(whooshing)
- Good morning, everybody.
Welcome to Science on Saturday.
I am Dr. Joanna Albala,
I'm the Science Education Program Manager
at Lawrence Livermore National Laboratory
and I'm really excited
to introduce this year's
Science on Saturday series,
Science in Space,
and it's really gonna
be out of this world.
So, I'd like to introduce
our speakers today:
Dr. Lars Borg, he's a cosmochemist
and works in the nuclear and
chemical sciences division
at the laboratory.
He received his PhD in
isotope geochemistry
from the University of
Texas in Austin, Dr. Borg.
(audience applauding)
And joining him today is Tom Shefler.
He's a physics and astronomy teacher
at Granada High here in Livermore.
(audience applauding)
And he got his undergraduate degree
from Western Michigan University
and then got a Master's from UC Berkeley
in astronomy and astrophysics.
And without further ado,
let's get started.
(audience applauding)
- Thank you very much for coming out
on a early Saturday morning.
It's a pleasure to be here with you.
As Joanne said,
my name is Lars Borg.
I work at Lawrence Livermore National Lab.
What you guys probably don't know about me
is I grew up in Livermore.
I went to East Avenue when
it was an elementary school.
I went to Almond when it was
also an elementary school.
I went to East Avenue when
it was a middle school
and I grew up Graduated
from Livermore High.
I also went to undergraduate at Berkeley.
So, I did all these things locally
before I started to go to Texas
where I worked at NASA for a while,
then at the University of
New Mexico where I taught,
and finally, I came to the
lab about 10, 15 years ago,
something like that.
So, I'm the local kid.
So, 50 years ago today,
the Apollo program was raging.
Apollo 11 and Apollo 12
had already returned
astronauts from the moon,
geologists and geochemists
from all over the world
were looking at those rocks,
and they were making
discoveries and insights
about the origins and evolution
of the Earth and the Moon
that were changing the very
foundations of the field.
And in celebration of this
tremendous achievement
of the American people
and people worldwide,
I'm gonna talk to you about
the origin evolution of the moon
as we currently see it today.
And this is going to be predicated
on a lot of work that I've done
at Lawrence Livermore
National Lab with my group
over the last 10 or 15 years.
But before I go there,
what I'd like to do
is try and put Apollo into
perspective a little bit.
This was the largest science
and exploration endeavor
ever undertaken by the human species.
It was unparalleled.
At its height,
it was using roughly 1%
of the entire gross national
product of the United States.
So this was something way
beyond what NASA is today,
way beyond anything that we've done
as far as a science-directed objective,
and it changed the world we live in.
The technology and the
thinking, the human species,
was changed as a result of this.
Obvious things are for example,
things like computers and cell phones
and things which were
technological derivatives
of things that were first just
stated in the Apollo program
and its predecessors Mercury in Gemini.
But it also changed the
way we think about things.
It engendered an entire group of people
to look at the problems
that we face as a species,
and realize that we have a
solution to some of these
and that's through technology
and science and education.
And that's what emboldened
people like myself
to go into these fields
and other people to invent computers
and do these sorts of things.
But it also changed the
way we look at ourselves
in the broader cosmos.
And this slide to the left
is an illustrative of that.
This is a photograph taken
by crew member Bill Anders,
who was on the Apollo 11 command module
as it orbited the moon before the landings
and he took this photograph
looking back at the Earth.
And what it did was it
put the human experience
into a broader perspective,
looking at this little bubble
of life sustaining Earth,
in this vast expanse of space
with a foreground of this
incredibly hostile moon.
And what he said in this,
I've quoted it up on the left,
it says, "We came all this
way to explore the Moon,
"and the most important
thing that we discovered
"was the Earth."
And that's very true from the
philosophical perspective,
but it's also true from the
geological perspective as well,
because by looking at
the geology of the Moon,
we gained insights into
how the Earth formed.
The Earth is a giant engine
that continually builds
itself and destroys itself
through weathering processes and recycling
that takes crustal rocks and
puts them into the mantle.
As a result,
if we wanna understand the
earliest history of the Earth,
we can't find any record
of it on the Earth.
So where do we go?
We go to the Moon.
And so this is what I'm gonna
talk to you about today.
One of the fundamental questions
that came out of this exploration was,
and goals of this exploration,
was to understand when did
the Earth and the Moon form.
And this has been a problem
that we've been working on
for 50 years.
And I think we found the
actual answer to this
in the last few years,
and I'm gonna present that to you today.
So, the general outline of this talk
is gonna go something like this.
I'm gonna begin by
describing the hypothesis
that we've developed
for the origin and evolution of the Moon.
This is based on the analysis of samples
that have been returned
from the Apollo program,
almost exclusively based on these samples.
There have been other information gained
from looking at remote sensing
data and theoretical data,
but the vast majority of the story
that I'm gonna present to you today
is based on the analysis,
the samples that were returned
by these six landed Apollo missions.
The hypothesis that is
developed has a built-in test,
and that test is based on
determining the ages of the rocks.
