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 1.16: Atmospheric escape
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The Science of the Solar System
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Unit 1: Water on Mars (week 2)
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Mike BrownProfessor
Planetary Astronomy
Is it possible that Mars could have lost one atmosphere of CO2 over its history?
You're going to get tired of my answers to these questions,
because they're all something like, maybe.
One way that you can lose an atmosphere is, it's kind of a slow trickle way,
is called Jeans escape.
Jeans escape happens from the top of an atmosphere.
At the top of the atmosphere, there are molecules of whatever molecules are up
there that are still gravitationally bound to the planet down here.
And they have some temperature, so they're all moving
at different velocities in different directions, and they all hit each other.
This is what happens in any typical atmosphere.
Eventually, though, the density, the pressures get so
low on top that you reach this place that we call the exobase.
The exobase is the spot from which, if a particle happens to be
going this direction, it will not hit another particle.
Now, there's still gravity, so it'll go up and
it'll come back down to the exobase, where it probably hits something.
If, though, it happened to be going a little bit faster,
it could have escape velocity and be lost from the top of the atmosphere.
This will happen sometimes.
For a given temperature in this top layer,
the velocity of the objects has what is called a Maxwellian distribution.
A Maxwellian distribution looks sort of like this, number of molecules and
their velocity.
And there's a peak, so most of them are going at this standard velocity,
but there's a high velocity tail, it's called.
Some of the molecules are going at very high velocities, and
some of them will have enough velocity to escape.
And you could say, for a given temperature at the exobase,
if things above this velocity are above the escape velocity,
then Jeans escape says that these things will trickle off into space.
It's a pretty slow process.
And if you calculated how much Jeans escape would happen over the history of
the solar system, you wouldn't get much loss coming from Mars.
So we need a faster way to do it.
There's sort of a different flavor that's called hydrodynamic escape,
although it's not that much different.
And in that case, what essentially happens is that
this temperature in through here gets artificially inflated.
Artificially is maybe not the right word.
This temperature gets inflated through some means, such as absorbing energy from
the sun up here, due to some sort of absorption in the atmosphere.
This gets heated up, and this entire layer can get a velocity that's more like,
this whole layer gets heat it up like this.
And the whole layer can be so fast that it basically escapes off into space.
As you can imagine, that can be a much faster process.
Hydrodynamic is often called hydrodynamic blow off.
There's no good reason to think that that happened very much on Mars.
A much more interesting, and perhaps likely, process goes something like this.
In the upper atmosphere, the molecules can be hit by high-energy ultraviolet photons.
These high-energy ultraviolet photons can ionize some of the particles up in here,
and leaving this one positively charged and electrons floating around everywhere.
And that happens on the Earth.
We have an ionosphere up high in the sky.
But on Mars, an ionosphere does something slightly different,
which is that Mars has no global magnetic field.
This is a really critical point, and
we'll talk a lot about this in subsequent classes.
With no global magnetic field,
the magnetic field of the Sun sweeps directly past Mars.
And if an ion is in a magnetic field, it likes to spin around that magnetic field.
If that magnetic field is moving, as the Sun's magnetic field does,
carried along by the solar wind, that can be stripped off and escape to space.
Of course, it can also be, instead of stripped off, then escaped to space,
it can be picked up and
slammed into the atmosphere if the atmosphere is thicker in through here.
And when it slams into the atmosphere,
it can hit other particles in the atmosphere and cause them to escape,
physically knock them off out into space, in a process called sputtering.
So this combination of solar wind, magnetic fields kind of stuff,
solar wind particles can do sputtering and ionization.
And the sputtering caused by these reentering particles
can induce some pretty severe damage to the upper atmosphere of Mars.
In a way that the terrestrial atmosphere is unaffected because of the strong
magnetic field that the Earth has, which protects us from these processes.
This solar wind process is particularly interesting.
Because it's thought that the solar wind in the past was something like 200
times stronger than it is today.
So there would have been a massive process of this scavenging
by the solar wind of the top of the atmosphere.
