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Lecture 3.14 : Pictures of asteroids
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En provenance du cours de Caltech
The Science of the Solar System
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Unit 3: Big questions from small bodies (week 2)
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Mike BrownProfessor
Planetary Astronomy
I remember the very first asteroid fly by and the images coming back from it and
the excitement over these.
And there have been so many now.
It's fantastic to have this many because we learn so much from each one of these.
But there's so many that I've even lost track about all these ones so
I can't even recognize them all the time.
Although, this one is a good one I recognize this is near Earth asteroid,
so this is one that comes close to the Earth and
it's not particularly big as asteroids go.
This is about 34 km across, and
it's sort of, this dimension is maybe more like 11 km across.
So, this is one of those small, euro asteroids, this one,
if it hit is, it'd be bad news.
These things are pretty big.
Is it going to hit us?
No. And what do you see?
Okay. Let's remember what it is.
Euro asteroid it's a collisional fragment from somewhere.
This particular one, I don't think we know where it is.
But since the time that it broke off, it was ejected from its parent asteroid,
you can see other things have happened.
Craters just like we know, love in craters other places big craters, little craters.
This is presumably a big crater that you can't even see the shadow outline of there
are thoughts that there may be basically big coherent blocks inside some of these
places that are sitting together and rubble that slides around on the outside.
They're pretty spectacular.
This actually is a series of images from that very first imaging of an asteroid.
This came from the Galileo space craft.
Remember I told you at the beginning of the asteroid part that
finding an asteroid to fly by is incredibly hard.
It's not like you have to go play Star Wars and
dodge between the asteroids as you fly out of Jupiter.
The Galileo space craft made a special attempt to image an asteroid and
Gaspra was the one that was chosen.
It's a S type asteroid, so inner part of the Solar System and
this is the series of images as Galileo got closer and closer and it's rotating.
It's kind of cool to see it rotating across here.
The size, it's not too different.
This one is about 20 kilometers across in this long dimension here.
And it looks kind of like arrows that we just looked at has sort of an odd shape.
Maybe there are blocks that are here.
It has craters all around it.
It looks like something that we can start to say,
all right, I think I understand what asteroids look like.
This is a spectacular one.
This is asteroid Lutetia, and it was returned by the E submission Rosetta.
One it's way out to the comet,
it of course tried very hard to take a picture of an asteroid.
And here's what it looks like.
Can you guess the size here?
It's kind of fun to look at this and say, okay,
we understand these things well enough to guess.
Is it bigger?
Is it smaller?
Is it the same size as those last two that I just showed you?
If I didn't know I.
I think that I would guess the right answer that its quite a bit bigger.
This is 120 kilometers across on it's long dimension here.
It was the 21st asteroid found.
It goes by the official name 21 Lutetia.
And it's the 21 asteroid found because it's big and its bright.
And I think you can tell that.
You can tell the craters are proportionally smaller
it's getting to have a more regular shape, sort of.
And what I find interesting is that it has these striations on it.
You see these stripes kind of here and here?
I don't know what these are.
But it's clearly a sign that there are some sort of global processes acting
on this body.
It's sufficiently large that it's beginning to behave
like a coherent something.
And I want to show you this one,
because this is the first asteroid image that was ever taken,
this was taken by the Galileo spacecraft as it was flying on its way up to Jupiter.
Remember I told you that to get, to close enough to an asteroid,
to take a picture of it, you have to try really hard.
And so, it tried, navigated really hard, and sure enough this is the asteroid Ida.
And the shocking thing about the asteroid Ida is,
you might have noticed, there's something else there.
This is its moon.
Its moon is called Dactyl.
It's about half of the size of that last one.
It's somewhere around 50 kilometers across.
The images, at least this image,
is not sufficiently detailed that you can see much.
But again, I think you're getting the picture of what these sort of surfaces
are starting to look like I'm going to show you two more,
just to show you some of the extreme versions.
This is the asteroid Vesta.
Remember Vesta is the one that I told you we know that it has an iron core and
it has differentiated, and it has had basaltic flows on the outside.
We know [LAUGH] this for a couple of really crazy reasons.
if you look really carefully down here at the South Pole, you can see,
I think it's on the other side a little bit better.
But you can see that there is the remnants of a pretty major crater here.
Some asteroid came smashing in through here,
almost big enough to break Vesta apart.
Almost big enough that we could have then found the pieces of Vesta
as they came to the ground, of the inside iron chunks.
Big enough, though, that we do find pieces of Vesta.
These objects that were excavated from this crater flew off into
a family that we can now see in the orbits around Vesta.