If we can measure the ages of these rocks,
it can test this hypothesis
for the origin of the Moon
and it can also tell
us the age of the Moon.
The problem is that the measured data,
the historical measured data,
that have been completed
over the last 50 years
really don't support the hypothesis.
And so this is very problematic
because it might mean that the chronology
that we've determined on
these rocks is incorrect
because of difficult measurements.
Or alternatively,
it might mean that the bloody model
that we've come up with
after 50 years is dead wrong.
So, we're gonna talk about
the chronology of rocks
and to introduce you to
the way rocks are dated,
Tom Schefler is gonna
give some demonstrations
to illustrate how
natural radioactive decay
can be used as a way to
obtain ages on actual samples.
And then finally what I'm gonna do
is I'm gonna show you the data
that's been completed by my group
that I think answers this question.
So, let me begin by giving
you a Geology 101 lesson.
This is going to be a version
of the origin and evolution of the Moon
as it might be presented
at a freshman undergraduate geology class,
minus the terminology
that is only specific to cosmochemists
who are a strange group anyway.
So, the first step in the
origin and evolution of the Moon
is depicted here.
What we think happens is
that there was a giant impact
of a body that we think is
roughly the size of Mars
with a protoearth that was
probably somewhat bigger
than the Earth is today.
The need for this
is based on the chemical
composition difference
between the Earth and the Moon,
as well as the physical properties
of the Earth and Moon today,
specifically something
known as angular momentum.
This giant impact was incredibly violent.
The planets were hit with such velocities
that the impactor was completely destroyed
and the Earth was lucky to stay intact.
What happened during this
impact was of course,
there was a breaking of material,
debris was shot into space,
much of the rock was
turned into gas and plasma,
that gas and some of the rock escaped,
and some of it was bound around the Earth
in a gravitational orbit depicted
in this illustration here.
This debris was captured by the gravity,
it was orbiting the Earth
at the same time, it's very hot,
gases are not bound by
gravitational forces
nearly as strongly as solids
and they have more kinetic energy
and so they're escaping.
So what's happening
is we're generating a debris ring of rock
devoid of volatile elements.
So, the next thing that happened
is this debris starts being
attracted to itself by gravity
and forming a bulk planet
which was to become the Moon.
Turns out if you have gravity
that is attracting materials together,
it gets very warm.
As it heats up, it starts melting.
So what happened at some point
early in the history of the Moon
as it was agglomerating,
or accreting is the
technical term for that,
it started to melt.
We don't know how much of it melted.
The general idea right now
is that the entire body
was probably mostly molten.
So, at some point right
after the giant impact,
the debris is gone,
the gas has left,
we've got a molten body
orbiting the Earth.
The Earth is also probably
going to be fairly molten too,
probably not completely melted.
So, thermodynamics tells us
that things that are hot get cold,
particularly if they're in space
and so this body then starts to solidify.
And it solidifies into the
moon that we've seen today.
And so this initial molten body,
which we termed the magma ocean,
solidifies fairly early in the
history of the solar system.
And the question that
we're trying to address
is when did this solidification occur?
This is a geologically dateable event
and this will tell us
when this whole sequence
of things occurred.
So, this is a movie showing
what we think happened.
There was a giant impact
forming a variety of debris,
the debris was,
much of it was ejected
out of the gravitational
field of the Earth,
some of it was maintained
within the gravitational
field forming debris,
the debris later accretes to form the moon
that generates a
significant amount of heat
which melts the moon
then the moon solidifies
forming what we see today
as the moon as we basically observe it.
There also was a period of heavy impacts
that occurred late in
solar system history.
I'm not discussing that
in the context of this
particular presentation.
Okay, so, in the next few
slides what I'd like to do
is I'd like to talk to you
about how the Moon solidified,
and this is important because
the solidification of the Moon
produces the rocks that
we're able to look at
and date today.
So if we can date these rocks
that formed as a result
of primordial solidification of the Moon,
we can get some sense
of when it solidified.
So to understand this,
we have to look at terrestrial analogs.
That's the way geology largely works,
is by comparing things.
So you look at the evolution of a body,
say an intrusive body
like something intruded into the crust
like the Sierra Nevada,
and it gives you insights
into the processes of solidification
that might be applicable
to something like a
magma ocean on the Moon.
This is a picture that shows
one of the closest terrestrial analogs
to solidification of a magma ocean.
On the left is a lava lake from Hawaii,
this is a lake that's produced
when molten lava is erupted
onto the surface and pools.
And then of course it
cools and it solidifies,
that solidification that occurs
can be modeled and understood
both in the laboratory
and by physical observation
of looking at these materials,
and it can be used as a
basis for understanding
what might have happened if a molten body
the size of a planet
like the moon solidified.
So, the next couple of
slides I'm gonna show you
this pie shaped diagram,
imagine this as a pie shaped
cross section through a planet.
So the entire planet
would be one and a half
times the size of the stage,
we're just looking at the interior
and the radius of the planet
goes from the point of the
triangle to the surface.
So, the body is initially molten,
it starts to cool.
