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Lecture 1.22: Thermal spectroscopy and mineral identification
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來自 Caltech 的課程
太阳系科学
310 個評分
從本節課中
Unit 1: Water on Mars (week 3)
與講師見面
Mike BrownProfessor
Planetary Astronomy
One of the other ways in which Mars has been studied other than just looking at
pictures and trying to understand the geology of what might have occurred,
is through a process called thermal emission spectroscopy.
We talked about neutron spectroscopy, Gamma-ray spectroscopy,
this is a different way of looking at photons coming off the surface of Mars,
something about what the surface is made out of.
First-off, thermal emission.
Well, we've talked a lot about thermal emission.
We've talked about the fact that Mars is at a temperature where it is
emitting as a black body with a peak somewhere around eight microns.
If you remember what my my plots of these black body emissions looks something like,
they do something like this, and they come back down.
Here's eight microns, and when I'm plotting it with these curved lines,
I'm plotting it with wavelength going in this direction on a log scale,
and intensity going on this way on a log scale.
It looks something like that. I said
everything emits as a black body given its particular temperature.
This is something like a 220 degree Kelvin black body, but I lied.
Sorry, everything does not emit at a black body given its temperature,
everything has slight modifications to the black body depending on what it's made out of,
unless it's actually a black body.
A body that is purely black and has nothing else going on on it.
But anything else that is made out of any specific material,
will have in addition to a overall black body shape,
it'll have these modifications of some sort,
and look like this, or like this,
or like this, depending on the material.
These wiggles, these changes in the emission,
which are actually changes in what's called
the emissivity of the material are characteristic of what that material is.
So, you can measure the thermal emission from the surface.
Look at the emissivity changing as
a function of wavelength and figure out what's on the surface.
The instrument to do this on Mars was the thermal emission spectrometer,
not a lame acronym.
This was on the Mars Global Surveyor,
not such a bad acronym either.
It provided the first up-close thermal view,
mapped the entire planet going around and around,
and moderately high resolution.
We don't have detailed views of things,
but we got to see the surface compositions
in addition to temperatures of different places,
how the temperature change with time of day,
telling you something about the thermal inertia.
As like if you're sitting around on a rock that's been sitting in the sun,
that rock can stay warm even after the sun goes down,
but a pile of dust that's warm in the daytime will very quickly cool down,
that's because of the thermal inertia differences and test could see those.
But we're going to concentrate on right now is what it
found about the composition of the surface of Mars.
The answer according to tests was the surface is are actually boring.
There are two basic things that are seen on the surface.
Here is now the spectrum, these are wavelengths.
Again, the peak of emission is somewhere around here, eight microns.
This instrument covered from eight all the way out to about 50 microns.
In this region here, the Martian atmosphere absorbs
the thermal emission coming up causing that green house effect.
So, we can't see the emission from the surface.
But here, the black data,
the wiggly stuff here,
is data from one type of surface.
It comes as a big broad dip in the emissivity across here at around nine microns,
and another dip over here at around 30 microns.
What is it? Well, the blue points here,
the smooth points going in through here are
a model of what surface would make spectra that look just like that?
The answer is volcanic stuff.
There's two different types of surfaces.
It's because there's two different types of volcanic compositions,
but the entire surface of Mars
where it's not covered with dust which obscures what's going on,
entire surface looks like it's just volcanic junk.
Let's take a detailed look.
Here are those two surface types.
There's two types of volcanic composition.
Okay. So, there's two types of volcanic composition,
and type two is mostly in the North.
Type one is Northern south,
tells you something about how the volcanic composition changed perhaps with time.
As you remember of course,
this is all Amazonian material up through here in the north.
This is older material down through here.
Very interesting, but if you were looking to
find potentially interesting regions on Mars,
you now realize that it's really just covered with big volcanic plains.
These regions in through here,
where there's not actually volcanic type one or volcanic type two,
it's because they're covered in dust.
Some of the school website are showing you jmars.mars.asu.edu,
where we're looking at the MOLA data,
the Mars Orbital Laser Altimeter, not a bad acronym.
We're looking at the MOLA data,
and every time I look at it, I love these outflow channels in through here.
But this website, in addition to having the MOLA data,
has a ton of other data on it too.
So, in fact, it has test data.
Let's take a look at some of those test data.
You can see where test is specifically looking for specific minerals.
How about low-calcium pyroxene?
Not a good one because there's really nothing anywhere.
How about high-calcium pyroxene.
High-calcium pyroxene is a volcanic product.
You can see that is the same as where those volcanic things are.
In fact, pyroxene crystallizes out of basaltic volcanic flows.
Basaltic volcanic flows are those dark ones that you would see in the Hawaiian Islands,
you would see if you look up at the moon,
the dark regions on the moon.
If you would actually look right here,
this is a big chunk of basalt that was found just outside of Death Valley.
On the outside of the basalt, it's very smooth,
you can't really see anything,
it's been weathered from all the rain in Death Valley.
Not much but enough, and if you crack it open, you can see,
and I'm sure you can't see it from here,
but on the inside, there are crystals inside there.
There are green crystals of olivine,
but there are also darkish,
slightly greenish crystals of the pyroxene.
This is the pyroxene that has been detected on Mars by tests.
Okay. What else can we find from tests?
Well, the other cool thing that you can look for is quartz,
and the answer is there's no quartz on Mars.
There's no quartz on Mars.
Why is there no quartz on Mars?
Why would we care that there's quartz on Mars?
Well, if we think about where we find quartz on the Earth.
Quartz on the Earth comes from big regions with granite.
Where does granite come from on the Earth?
Granite is essentially a product of the distillation of the continental crust.
