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 1.27: Mineralogy on Mars, Part 2
Loading...
Из курса от партнера Caltech
The Science of the Solar System
295 оценки
Из урока
Unit 1: Water on Mars (week 3)
Познакомьтесь с преподавателями
Mike BrownProfessor
Planetary Astronomy
We'll step through just a few examples by zooming in with this high-resolution data,
looking locally, looking at local stratigraphies, local mineralogy.
And we'll kick it off with that first earliest environment,
the deep phyllosilicates.
Which, as we'll discuss here, indicators of hydrothermal,
elevated temperature, interaction of water with rocks.
So how do we get deep?
No major drilling systems on Mars, so the main way is by looking at craters.
So for example, we can look at this 25-kilometer crater that
punches through the Hesperian Syrtis Major Lavas.
And then we can make maps using our spectra to map out band depths,
absorptions, and map out particular minerals.
And you can see that this crater actually has an amazing exposure of different
hydrated phyllosilicates.
And other kinds of hydrated silicates associated with the central peak,
the surrounding structure, and the walls.
We can zoom in, take a closer look.
Some of the phyllosilicates are in these breccia blocks in the central peak.
And the knobs have these zeolite minerals in them.
We can repeat this exercise for
craters all over the surface of Mars checking out their mineralogy.
So now we'll go here into this 40-kilometer impact crater.
And what you can see over to this side is that, so here,
rather than looking at the central portion of the crater, we're looking outward
to what is clearly ejecta that's been then thrown from the crater.
So there's different kinds of hydrated silicate minerals that you see.
Potassium, aluminum clays, as well as prehnite and chlorite.
We can zoom in and take a really close look at the morphology,
pairing the mineral signatures.
And you see they're associated with these chains of rocky,
blocky ejecta thrown out, or what are the carriers of the signature.
So here for example, you can see that this data spectrum
is a mixed signature of prehnite, indicated by this particular absorption
features here, as well as the overall feature of chlorite, okay?
For example, and likewise with the other materials in the scene.
So that's how it works, that's how we map it.
And what do these rocks look like?
Well, when I say that there's chlorite and
prehnite, I don't mean that 100% of the rock is.
What we're doing here is we're ratioing areas where there are minerals of interest
to less interesting spectral regions.
So we're pulling out what the difference is.
And the difference may possibly be that these rocks, that we're identifying
the chlorite and prehnite, have veins where we see these, now,
Why is this important?
Well, this is important because certain minerals act as indicators.
So this is a phase diagram of the stability as a function of pressure and
temperature of, for example, prehnite.
And I've made gray here the particular zone in which prehnite is stable.
So it is an indicator of low-grade metamorphism because it only forms at
temperatures between about 200 and 400 degrees Celsius.
And also at fairly low pressures.
So it's a low-grade metamorphic hydrothermal type mineral.
So there are hundreds of impact craters
over the surface of Mars that have these phyllosilicate signatures.
And one can repeat this exercise of going in and
looking at the detailed mineralogy for each of them.
When you do that, you can start to build up statistics.
And so this pie chart here are the clays and
craters that were deep in walls that we're talking about at the moment.
And you can see, in overall statistics, we have iron, magnesium,
smectites, chlorite, and prehnite occupying some of that.
Along with materials such as zeolites, like analcine, silica.
Collectively, these data of place us in this particular field.
So some of these made have formed, for example, some of the smectites.
We can't say very much about the temperature of formation,
just having identified them.
They occupy this whole range.
But some of them, like phrenite and
like some of the zeolite minerals, are definitive indicators of temperature.
So we can place ourselves on this field.
And we can say that these deep crustal phyllosilicates
formed at higher temperatures.
And also presumably in the subsurface because we don't
anticipate that the surface of Mars reach temperatures of 400 C.
So that's not the only way of getting at what's going on in the past.
The other is by looking at key stratigraphies.
So this is a schematic that will appear later on, too.
But we're going to work our way through the building of this particular
stratigraphy here.
Nili Fossae/ NE Syrtis with its clays on the bottom, carbonate and
aluminum phyllosilicates at the top, along with olivine sulfates.
And then heading into lava flows that aren't altered.
And this is something one can do in multiple regions of Mars.
But for the purposes of this talk, we'll restrict it to one.
So Nili Fossae occur here, the crust is just to the side of the Isidis Basin.
And we're going to be looking at this fracture system which are tectonic
down-dropped graben.
So zooming in, so there, one thing you can do is you can map out minerals coarsely.
And this is an overall picture of the abundance of olivine.