The first thing that solidifies
is going to be the thing that transitions
from liquid to solid at
the highest temperature,
and that turns out to be metal.
So the first thing that
solidifies is iron and nickel
which form a core.
The core of the moon is
roughly about 300 kilometers.
It's very, very small.
In fact, until about five years ago,
we weren't even sure
that the moon had a core.
As this metal is subtracted
out of the liquid,
the liquid becomes depleted
in those components
that go into the core,
so the liquid becomes
depleted in iron and nickel.
The other elements that
don't go into the core
become progressively enriched.
So the rest of the material are
something we call silicates,
which are characterized by
high abundances of SiO2.
But there's a variety of other elements
that follow that including
titanium, aluminum,
magnesium, calcium, so on, et cetera.
So imagine you have a liquid
body of liquid silicate,
so sort of like liquid glass,
below a solid metal core,
above a solid metal core.
So, this liquid continues to solidify.
The next thing that
crystallizes out of this magma
is a mineral called olivine.
If you go to Hawaii and
you pick up a rock there,
you'll see a bunch of
little green minerals,
they're sort of olive green,
that's all of olivine.
You've probably all seen
it not knowing what it is,
maybe some of you do.
But it is a material that is
rich in magnesium and silica.
It takes up about 50% of
that silica molten liquid
of the magma ocean.
So, that liquid now becomes
depleted in magnesium
and it lowers its temperature
so another mineral starts to crystallize.
In this case,
it's a mineral called orthopyroxene,
which you've probably also seen,
it's also in Hawaii,
it's a little harder to find.
It's sort of a honey brown kind of color.
It's pictured here it's
also a magnesium silicate.
The next mineral that crystallizes
is called plagioclase.
Plagioclase is something that
you see all over the place
in the Sierra Nevada.
If you grab a rock, a granitic rock,
the white mineral in there is plagioclase.
It has calcium and aluminum in it.
But this material is fairly light,
it's less dense than the
liquid it crystallizes from.
So as it solidifies,
instead of sinking like the
olivine and orthopyroxene
it rises to the surface
and it forms early,
what we call a primordial
crust to the Moon.
If you go and look at the Moon today
and you see white spots,
that's what you're looking at,
you're looking at that
plagioclase-rich material.
Crystallization continues,
the next mineral the
crystallizes is clinopyroxene,
this is a magnesium
calcium bearing mineral.
Something strange is starting
to happen here though,
on all of these minerals
I've put the density.
So the olivine has a density of about 3.3,
as does the orthopyroxene,
the plagioclase rises to the top,
it has a density of 2.7.
Everything is in harmony
in terms of density and
gravitational stratification.
But if you look at the clinopyroxene,
it has a density of about 3.7
and yet it's thought to
crystallize relatively high
in the magma ocean.
So we're starting to build
in a density instability here
into this magma ocean.
The next thing that
crystallizes is ilmenite,
it's an iron titanium oxide.
And the gravitational
instability gets even worse
in this case,
because this is really dense stuff.
It has a density of about 4.5
and it's at the very top.
And so we've got a series
of minerals idealized here
that are in a pile
in which they're gravitationally unstable.
Now we've also got a little bit of liquid
that hasn't gone into any
minerals and it remains.
And geochemists being
geochemist call it KREEP,
K-R-E-E-P
and that's because it contains potassium
which is on the periodic
table the letter K,
it has rare Earth elements,
the things that make Priuses work, REEs,
and it has phosphorus in it.
So, being the imaginative
fellows that my colleagues are,
they came up with the name KREEP.
But basically,
this is the stuff that
doesn't go into olivine,
or pyroxene, or plagioclase, or ilmenite.
It has all the elements
that aren't in the
previously formed minerals.
So, at the end of this
crystallization of the magma ocean,
this is the general outline
of what we think the moon
might have looked like.
It had a metallic core,
the core is relatively small.
It has a mantle composed
of a variety of minerals,
mostly magnesium silicates,
including olivine, pyroxenes,
also is interspersed with ilmenite
and it has a primary crust
that's dominated by
the mineral plagioclase
because it's fairly buoyant.
And in between,
there's a bunch of stuff that's enriched
and a whole bunch of elements
that are rather obscure
but really important to
cosmochemists that is called KREEP.
And as I said before,
this is very unhappy because
it's gravitationally unstable.
So, what happens is everything
starts mixing around.
So to give you an analogy
of what this might be like,
imagine building a house of cards
and then trying to place
a basketball on top of it.
What's gonna happen is the house of cards,
you're gonna go all over the place
and the basketball is gonna go down.
Well, the same basic thing happens here.
The ilmenite is the basketball,
so is the clinopyroxene,
it drives to the bottom,
it displaces wider minerals
that are forced to the top
and so this entire mantle mixes around.
The plagioclase stays up top
because it's more buoyant than everything.
The KREEP, because it also has
radioactive elements is hot,
it stays at the top,
the core has a density of about nine,
so it's uninvolved it doesn't care,
and everything else is mixed up.
So, at the end of this primary
differentiation of the moon,
we have a primary crust,
a mixed mantle of material
and a metallic core.
