Hopefully during this semester you will get a chance to look at the Moon
through one of the telescopes at the campus observatory. Before we talk about
the Moon, let's get some numbers out of the way: The equatorial radius of the
Earth is 6,378 km, while the Moon has a radius of 1,738 km. The mass of the
Earth is 5.97 X 1024 kg, and the mass of the Moon is 7.35 X
1022 kg. The density of the Earth is 5.5 gm/cm3, while
the Moon's density is only 3.3 gm/cm3. The average distance from
the Earth to the Moon is 384,000 km, and a properly scaled model is shown
One glance, and
you quickly can see how different the surface of the Moon is from the Earth:
It has a very large number of round "craters", and large flat, dark plains
called "Maria" (from the Latin for "seas", Maria is plural, "Mare" is the
singular). The origin of most of the lunar features
are due to a single process: bombardment of the Moon by large rocks, boulders,
asteroids and comets. As
the solar system formed, there was an abundance of material left over, most
of this was in the form gas and dust, and small "rocks". But there were
some large bodies also. Eventually, all of this material ended-up crashing into
the Sun, or onto some other planet or moon (note that comets and asteroids,
which we will talk about in upcoming classes, are believed to be left over
material from the earliest days of the solar system).
It is this process that has shaped the surface of the Moon. The Moon has
no atmosphere, thus when the Sun is above the horizon, the surface is hot
(about 100C, the boiling point of water on Earth), but when the Sun sets it
gets very cold (-173C = 100 K)! As shown below, the presence of an atmosphere on Earth
traps the heat:
A brief digression on temperature scales. Astronomers (and all scientists)
use the "Kelvin" temperature scale system. Because many equations that describe
real physical processes (like gas pressure: P = NkT/V) have a "T" in them,
if the scale suddenly switches between positive and negative temperatures,
the equation would suddenly switch signs--obviously air pressure does not
go to negative pressure between +1 F and -1 F, or go to "0"
at T = 0 F. So we need a temperature that does not have negative values or
a zero that can be encountered in the physical world. The chosen system is
"Kelvin". The size of the Kelvin degrees are the same size as those of the
Celsius system,
which are larger than Farenheit degrees [K = (F + 459.67) x 5/9]. Here is
a comparison:
On the Moon, a small fraction (< 10%) of the sunlight is reflected into space,
the rest is absorbed by the solid surface, heating it to fairly high
temperatures. Note that the Earth has a lot of cloud cover, and it reflects
about 40% of the incoming sunlight, keeping us cooler than if all of the
sunlight hit the Earth's surface--but it also traps heat in, keeping us warmer
Once night falls on the Moon, all of the gained heat is radiated off into space--one
of the heat transfer mechanisms we mentioned a few classes ago. Shortly,
we will talk how this cold might trap water in the form of ice deposits near
the poles.
Because the Moon has no atmosphere, and it
does not have active volcanoes or plate tectonics, there are no processes
to erase the craters left over from the bombardment phase. Thus, the surface
of the Moon reflects the violent history of the early days of our solar
system. Note that the Earth probably suffered even greater numbers of
craters, but the erosion processes here erased MOST traces of such
events (not all, however, see below).
Let's look at a lunar crater more carefully. Here is the lunar crater
Copernicus (named after Nicholas):
If you look carefully at this picture, you will see lots of radial features
that emanate from the crater ("crater chains"). These smaller pockmarks are from material
blasted out of the crater that fell on the surrounding surface creating
hundreds of smaller craters. Copernicus is about 93 km across. Note how
the surrounding plains have been disturbed by the impact. Near the center
of the crater you can see some small mountain peaks. Here is a view taken
by the Apollo 17 crew of Copernicus from a different angle than we can see
Now we can see the central hills/mountains more clearly. These peaks are
about 1 km in height. So how does all of this structure form? Here is the
basic process:
As the meteorite impacts the lunar surface it first compresses and heats
the surface rocks, this then "rebounds" in a giant explosion that excavates
the crater. Some material is ejected outwards forming rays and crater chains.