And other folks have hypothesized that these are perhaps olivines that
are accumulates from impact melt sheet.
Perhaps they are komatiitic magnesium-rich lavas that came out
after the Isidis impact.
Regardless, they're circumferential to the basin.
But we see that they actually control
to some extent the secondary minerals that we're seeing.
So there are two minerals, one is a aluminum clay, kaolinite.
One is a magnesium carbonate, and you can see their distribution here.
So now let's just zoom in and get a picture of what's going on.
So zooming in to this fracture, and now mapping the various minerals by color.
So what we see, we can probably zoom in actually even closer to here.
So here's what's happening.
So we have this low, you can pick out at least three
different stratigraphic units here along with something like sand.
That's not a rock in stratigraphy.
So here you have this bluish-tone material with ridges running through it and
these blocks.
That's our iron magnesium phyllosilicate.
On top, this is a olivine-rich unit.
But where you see the greeny tones coming in,
that's where it's altered to carbonate.
And then all of this, working up the stratigraphy,
is capped by this mafic basaltic cap.
So here's what the spectral signatures look like of our iron magnesium clay,
or olivine, or magnesium carbonate.
If we were to draw this stratigraphy in cartoon version,
it would look something like this.
And I'm just highlighting here some of the features, some of the ridges in this
phyllosilicate unit that indicate fluid flow, mineralization.
And then erosion of the surface led to these existing today as ridges.
And where you see the white arrows,
we actually have these huge breccia blocks from impact.
So this indicates that the oldest, deepest crust of Mars is, as we might expect for
an early planetary surface, heavily influenced by impacts.
So now let's move elsewhere.
Let's migrate to check out these kaolinite, these aluminum clay layers.
So we're going to zoom in to these two craters.
And our crater of interest is this guy up here, but
we'll compare it to a nearby fresh one.
And when we do that topographically,
we notice that our crater of interest here is over a kilometer shallower.
And that's because it's been filled with sediments.
But it's not just been filled, it's also been eroded.
And you can see that the crater here with a breach channel out
the northern bit of it where water drained from there.
But erosion has exposed a nice area here that we can check out and
look at at high resolution in CRISM.
So here we're mapping out now iron magnesium minerals with aluminum minerals.
But zooming in still further, you can see here that our aluminum clay is a very,
very thin unit no more than a few meters thick above our iron magnesium
thick sequence of rocky, blocky sediments.
And here's our cartoon stratigraphy here, okay, so what's going on?
Well, a logical explanation for how you get these mineralogies
in this particular setting is that we had, in contrast to deep hydrothermal systems
flowing through the crust, here we have evidence for top-down weathering.
That is, starting with basaltic parent materials,
these are partially altered in some cases to iron magnesium smectite.
Perhaps in some cases underground, perhaps in some cases near the surface.
But then those materials sitting near the surface are subject to enhanced leaching.
Either from greater amount of water throughflow,
exposure to more acidic conditions.
And so they lose the calcium, magnesium, and iron ions out of the system,
leading to this residual Kaolin aluminum clay-rich material.
In contrast, where we have olivine, and we'll get to this serpentine in a moment.
Where we have olivine, the magnesium from the olivine reacts with atmospheric CO2.
And so we get these magnesium carbonates where olivine is at the top of
the stratigraphic section.
So top-down weathering with whatever the original lithology is, mafic or
ultramafic, exerting a control on what the alteration minerals are.
Okay, so we worked our way in the system
only through part of the stratigraphy here.
If we headed southward, we would also see the evidence of sulfates coming in just
beneath the Syrtis Major lava flows.
So you can repeat this exercise all over the planet to construct the columns that
we have here.
And there are certain trends that you can see.
The oldest, deepest layers have iron magnesium clays.
They're partially altered, some of them have no alteration at all.
We just have high and low-calcium pyroxene, olivine, primary minerals.
Toward the top of the section and
this line here connecting them indicates the approximate
location of the Milwaukeean Hesperian geologic boundary in each location.
Toward the top of the section, we have these aluminum phyllosilicates that
indicate this near-surface weathering and leaching.
And then you see that, in most places across the planet,
when we cross into the Hesperian, there are unaltered lavas or ashes on top.
There's very little sign of hydrated minerals.
There are a few sections though where this is not the case.
For example, Meridiani, where the rovers are now,
there are sulfates at the top of the stack.
And you'll hear it from John Grotzinger about what we're discovering at the bottom
of this section now, as the Curiosity rover climbs through Gale Crater.
So just as a difference here,
transitioning from these very early clays into the sulfates.
We'll actually jump over to the other side of the planet here and to Meridiani.
