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 1.23: Geology 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
Shortly after that discovery of hematite in that one peculiar
region on Mars, Meridiani Planum, NASA was in the process
of trying to decide on landing sites for the two
Mars exploration rovers that were supposed to land on the surface.
Spend 90 days looking around for evidence of water, of course.
The discovery of that large flat region of hematite.
And you remember hematite, one of the ways
that you form hematite is, is through water deposits.
Meridiani Planum was an excellent spot to go try to land a rover.
The other rover was landed on the opposite side of Mars.
And we're actually not going to spend much time talking about that one.
But today we're going to show some of the results From
that rover, the Opportunity rover that landed there in Meridiani Planum.
It launched the way they always launch, it got to Mars the way they always
get to Mars, it opened up a parachute the way they opened up a parachute.
And then a crazy thing happened, but let's, let's just watch.
In fact you can watch the whole thing here on YouTube.
Just look for How To Get to Mars.
Very Cool, because it's very cool.
But this is the part that I, I remember distinctly sitting in the audience,
watching someone give this presentation early on
before they landed, and they showed this part.
They showed you can see that the, that the the parachute is up here.
The eventual lander is right down here, it's on a
tether from the, the landing rocket, which is right here.
And the presentation got to this point, and, well, let me show you what happens.
It's starting to go down, it's going down,
and suddenly, it explodes into giant air bags.
The retrorocket goes off, and, they dropped it, and it rolls
across the surface of Mars, coming to a stop, and eventually unloading.
I saw that presentation and I thought, that is
nuts, there is no chance this is ever going to work.
I have that reaction a lot to these crazy ideas of landing on Mars.
So far, I'm always wrong, in this case, I was totally wrong.
It worked, it landed on Mars.
It bounced, it rolled, and this particular
one rolled into a crater, and landed there.
And when the rover first unfurled, opened up
its eyes, took a panoramic of the entire crater.
It was spectacular, spectacular in a very specific geological sense.
Spectacular in the sense that everybody who I knew
who was waiting to see these images come down, the
planetary scientists who I, I share the hallway with Saw
the first picture come down and there were audible gasps.
The audible gasps be, were because of these bright features all
through here and what look a lot like layers, stratigraphic layers.
In fact, lets zoom in on some of these over here.
Here we'll go zoom in there and see what you see.
Look at these layers that come, around in through here, those are pretty cool.
How about this right here?
Layers upon layers, maybe even different layers down here.
Then up here.
Way over in here.
Layers, layers, layers, a bottom unit with different layers.
What causes these big flat layers?
These big flat layers are caused by some sort of sedimentation.
These are clearly sedimentary rocks, as opposed to just primary.
Volcanic rocks that have been seen everywhere else across Mars.
This was the single most spectacular spot on Mars that anyone had ever seen.
And it was the middle of this crater that
this crazy thing in airbags happened to bounce into.
The Rover that was supposed to last for 90
days is still alive more than ten years later.
And it has explored that crater in depth, a nearby larger
crater in depth, and many other things on the surface of Mars.
And I'm going to show you some of the,
I think, most spectacular results of the science that
came out of those early explorations where The rover
would get up close to some of those sedimentary units.
Take pictures, do chemical analyses that we'll talk about in the next lecture.
And make interpretation of what was happening
on Mars in these very specific regions.
I'm going to show you a series of images from this one paper by John Bradsinger.
Remember that name, because you'll meet him here in a few lectures.
And these images are not from that first crater, which was called Eagle
Crater, but this is from the, the medium sized crater nearby, Endurance Crater.
At Endurance Crater there was this beautiful cliff
side of strata which was called the Burns Cliff.
And this one paper presents a detailed
analysis of the geology, of this Burns cliff.
And let's look at what we can figure out
just from looking at some of these large scale images.
Here's the entire sequence of the Burns cliff, and the first thing you should
notice on the sequence is, that there
are perhaps three different different sorts of surfaces.
There's something down here
that appears to be different from whatever is here, and at the
top here, see this dark band that goes across here like this?
