Learn about the science behind the current exploration of the solar system in this free class. Use principles from physics, chemistry, biology, and geology to understand the latest from Mars, comprehend the outer solar system, ponder planets outside our solar system, and search for habitability in our neighborhood and beyond. This course is generally taught at an advanced level assuming a prior knowledge of undergraduate math and physics, but the majority of the concepts and lectures can be understood without these prerequisites. The quizzes and final exam are designed to make you think critically about the material you have learned rather than to simply make you memorize facts. The class is expected to be challenging but rewarding.
Lecture 3.09: Meteorites and the beginning of the solar system
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From the course by Caltech
The Science of the Solar System
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From the lesson
Unit 3: Big questions from small bodies (week 1)
Meet the Instructors
Mike BrownProfessor
Planetary Astronomy
A nearly unique aspect of small bodies is
that, unlike almost anything else you can think
of in the astronomical universe, we can actually
pick up pieces of them on the ground.
Sometimes you're walking along and you just
find a little piece that looks like this.
This is a little chunk of iron meteorite that I actually found on the ground.
I know it's a chunk of iron meteorite,
because I was looking around a region where
I knew there was a lot of iron meteorites, and because it's a chunk of iron.
If you get out a magnet, you can see that this is a chunk of iron.
I have another one.
This is a, a prettier piece of iron meteorite.
Someone gave me this one.
I actually have cufflinks made out of iron meteorites.
Okay, that makes me a geek, but you already knew that.
You can't really go around picking up other things in
the universe that just happen to land on the Earth.
Things that land on the Earth we have the ability to do amazing experiments with.
We have the ability to bring them into laboratories.
Measure things in incredible detail.
And the amount of stuff that we've learned from these little
chunks that land on the Earth is, is really, truly amazing.
One of the things that we know because of
these things is when did the solar system form?
We've talked about this before.
I've already mentioned that the solar system is 4.5 billion years old.
I've never really told you why we know it's 4.5 billion years old.
And the answer is things like this.
Well, it's actually not things like this.
This is an iron meteorite.
It's not from iron meteorites.
It's from a different type of meteorites.
Let me show you what these look like.
This is a different kind of meteorite.
It's a, it's called a chondritic meteorite.
We'll talk about those in detail in a few lectures.
But the important things that I'm going to talk about in this particular meteorite
are these little white specks that you might be able to just barely see.
Little white specks all over the place on this particular meteorite.
These little white specks, if you look at them
chemically, you realize that they are full of calcium.
They're full of aluminum.
They're full of minerals made out of calcium and aluminum.
And in their very clever way, people who
study meteorites have called these things CAIs, calcium-aluminum inclusions.
Inclusions because they're not part of the overall background of this meteorite.
They're little bitty fragments that sit inside there.
For many years it was thought that calcium-aluminum inclusions were the
very first solid materials that ever formed in the solar system.
And, and let me show you why that was not an unreasonable thing to think.
I'm showing you what is called
a condensation sequence for the solar nebula.
And I apologize for the sort of crummy picture here.
And it's because it's from an old textbook that I still like, I find
very useful by John Lewis, called Physics
and Chemistry of the Solar System from 1995.
So, I actually had to scan this image which
is an astounding thing to have to do these days.
But here, let me show you what's happening in this one.
We have a temperature scale here in kelvin.
And it goes, if you can't read it, this is a one, this is a 2000.
2000 degree kelvin, very hot.
Very, very few things are not molten or sublimed into gas at 2000 degrees kelvin.
What this is trying to show you is what
would happen if you took the composition of the solar
system and you heat it all up into a
very hot gas like presumably the original protostellar nebula was.
And particularly close to the sun, it would be even hotter.
And the way we thought that the sun and the
planets formed is that this, there was this protostar in here.
There was this disk of gas increasingly moving out through here.
And it's hot, it's very hot all throughout here.
It slowly cools.
Particles slowly form.
That dust coagulates.
We form planets.
The very hottest times there would have been
very few solids, particularly in this close region here.
As it cools, what's the first thing that condenses out?
Well, you can think of it.
It's the opposite process of if you had a big chunk of material and ask yourself
the question as you heated that material up,
what would be the last thing that didn't vaporize?
The first thing that condenses out, the
last thing that didn't vaporize are aluminum oxides.
Aluminum oxides are still solid up at
2000 degrees kelvin, where almost nothing else is.
A rock would not be a solid at 2000 degrees kelvin.
It would melt.
Aluminum oxides, and calcium, and iron, these things
are the highest temperature things that are still solids.
So, you could imagine.
Remember what I just told you?
These things we were looking at were calcium-aluminum inclusions.
Calcium and alumun, aluminum are, come from the hottest
times when they're the first things to condense out.
It would not be a surprising thing if it were true.
Let's just follow the rest of the
sequence, just for fun, to see what happens.
As the temperature drops, you can see
this, this line shows where they become solid.
They go from the gas phase to solid.
Nickel, iron and nickel happening through here.
Magnesium things, like magnesium SiO.
These are rocks, SiO2, SiO2.
Anything with silicon like this.
These are what we call rocks.
Sodium potassium, they're, they're coming down through
here, but finally we have things like water.
Much colder temperatures.
These are temperatures like where comets would have condensed water.
ammonia, methane, and then noble gases way out through here.