And this process is called overturn.
So, the geology of the
Moon isn't finished here.
There's still things that happen.
This involves secondary geologic processes
that typically aren't affiliated
with the primary crystallization
solidification of the body.
And in this case,
what happens is we have interjection
of molten material into
that primary crust.
And this is termed a secondary crust.
It's a group of rocks
that are sometimes called
the magnesium-suite.
But these rocks are thought
to be significantly younger,
or somewhat younger,
than the rocks that they're intruded into.
You can't intrude into something
if you're older than it
because it wouldn't exist.
So, in geologic terms it's a stratigraphy,
the primary crust should be older,
the secondary crust should be younger.
Again, geology continues to occur.
So, after these rocks are
intruded into the primary crust,
a series of basalt flows
are erupted onto the surface of the moon.
So, if you're looking at the moon today,
it might look something
similar to what you see here,
that plagioclase-rich material
is represented by this white stuff,
and the dark material are those basalts
that represent mantle melts
intruded into large impact basins
and they're represented
by these dark areas
in the center of the of the diagram.
Galileo looking at this
in the 17th century
called these the mare,
thinking they might be have
some relationship to the ocean.
Okay, so this model that I just presented
is based on the
geochemistry and mineralogy
of rocks that have been collected
primarily from the Apollo missions.
There are a few lunar meteorites
that have been collected,
but the vast majority
of what I just presented
is based on the analysis of rocks
determined or collected
from these missions.
There were six Apollo missions
that flew from 1969 to 1972,
Apollo 11 through 17,
they returned approximately
738 pounds of rock.
That rock has been continually curated
at the Johnson Space Center since 1969.
And this is the basis,
and I should add that those rocks
have been studied continuously
for the last 50 years.
By dating the appropriate rocks,
we oughta be able to test the
model that I just presented.
And one of the problems
is that when we do that,
the chronology does not support the model
and this is gonna be the basis
of much of what we're
gonna talk about today.
Before I do that,
I wanna demonstrate
exactly what it looks like
to get a rock from the moon.
So this is a panoramic composite image
from the Apollo 16 landing site.
So this lander landed,
was the one mission that
landed in the highlands.
So this is one of the
plagioclase-rich areas.
This image is taken from the lander
with the sun the background
showing the shadow.
If you look at the center of the diagram,
there is a note with a
little box around it.
This is sample 60025.
This is actually one of
the most famous samples
from the Moon.
By way of an aside,
you can tell where
individual rocks are from
by looking at their numbers.
So all Apollo samples
have a six digit number.
The first number represents the mission.
So, six means this is from Apollo 16.
The second number represents the station
from which it was collected,
the landing sites for the Lambs
were by convention, station zero.
So you know that this rock was taken
from a site that was right near
where the lander came down
and you can see it in
situ or in place there.
If you look at the blow up,
it's over there.
You can see a tire track of the rover
where it almost got run over.
To put this in perspective,
it's probably a little bit bigger
than the size of a golf ball.
Okay, so Tom's gonna help me
show you some of the principles
behind how to date rocks,
and then we'll go on and
show how age dating of rocks
can be used to address
some of these issues
regarding the origin and
evolution of the Moon.
Snap button.
- So to understand how
do we look at a rock
and tell how old it is,
we want to understand
how certain materials
can be radioactive.
What does it mean to be radioactive?
Well, to talk about radioactivity,
we have to do a little
bit of basic chemistry.
So, all chemicals are made up of atoms.
An atom has a nucleus at the center,
the nucleus contains
particles called protons
that are positively charged,
neutrons that are neutrally charged,
and then this nucleus is surrounded
by a cloud of negatively
charged electrons.
What element you have
is determined by how
many protons you have.
If you have one proton, you have hydrogen.
If you have two protons, you have helium.
If you have 42 protons, you've
got molybdenum, et cetera.
So for example, however,
you can have more than one kind of helium.
Most helium, the kind of helium
that is in party balloons
or makes your voice sound funny,
most of that helium has two neutrons,
but you can have helium
with only one neutron
and it's still helium because
it still has the two protons.
The fact that you can have
different types of atoms
with the same number of protons
but different number of neutrons,
this is what we call isotopes.
Well, it turns out that certain isotopes
of certain elements are
radioactively unstable.
One of the most commonly known
ones is a isotope of carbon.
Now, carbon is an atom
that has six protons,
and most carbon atoms
also have six neutrons.
But a rare type of carbon,
a rare isotope of carbon
called carbon-14,
has eight protons
that has a couple extra neutrons.
We call it carbon-14
because it has a total of
six particles in the nucleus.
And it turns out
that this particular isotope
of carbon is not stable.
At some point, every carbon-14 nucleus
will undergo a decay,
we call the particle that is
ejected the radiation particle,
in this case it's a beta particle.
And it results in, what did
we call, the daughter nucleus.
In this case it happens to be nitrogen.
Now, if I have an
individual carbon-14 atom,
I have absolutely no idea
when it's going to decay.