Is another contact, above which there's very different material on top.
We're going to talk about these three
different regions as these three regions are
going to correspond to three main environmental
conditions that these rocks found themselves in.
First, let's look at just this bottom layer, in fact, the
contact between this very bottom layer and the next layer up.
Here's an image for another point of view of that contact.
And the contact is right across here, this is that middle unit.
You can see the upper unit up through here too, dark, it's always dark stuff,
with light stuff right on top of it and it goes to the top here.
And while this middle unit is a series of relatively straight layers in
through here, notice that this bottom unit has What is called cross bedding.
There are surfaces like this that intersect other
surfaces like this, that intersect other surfaces like this.
Cross bedding, in terrestrial geology, is typical of
what you find in things like dune fields.
Here is, for example, an ancient dune field
in Zion National Park, National Park Service picture.
And these cross beddings are things like this,
where you have the layers come in like this.
Cross layers like this, layers like this.
You can very clearly see the patterns going up and crossing.
You can certainly see why it is called cross bedding.
In cross bedding in general forms as I said before from things like dunes.
Imagine if you have a dune, a series of dunes, dunes crest that look like this.
The way a dune works is that sand it gets
blown eroded off the top here blown up to the top.
And falls back down on the side, and deposits here.
As it deposits here, it forms new layer, and a new layer, and
a new layer, and a new layer, and the same thing's happening over here.
New layer, new layer, new layer.
And, eventually, these new layers, will intersect, as
these dunes continue to march across here, and
you get this cross-bedding of things at this
angle, things at this angle intersecting and continuing on.
This is exactly the sorts of things that we're seeing at Mars in that bottom layer.
What forms cross-bedding like that?
Well, wind-blown dunes.
To get that cross-bedding, you need the surfaces to be easily erode And
deposit on the other side, and that is exactly how dunes are made.
That's exactly the process that makes that cross-bedding.
An interesting thing happens with that cross-bedding though.
Again, here's that contact between the, the lower and middle unit.
And notice it's not the best image to see it in but you can sort
of see that these, this cross bedding goes right up to this edge and basically stops.
What you should imagine might be going on is that there
was a surface of dunes being cross bedded like they normally are.
And for some reason the tops of the dunes all got cut off at some subsequent date.
Why are the tops of the dunes all get cut off?
And why would they get cut off at a particularly straight line like this?
These sorts of things are seen on the Earth when the
water table rises up to a certain level, the water table rises
up to right here in an, in an, dune field And cements
those grains into place, hardening them, making them harder to blow away.
The dew, the water level never rises up to here, or even higher.
It could've gone quite a bit higher.
The water level never gets any higher than this.
The wind then subsequent, subsequently comes, and removes this entire surface,
in a period of what's called erosion, erosion by the wind.
This is the first evidence we've heard of actual water at this site.
But it's a different sort of water than we've been
talking about in almost every other evidence we have for water.
Here its just everyday typical ground water
that rises and presumably falls back down again.
Well it certainly was low at the beginning because dunes like
that don't form in the presence of water table like that.
We'll see exactly what does happen in the
presence of a water table, in just a second.
Okay, so, we have this flat layer with the dunes cut off
that looks like something that would be formed because the water table rose.
Let's see what happend after that process.
Here's a small section from that middle unit where the water table had risen,
the wind had blown away the dunes
that were there, and then, something else happened.
Now, there's no way to know how much time went between, the
wind blowing away what was there and whatever else happened after that.
That's one of the problems in geology, there are big regions of
time that are lost when things get blown away or washed away.
But, whatever happened next looked something this.
This is a pretty small area, and you see these fine layers,
and you see these fine layers are a little bit lightly cross-bedded.
There are very low angle cross-beddings all throughout here.
It's not nearly as severe as we were seeing in that, in that lower layer.
And this is the sign, not of dune formation, but of, of simply sand sheet.
You can imagine, a big, flat area, that's
just getting covered with sand, and re-covered with sand.
Slightly mounded in one direction, slightly mounded in the other.
This is exactly what forms in those sorts of things.