Other material falls back in. Sometimes the central part rebounds and forms
a central peak (or complex in the case of Copernicus). Here is the basic
structure of craters:
Breccia is a type of rock made from the "cementing" of lots of broken bits
of rock together
(. Here is a simple crater (Moltke):
Here's another more complex crater (Euler):
Simple craters result from smaller meteorites impacting the Moon, while complex
craters are the result of large impacts. In general a meteorite that has
a diameter of 1 km will blast a crater that has a diameter of about 20 km.
Why such big explosions? Because meteoroids
(what a meteor is called before impacting a planet or Moon) are traveling at very high speeds, from
10 to 70 km/s! Compare this to the highest velocity bullets that travel at
1 to 2 km/s. There is a lot of energy in these rocks out there orbiting around
the Sun, and the amount of this energy depends on the speed of the collision:
Kinetic Energy = 0.5mv2 (one half m-v squared, where m is the
mass, and v is the velocity). So, the faster and more massive an object is
when it hits, the larger the explosion. Remember the Earth (and Moon)
are traveling through space with a high velocity:
Sun-Earth distance ("one Astronomical Unit") = 149,600,000 km, so the circumference of the Earth's orbit
is 2*pi*149,600,000 = 940,000,000 km. It takes one year to complete an orbit,
or 365 x 24 x 60 x 60 = 3.15x107s in a year, so the average velocity
of the Earth is 30 km/s! From Kepler's laws we know that all objects near
the Earth travel at a similar velocity---it is just their directions that
are different. So, you can see that the impact velocities are going to be
very high, and the resulting explosions have hundreds or thousands of megatons
for even relatively small impactors.
The most dominant features on the
Moon, those that are visible without a
telescope, are the lunar maria. In this (nearly) full Moon image, the Maria
are the large, dark regions:
Here is a close up (below) of one of them, Mare Serenitatis (the Sea of Serenity, it
can be located in the full Moon image above--it is the roughly circular dark region above and to the right of center):
As this picture shows, the Maria are quite smooth---almost free of craters.
[Mare is the sigular of Maria, so an individual "sea" is called a "mare."]
The number of craters a particular
region has indicates its age: more craters means that it is older. Why?
Well, back when our solar system was young, there was a bunch of junk left
over from the formation of the planets. This junk (rock, dust, etc.) was
eventually "swept-up" by the planets and the Sun. So, the amount of material
floating between the planets declined with time. In the early times, the
planets were being bombarded with meteors, but this died off fairly quickly,
and fewer, and fewer meteors were left to cause impact craters. Thus, the
number of impact craters declined with time. Because maria have few craters,
they MUST be younger than the surrounding regions. But, you ask, how is
a maria formed? The same way the smaller craters are formed: by impact. But
now the impacting body is very large, and it cracks the crust of the Moon and
lava from the mantle of the Moon leaks up to the surface and floods the
surrounding terrain. Here is a maria impact cartoon:
The cracks in the crust (the red/orange lines in this drawing) show the molten
material from inside the Moon making its way to the surface so as to
flood the surrounding regions, and to fill in the large hole created
by the impact. The bodies responsible
for creating Maria are many kilometers in diameter. As the text book notes,
a meteor that is 1 km in diameter creates a crater that is 10
to 20 km wide,
and 1 km deep. The largest lunar maria are over 1,000 km across! It would take
a body about 100 km across to make such a large feature. With the Apollo
missions it was possible to actually date the rocks located both within the
maria, and outside the maria, in the brighter regions called the "lunar
highlands." In the lunar highlands, a region of numerous craters, the rocks
were found to be about 4.2 billion years old. The rocks from the maria,
however, are somewhat younger: 3.8 billion years old. Obviously, as the solar system
aged, the smaller rocks and boulders coagulated into larger bodies, and these
larger bodies were "accreted" by the even larger moon-sized and planet-sized
objects. Here
is another Moon image with the Highlands labeled:
Here is a close-up showing how rough the highlands actually are:
Craters upon craters! There was some trepidation in sending a manned mission
into this region due to a fear that the surface would be too rough to safely
targeted the highlands so that we could confirm the
hypothesis that the highlands are older.