So reminder, Meridiani is this location here, our beacon of hematite.
So the rovers landed, an interplanetary hole-in-one here.
And they extensively traversed this crater,
leaving our tracks behind as we explored this outcrop.
And here, we're able to ground truth what we were able to see from orbit.
We're only able to do this in a few locations where we've sent rovers.
But in this case, so
the hematite signature was actually carried by these small concretions.
But you can see this interesting sedimentary rock here that has these
hematite concretions.
As well as these void spaces where there used to be crystals.
And one can use the Mössbauer instrument combined with the chemical
instrument on the rover to build the mineralogy.
Which is actually about 35% sulfate hematite in these concretions.
And then the remainder is our amorphous silica and
allophane alteration phases, with maybe a little bit of primary feldspar left.
The overall picture, based on the traverses of the rover,
is these sulfates that we're visiting in the late Noachian, early Hesperian
are basically small paleolakes that are in-between dune sediments that form
from ground water upwelling, becoming acidified as it nears the surface.
And then slightly changing groundwater chemistries over and
over again coming through the rock, flushing through.
Leaving and coming back with slightly different chemistries,
lead to the formation of this concretions.
As well as the development and dissolution of minerals that formed previously.
And it's only by landing at these sites that they we're able to discern these
really fine-scale details.
From orbit, our inferences are coarser.
So collectively, though,
we can now tie with this understanding from the ground from Meridiani here.
We can tie this together with similar mineralogy and
sedimentary deposits that we see from orbit.
And then we can start to model geophysically what's going on.
So this shows the correspondence between a groundwater
upwelling model developed by Jeff Andrews-Hanna.
And the fact that it correlates with many of the places where we see iron oxides and
sulfates together across the planet
in the interior layer deposits of Valles Marineris here.
The thickness of evaporites matches well with places where we see sedimentary
deposits with sulfate evaporites from orbit.
And likewise, where the rovers are.
So we've talked through this system, we've talked through two stratigraphies.
We've talked through the valley networks.
So we talked through these early environments exposed at
Nili Fossae NE Syrtis.
We've talked about the sulfates that appeared later in the record.
And collectively, now we can start to say certain things about what's going on.
That this earliest deep phyllosilicate period,
this is a period of hydrothermal alteration.
And where the action was
largely in the subsurface in terms of water interacting with rock.
And impacts played a huge role, both in exposing the material and
perhaps also generating part of the heat to alter it.
Stepping forward in time, we see, especially toward the end of Noachian and
Hesperian, the influence for surface waters in forming these carbonates and
forming these aluminum phyllosillcates.
And then also generating these fluvial systems across the planet.
Stepping later in time to places like Meridiani,
we see that the surface water system is starting to go away.
Not only morphologically with the fluvial networks, but also mineralogically,
there's a change in the system.
We don't have clays forming anymore, instead, we have evaporite salts.
And most of the indications are that the mineralogy forms from groundwater flowing
through the system, rather than any long-term existence of lakes.
So if one were to make a cartoon schematic of maybe what this looks like
in terms of Mars' environments,
in terms of thinking about what kinds of liquid water environments are available.
That is, if we step into the Noachian area, so this lower portion,
both the pink and the orange, indicate sub-surface type aqueous environments.
Hydrothermal environments that appear to have largely gone away by the Hesperian.
As well as groundwater systems which continue at smaller and
smaller frequencies of occurrence into the Amazonian.
And the cartoon blue bumps here are meant to indicate that liquid water is
episodically available.
Perhaps most so where we see fluvial valley networks.
Where we see aluminium clays at the Noachian/Hesperian boundary.
So this mineralogical data now is added to this understanding that we had
in the 90s based on morphology on valley networks and
outflow chancels to really build up what were the environments like.
And I think that testifying really to the importance of groundwaters in addition to
the surface morphological features of the water that we can see.
So I hope this now gives you a sense and an appreciation of then,
when we look at this complicated dot map of where the minerals are on Mars, that
each of these dots, in fact, has a really detailed geologic story to be unraveled.
And by piecing these together,
by carefully selecting where we land to verify what we're seeing from orbit,
we can build a better picture of Mars' geologic history.
And really unravel what these aqueous environments were like
during Mars' first billion years.
Ознакомьтесь с нашим каталогом
Присоединяйтесь бесплатно и получайте персонализированные рекомендации, обновления и предложения.
Coursera делает лучшее в мире образование доступным каждому, предлагая онлайн-курсы от ведущих университетов и организаций.
© Coursera Inc., 2018 Все права защищены.