And the very quick erosion of, if there were a massive atmosphere out there.
Unfortunately, like many of the other things here, it's difficult to calculate
exactly how much material was pulled off through these processes.
Because it depends in detail, how much solar wind was there?
What was the strength of the magnetic field?
How does the interaction really work?
We have theories, but
we've never had any measurements of this process happening anywhere in space.
And so there are two approaches to try to figure out
if something like this really happened.
The first approach is to look for evidence that Mars lost a lot of atmosphere.
How could we find that?
One really powerful way is to look at isotopes.
Now here, we're not talking about radioactive isotopes, like we were before.
We're talking about the fact that chemically,
two different isotopes of the same molecule react exactly the same.
But one of the isotopes will be a little bit heavier,
because it has extra neutrons in it.
That little bit extra mass can make a big difference in these processes of escape.
For example, in the Jeans escape process that we looked at,
a heavier isotope will be going slower at the same temperature.
So less of it will escape.
Hydrodynamic escape, it can be the same way.
And even in the sputtering process,
if you knock into molecules, the ones that are heavier will be going slower.
So in all cases, if you lose an atmosphere,
you preferentially lose the lighter material and retain the heavier material.
Now, these differences are not very big.
And yet these are the sorts of things that if you can measure them very precisely,
you might have a good measurement of what's going on.
They've been a couple of attempts to measure isotopic ratios in the atmosphere
of Mars.
And let me summarize the data for
you in this nice NASA-generated press release plot.
Except just to keep you in suspense,
I removed all the interesting parts of the plots.
And I will slowly reveal them, to your astonishment.
First off, let's think about what we could use to make this measurement.
So there are many different
molecules in the atmosphere that you might consider measuring.
A particularly nice type of molecule is a gas that does not react
with other molecules.
Why do you care about one that doesn't react?
Because chemical reactions also favor one type of mass over another type of mass.
Fractionate is the word that we use.
If chemical reactions are the cause of fractionation,
then you're not learning anything about mass loss in the atmosphere.
So you'd really like something that is completely inert.
You can't get much more inert than something like argon.
Argon is a noble gas.
It has no reactions with anything, and
it just sits there in the atmosphere minding its own business.
Nice thing about argon, it's main isotope is argon 40,
Which almost all argon has.
But there's also an isotope that has a weight of 38 and
one that has a weight of 36, which are much more rare.
You might think, well, why would you measure the ratio of 36 to 38 instead of
something that's rarer to something that's very common?
The answer is because these two have nearly the same abundance.
They're only different by about a factor of five,
whereas these are different by factors of 1,000, 10,000.
And so it's difficult to make a measurement at the same time
between something that has a factor of 10,000 times more than something else.
So measuring these two things at the same time, the abundance of these two things at
the same time in the same thing is a much more easier observational task.
How do you do this observationally?
You do it through some sort of mass spectrometer, which, for example,
ionizes the argon, sends it through a magnet.
And that magnet causes the trajectory to bend.
And it bends a little bit more if it's lighter and
a little bit less if it's less light.
So where that argon hits on a detector tells you its mass, and
you can then measure the mass ratio.
This technique has been done on the terrestrial atmosphere, it's right here.
And interestingly, it's been done on the Jovian atmosphere, and
even on the solar atmosphere.
How do you do the Jovian atmosphere?
Well, this was done from the Galileo probe that actually went
into the Jovian atmosphere, measured this ratio.
How do you do it for the Sun?
Well, we didn't send things into the Sun to measure it.
But we did have a spacecraft, Genesis,
it was called, that sat out in space, collected solar wind particles.
Brought them back down to the Earth, brought them back down to the Earth,
forgot to deploy the parachute, and crashed into the Earth.
But materials was still recovered, and things like
the 38 to 36 argon ratio were measured for the Sun, for Jupiter, for the Earth.
Sun and Jupiter are taken to be the primordial abundance
of argon 36 to argon 38 in the initial Solar System.
And it's nice, they're about the same value.
How the about the Earth?