Here is a map of where the Apollo missions (green triangles) landed (as well
as some unmanned probes. Click
for a bigger image):
see recent images of the Apollo 17 landing site taken by a NASA probe.] Speaking of Moon landings, we also get information on the lunar surface, showing
it to be very powdery, as can be seen by the amount of dust
kicked-up by the rover:
and shown by the footprint:
This is due to the bombardment of the surface by "micrometeorites", very
small grains of dust and rock that hit the surface because the Moon has
no atmosphere. Over billions of years, this pulverizes the surface.
we got to the moon there was quite a bit of fear that there could be many
meters of this stuff on the surface, and any lunar landers would sink out of
sight. But on average there are only a few cm's of this powdery dust.
The Moon has other geological features, including evidence for lava flows,
such as this collapsed "lava tube":
Similar features are seen on Earth, here is a lava tube cave:
Somtimes, after crater formation, cracks can develop in the surface due to the
cooling of the lava that flowed in:
And, longer versions of these, which are faults, called "graben" can also be seen in the maria:
Faults are also seen on the Earth's surface, like the San Andreas fault pictured
but in the case of the Earth, most faults have their origins in tectonic
activity--most of those on the Moon are due to fractures caused by impacts,
or by the shrinking of the lunar surface.
Two volcanic domes:
While volcanism is an ongoing process on the Earth, most of the evidence shows
that nearly all of the volcanism on the Moon ended some 3 or 4 billion years
ago. This was shortly after the Moon formed, and was probably due to the
inner portion of the Moon still being hot, and some of this leaked to the
surface. There is no evidence that the Moon ever had plate tectonics.
Interestingly, since the Moon's gravity is about one sixth that of the Earth,
the smoothness of the lunar maria imply a rather low viscosity ("runny") for the lunar
lava flows, and explosive eruptions (if they were to occur) could eject
material to much greater
distances than similar events on the Earth. The material from which the Moon
is comprised is very water poor, and water (actually steam) is one of the
drivers for explosive eruptions on the Earth, like these "composite" volcanoes
on the Kamchatka penninsula:
The two main volcano types are composite and shield--Hawaii is a shield
volcano, Mt St Helen's is a composite (a go
for more on earthly volcano types).
The Moon is almost water-free,
thus any volcanic eruptions that occurred were probably less dynamic.
It is interesting to compare lunar craters with some of those on Earth, here
is a sample:
The topmost of these three is Berringer ("Meteor") crater (1 km diameter, 170 m
deep) in Arizona (near Winslow) that
has an age of about 49,000 years. The middle one is Wolfe Creek (0.8 km
diameter) in western Australia with an age
of about 300,000 years, and the last is Manicouagan (70 km in diameter) in
northern Quebec at about 210 million
years old (and the oldest easily identified crater on Earth). Thus, like the
Moon, the Earth has suffered from meteor/asteroid impacts---it is just our
erosion processes (tectonics/weather) that erase the evidence for these
Composition of the Moon
What is the composition of the Moon? Here is a table comparing the Moon to
the Earth:
At first glance, their compositions are completely different. We can see that
the Moon is iron-poor, but oxygen rich. Recent space probes (e.g.,
Clementine) have started mapping the global composition of the Moon (there
is renewed interest in the Moon, and even the fledgling Chinese space program
is sending probes to the Moon). Here are
two maps of the distribution of iron and titanium on the Moon:
If you compare the iron distribution to a picture of the full Moon,
that it follows the distribution of maria. Why? Because the maria are
comprised of lava that flowed from the interior of the Moon to the surface.
Iron is dense, so it naturally drifts towards the center of massive objects
("diffusion"), so there is more of it in the mantle and core of the Moon.
Water on the Moon?
As we have just discussed, the lunar surface is very dry---there is no
atmosphere, so that water cannot exist in liquid form. Now, it is possible
to have ice on the Moon. Except, we just found out that the surface of the
Moon gets very hot during the "day" (remember a lunar day is about two weeks
long). Thus, ice could not exist anywhere on the surface that gets
direct sunlight. Are there places on the Moon that never see sunlight? Yes!
The Moon is covered with impact craters. These craters can be several kilometers
deep. Thus, the floors (or parts of the floors) of some craters near the North
and South poles of the Moon never see sunlight! These areas are extremely cold,
and thus can become a "cold trap." Any molecules of water that happen to
be passing by can get frozen to the surface. This is like pulling a beer mug
out of the freezer---it instantly gets covered in frost. What is happening
is that the water vapor in our atmosphere is freezing to the surface of the
mug. If humans ever want to live on the Moon, they will need a source of water.
