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 1.26: Mineralogy on Mars, Part 1
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Del curso dictado por Caltech
The Science of the Solar System
295 calificaciones
De la lección
Unit 1: Water on Mars (week 3)
Conoce a los instructores
Mike BrownProfessor
Planetary Astronomy
So, hi.
I'm Bethany Ehlmann, Assistant Professor of Planetary Sciences at Caltech.
And I'm going to be talking to you today about the emerging exciting story about
the, the minerology of Mars.
So, if we, if we take a look at this first image here, this is something you,
you've already learned about and you're probably familiar with.
So, so here I've taken the topography of Mars and I've tilted this topographic
globe, so we're staring down into the northern plains.
And of course, you can see these amazing outflow channels,
valley networks heading into the plains.
For all the world it looks like, there, there should have been an ocean here.
So this was the, the view of water on Mars about 1997.
Abundant, morphologic evidence of water.
But, but paradoxically up until the mid-90s,
there's actually very little mineralogical or chemical evidence that,
that any water had, had interacted with the, the, the rocks.
So, if we have an ocean on Earth,
we see large carbonates building up precipitating at the, at the base.
And if we look elsewhere in the crust, we see clay is forming from weathering.
Well, none of this was apparent on Mars.
But this all began to change with the, the next generation of mineralogical
instruments that, that kicked in and headed to Mars in the last 90s.
So the first of these was the thermal emission spectrometer.
And you can see here go here.
[LAUGHS] Says, this, this spot of, of hematite that was identified from orbit.
Using the thermal emission spectrometer and
indeed we did go there with the opportunity rover.
So this was the first hint.
This, this oxidized iron was the, was the, and
it was associated with sedimentary units was the,
was the first hint of a possible chemical mineralogical story to water on Mars.
So before I, I, I go back to the story of Mars itself, I sa,
thought I'd spend a little bit of time to explain how we know what we know in terms
of miner mineralogy and what the uncertainties are.
So, so this graphic here describes what it is exactly that we
are sensing when we're mapping the mineralogy of Mars.
And it's these molecular vibrations in, in mineral structures and
you can have different senses of motion here from, from various stretching of,
of, of atoms bonded together to scissoring and rocking motion.
So, each of these have a characteristic energy dependent on the bond length and
on the mass of the, the atoms involved.
So for example, this is a clay mineral.
Clay mineral is a member of the phyllosilicate family and
it's called phyllosilicate,
because like a sort of layered phyllo pastry you have these different
layers built up one right after the other in a particular characteristic unit.
So so here, for example, you see silica aluminum tetrahydro,
octahedral layers and then an inner layer that is the carrier of the water.
So your tetrahedral layers with silicon oxygen, sometimes aluminum substitution.
This octahedral layer or different canions substitute in this oxygen and OH.
Tetrahedral layer, inner layer and then this structure repeats over and
over again.
The, then, how this how does this translate to, to spectral features and
spectral properties?
Well, so in the mid-infrared wavelength region and here,
I'm talking from about seven microns to about thirty or so.
You can see, for
example the characteristic features of these different types of minerals.
So for example, this big emissivity minimum here,
corresponds is about between eight and about ten microns.
So this is a major stretch and it's related to SiO.
It's an SIO stretch.
And where it is located is actually,
dependent on the degree of polymerization in the silicate.
So, so olivine is sort of the end number here.
The longest wavelength SiOH restrolin feature and
quartz here on the shorter wavelength.
And you can see there is also some structure in there.
So, so, using mid-infrared, you can track out very nicely the silicate minerals and
identify these different compositions.
And whether your rocks are masic iron magnesium rich or they're,
they're more felsic with a greater degree of of framework polymerization in
the silicates and this is all on the basis of this particular absorption feature.
You can see that this feature is absent in calcite and gypsum.
Where instead there are features related to the presence of carbonate and sulfate.
Similarly, in, at shorter wavelengths, in this case, the visible near infrared.
We'll head down to the corner here and
you can see that there are also molecular absorptions.
In this case, this, this wavelength region is highly sensitive to the presence of,
of hydroxyls as well as, as well as H2O.
And I'm just showing for a bunch of clay minerals here, how you can distinguish
based on the band position, what particular kind of mineral you have.
Whether you have aluminum, OH, what it.
