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Lecture 1.24: Geochemistry from the Opportunity rover
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The Science of the Solar System
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Unit 1: Water on Mars (week 3)
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Mike BrownProfessor
Planetary Astronomy
In the images that I showed you in the last lecture.
Some of the most intriguing things that you could see,
were things that were noticed in the very earliest images were these small spherical
things that were immediately called blue berries.
They were called blue berries because they look blue in the false color that's used,
they are not really blue.
But let's take a look at some of those pictures again.
Here they are as you'll remember in the upper parts,
this is where those wavy forms were that look like they were forming damp regions.
Damp stuffing through here, and blueberries, blueberries,
blueberries, blueberries everywhere.
What are these things?
Well there are a couple of experiments on these rovers that were designed to be able
to measure the compositions of small scale things.
One of them was a miniature version of the thermal emission spectrometer,
Mini-TES, in fact it was called.
And the problem is that Mini-TES,
when it looked at a field it looked at a pretty big swaf.
And so if you really wanted to know what the blueberries were and
you looked like this.
You'd be getting some of the spectrum of the blueberries, but
some of it of this stuff.
If you could look here, it's very difficult to isolate the blueberries.
Except that there was this one nice location where the blueberries did a good
job of isolating themselves.
This region came to be known if not surprisingly as the berry bowl.
And what happened was that a spectrum, a thermal emission spectrum was taken
right here and another thermal emission spectrum was taken right here.
And the two were compared and it's great because you have the same background rock
in both locations and the only difference is blueberries.
And what are the blueberries?
Hematite, these are the main source of all that hematite that was seen from space.
These funny looking nodules that are strewn all around the crater.
In the images that you see here, they're in that upper unit of this stratigraphy.
But if you look back at least at the stratigraphic column that we looked at
last time, you see that they're everywhere.
They are the hematitic concretions something, something, something.
Those little black dots, the little black dots are down here in the lower unit.
They're in the middle unit and they're in the upper unit too.
They are infusing this entire region in through here.
What are they?
Well it's hard to know when you just see these little blueberries.
Little blueberries could be things that came in from above,
maybe an impact happens.
These things get strewn all over the place.
They land all over the place.
They could have been formed there.
How do you know?
One of the nice ways in which this was figured out was by looking in
detail with a microscopic imager.
A little, almost like a little handheld lens, and
looking at some of the blueberries in place.
And there are many examples of this sort of image, where you can see this.
But this is one that what I want to show you where you can see that this is
a region that's limited.
It has layers going across here in the small scale picture.
You can't see them as much and here is a layer going across here like that.
And if you look very carefully the layers continue thrpugh the blueberry itself,
parallel to the layers on the outside.
This happens over and over again, and this would not happen unless
this blueberry will form inside of these layers.
These blueberries and their hematite.
These blueberries are now thought to be what are called concretion.
Concretions are things that form when water is flowing through this unit.
And a small bit of hematite will precipitate out and
it will grow rings of hematite around it.
And in fact, there are some blueberries that have been broken open.
And you can see inside almost like tree rings on the inside as these
concretions have grown.
So they're everywhere, they're not just up in this upper layer that was thought to
form in a damp environment.
They are in a very dry environment down here of the dunes.
They're in the semi moist environments up here of the sand sheet.
They're everywhere through there.
Why are there blueberries throughout?
It's because they were formed not at the time that these layers were put down.
But they were formed after the fact as the water level rises up through here,
precipitates out these blueberries throughout the entire column.
So these are much later than these initial dunes.
If you would come back when they had dunes on the surfaces,
you'd have found no blueberries.
But again, an indication of this water table rising and
lowering through this region.
What else do we know about the chemistry and the minerology of this region?
There was yet another instrument onboard the rover called,
the mossbauer spectrograph.
The way a Mossbauer spectrograph works as it puts, there's a rock right here.
It puts the unit right up next to the rock, so
this is on the robotic arm of the rover.
It puts the unit right up next to the rock and it sends gamma rays into the rock.
Gamma rays, once again very useful things, and
those gamma rays interact with the nuclei that are there inside.
Every type of nucleus will absorb and re-emit the different types of gamma rays.
And this particular Mössbauer spectrograph was tuned to send
out gamma rays that are absorbed and emitted by iron.
And taking that iron and putting it in different minerals
slightly changes the energies of the gamma rays that are absorbed and emitted.
So the way the Mössbauer spectrograph works is the energy level of
the gamma rays being emitted is scanned over a range.
