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 1.18: Gamma ray spectroscopy and subsurface water
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Del curso dictado por Caltech
The Science of the Solar System
289 calificaciones
De la lección
Unit 1: Water on Mars (week 2)
Conoce a los instructores
Mike BrownProfessor
Planetary Astronomy
So far, we've used some very tangible evidence to look at the
presence or the possible presence of water on Mars in the past.
We've used things like images of drainage systems in canyons.
We've used measurements of elevations that show where things would have flowed into.
As missions to Mars have
gone on, increasingly sophisticated instrumentation has
flown to Mars and investigated these questions in even more detail.
One that I think is really important but takes a little bit
of explaining is the use of gamma ray spectroscopy and neutron spectroscopy.
These aren't things that most people are intimately familiar with or find intuitive
right away, so let me take a second to explain how they actually work.
Okay, well here at least is NASA's attempt to explain how it all works.
The process starts when the surface of a body is hit by these cosmic rays.
What are cosmic rays?
You probably heard the term cosmic ray before, and
maybe have not really thought about what it means.
Cosmic rays are very high energy ionized
particles that are coming streaming from space.
From the sun, from the center of the galaxy, from many other places, and
they are penetrating through everything all the
time, including your head right this very minute.
Cosmic rays are the sorts of things that maybe can cause long term
damage to your DNA by breaking apart some of the, the bonds, and mutations.
I see cosmic rays a lot when I use a digital camera to take pictures of the
night sky, and if you, you look very carefully
at those dark regions of the night's sky, you
see, you see bright spots every once in a
while, and those bright spots were cosmic rays that
have gone through and lit up the detector, not
the sky, but the detector and that's bad spot.
Cosmic rays are sufficiently energetic that they can hit
a nucleus of an atom and knock off a neutron.
So a lot of this that we're going to discuss
here, we're going to talk about the, the nuclei atoms.
And we often think about atoms, they're a bunch of neutrons, bunch protons,
and surrounding all of these are a bunch of electrons at different energy levels.
And most of the time when, when we think about the interaction of these atoms with
radiation, we think about things like electrons changing
energy levels, or electrons getting lost and ionization occurred.
When an electron changes energy levels, a photon is emitted,
and the photon has a wavelength of that particular energy level.
Same thing happens within the nucleus, that we don't usually think about it
this way unless we're nuclear physicists, but
protons and neutrons also have energy levels.
Protons and neutrons can be removed from the nucleus, and when that
happens, and when protons and neutrons change energy levels, they emit photons.
The photons that they emit in this case are extremely
high energy photons because these things are bound very, very tightly.
And these are gamma rays.
There is the symbol for gamma right there.
These are gamma rays that are emitted when these processes occur.
Okay, so again, let's restart.
Cosmic ray comes in, hits a random nucleus, we don't
care what it is, and it knocks off a neutron.
This neutron goes blasting around and a couple different things
can happen to this neutron as it goes blasting around.
First thing that can happen is a collision with another
nucleus and when this collision happens with the other nucleus,
first the, the neutron bounces off essentially but as it
bounces off it excites the nucleus to a higher energy state.
Nuclei don't like to be in high energy states for very long.
So that energy state decays back down.
Boom, the gamma ray is emitted.
The gamma rays that are emitted through this
process, just like the photons emitted when electrons
move in energy states, the gamma rays that
are emitted are very distinct for each nucleus.
So by looking at the gamma rays that come off, you can not only tell that
things have been happening, but you can tell
exactly what the collision was that occurred down here.
Another thing that can happen is, after a few
collisions and it slows down a little bit, the,
neutron can actually get captured by the nucleus and
again, the proton, gamma ray photon, will be emitted.
The other way to get gamma ray photons is from radio active decay.
Gives off these gamma ray photons, you have to figure a way
of figuring out which is which, but these come at specific energies also.
So this is the gamma ray half of the gamma ray spectrometer.
But remember there's also a neutron spectrometer that is that you actually
don't just watch the gamma rays come up, you measure the neutrons themselves.
