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.