Magnesium OH is all about tracking the positioning and
you can see also that this region is sensitive to, to carbonate.
The other bit of information in this wavelength region is related to iron
oxidation statements.
And this is important for understanding the degree to which primary minerals have
transformed into secondary minerals.
So here you see, our pyroxenes and olivines classic rock forming minerals.
In comparison to these iron three plus charge transfer absorptions with a very
strong absorption down here at longer wavelengths due to iron three in the site.
So that's how we do what we do and these are just a few specifications of the,
of the instruments that we'll be talking through the data of.
So as you can see, part of the reason for, for this this,
this enhancement in the number of, of minerals available and in the, and
as we'll see in terms of the number of aqueous environments identified is
the fact that we're going from increasingly,
to increasingly higher spatial resolution.
So from a kilometer spot says, to discover these large hematite
bearing sediments that Maradiany in the late 90s early 2000s all the way to now.
Our capability is at 18 meters per pixel and in associated with images.
So that for each image acquired every single pixel has a spectrum, like this
in the middle from which we can, we can make the mineral, mineral identification.
And importantly also,
relate those mineral identifications to particular geologic units.
This is a bit of an eye chart and it's not intended to be memorized, but I,
it's worth spending a moment to digest what's, what's going on here.
So these as, as of the time of this lecture, sort of the,
the most up-to-date listing of all of the secondary alteration minerals
related to water that have been discovered on Mars.
And remember, as of about 1997, we didn't really know that any of these,
except perhaps some of the iron oxides in, in Mars dust, red Mars dust were present.
So this is all,
all very new and and we're still trying to figure out what it all means.
But, but so here's phyllosilicates, our clay minerals.
And you can see that there's a diversity of types that, that can be identified,
distinguished and mapped.
Other types of hydrated silicates and we'll get into their significance in a bit
along with our carbonate and sulfates salts as well as chloride minerals.
So, okay.
Why do we care about all these minerals?
There's, they're, they have these complex names, complex formulas.
Well, we, we care ab, about these, because they indicate something fundamental
about the geochemical aqueous geochemical conditions at the time of formation.
For example, was the, did the water have a high or low pH, acid or, or alkaline.
What was its oxidizing potential of the fluid?
What was the activity of different ions and solution?
All of this goes to the question of what was the environment and
was that environment habitable?
So this is a schematic for just two of the parameters that go into figuring out,
like, what your fluid chemistry was.
And so here you can see our Eh and
pH with a number of different minerals that serve as indicators.
So for example, if you see these iron and aluminum sulfates,
you know you're dealing with an acid oxidizing environment.
Whereas if you're in the carbonate realm, you're at higher alkaline type pHs.
Okay. And then by pairing these mineralogists
with, with the actual structure of sediments that is doing geology.
Once you've figured out the mineralogy, you can figure out.
Okay.
There was water on Mars, but was that water in a hydrothermal system?
Were we snowpack said lake systems.
We're, we in evaporative playas like you get in the desert Southwest today?
Or we're, we're deep in the subsurface with fluids flowing through rocks.
So this is the state of science right now in terms of water on Mars,
the mineralogic and chemical perspective.
And so you can see that there are a number of locations of phyllosilicates.
Also, you can see that they're largely within the southern highlands of Mars and
most region.
They're not in the really young volcanic regions and
there are few exposures in the younger northern plains, but
these are mainly actually locations where there are large impact craters that,
that, that punch through that punch through the,
the the Hesperian and Amazonian age materials.
So, so these phyllosillicates are largely correlated with where we see exposures of
Noachian terrain.
You can also see that the story for, for salts has got interesting.
Not only do we have sulfates and they're still largely restricted to vallis,
marinus, maritioni, with the few, exceptions.
But we also have the addition of chloride salts and the addition of,
of carbonate salts as well in different geographic distributions.
So this is where we're going to end up.
But I, but this is I think one of the best understandings of the current
understanding of the evolution of environments on Mars.
Where many environments persisted for long periods of time, for example, throughout
the Noachian epoch or even spanning the Noachian and Hesperian epochs.
But there is certainly a trend with, with time and
the way that this was built up was, was not sort of by considering Mars globally,
but by zooming in with this high resolution data, looking locally.
Looking at local stratigraphies, local mineralogy,
using crater counts to understand the timing of it all and
then building this up, this, this global understanding.
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