How does that work?
Again, you have the cosmic ray collision here and
again a nuetron can go blasting off and sometimes, that
neutron can simply escape the surface without interacting with
anything, when it does it will go off into space.
It can however have this series of collisions like we looked at over
here and looked at over here, and these collisions slow the neutron down
until it is finally going at a speed that's similar sort of to
the background speed of neutrons, the temperature,
the, the thermal energy of that surface.
So, if you were looking at neutrons coming off
the surface due to these ga, cosmic ray impacts, you'd
see fast neutrons, and then you would also see
a series of much slower neutrons coming up through here.
One of the really important things that we can
measure is, the ratio of fast neutrons to slow neutrons.
What is that ratio going to tell you?
It's really going to tell you how much slowed down these neutrons get.
Why do we care how much slowed down the neutrons get?
Well for one really good reason which is the
thing that slows down neutrons the best, is hydrogen.
Why is that?
Well neutrons can hit any thing that's in
there, any, any nucleus that's in there, but
if it hits a heavy nucleus a silicon
nucleus, an oxygen nucleus, things that are fairly common.
If it hits one of those very heavy nuclei, it just bounces off with the same speed.
It gives a little bit of speed to the other nucleus, but not much happens.
It's like taking a superball and throwing it at a bowling ball.
That bowling ball doesn't move.
Super ball just bounces off at exactly the same speed.
If the neutron however hit something that it's about it's same mass, now
imagine taking your super ball and throwing it at well, a super ball.
What happens?
The super ball bounces off sure, but that other
super ball comes off at about the same speed.
The overall energy of the super balls has been decreased.
And then, each of those is going to collide and turn, and it's going to create
this thermal cloud of hot neutrons, but
not these very fast neutrons in through there.
So, the amount of cooling, in through here,
depends critically on the amount of hydrogen, in particular.
What has hydrogen?
Well I'll give you a hint.
You can drink it.
Although not in the case of mars, cause it's going to be mostly frozen.
Let's look first at some of the gamma ray data that
came down from the very first science paper that was written showing
what the gamma ray data looked like, and here you see
an actual spectrum, gamma ray spectrum, energy in the kiloelectron volt range.
That's very very high energy photons.
And you see three spectra from collected
gamma rays over different regions of the planet.
First, you see the North Pole, and
basically, you see a little bit of nothing.
You see something right here for aluminum, and
a little peak right there where hydrogen should be.
And maybe nothing else.
That's North Pole raw of 75 degrees latitude.
The middle, nothing, nothing, nothing, maybe a little
bit of hydrogen right there, nothing, nothing, nothing.
And the South Pole, south of 60 degrees, nothing, nothing,
nothing, one big signal in through there, and, and out.
Ton of hydrogen emission.
Gamma rays that came from hydrogen being emitted at the South Pole.
It's important to know that the gamma ray spectrometer is
only sensitive to the first, maybe, meter of the surface.
It doesn't penetrate any more deeply than that.
And this was northern winter.
So the North Pole, had a CO2 frost
covering the surface of the otherwise water ice cap.
So you don't see the water that we actually know is up at the cap there.
But in the south where there's no CO2 frost you see a ton of that.
Let's compare now the strength of that gamma ray
emission as a function of latitude on the planet.
And you'll see just as I was talking about,
here is the gamma rays on this points here.
Ton of gamma ray emission from that hydrogen
line, starting at something like 45 degrees south.
Sort of flat across through here, and the beginning of a rise and then a big drop.
This big drop is because this is where the C02 ice cap is, frost cap is.
But it was starting to go up again perhaps it might continue up like this.
And in fact, at later seasons it would seem that it does continue up like that.
Let's compare that now to the neutrons.
Now, remember what the neutrons do.
The neutrons are attenuated by hydrogen nuclei.
They are slowed down by hydrogen nuclei.
So if you look at the number of these thermal neutrons, and epithermal,
don't worry about what those two mean, but they're not the fast ones.