Well, the Earth is down by a little bit, but really not very much.
There's no evidence that the Earth has lost very much atmosphere.
Okay, I'll let you see the first measurement.
The first measurement came from the Viking lander that got to the surface of Mars,
measured the atmospheric composition, measured the isotopic ratios.
And sure enough, well, it's either higher than the Earth and
Jupiter and the Sun, or somewhere in between.
Not a useful enough measurement to really say anything.
The next measurement came from a pretty novel source.
It's from a meteorite that was found on the surface of the Earth
that is thought to be from Mars.
That has bubbles trapped inside of it that are thought to be the atmosphere of
Mars at the time the meteorite was blasted off the surface of Mars.
It's a pretty crazy sounding idea.
But people had found these meteorites that were different from
meteorites that had been found that looked like they come from the asteroid belt.
We'll talk a lot about this later on in the class.
But they looked nothing like the normal meteorites that had been found.
And they looked a lot like things that might be coming from Mars.
And people calculated that, yes indeed, you could get meteorites from Mars.
An asteroid comes in, smashes into Mars,
material is shot up into space, travels through space, lands on the Earth.
And in fact, as it's blasted off the surface and slightly molten,
it can capture gas bubbles from that atmosphere.
So there were gas bubbles in these meteorites.
People measured the isotopic composition, and look what they found,
inferred from Mars meteorites, way down here.
Significantly lower than Sun and Jupiter, and significantly lower than the Earth.
Now, you're welcome to be a little bit skeptical about the Martian meteorites.
There are a lot of things that have to happen right that the atmosphere has to
get captured correctly without doing any sort of fractionation.
There can't be any diffusion, any loss of that atmosphere over time as
the meteorite is in space or the meteorite is sitting on the ground in the Earth.
And all that story has to be true, and so you could still say,
well, I'm not sure I totally buy this.
And so the most recent measurement, actually just happened last year,
came from the new Mars Curiosity rover.
The Mars Curiosity rover has a fabulous little mass spectrometer on it.
It's called SAM, Sample Analysis at Mars.
And one of the things that it did early on was grab some atmosphere and
try to measure these isotopic compositions.
And the answer is?
Boom, everybody was right before, by the way.
Here's sample analysis on Mars is right here.
It is perfectly in agreement with the Viking data and, well, close to
being in agreement with the Mars meteorite data, I'll call that close enough.
And we have a good measurement right down here of about a ratio of four,
compared to the ratio of five and a half.
How much mass was lost?
It's one of those things that's hard to tell.
[LAUGH] I'm sorry, it's my answer to almost everything.
It seems like at least half the atmosphere had to be lost.
And perhaps up to 90% of the atmosphere had to be lost to reach
low values of this ratio.
Moreover, we know some of the ways in which it had to have been lost.
One of the things that does not work is that hydrodynamic escape.
Remember, hydrodynamic escape is where an entire layer
sort of goes off at the same time.
That doesn't change the isotopic ratio.
Jeans escape changes the isotopic ratio,
but Jeans escape is so slow that it really can't have had that much of effect.
So this is suggesting that that process of ionization, solar wind pickup, that
sputtering really did have a substantial effect on the Martian atmosphere.
How much?
We'd like to know.
One way to know is to go there and
measure how much of the atmosphere is being lost today.
And in fact, this is exactly what the Mars MAVEN spacecraft is designed to do.
MAVEN, which stands for, you can guess that's probably Mars,
that's probably atmosphere.
And then I get confused, but this is, and volatile evolution with an N.
Okay, kind of a lame acronym, but most acronyms are kind of lame.
MAVEN measures specifically these sorts of processes to see what's happening today,
to try to infer what really happened to Mars in the distant past.
Right now, MAVEN is in orbit around Mars.
And it's on this very cool orbit where it dips into the atmosphere for
a portion of it.
It measures what's going on in there.
It looks to see things that are coming off over here.
It looks to see what's in the atmosphere there.
It's been there for a while.
The results should be coming soon, and I would just say, stay tuned.
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