Radioactivity is spontaneous
and it is random.
But if I have a large sample of them,
I can study it
and people have studied carbon
and realize that the probability
of a given carbon-14 decaying
is about 50% probability to
decay if you give it 5730 years.
So we call that the
half-life of the carbon.
So if I start with a
million carbon-14 atoms,
and I wait nearly 6000 years,
I don't know which
500,000 are gonna decay,
but by the end of that
what we call half-life,
we can be pretty sure
that I'm gonna be left
with half a million.
So, we're going to
demonstrate a half-life decay
and you all are going to help me.
So you were given a coin when
you came in this morning,
so everybody go ahead
and get out this coin
and also stand up stretch.
Get out of your seats,
bend your back.
So, congratulations all of
you are now unstable atoms.
(audience laughs)
Now, we're in a moment going
to experience one half-life.
And sir, I don't know you,
I don't know how unstable you are or not,
I don't know if you're
going to decay or not.
But if there's say about 400 of you,
what we're gonna do is we're
going to flip our coins
and if your coin lands
Science on Saturday up,
you're going to decay.
So I don't know who's gonna decay,
there's about 400 of us in here,
I can be fairly certain that 200 people
are about to sit down.
So let's all experience a
half-life, flip our coins.
And if it's a Science on Saturday,
congratulations, you have decayed,
you may now sit down and enjoy stability.
(audience chattering)
So we're now left with,
we've just experienced one half-life,
so half of you have decayed,
half of you are not.
So, I think enough time has elapsed
we're going to experience this again
so already flipped your coin again,
and I don't know which
half of you just sat down,
but I can estimate about 100
of you just sat down again.
If we do this one more time,
if there's 100 of you standing,
I'm going to expect about
50 of you to sit down.
Let's do it one more time.
(audience chattering)
So, every time a half-life elapses,
you lose half of what was left.
So everybody still standing,
you survived three half lives.
So some of you're gonna survive a fourth.
So, let's say somebody knew
that we were going to do this activity
but they were late getting
here to the Bankhead Theater,
and they walked in right now
and they see a handful
of people still standing.
They would say to themselves,
shoot, I missed a demo.
I can tell the demo has already happened.
Time has elapsed.
That's what geologists can do.
We can look at a sample we can say,
"Oh, there's not many
radioactive isotopes left.
"This must have undergone
many half lives."
So as mentioned for carbon,
the half-life is 5730 years.
So if you start with a
certain amount of carbon-14,
after 5730 years,
you're left with half of it.
After two half lives,
you're left with half of
half of it or a fourth of it.
After three half lives,
you're left with half of a half of a half,
or an eighth of what you
started with, et cetera.
So we could look at materials
and find the ratio of how
many carbon-14s do we have
to what we think we started with,
and that tells us how
old that material is.
Now note with carbon-14,
the half-life is about 6000 years.
Once you get past more than
somewhere between five to 10 half lives,
you have lost so much of
the radioactive isotopes
that you've started with
that it starts to lose its power
to tell you the age of something.
So, radiocarbon dating is pretty good
for up to about 30,000 years.
So if you find some pottery or cloth
in an archeological dig,
then it can be useful for dating lead.
But if you want to date something as old
as the Earth in the Moon,
you're going to need an isotope
with a much longer half-life
than what carbon can provide.
Fortunately, there are other
things other than carbon
that undergo radioactive decay,
for example, uranium come around here.
So this is a Geiger counter.
This is a device
that detects the actual
radiation from decay.
So I'm gonna start with
these bananas over here.
So, you might hear the occasional click,
and that's because potassium,
one of the things that
makes bananas nutritious,
is also radioactive.
So how many of you had bananas
for breakfast this morning?
Well, please do not worry,
you are neither going to
explode from radiation,
nor are you going to turn
into the Incredible Hulk.
The amount of radiation from
a banana is pretty negligible.
The amount of radiation however,
for something like uranium
which was used in the
paint which made this cup
that was made in the 1930s
is pretty significant.
So, this is something from
a brand called Fiestaware
and we're reading the
incredible amount of radiation
from the uranium that's in that paint.
Anybody like a pot of tea?
(audience laughs)
So, Dr. Borg is gonna tell us
about another radioactive isotope
that he is used to determine the date
of samples from the moon.
- Thanks.
Yeah, so as Tom showed you,
there are elements like uranium
that decay relatively quickly.
There are also elements that are relevant
for dating very old things
so you need very long half lives.
So uranium is one of the
systems that can be used
because there are some
very long decay products
but the one that we use primarily
is the samarium-neodymium system.
These are rare earth elements.
Samarium abbreviated Sm,
neodymium abbreviated Nd,
has a half-life of 106 billion years.
So to put that in perspective,
we think the solar system
is 4.567 billion years old.
So this is roughly 18 times longer
than the age of the solar system.
So, samarium is present everywhere,
but it's decaying at such a low rate
you couldn't detect it
on a Geiger counter,
it's even worse than
potassium in that regard.
So, this is a slide that I've constructed
to sort of show you how you
date a rock using this system.