And we can actually trace the path those family
members take through things the effect as they get into the residences and
they come screaming down into the Earth.
We have picked up pieces of Vesta with our hands.
It's kind of an astonishing thing to pick up pieces with our hands,
send a spacecraft there to verify that is was really true, and it was really true.
This is, I should mention, about 500 kilometers across.
And you can see there's some pretty major impacts in through here.
People call this one the snowman for obvious reasons.
You can't see from this particular view that there's a nice ridge
around here again.
This is starting to behave like a coherent body.
This is not surprising.
It's been melted on this inside.
That's the sort of thing big coherent bodies do.
This is one of the reasons why when things start to get round like this,
when they get big enough that they get round, they maybe have melted.
There is this reason why there is this special class of objects called dwarf
planets, I'm sure you've all heard of it.
Vesta, it's officially not a dwarf planet because it's not sufficiently round.
Dwarf planets are sufficiently vague.
If it makes you happy to call Vesta a dwarf planet, I'm happy for
you to call Vesta a dwarf planet.
We'll talk more about that in a following lecture, why there are these things called
dwarf planets versus things that are called planets and why anyone should care.
But for now, you can see that this transition is occurring.
As things get big, they start to behave in this more coherent way.
After exploring Vesta, the Dawn spacecraft left orbit around Vesta and
flew to Sirius, and went into orbit around there and
started taking these spectacular images.
Sirius, at 950 kilometers across,
is by far the largest object in the asteroid belt.
At this size, it's quite clear that things look different.
What looks different?
Well, one, it's round.
This is not sort of round like Vesta was.
This is definitely round.
You would call this a definitive dwarf planet.
If you look at these images though,
there are other things that might catch your eye.
I look at, for example, look at this big crater right here.
And some of these other large craters through here and these.
They look, I would call them muted.
They don't look like fresh craters that have been impacted into rock.
They look like things I don't know.
Maybe they look like things that have impacted into mud or
into stiff clay or to something.
And that was one of the first clues that the surface of Sirius is very different
from the surface of Vesta.
Now, we think it's true that Sirius has a surface that has
a mixture of sort of clays and waters.
There's a probably more rocky core down here but clays and
waters in through this part of it.
There's some other interesting things going on that tell that there are some,
another story about water on Sirius which is again this is serious.
This is an asteroid it's not very big.
So the asteroid belt well inside what we would think of as the ice line.
But for the crater there's evidence that interior might have water in it and
then there's even better evidence.
Look at this crater here and this crater here and in this image you
couldn't tell right away but it looks like there might be something going on.
Some bright spots in through there.
Let's look at some of those in more detail.
Here's the crater that has the best of these bright spots.
Look at that, right at the center of the crater, dead center of the crater, and
then off to the side there's one too, and here's the crater.
This bright spot, this brightest spot on Sirius has been known for a long time and
many of us speculated that it's going to be in the middle of the crater.
And fact, I would tell people all the time, when you go and
see what the bright spot is, it would be a patch of ice in the middle of the crater.
And my idea being that you had ice just below the surface, something comes in,
excavates the crater and you excavate down to that ice layer and there it is.
So, when I first saw this image and
saw that it was right in the middle of the crater, I was like ah-ha!
I was right, see?
And it turns out,
very best images show something very different from what I expected.
It's not that it's excavated down into an icy layer, this is that same thing now.
Here's the one bright spot in the middle of the crater,
and this is the one off to the edge.
It’s not that it is excavated down, this is a peak in the middle of the crater.
And if you’d look at this, you could almost kind of tell what it is.
It looks a little bit salty, it looks a little bit crusty.
This one you can’t tell as much, but
this looks like a little mountain of evaporitic salt.
And the reason it looks like a mountain of evaporitic salt, is because well,
it's a mountain of evaporated salt.
It is Sodium Carbonate, which is Na2CO3, and
sodium carbonate is a salt that you might get if you had a dry lake bed,
evaporating, and you would have the salts leftover after the water left.
How do we know this is sodium carbonate?
We know it's sodium carbonate not from just looking at it and saying wow,
doesn't that look sort of like sodium carbonate?
But in fact, from spectroscopy, in addition to having these beautiful images
it could take, would look at this spot right here, get a spectrum of it, and
see the distinct signature of sodium carbonate in the spectrum.
So this is definitive, this is sodium carbonate and it is a salt that
presumably comes from, there must be water that would be coming up underneath, coming
through these cracks, you can really see the cracks over in here actually nicely
where the water probably comes up through, it gets to the surface and
then evaporates, but it leaves behind this carbonate.
