magnetic field, as we will 
see, and atmospheric protection will be 
dissociated by the sun's ultraviolet 
radiation, and so, for example, if 
there's any water vapor on the moon 
it'll, the molecules will be broken up by 
ultraviolet radiation. 
The hydrogen atoms will be unbound and 
they will escape to space. 
The temperature on the moon, 
well, the moon is kind of like the easy 
model of the Earth we studied before. 
It has an actually, a very small albedo, 
and it's roughly a black body. 
There's no greenhouse effect. 
So the temperature we expect will be a 
little bit under 300 degrees. 
It's in the same orbit around the sun as 
Earth is. 
The thing that the, moon, that the 
absence of an atmosphere does is that 
there's no mechanism for shifting heat 
from the day side of the moon to the, 
night side of the moon, remember that 
lunar days and nights are two weeks long. 
And so you develop a huge temperature 
differential. 
Temperature is as high as 370 Kelvin on 
the day side, as low as 100 Kelvin on the 
night side. 
And in the deep dark shadows of some 
craters where the sun literally never 
shines the coldest temperatures known in 
the inner solar system have been 
measured, down to 35 degrees Kelvin which 
is very cold indeed. 
there's clearly no water on the moon 
because water as we saw requires 
atmospheric pressure to maintain it, but 
we have very good evidence that in some 
of these 
craters of eternal shadow. 
There are significant quantities of water 
ice, presumably imported on the same 
large bombardment that imported water 
into the time of the Earth and 
the crust of the moon is very old. 
We see that it carries the traces of 
impacts generations in billions of years 
of impact on Earth. 
That same amount of impacts at least the 
larger ones must have occurred. 
On Earth this record has been erased by 
tectonic activity. 
Since the Moon ceased geological activity 
it's in some sense is a museum of the 
history of the solar system for the past 
four billion years or so. 
And in the Highlands the effect of these 
impacts been basically to cover the 
entire surface of the moon with a thick 
layer of very fine dust, which is 
basically rocks that have been pounded 
down to dust by impacts and then also the 
effect of ultraviolet radiation and this 
is the regulate, the fine dust in which 
that beautiful footprint we saw was, 
created and so we can use the moon as a 
museum of history and when we put 
together the ability to date crater and 
order craters by their size and their 
relative position. 
Together with the fact that we have lunar 
samples whose provenance is known, and we 
can radio date them we have 
we can now identify which craters were 
created when, and then use that to relate 
that, find the, dates of other craters, 
and we have 
Bombardment history we can construct a 
history of the rate at which the moon was 
bombarded starting from the present and 
the most recent craters that we see and 
into the older craters that we found. 
And we see that the rate of lunar 
bombardment and you see that none of this 
is perfect. This is, science is an 
approximation to truth, has been 
relatively constant for the past three 
billion years or so but it appears that 
about 3.9 billion years ago there was 
phenomenon first described as a lunar 
cataclysm, now it's known as the age of 
heavy bombardment. 
The Nice model very nicely explains it. 
As you can see, not everybody agrees 
that, that is what happened. 
But we have evidence there that the moon 
was heavily bombarded about 3.9 billion 
years ago and 
there are reasons to imagine that perhaps 
this was in fact a spike, as in these 
models would suggest. 
One of the main reasons we believe in the 
predictions, is that it agrees with this 
dating of moon rocks, there's more detail 
than is apparent in what I have said. 
careful geologists have been studying 
this thing very carefully over decades. 
This is what we learn from the lunar 
surface, what goes on inside. 
The moon melted, it's a round object, 
there's chemical differentiation. 
There is a core, but the lunar core is 
anomalously small. 
the moon's total density is not much 
larger than that of rock. 
once we conclude that it has a very small 
core. 
we study the moon's internal structure 
the same way we do the Earth's. 
There are moonquakes, in this case not 
generated by plate tectonics because 
that's not happening, but mostly by the 
tidal forces of Earth causing internal 
friction. 
and we see that the moon's core is 
anomalously small. 