So, samarium 147 is the daughter product.
It decays to neodymium 143
which has a half-life again,
as I said 106 billion years.
The way we date rock is you
start off with the rock,
you break it into little bits,
and you separate all
the constituent minerals
that are in it.
So you separate out elements or minerals
like plagioclase, and
olivine, and pyroxene
into little piles.
And this is done excruciating pain
by crushing the rock and moving minerals
that are roughly the diameter
of five or 10 human hairs
in the little piles.
And these piles have to
be actually big piles
in order to measure the
thing with enough precision
because there's not much of
the daughter product produced
because the system has
a really long half-life.
So, if you look at the upper left,
that's a sort of a cartoon illustration
of a hypothetical rock.
The white is the plagioclase,
the red is the olivine,
the green and the yellow
are the pyroxenes.
You've separated those things out.
And then what you do is you analyze them
and you determine
the neodymium and samarium
isotopic compositions
of these materials.
And you plot them on
this plot on the left,
which is called an isochron diagram.
So, for the sake of argument,
imagine that you have a rock
that's crystallizing in Hawaii,
let's say yesterday,
and it produces these minerals.
Each of the minerals
has a different affinity
for samarium and neodymium.
They're different elements,
some minerals like samarium more,
some minerals like neodymium more.
On the other hand,
the minerals don't care
which isotope of neodymium
goes into them.
They'll take neodymium
143, or neodymium 144,
or any other isotope of neodymium.
They don't care,
they're really not very finicky.
So, if I took that rock from Hawaii,
I divided it into its
constituent minerals,
and I measured the
samarium-neodymium ratio
of those minerals
and the neodymium isotopic
composition of those minerals,
I could plot it on this isochron diagram.
So, the lower axis is the parent isotopes,
samarium 147 ratio to a stable
neodymium isotope of 144.
It's basically a proxy
for the amount of samarium
and neodymium in the mineral.
At the time that the rock forms in Hawaii,
all the rocks will have different,
all the minerals have different
samarium-neodymium ratios,
but they'll have the same
neodymium isotopic composition
that's represented by the
vertical axis on that diagram.
It's neodymium 143 which is the parent,
again, normalized to
that same stable isotope
of neodymium 144.
Plagioclase doesn't care
which isotope goes into it
so it has the same
composition as the olivine
and so pyroxene, so on et cetera.
So if I analyze that
rock and erupt it today,
it would lie on a flat
line on that diagram.
However, what happens is samarium
starts to decay to neodymium.
So we start losing samariums
out of the minerals.
And for every samarium we lose,
guess what?
We gain one neodymium.
So as a result,
these minerals start
moving on this diagram
depending on their age.
So that originally flat
line starts rotating
as we lose the samarium and
move back towards the origin
and we gain a neodymium.
So, the slope of this line
is proportional to the age.
And basically we can measure
the slope of the line,
divide it by the decay constant,
which is basically the
half-life of the system,
and we can calculate an age.
It's really that simple.
This is algebra one kind of math,
maybe even less, almost arithmetic.
So fairly simple,
but this is how we date things.
So, the next question is
we wanna date the moon,
we wanna test this model,
what are the materials do we date?
So in the center of this diagram
is an inset showing you what we think
of the stratigraphy of
the moon as it exists,
or the geology of the moon
as we think it exists today.
So the first thing that should form
in our magma ocean model
that I presented earlier,
is that we expect this mantle
material to form first,
almost contemporaneously with this,
we expect the primary
plagioclase-rich crust
represented on the top
and the number two to form.
The next thing that should form
should be this KREEP-rich material
that contains all the junk
that doesn't go into any of the minerals
that crystallized previously,
these three things should
form almost contemporaneously
with one another
because we think the magma
ocean cooled relatively quickly.
You have a very hot body
near absolute in space,
which is near absolute zero,
so cooling should be very quick.
And then finally what should happen
is we should inject magmas
of the secondary crust
into this primary crust.
So, samples of these materials
are represented in the Apollo collection.
On the upper left here is rock,
it's a very famous rock,
It's called 15555.
It's from Apollo 15 and it's mare basalt.
This rock is a melt of
that mantle material
and so we can determine the
isotopic systematics of it
and its brethren from other locations,
and we can determine
the age of that mantle.
The second rock on the
left down, number two,
is another fairly famous
Apollo rock from Apollo 16.
It's called a throne anorthosite
but basically what it is,
is it's some of that
plagioclase-rich material
that floated to the
top of the magma ocean,
represented by the white crystals
on the diagram in the middle.
The bottom rock on the
left is from Apollo 14.
Turns out Apollo 14 rocks
have most of the KREEP-rich material
that's present on the moon.
So there are some ways
we can extract the age
of KREEP by analyzing rocks like this.
And then this rock over
here, the Apollo 17 rock,
number four,
of the lot it's also
one of the most famous
it's called the troctolite,
and troctolite is a place in Norway.
It's not from Norway.
But this rock represents
some of this crustal material
that has been intruded
into the primary crust.
So by dating that we apply the minimum age
for the formation of magma ocean.