If you take the continental crust of the earth,
you subduct it below other stuff,
you heat it up, it distills out,
the light materials come up to the top, volcanoes can occur,
and where those big magma chambers freeze out,
you get huge chunks of granite.
They're close to me,
the Sierra Nevada are these huge,
batholith is the name.
Geologists use this huge region of of granite.
In these huge regions of granite,
you can have giant crystals of the things that granite is made out of.
In fact, the biggest crystal of quartz I ever
found in my life was just below Mount Whitney,
at the highest peak in the Sierra Nevada.
I took it because it was so cool.
This is a pure chunk of quartz.
This crystalized inside this giant volcano.
But essentially it distilled from old continental crust that was subducted.
None of this exists on Mars. Why not?
No plate tectonics, no subduction.
None of this volcanic eruption is from the distilled crust of dead plates.
We can keep on looking, and keep on looking, and keep on looking,
and we won't see anything interesting unless we accidentally
touch this one tests hematite.
What's going on here? Suddenly, we're looking at this global map again and dead center is
this bright region that shows up in this one very strange mineral, hematite.
Hematite, single bright patch of
hematite randomly placed in the middle of the planet. Seems fishy.
First question is, do we believe it?
If you go to one of the early papers where
the claim was made that hematite is detected on Mars,
you can see some of the original data and they look like this.
This is the typical spectrum.
Here again, this is tests seven microns to 50 microns.
This is the absorption due mostly to CO2 in the Martian atmosphere.
So, you don't get any information through there,
and what do you get?
Well, typically Mars looks like this.
These are those volcanic features that we saw before,
they go like that.
Just what we knew.
What do you see in this one region which is allegedly hematite?
Well, it does about the same here and then it does that.
That's the hematite. Do you believe it?
Well, first let's look to see why it was identified as hematite.
The only way to really do this unfortunately is to go to your laboratory and
make similar measurements of many other things and see if anything else matches.
Here's the spectrum of Mars again and this is with the background spectrum,
the typical spectrum taken out,
and here are those two lines.
Lines meaning those dips in the emissivity that are seen.
In a laboratory, well,
doesn't look like maghemite,
doesn't look like magnetite,
doesn't look like chromite.
The only thing it looks like is this hematite.
This is why the identification was made.
Of course, I'm not showing you
a million other minerals to rule those out and that is one of the problems here,
which is that there might be other things that have been not
considered that are what's causing these features instead.
I will admit, I saw this presentation at a scientific conference
early on and I was pretty skeptical.
That's how scientists are,
they see something, it's a pretty spectacular claim.
Hematite, we'll talk about why it's spectacular in a minute,
and you immediately think, "Yeah,
but what about all those other things that it might be too that are less spectacular?"
I'll tell you, as you'll see in the future.
While it's a good idea to be skeptical,
it's also a good idea to admit when you're absolutely wrong.
It's really hematite, and we'll see
the hematite up close and in person here in lecture two.
But before we do that,
let me at least tell you what is hematite anyway?
Hematite is a grayish rock.
It's an iron oxide.
Chemically, it is Fe2O3
and I think it's what this is made out of. Here we're seeing these.
These are those incredibly strong magnets,
you throw them up in the air and they make that
crazy sound and you can spin them around and
this grayish metallic substance is clearly iron.
It's clearly magnet, I think it's hematite.
I can't actually guarantee it. You know how we could tell?
We could take and do a thermal emission spectrograph and measure
the spectrum and we would see these two lines.
But I don't have one right here,
but I think this is what hematite looks like and I think this really is hematite.
That's great. Why would it be on Mars?
Well, there are couple of ways that you can get hematite to form,
but the most common way on the Earth is by having
iron in surface materials and we know that Mars has iron in surface materials.
That the reddish colour of Mars is generally rust iron oxides.
Have iron in surface materials and then have water,
potentially hot water circulating through
those surface materials leaching out the iron and then iron oxide,
the hematite precipitates out.
Places that you get a lot of hematite or places
like Yellowstone National Park here in the United States,
which has hot bubbling springs and
iron-rich rocks and you get layers of hematite precipitating out.
Why is this spectacular on Mars?
I think now it makes sense.
If you have hematite over a fairly large region that is
suggesting that you had liquid water circulating in this region,
not just flowing through quickly,
not just raining down but liquid water that was
living long enough to cause these chemical changes.
Let's look at the place where the hematite is supposed to be one last time,
and see if there's anything else we can find about it.
All right, there it is.
We're going to zoom in on the hematite.
For fun, now that we're looking at it,
it's as you can see pretty low resolution,
we're going to go from the hematite,
let's look at the MOLA shaded relief.
It is as same as MOLA colour except that
this is a flat enough region that the MOLA colour doesn't show anything.
So, it'll just show us the background very nicely. What do we see?
Well, mostly we see around through here pretty cratery terrains,
rugged, everything looks uniform except for this region.
Look at this region right through here,
it's smooth it looks like it's a region that has had something different happen to it.
There may be some fresh impact craters happening in there too.
Let's go back to the hematite and you will see astoundingly that is the hematite region.
This whole region that you see here is this smooth region that you see here.
The correlation is perfect.
Now, you can be skeptical when you see something in
the data in the thermal emission data and you're
not quite sure it's the right identification,
but when you see a nice correlation between geological features like this,
and something that's being seen in the spectra,
you know you're onto something.
In fact, this is exactly what you would expect if you
had a region with large bodies of standing water,
circulating, water it looks smooth,
it looks like these craters have been degraded by smooth water.
It looks like there's ponding that could've happened in through here.
It's pretty spectacular looking in a pretty special looking place on the surface of Mars.
It was sufficiently special that it was one of the two places,
that one of the early Mars Rovers decided to go and check out what's really happening.