Why do you form sand sheets, instead of dunes?
Well, one reason is, if you are slightly wetted as you're being deposited, and
so you can no longer make the dunes my marching up, and eroding and redepositing.
You just make these flat sand sheets that are kind of stuck together.
They don't erode as the wind blows nearly as much.
As that middle layer was growing up sheet by sheet by
sheet, the water table would've been close to the bottom of it.
Just enough to keep it slightly damp, or perhaps
precipitation was keeping it slightly damp, but, but very little.
Not, we're not looking at large lakes, we're looking at slightly damp sand.
Let's look now in detail at that contact
between the middle unit and the upper unit.
Remember how it worked is that the, there was
a dark region here, the dark region in through here.
This is that region of very fine layering,
very slightly dipping in one direction, and the other.
And then it gets darker up through here and
very clear transition to something different happening up through here.
Darker, in this case, is attributed to recrystallization
and the fact that there was water, now actual
water table all the way up to here at this point and water very close to the surface.
And these materials, which you can see are only a couple tens of centimeters.
Deep through here are seeped in water and they are
being modified by the water and that's why they look different.
The chemically they're the same as these.
We'll talk about chemical compositions next time.
They're the same as the stuff down below.
But they've been altered and cemented by the affects of water.
Finally let's look at the upper layers.
The upper layers are where the interesting action takes place.
And you can see exactly what I'm talking about here in these pictures.
There are things like this, where you, you no longer have
nice straight layers but you have what I will call mushy layers.
Things that just don't quite make it as nicely as they would have.
Look at these in through here, there's, there's ripples.
There are indentations in layers like that, and this
is exactly what it looks like when you have
the sort of deposition, not on a lightly wetted
surface, but in kind of a, a wet surface.
Not a big standing lake.
You're not, you're not depositing into the bottom
of a lake, but you're depositing onto a
damp Surface you get layers that look just
like this or even like this up through here.
Of course, I should point out these things
that look like blueberries all over the place.
We'll talk a lot about those in the next lecture.
So we're not going to talk about them right now.
But there going to be a very important part of the story.
Let's put all that together and draw a picture of what that might look like.
And here's the interpretation of those three units in what's going on.
Here's the lower, cross bedded dune field, cross bedded sandstone.
It looks just like that these were formed in a dry region, sand blowing around.
Right above with a really straight contact are these very low angled
strata, that look like they were formed in a sort of damp region.
And then here's the dark zone of recrystallization, capillary fringe of
the water table, and finally up to right above that wavy bedding.
Wavy bedding is the one that I like, also festoon cross-lamination, don't
worry about that so much, that's actually something from the water actually flowing.
And the interpretation is that these, this is a
dune field water table rises up and cements this part.
Wind blows the rest of it up here, and then you're a, a, a playa, a
dry, nearly damp, slightly damp lake bed that slowly deposits these very flat
layers Over time, the water table rises up even higher to here, just at the very
top of it, just, just barely penetrating the, the upper unit right here.
And, as things get deposited in through here, there's enough water, either
coming from above or below that you make these, these wavy beds.
You make these festoon cross laminations.
You're in a region, perhaps you're not in
a region where it's globally wet, versus globally dry.
But perhaps you're in just a intra-dune region.
Inter-dune region where you have dunes here, and dunes
here, and you have maybe in between some dunes
a low depression, where you have a flat plya
lake surface, like you can sometimes see on the Earth.
So what does all this mean?
Well, all this means, that there was water.
We knew there was water, so, is this a big surprise?
The answer is it's actually really quite
exciting because everything we've seen so far
has been evidence for, extremely fast erosion
of things with water or seepage from underground.
This is a, a sequence of, of a long period of water
table moving up, water table moving down dunes, intra dunes, sand sheets.
This is a complex history that happened over, well, we
don't know how long and we don't know exactly when.
But this region on Mars clearly has
a geology that is intimately associated with water.
And in the next lecture we'll talk in detail not just about the geology that
you can see but we'll talk about the
chemistry of the rocks that were found also.
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