And its mantle, very thick. 
the moon lacks a geodynamic magnetic 
field. 
There are traces of magnetization on the 
surface, but no global magnetic field. 
this, this has to do with both the slow 
rate of rotation, and the fact that it's 
essentially done cooling. 
And while there is a fluid inner core. 
A solid inner core and a fluid outer 
core. 
There's not enough convection heating, to 
drive a magnetic field. 
And 
the last important question obviously is 
where did this thing come from? 
How is it that earth of all the 
terrestrial planets has a moon that is so 
ominously large compared to the moons of 
other planets and 
even compared to the Jovian planets no 
planet has a moon so large by comparison 
to the planet. 
it turns out that the isotopic 
composition of the moon suggests that 
it's basically a piece of Earth. 
The relative concentrations of various 
isotopes of oxygen turns out to be a 
sensitive trace of where in the solar 
nebula something was formed. 
on the moon they track those on Earth so 
precisely that the conclusion is that 
moon is literally a piece of the Earth. 
how did a piece of the earth come to be 
in orbit? 
Well, the most likely theory is that 
there was what was called a giant impact 
early in the earth's history, before the 
solar system got cleaned up 
something either the size of Mars, or 
perhaps a little larger, crashed into the 
Earth, 
The impact vaporized large fractions of 
both objects. 
And if you get the dynamics of the 
collision right, as we will see. 
Then the, the impacting object, which is 
not part of Earth, and therefore doesn't 
have the right isotopic ratios, will be 
absorbed into the Earth's core, and 
mostly, bits of the mantle of Earth, will 
be spewed out into space, and this would 
explain both why the moon is. 
Poor in iron relatively, and not that 
dense because it is made up of the ejecta 
from this collision, whereas the core, 
the heavier materials sank into the core. 
We should look at this next simulation 
and see what it does. 
This is a very recent simulation. 
This is a recent version of the giant 
impact hypothesis that created the moon, 
and what we see here is that in this 
simulation, the Earth starts out with a 
rotation period, a day, a sidereal day, 
of 2.3 hours. 
This is something to pay attention to. 
Remember that near earth satellites orbit 
the earth every 90 minutes. 
the earth is rotating so fast that rocks 
are almost in orbit. 
The earth is on the verge of falling 
apart, in the way the simulation works. 
This is important because it is the 
reason that this impacting object with a 
mass, only 5% of Earth's mass, crashing 
into earth, essentially head on. 
leads to a cataclysmic collision the 
impactor is essentially absorbed by the 
earth and within 24 hours there's this 
cloud of what was formerly the earth's 
mantle scattered around the Earth the 
Earth's 
and, and out of this cloud, we now have 
protoplanets and the standard accretion 
disk model the moon will eventually form. 
Note that the moon forms in these models 
very close to the Earth. 
we've talked about George Darwin's theory 
about how the Earth is transferring 
angular momentum to the moon and slowing 
down. 
Earth's day, by the end of creating the 
moon was about 
2 and a half hours in this model and then 
over time it turns out the entire 
Earth-moon system exchanges angular 
momentum with the sun because of the 
sun's tidal forces over tens or hundreds 
of thousands of years. 
the moon settles down into it's nearly 
circular orbit about the Earth and then 
from this moment on, and the Earth has 
developed a day that is relatively 
generous five or six hours. 
And then the process from here on is the 
tidal 
a transfer of angular momentum from the 
Earth to the moon which mush, moves the 
Moon farther away from Earth and leads to 
the, 24 hour day that we observe today. 
one of the. 
Nice things that has been verified is 
that, astronauts on the moon left 
reflectors there. 
We can measure the distance to the moon 
very precisely. 
And indeed, the moon is receding from 
Earth at a rate of about three 
centimeters per year. 
as a told you once, total solar eclipses 
are something to enjoy because they won't 
last.