And the gamma rays coming back are examined to see which gamma rays come
back.
Because they were absorbed and remitted versus just passing on through the rock.
From this you can tell, not just that there's iron there, but you can tell every
little flavor of iron that there might be, what kind of mineral that iron is in.
The way you do that of course is, you take your Mössbauer spectrograph on the Earth.
And you do the same thing on known minerals on the Earth and
you can identify them.
So what does it look like when you do this?
Here's some of the first data back from the Mössbauer spectrograph.
And let's look to how it works, on this axis is velocity.
It seems a strange thing to have.
But this is the equivalent velocity that the gamma ray has been doppler
shifted by millimeters per second.
Very small change in energy, but the energy states that the nucleus are very
sensitive to the state of the iron as a whole.
And you can see when you send out the gamma rays at zero velocity,
some of them are emitted and reabsorbed.
There's a big peak when you send them out at slightly negative velocity.
Slightly less peak over here and then a series of peaks out through here.
What are these peaks due to?
Well every single mineral with iron in it will have peaks in
different locations, and these peaks here, here,
here, and here are due to a mineral called jarosite.
And you can see that they look just like the peaks
that are seen in the Mössbauer spectrograph.
The Mössbauer spectrograph is an incredibly sensitive device for
finding these things out.
There's also a little bit of a peak over here, Hm,
my guess is hematite and a little bit of hematite is seen here.
This was not looking at a region with blueberries in particular, so
there's some hematite just scattered throughout.
But let's look carefully at what jarosite is.
Jarosite has a complicated chemical formula.
It looks like this, here's the iron, we knew there was iron was in there.
And sulfur, sulfate in particular is a main component.
And then there's also either potassium or sodium or H30,
or anything else with a plus one sitting in front of it.
Attached to all this is some hydroxide and in fact, notice that there's Hs in here.
Those Hs in things like jarosite very well might be the presence of that
hydrogen that was detected by the initial neutron spectrometer.
The key thing about jarosite really is the sulfate.
Jarosite forms in an acidic sulfur rich environment.
In fact, the places you find jarosite on the Earth these days is in mine waste,
highly acidic water seeping out of mines with sulfur rich regions around
them turn into these jarosite like minerals.
And in fact one that's often used as a analog for
marsh environment is the Rio Tinto in Spain, which Tinto, red.
You can see why it's called the red river here.
It's so much iron in the river that the whole river runs red.
You can see essentially rust throughout the bed of the river.
This is a heavy mining region, highly acidic environment.
Meridiani Planum may have looked something like this,
maybe not quite the flowing river.
But the general enviroment like this might well have been what it was like,
at the time that all these features were being formed.
One other incredibly important thing that was discovered from the chemistry of
the rocks was that this lower unit was made out of the same sulfur rich material
that the middle unit was made out of.
Which is the same sulfur rich material that the upper unit is made out of.
These sulfates, these iron sulfates, these jarosites, these are salts.
These are the sorts of things that you get in a playa.
A shallow lake that evaporates leaving behind,
the salt if you have sulfur rich water.
We know that that happened up through here, but it happened throughout.
The interesting implication is that these dunes,
well we think about this dune period as the dry period where it's making dunes.
Yes, it's a dry period in this precise spot,
but the dune material is evaporitic material.
It is the stuff coming off a playa that might be right next door,
might be a 100 kilometers away.
But it's a locally dry region at a place where there are wet playas
either at the same time or previously.
So the overall interpretation of this region,
of Meridiani Planum is not that we are looking necessarily at a dry period,
a middle period, and a wet period with water going up and down between both.
Perhaps looking at spatially variable regions,we are looking at something
that might form dunes in some places.
And there'll be some slightly lower spots that form
damp regions between the dunes and even lower spots that form wet regions between
the dunes as the dunes move around as the water table moves around.
These different regions can become dunelike, can become interdunelike, and
we're seeing something like that in this.
What do we now know?
We now know that this region at least had water intermittently throughout it,
throughout the column, sometimes at the surface.
And that it had a very sulfuric composition,
much like that Rio Tinto that we saw at Spain.
It's a great story.
I think that sending the Mars Rover to this one spot and
putting together this geologic history, this one spot,
is incredible contribution to understanding the overall history at Mars.
And it leads directly to the current day of sending an even bigger rover
looking even in more detail at a different spot.
And we will talk about the geological history there in a few more lectures.
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