Currently it costs $25,000 to launch one kilogram into Earth orbit. Humans
need about 2 liters of water (= 2 kg!) per day. You can guess that even with very
efficient recycling, we need lots of water to sustain life on the surface
of the Moon. Thus, finding a cheap, local source would help make a Moon base
feasible. This was the reason behind the : to crash a big object into the surface of the Moon
to see if there was water mixed into the soil ( is a little video on the mission). The mission
crashed a projectile into the crater "Cabeus":
And here is a sequence of pictures (flipped from previous image) centered on the impact site, showing
the tiny little blip that was caused by the impact. Fortunately, this was
enough for the instruments on the space craft to detect water ().
The Formation of the Moon
So, where did the Moon come from? At the time of the Apollo missions, there were four
main ideas about the origin of the Moon: 1) the fission hypothesis
(Earth was spinning rapidly, and a large chunk broke off), 2) the "capture"
hypothesis
(the Moon formed elsewhere, and was captured after coming too close to the
Earth), 3) the "condensation" hypothesis (the Moon formed in place from material
left over from the Earth's formation), or 4) the "giant impactor" hypothesis
(a huge body, about the size of Mars, hit the Earth, and the material
liberated from the Earth's surface coalesced into the Moon). It is the
last idea that is now believed to be correct. New (computer) models for the
fission hypothesis show that requires an extremely rapid rotation rate of the
Earth, and this is not observed (the Moon would have also ended-up orbiting
in the same plane as the Earth's rotation--that is over the equator, tilted by
23.5 o to the Earth's orbit).
The lunar rocks also show evidence of tremendous heating. Not possible in
the fission model. The capture hypothesis does not work because it is next
to impossible to capture such a large body without perfect conditions. The
lunar composition (low density of 3.34 gm/cm3) would imply that it
had to form out near Mars (which has a density of 3.9 gm/cm3),
migrate inwards, and get trapped by the Earth. Here is a table showing how
the planets change in density with position in the solar system:
A body coming from outside the Earth's orbit would have way too much energy to
get captured by the Earth. The problem with the
condensation model was that if the Moon formed from the same material
as the Earth, it should have the same overall density and composition.
It does not, it has a lower density and is iron-poor. The giant impactor
theory gets around all of these problems: the impact of a Mars-sized planet
with the Earth would remove mostly crustal/mantle material that is low in
iron, this explains the density. Analysis of the Moon rocks shows they
have a similar composition to the crust/mantle of Earth:
Finally, the
tremendous heating apparent in lunar rocks can be easily explained by the
heat of the impact. Here is an artist's view of the impact:
The material blasted from the Earth and from the disrupted impactor during
this impact eventually coagulates
and forms the Moon (for more on this theory, including new computer models,
So when did this occur? Since the Moon is iron-poor, this tells us that
that most of the iron of the Earth had already found (diffused) its way to the core.
This indicates that the Moon must have formed well after
the Earth.
(based on isotopic ratios of Tungsten) suggests that the
moon formed somewhere between 60 million years and 100 million years
after the Earth formed. One of the outstanding questions is why there
is little trace of the impactor---models suggest it should comprise most of
the ejected material. But new models suggest that the Earth and proto-moon
material were enveloped in a "silicon vapor" cloud/atmosphere, and there
was sufficient mixing to drive the final lunar composition to be similar to
the Earth's mantle/crust.
While the formation of the Moon was violent, not long after formation (a
few hundred million years), the Moon became a much quieter place.
As the bombardment phase of the early solar system ended, fewer and fewer
impacts were occurring anywhere in the solar system, but some of those that did
occur involved large bodies creating the maria, here is a time sequence
as described in Fig. 49 of chapter 9:
The rate of cratering of a surface allows us to estimate the age. Here is
a plot of the cratering rate that was estimated shortly after the return of
the Apollo 12 data:
Because of its small size and low density, the Moon actually cooled very
In the early stages, the heavier elements (iron or radioactive species)
diffused to the center, and there was probably a liquid core, and a small
amount of heating--including that from heavy meteorite bombardment.
Within a few hundred million years, the Moon started to solidify.