The fast ones just come straight out.
These are ones that have been cooled down
by interaction with hydrogen, and look what happens.
Relatively flat, relatively flat.
And then at the same place, this goes up, this go plummeting down through here.
Same place, this starts to go up, this goes
plummeting down, and then who knows up through here.
This is again the problem with the polar
cap, and again, observed over many different seasons.
You could see that there's plenty of water here.
You can take these measurements, and now turn it into a map.
We call it water on Mars in the upper meter of Mars.
But it's really of hydrogen atoms in the upper meter of Mars.
And it looks like this.
Again, favorite regions.
Here's the Tharsis bulge, our favorite three volcanoes.
Here's Valles Marineris.
So now you sort of know where you're looking.
Here is the northern plains, southern highlands, and a ton of
water from sort of 50, 60 degree north all the way up.
A ton, by a ton let's see what this means.
This means something like greater than 30% of the material in this layer.
60 degrees.
Greater than 30% of the materials is hydrogen, is presumably water.
Down here again 60 degrees.
There is a lot of water.
In the crust, even in these regions, in through here, which are
never covered in polar caps, never, never get very cold at all.
We still see regions with, sort of 8-10%.
All these blues, 10% water.
10% in through here, in through here.
Some of the areas are very dry.
These 2% values.
Some of these high lands in through here.
Some of these equatorial regions.
But, there is a lot of water, a lot of, some
type of water in the upper meter of the surface of Mars.
What type of water is it?
Well, it's not liquid water.
The upper meter of the surface of Mars is
still too cold for liquid water to be stable.
The answer is probably some surface ice.
We're used to the idea that the north polar cap is a
big mound of ice, but we're not used to the idea currently,
that this ice extends all the way down through here into these
regions, and that, that some of that subsurface ice could even be equatorial.
Is it true?
Well, how would you find out?
I will give you two ways that it was found out that, yes, indeed, this is true.
First way, fly to a polar region dig, see what you get to.
The phoenix lander landed at about 65 degrees north in
this region here, in one of these very blue areas, where
there's something like more than 30% water in the upper meter,
and it landed and it dug, and here's what it saw.
When this picture came up from the Lander, scientists involved hemmed and
hawed and argued about the different possible things it could possibly be.
But I tell you, you don't necessarily have to argue
too much, you can look at that and, if your first
guess is that it looks like ice just below the
surface of this dusty cover service, your first guess is right.
Let me zoom in a little so you can see a little bit better.
I mean just looking at it right here you can see regions that are, that look
like hard scraped snow in through here and even a little bitty layer of ice that
is maybe even melting the thin part through
here melting through here since it's been exposed,
that's water ice and it is just below the surface over a big chunk of the planet.
I promised you I would show you two, here is
another way we know how extensive that water ice is.
We have enough space craft in orbit around mars
these days staring down that we can find things like
fresh impact craters, regions where we had images and
there was no impact crater, and come back and boom.
There's a brand new impact crater that wasn't there before.
Here's a couple of craters that are curved at about 45 degrees north latitude.
45 degrees north latitude is not that region where you're
in the 30% water, you're in the ten's of percent,
not very much necessarily in that first meter, but slam
a meteor into the surface and what do you see?
You see these bright spots right here,
these are the first images, these bright spots.
The crater is right here, the bottom of the crater has these nice bright spots.
What are they?
One guess.
Yeah, look, look what happens over time.
This is about a three month sequence and, at
the end of three months, you see the craters.
You don't see those white spots at all anymore.
You have seen that they've faded through time, like this.
And, that's because, the ice that's in the bottom of these craters.
Ice is not stable on the surface, it evaporates.
But if you can cover it with a little bit of dust, that ice is stable.
So the gamma ray spectrum graph, the neutron
spectrometer and these other sorts of images have
given conclusive evidence that sub surface ice is
pervasive, even at moderately low latitudes, on Mars.
In the next lecture, we'll look at some evidence
to see really how pervasive that subsurface ice is.
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