So, all these materials can be dated
and that's relevant for determining
what the age of this this body is.
So, the magma ocean model
hypothesis makes this prediction,
that these materials
should fall and yield ages
with this general systematics,
so that the slide on the right
is so called Caltech diagram,
which has a lower axis of something
versus a vertical axis of nothing,
but it represents the point
at which various materials,
the ages we predict
from various materials.
So the mantle material
should be fairly ancient,
should be the same age as
the primary crust and blue.
The KREEP-rich material
should be the same age
represented in green.
The individual age determinations
are gonna have variable
amounts of uncertainty
associated with them.
And as a result,
individual ages should
have some slop in them,
but basically,
we should get a continuous age
from all these systems.
And the general idea, at
least as of 10 years ago,
was the moon was very old,
something like 4.5 billion years old
so we expect it to be old.
The secondary crust which is
intruded into the primary crust
should be younger.
And since this intrusion process
is not limited by heat flow,
we would expect to see a range of ages.
This is the actual data that we have.
The range of ages that we have
are problematic for a variety of reasons.
The mixed mantle rocks
are obscenely young,
and they don't agree with one another.
The primary crust is both obscenely young
by geologic standards
but also we have values that
are as old as the solar system.
In fact, some that oldest point
is actually older than the solar system.
There's a problem.
The the KREEP-rich materials
also show a range of ages.
So when we look at these ages,
we see no systematics.
If you look at the secondary crust,
it shows a range which is fine
but it's also almost as
old as the solar system.
So we clearly have a problem here
associated with either our
model, or the chronology.
And the question is which?
And so it's illustrated
on this slide here.
Which one of these is the problem?
Have we measured the ages improperly?
Or have we developed a model
that's complete garbage?
And so this is the question
that we're trying to address.
And Tom is gonna show you
how you can look at geologic
and scientific problems
when you have issues like this
and how you can best test them.
- So I have some student volunteers,
if you could come on out,
watch your step there's
some cables down there.
So first of all,
thank you very much for coming out
and helping me today.
If you could just line up
in front of the table here.
So I've given each of you a
little whiteboard and a marker
and I'm gonna ask you two questions
and I want you to write your answers
to the questions on the whiteboard.
So first,
I would like you to write
a number on the whiteboard
that is within five years
of your current age.
And then underneath that,
I would like you to write your
current grade in high school.
And go ahead and hold
these up to the audience
and everybody to see.
So, just kind of going
down the list 13 years old.
and 10th grade,
you must be somewhat advanced.
13 and you're a senior,
you must be a prodigy sir.
I'm very, very impressed.
Similarly.
11 year old in 11th grade that's,
I assume therefore you
started kindergarten
before you were born.
21 year old in 12th grade
and 19 in ninth grade,
good for you for sticking it out.
(audience laughing)
So, this data seems a
little bit confusing to me,
a little discrepant,
I've taught high school for 20 years,
I thought I had a pretty good idea about
how old is your typical sophomore,
how old is your typical freshman,
and junior and senior, et cetera.
So, one of two things is happening,
either I'm totally screwed up
in the relationship that I think there is
between how old you are
and what grade you're in high school,
or there's something
wrong with the data here
and these ages are actually not correct.
So, this level of confusion
this kind of conundrum or puzzle
is what geologists and people
who are dating moon rocks
in trying to make that
jive with accepted models
of lunar formations.
This is the level of
conundrum and head scratch
that they were facing.
So thank you very much.
(audience applauding)
- Yeah, that's a perfect illustration
of the problems that we've
had in the last 50 years
of trying to understand
the chronologic data
that I just showed you.
Some people would argue that
the ages are all perfect.
And of course,
if you're a scientist
and you spent the last year of your life
generating one of those ages,
guess which category you fall into?
My age is perfect.
I got it right.
You guys, you're morons.
You've been sticking your
fingers in the beakers
and you've been making lots of mistakes.
But then there's another
way you can get out of that,
you can say well,
you thought you were
dating the age of the moon,
but maybe the rock you dated
really isn't a primordial
formation product of the moon
and that the model that we
have is completely wrong.
So maybe Tom's model that
understands the age of somebody
based on their grades is fallacious,
or maybe the ages are wrong
because the kids chose ages
that had no bearing on their true ages.
So, what we started to do
was look at the ages
that have been generated.
And so I wrote a paper
about five years ago
that basically reviewed all the ages
that had been done up to that point.
And this is a summary slide
of one of the main conclusions of this.
So again, same Caltech diagram,
same four materials that were dated,
but what I did was I went
and I looked at rocks
that have been dated,
the same rocks have been
dated by different groups.
And guess what?
They didn't agree.
So you give Bob a rock, and
Mary a rock, and Sam a rock,
and they all come up with different ages
and that's what's
illustrated on this diagram.
These pink spots, or pink data points,
all should yield the same age.
They should be what we
call concordant and agree,
and they aren't.
And so this gives us the hint
that maybe it's not the model,
maybe these ages aren't right.