In the early days, the Moon probably had a liquid interior. But now, the
Moon is believed to be solid all the way to the core:
Here is a slice through the Earth (left) and Moon (right) showing the (to
scale) sizes of the components that comprise the internal structure of the
two bodies:
Because the Moon is completely solid, there is very little geological activity
on the surface of the Moon. Any volcanoes the Moon had died-out shortly
after it formed. To have volcanoes you need a hot, liquid interior. The Moon
does not have this, thus the Moon's surface has remained relatively unchanged
by geological activity for the last 3.5 billion years. So, as the Moon slowly
cooled, it shrank. There has been a bit of buzz about the "incredible shrinking
Moon," showing that it was an ongoing process for much of the history of
the solar system, and probably stopped about 1 billion years ago. Go
for a recent press release from NASA.
Why doesn't the Moon
have an atmosphere?
The reason the Moon has no atmosphere is that it the
gravity is too weak to hold on to any gases that might have been present
during the time when the Moon had volcanic activity. The Moon and Earth
are relatively close to the Sun, and the heat of the Sun warms the atmosphere,
and it expands. The Moon's gravity is much too weak to hold onto an atmosphere.
Thus, without erosion from wind and rain, the Moon's surface does not change
(except for----meteorite impacts).
Tides: The Influence that the Moon Has on Earth
For those who grew up near the Ocean, or have visited a costal region,
you are probably aware of "tides." The gravity of the Moon pulls on the
The oceans, being liquid, respond more easily to this force than solid ground,
and a bulge is created that rotates around the Earth twice per day, and the
water level on the coast rises and falls:
Astronomical Devices
Some of the images in previous lectures were taken through a telescope (and you
will see more in the coming weeks). Telescopes are used to make objects both
brighter, and bigger. Before we begin our exploration of the more distant
reaches of the Solar System, we need to take a brief excursion to talk about
optics and telescopes. As
we noted a week or two ago, Galileo is given most of the credit for being the
first to use a telescope for astronomical purposes. But glass lenses have
been around since at least 1100 AD, and eyeglasses seem to have been
developed in the 13th or 14th century.
What good is a telescope?
First, let's look at something you are familiar with, your eye:
Your eye has a pupil and a lens.
So, how does a lens work? Well, when light encounters the glass, its speed
slows down---light travels more slowly in glass then in air, or in empty
space. Thus, if light encounters a tilted piece of glass like that shown
it is bent. Think of a marching band trying to march in a line
and then executing a turn while trying to remain in a line--those on the
outside tip of the line have to walk faster while those on the inside edge of
the turn are not moving at all. Now picture a marching band that is in a
diagonal line formation marching up a football field, but when they get
to the 40 yd line they encounter thick mud. The first band members that hit
the mud all of a sudden march more slowly, while those in the dry portion of
the field keep marching at the same speed until they hit the mud. This causes
a slight bending in the line as they go through the mud. So in glass, the bottom
portion of the light beam in the figure above
hits the glass surface first, slows down first, and this causes a turn
in the beam.
In the eye the lens forms the image (upside down!), while the
pupil adjusts in size to allow you to let in more light when it is dark,
or less light when it is bright. The size of the pupil is what controls how
faint of an object you can see. For example, owls have large pupils to allow
in more light since they hunt at night. Cats have sensitive eyes, so when
it is bright out, their pupil contracts to a very narrow slit (for more
on the nature of the human eye,
.). The size of
the lens is the controlling factor for how faint a light source you can see.
That's the first important property of telescopes---they "collect light".
Looking through a telescope is like replacing your small eye lens with a much
bigger lens. The second important property of a telescope is its ability
to magnify objects to see finer detail. The amount of detail that you can
see depends on the size of your pupil---or the diameter of the lens of
a telescope. This is very similar to parallax. The larger the baseline used
in a parallax measurement, the smaller angles (finer details) you can
measure. The two extreme edges (diameter) of a lens are your "baseline"
for creating detailed images. That is why big telescopes are valuable,
they collect light, and allow you to magnify an image to see fine detail.
The amount of light a telescope collects depends on its area. For example,
the largest dilated human eye pupil has a diameter of about 8 mm. Therefore,
it has an area of 50 square mm (& R2!) A one inch telescope,
like Galilo had, has an area of (d= 1 inch = 25mm) 490 square mm. Galileo's
telescope collected 10X more light than the human eye, allowing him to see
stars that were 10X fainter than those visible with the naked eye. The
largest telescope (Keck) has a diameter of 10 meters (10,000 mm), it
collects 1.5 million times as much light as the human eye.