The high temperature sequence in through here, the rocks
of the inner solar system and these calcium-aluminum inclusions.
Okay, so, calcium and aluminium inclusions may have
been some of the first things to form.
And so, if you could go find some of those calcium-aluminium inclusions and
figure out their date, you would know how old the solar system was.
So, how do we figure out how old one of these calcium-aluminium inclusions is?
We're going to use the technique of lead-lead dating which
relies on the fact that uranium-238 decays eventually to lead-206.
Lead-206 is stable.
Uranium decays through a series of other unstable things.
And uranium-235, that's a five, decays
eventually to lead-207, which is also stable.
The uranium-238 timescale, the half-life for decay, is
something like 4.5 billion years, which is a
conveniently really long time, not dissimilar from the
age that we'll find for the solar system.
And uranium-235 has a different half-life, which is 0.7 billion years.
So, imagine what happens to a meteorite, or, or
asteroid, or anything else in the, in the universe.
It starts out with a certain amount of uranium-238
and 235, whatever that original amount was, but that decays.
That's a 3.
I can't write today.
So, let's see how this works.
You take a meteorite and it started out, and it started out with an initial amount
of uranium-235 and uranium-238, an initial ratio there.
And that initial ratio changes with time.
How does it change?
Well, 235 decays pretty quickly and becomes lead-207.
And more slowly, 238 decays and becomes lead-206.
So, you could figure out that ratio of 207 to 206 lead is related
to both the initial abundance of this and the amount of time that has passed.
Because we can measure these isotopic ratios very
precisely on a rock that you get into
a lab on the Earth, we can measure
very precisely when these things started their radioactive decay.
Let's look at some real data which you measure and see what we think this means.
Okay, let's look at what's really being measured here.
So, it's a ratio of lead-204 to lead-206, and lead-207 to lead-206.
Remember, the 207 and 206 are the end
products of the decay of uranium, 204 is not.
204 is here because it tells you how much lead was in the material to begin with.
Something that has a lot of lead to begin with, well,
it's going to have a lot of lead-206 and a lot of lead-207.
Something that didn't have a lot of lead
to begin with will not have nearly that amount.
What we're looking for is the extra lead that was caused by the uranium decay.
So, let's look how this works.
Here.
Let's look for things that have large ratio of lead-204 to lead-206.
These little spots, thesee, these dots on the graph are different
parts of a particular calcium-aluminum inclusion that was measured.
And you can look at different parts, and they have
different amounts of lead in them, different amounts of the ratios.
And you can even look way down in this thing that's blown
up here and all these different parts have been measured very well.
So, here's a part that was measured to
have high abundance of lead-204 compared to lead-206.
It also has a high abundance of lead-207 compared to lead-206.
Simply knowing this number doesn't do you any good,
because you could've just found a chunk of the solar
system that happened to have a lot of lead-207 and
a lot of lead-204 and doesn't really tell you anything.
What tells you something is to then go
in the same calcium and aluminum inclusion and look
at spots that have less lead-204 and less, and
less, and less and less and less and less.
And in each case, where you have less lead-204, you also have less lead-207.
Those two things are that radioactive decay product.
And what you're really interested in is not these absolute values
here, but you're interested in this slope of this line here.
A slope could go like that.
A slope could go like that.
And it depends, you, do you have more 207 than 204 on a relative basis?
Do you have less 207 than 204 on a relative basis?
And because those are the products of that
uranium decay, you can then figure out exactly
how long that uranium has been decaying and
what the age of this calcium-aluminum inclusion is instead.
What's the age I found here?
4567, it's the easiest number in the world to remember.
4.567 times 10 to the, should have been an
8, but it's 10 to the 9 years, billion years.
4.5, we always say 4.5.
It's really 4.567 billion years is the oldest age
that's found, and it's found for these calcium-aluminum inclusions.
This same paper measures the ages of other calcium-aluminum inclusions and some
other parts of meteorites, and let me show you what those are.
Here's that one that we just looked at, 4.567 something or another.
It's got a little bit of uncertainty but not bad.
Calcium-aluminum inclusions here in red all are in this
time period right through here, this earliest time period.
And up here, they call it brief epoch of CAI condensation and melting
right here at the start of the collapse of the cloud that is the solar system.
The other parts of the meteorite are now, we're, the
CAIs are those little white flecks, but the rest of
the meteorite is full of what are called chondrules, and
we'll talk a lot about chondrules in a subsequent lecture.
For a while, it was thought that chondrules were significantly older.
By significant, I mean millions of years older.
The chondrules didn't form until much after the CAIs had condensed.
This paper is the first to show that the, the some
of the assumptions that were made in the previous analyses were wrong.
And in fact, chondrules are forming at the same time as calcium-aluminum
inclusion, but the chondrules form over an extended period of time in through here.
[SOUND] What are these times?
Well, here we go.
4.5, 6, 7 billion years ago, and this is a couple of million years.
There's a 3 million year time period from
here to here where the chondrules are being formed.
These time periods, we ac, these time periods actually mean something to us now.
If you remember, Jupiter forms over something like a 3
million year, time year period, ten million year time period.
This is the time in which there is gas in the nebula.
Much longer time period is when the terrestrial planets are forming.
When these great oligarchs have come together and are starting to collide.
But these meteorites, these things that are delivered to the Earth, were
largely put together out of materials that were in this very earliest time.
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