Having spent 22 years dating these rocks,
I can tell you it is a labor of love,
it is extremely difficult to do this.
When this was first began in 1970
with the technology of the day,
it was almost an insurmountable challenge.
So, what we did was we
got my group together
and just so you guys know,
I had to put this thing down the left
that says that they don't
really look like that.
But they do (laughs).
So, that's a portrait of
Greg over on the right,
he's in the audience somewhere, I'm sure.
Anyway, we decided,
we got together we said,
well, what can we do?
Well, we've got 20 years of
technological development.
At the lab,
we have the state of the
art chronology center
that can go back and look at rocks
and date them with fidelity
that could not have been
even dreamed of in 1969.
And so what I'm gonna show
you in the next few slides
is a result of our work here,
and I'mna go through
each of these components,
one through four,
and show you the ages that
we have determined on them
and they'll show you it paints
a fairly concise picture
for the age of the Moon.
So, this is a isochron
plot that's used to date
the formation of the mantle.
This is samarium-neodymium,
it's very similar to the
plot I just showed you.
The data points are analyses
of individual mare basalts
that we've done at Lawrence Livermore.
Various types of rocks
are represented here,
rocks from all over the
moon are represented here.
And in fact, every mare
basalt we've ever analyzed
is represented on that plot.
The slope of that line
corresponds to an age
of 4331 million years,
or in other words 4.33 billion years.
So, we've also gone back
and looked at that primordial
plagioclase-rich crust,
very difficult material to date,
it's seen impacts,
it's been heated and cooled,
it's had melts injected into it
as a result of large
base and forming impacts.
It's a tough thing,
but we've managed to figure
out ways to date this.
If you look at the upper left,
there's that sample I'd show you.
I promised I'd show you again, 60025,
taken from the Apollo 16 landing site.
Below it is a sample 60016
which was taken nearby.
Below that is a sample 62237,
which we recently dated like
last year from station two.
The isochrones are
presented in cartoon form
next to the diagrams.
The point that I want you to note
is the summary of the
ages over on the right.
So in other words,
ignore the data, look at the words.
The oldest sample we have is 4.37.
The youngest we have
is 4.30 billion years,
we have another rock
of 4.36 billion years.
If you take the average
of these three rocks,
and these are all three that we've dated,
we get an age of 4.34 billion years.
The age we got on the
mare basalts was 4.33.
Yay, we're seeing the same thing.
So, could we be so lucky to
see that in other things?
So, we've gone on and we've
dated the KREEP-rich material.
That KREEP material has to be dated
using a very different technique
and in the interest of time and boredom,
I'm not gonna show you how
that chronology is done.
But suffice it to say that
we've done it two times.
The first age we got
was 4.35 billion years,
the second age we got
was 4.39 billion years.
So again, we're seeing
that same age recorded
in this last vestige of
solidification of the magma ocean.
We've also gone and looked at the age
of the secondary crustal material.
So like the previous slide,
the insets represent pictures
of the various rocks we've dated.
The top one is from Apollo 15, 15445.
The middle one is that very
famous rock I told you about,
called the troctolite 76535,
Apollo 17 station six.
The lower one is from a large boulder
at Apollo 17 at station eight
and the ages we have
obtained are 4.33, 4.31
and 4.33 billion years.
So this yields an average age
of about 4.32 billion years.
So, in summary,
we have an age for the mantle
that we determined on
that very first slide
I showed you of 4.33 billion years,
the average age we've determined
represented by number
one on this Caltech plot.
Number two, is the age of the anorthosites
that have an average age
of 4.34 billion years.
Number three, is the age we've determined
on the KREEP-rich material
which has an average age of
about 4.37 billion years,
and then the magnesium-suite rocks
give an age of about 4.32 billion years.
So, by going back and
looking at modern techniques,
we've maintained a series
of concordance ages,
which are what's predicted
from the origin and
evolution model for the moon.
And so we've seen a agreement
between the geology that
suggests how the moon formed,
and the chronology that
suggests when it formed.
So, that's the takeaway message,
that's the age of the moon.
So, if you have any forms to fill out,
this should be a question
that you have to answer.
And that's the answer, okay?
This has some very profound ramifications
for the origin and evolution
of the Earth-Moon system.
It indicates that the moon formed,
and the Earth formed very
late in solar system history.
So the impact that produced
the Moon and produced the Earth
had to occur something
like 250 million years
after the beginning of the solar system.
We think that planets form
based on analysis of meteorites
within the first couple million years,
one of the people in my group
dated the formation of Jupiter
two million years after
the ignition of the sun.
So planets formed very early,
but the Earth and the
Moon form really late.
So we formed by a unique process
by solar system standards.
In the vernacular,
this is called a stochastic process.
And this is consistent with
something strange happening
like a giant impact.
So, this demonstrates that
the Earth and the Moon
likely indeed formed from a giant impact
because if we had an age
that was as old as the solar system,
there would really be no need
other than the physical
angular momentum requirements,
to generate the Earth-Moon
system by that process.
So with that, I am done.
(audience applauding)
(upbeat music)