During the late 17th and early 18th century, telescope technology was
quickly improving. As with Galileo's primitive telescope, nearly all the
instruments of that time were "refractors". Refractors use lenses to bend
the light to a focus where an eyepiece (similar to a little microscope)
magnifies the image:
The problem with early refractors was that they used only a single glass
lens, and this arrangement could not bring all colors of light to a common
focus. Thus, everything you looked at had blue and red images that were
not in focus. This is called chromatic abberation:
To counteract chromatic abberation, the first telescopes were made to be
very long.
Some of these telescopes had small lenses, on the order
of 3" in diameter, but were more than 20 feet long (the longest ones
approached 75ft)! This made them difficult to use, and discoveries made with
them were rather slow in coming. Though,
used one of these primitive telescopes to discover Titan,
the largest Moon of Saturn, and to figure out the true nature of Saturn's
rings (that's why the
that entered Titan's atmosphere in 2005 was named
"Huygens").
It was discovered later that mixing two (or more) types of glass could counteract the chromatic aberration, and all modern telescopes and camera lenses have
more than one lens:
One of the major contributions of Isaac Newton was his book on optics
entitled Opticks, published in 1704. One of his proposals was to
use a curved mirror to focus light to eliminate chromatic aberration. Newton
knew that all light, no matter its color, suffers an identical reflection,
thus a telescope using a mirror as its objective would eliminate most of
the chromatic abberation suffered by refractors. Using mirrors, telescopes
could grow in light gathering ability without becoming too large to manage.
One of Newton's designs for such a telescope still bears his name:
Besides providing better image quality, telescopes with a single curved
mirror were easier to produce than refractors (that have two curved surfaces).
They were also cheaper. One of the first people to successfully build and
use these types of telescopes was
(1738 - 1822). A drawing of his largest telescope
is shown, below.
Herschel is best known for his discovery of Uranus in 1782, a discovery
that would give him a secure future as "astronomer to the king". Herschel
also discovered two Moons of Saturn, as well as two Moons around Uranus.
He also observed binary stars, and showed that their orbits followed Newton's
laws, and thus these laws appeared to be "universal" (that is, they worked
everywhere in the Universe the same way as on Earth). Currently, the world's
largest telescopes are the 10 m Keck telescopes:
Because it is extremely difficult to make a single piece of glass that is
10 m across and optically perfect, the Keck mirrors are segemented:
The next generation of mammoth telescopes are being planned, like the "Thirty
Meter Telescope":
The desire for larger and larger telescopes is due to the need to detect the
weak, faint objects found in astronomy. But larger isn't all that we need,
going into space allows us to get above the distorting atmosphere. Here is
an example:
why the Hubble Space Telescope was launched:
But techniques are being developed to allow ground based telescopes to
reduce the affects of our atmosphere ("adaptive optics"), where a flexible
mirror is used to improve the image.
Most cartoons you see have astronomers looking into eyepieces and making
drawings. But just about all astronomy now uses digital detectors like
those used in your digital camera:
The only difference is we use better quality detectors, bigger detectors
(>60 "megapixels"), and chill them to
extremely cold temperatures to get rid of electronic noise. But even with
simple webcams, you can get amazing quality--here is comparison between a
typical snapshot, and an image made by processing a short movie of Saturn
using a webcam at our campus observatory:
Inverse Square Law of Light
Astronomers build bigger and bigger telescopes to collect as much light
as possible. This is because the brightness of an object
falls off as 1/R2, that is if you double the distance, the object
is only 1/4th as bright, because the light spreads out as
it travels:
Note that any sphere drawn around a star (or a light bulb in this diagram)
intercepts all of the light emitted by the star. The area of a sphere
is 4piR2. If you double the radius (R), the area quadruples
(22 = 4). The intensity of the light passing through a square
centimeter drops as 1/R2. In the figure, the radius of the
inner sphere (or hemisphere) is 1, the next one has a radius of 2, and
the next one has a radius of 3. The areas of the spheres go as 1, 4, and 9
square units, as is highlighted.