In fact, as it's evaporating,
it probably builds this mound of carbonate in through there.
Really, just absolutely stunning.
One other feature on that I would say is
also absolutely stunning is this bizarre looking mountain.
Now, let me make sure you get the perspective,
because your eye can sometimes flip this.
This is a mountain.
And this is a crater.
This mountain, it almost looks like a barnacle on the side of Sirius.
It's unlike any other mountain that's seen anywhere on Sirius.
[LAUGH] It even almost looks like you could have taken the material from this
crater and turned it upside down, and planted it on top of there and
that's what you get for this mountain instead.
It is a ridiculously tall mountain and again there's nothing else like it on.
Let me show you an exaggerated view of what it looks like but
still give you a Sirius.
Here the vertical elevation has been exaggerated by a factor of two.
So it's not quite as tall as this.
It is 6 kilometers tall.
6 km tall, something like 4 miles.
Something like as tall as the tallest peak in the Andes, except it's on this tiny,
tiny little dwarf planet, it's a very strange form.
It is now widely accepted that this is an ice volcano.
Ice volcano?
That's cool.
Ice volcano in that, there are ices, there is water underneath the surface,
and that water is being pressed up just like we saw in that previous crater.
That water is being pressed up, it comes out and it freezes onto the surface,
just like magma freezes on the surface on a threshold volcano.
And it builds this mountain up.
And we now also know that mountains like this on Sirius that are that big
because this surface is that sort of, it's not mushy,
but it's certainly not as strong as a strong rock surface.
That surface can't support something as big as this.
So this volcano is going to collapse back down
into the surface on a relatively short time scale.
So, this whole feature is probably something like
one billion years old or younger.
Okay, a billion years is a long time, but
that means that Sirius has what we would still call real geological activity
all the way out for something in the last billion years.
There's something in there that's continuing to maintain that activity, and
that's a pretty amazing thing.
We're still just digesting all of this information about Sirius,
and much more information will be coming in the future.
This is asteroid Itokawa.
And it was visited recently by the Hayabusa Spacecraft from
the Japanese Space Agency.
And I'll show you something xxtremely cool that [INAUDIBLE] in just a minute.
But first let's look at first off how big is [INAUDIBLE]?
Remember Vesta was about 500 kilometers across.
This is something more like 500 meters across.
This is the size of a city block or something.
And what does it look like?
Looks kind of like an otter to me.
There's his hand sitting here, and his face and its eyes.
But okay, it's probably not a space otter.
But it sure doesn't look anything like any of the other things we've looked at.
Do you see the crater?
I don't know, maybe, is that what this thing is here?
What does it really look like?
It looks like a bunch of debris.
It looks like a bunch of rubble.
With maybe dust covering some of the parts.
Maybe there's a coherent chunk here, maybe here, maybe not.
The smallest things are both chunks of colesions that occur, but
this looks like it was re-accumulated chunks, maybe from the original collision.
Maybe from when these things go by, because it's a asteroid,
goes by the Earth, tidal forces can move things around, change the shape of it.
A very, very strange looking thing.
And what did we do with this very, very strange looking thing?
The Hayabusa spacecraft touched it.
It actually sampled a little piece right through there.
This thing is so small that it has essentially no gravity.
And so you can just fly next to it, touch it, collect a sample, fly back.
It's not landing.
It's really just touching and this sample has now
been returned to laboratories on Earth and it's being actively investigated.
Quite a fantastic thing to get the first sample back from one of these asteroids.
Now, of course, we get samples from asteroids all the time.
They look something like this.
But being to able to get the ones that are in space, haven't
gone through the Earth's atmosphere that's a pretty fantastic thing.
There's another big mission that's a sample return mission called OSIRIS-REx
and if I have complained about lame acronyms before this takes the cake.
It looks to me like this is something about an Egyptian dinosaur maybe.
I don't know.
I can't even tell you what all this stands for Regolith Explorer.
Sample return.
I don't know.
Who knows.
It's a dinosaur that flies up into space, and it actually goes to an asteroid and
picks a sample, knowing what it is that it's picking, knowing the asteroid's going
to, and brings a pretty big macroscopic sample back to the earth.
It'll be, again, in the future.
We're starting to get more and more of these samples.
We're starting to get more and more of these images of asteroids.
We're starting to do more and more of this global spectrocity, to see where all
these objects, what they're made out of, where they're distributed in space.
In the last few lectures of this unit, we'll try to put all those
pieces together, to learn what these small bodies really tell us.
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