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Lecture 3.01: Introduction to the small bodies
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来自 Caltech 的课程
太阳系科学
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从本节课中
Unit 3: Big questions from small bodies (week 1)
与讲师见面
Mike BrownProfessor
Planetary Astronomy
We're going to spend the next two weeks talking about the smallest
bodies in the solar system.
And specifically how we can use these smallest bodies
to answer some of the biggest questions in the solar system.
Two weeks is a short amount of time to do this.
I could spend an entire class talking about this.
I could spend years talking about this, because we have finally hit the part of
the class where this is what I actually do.
I spend my research time here at Caltech, and my time going out to the telescopes,
trying to answer these very questions that we're going to be talking about today.
So I'm very excited about this part of it,
and I'll try to keep it down to a manageable two-week level.
First, I want to introduce you to what small bodies there are in the solar system
that we're going to be talking about.
You've probably heard of all these.
Let's take a look at some of them.
When I say small bodies, I mean anything other than the eight planets.
And one of the first that comes to mind is a comet.
Comets, spectacular comets look something like this.
This is comet Lovejoy, a great name for a comet.
And this is a picture that was taken by an astronaut
in the International Space Station,
just taking his camera, taking a snap outside of it.
So you see, this is the Earth's atmosphere at the top of it here.
And you see this comet, beautiful tail being seen through part of the Earth's
atmosphere and then out into space, really just spectacular.
Now this comet is much further away than the Earth.
It just happens to be that they're looking at it through that atmosphere there.
We'll talk about comets, where they come from, what they're made out of,
what they tell us about both the early solar system and the solar system today.
And in addition to comets we'll spend some time talking about asteroids.
Here's a picture of the asteroid Ida.
And if you look really, really, really, really, carefully,
you see this little dot down here.
This is the moon of Ida, it's called Dactyl.
Now asteroids are in a band between,
mostly between the orbit of Mars and between the orbit of Jupiter.
And this picture of Ida was taken by the Galileo spacecraft.
Remember the Galileo spacecraft?
The Galileo spacecraft was the one that went to Jupiter and
went into orbit around Jupiter.
As it goes to Jupiter and goes into orbit, it has to go through the asteroid belt.
Now if you've seen Star Wars or any other science-fiction movie,
the way you go through the asteroid belt is you go zooming through there,
dodging the asteroids as they go, because they're about to hit you.
Not true at all.
Ida here is the first asteroid that was ever imaged by a spacecraft.
Even though Galileo was not the first spacecraft to go to Jupiter.
And that's because it is nearly impossible to find an asteroid that you're going to
travel close to in the asteroid belt, if you are flying from here to Jupiter.
So the Galileo mission controllers and JPL had to work really hard to see if they
could find an orbit that would eventually get them to Jupiter.
Left the Earth and actually swung by a couple of other planets at the same time,
to get gravitational boosts.
And just barely found one that could make it to Ida,
and got this very first image of an asteroid from a spacecraft.
Now there are other images of asteroids from spacecraft because sometimes we
actually send the spacecraft directly to the asteroid.
The Dawn mission has recently visited the asteroid Vesta,
it's in orbit around asteroid series.
These are the two biggest asteroids.
Very exciting to see what's happened and going to happen there.
And there have been spacecrafts that have flown into some smaller asteroids too, but
this was the very first.
And it was because the navigators tried really, really hard
to find an asteroid in that very sparse region of space between Mars and Jupiter.
And while most of the asteroids are between Mars and Jupiter,
they don't all stay there.
There are near Earth asteroids.
These are the things that people worry about maybe hitting us.
We'll talk a lot about that in a few lectures.
And there is just general debris in the near Earth area and
sometimes that debris hits the Earth's atmosphere.
And when it does, you get a spectacular meteor shower or
a single meteor or a shooting star,
these sorts of things that you can see almost any night that you look up.
You can go and look at a particular meteor shower, and see a lot of them at once.
But any clear night that's dark or
the moon is not out, get out there, start looking around and
you will start to see some of these streaks going across the sky.
They tend to be very small, sand-sized things, dust-sized things.
Although in some of the bigger meteor showers,
you can actually get some substantial chunks.
And of course, occasionally, the chunks hit the Earth.
Usually when they hit the Earth,
they're just small meteorites that maybe people can go find and pick up.
Occasionally, very occasionally, almost never,
they do damage usually in Siberia for some reason.
And many of you probably remember recently where a meteorite,
a fireball exploded over the Russian town of, I can't pronounce it, but
I'll write it, Chelyabinsk.
Exploded high in the atmosphere, sent out a shockwave throughout the atmosphere and
broke windows, lot of people took videos of it.
It actually caused some injuries, an incredibly rare event.
Not one I think we should be worrying about, but
nonetheless these things do happen.
As we move to the outer part of the solar system,
the outer part of the solar system too has populations of small bodies.
And I'm going to now draw where some of these things are.
And I'm not very good at drawing it to scale, but I'll try.
The sun is here, put the Earth here at 1AU, Mars here at about 1.5,
Jupiter's here at about 5, one, two, three, four, five.
So Jupiter's out here.
So the asteroids are all in this region and through here, very sparse,
as I said before.
Some of the asteroids, and we'll talk some more about these,
some of the asteroids actually co-orbiting with Jupiter.
They have the same semi-major axis as Jupiter which is 5.2 AU.
And they're in little chunks ahead and little chunks behind Jupiter.
This is where Jupiter is.
And these are called the Trojan asteroids.
And we'll talk about Trojan asteroids where they came from and
what they tell us about the solar system.
But those are, well actually one of the things it's interesting is because they're
in this transition between what we think of as the inner solar system
inside of Jupiter and the outer solar system outside of Jupiter.
And so it's an interesting question.
Are these Trojan asteroids more like the asteroids, or
are they more like these other populations?
We'll talk about here in a minute.
So there is Jupiter, Saturn, Uranus, Neptune and I'm going to
now compress space because I don't have space to draw my entire solar system.
Here is everything out to Neptune.
And beyond Neptune is this region of space that I spend a lot of time studying,
which is called the Kuiper belt.
The Kuiper belt, you can think of it as like the asteroid belt,
a bunch of small bodies out there.
And the difference between the Kuiper belt and the asteroid belt,
in addition to just where they are.
You might guess is that the asteroid belt is predominately rocky material,
just like the terrestrial planets are predominately rocky material.
The Kuiper belt is predominately icy material.
And when I say predominately icy material, I mean mostly water ice,
although, as we'll see in a minute there are other interesting ices that
happen when you get to the cold, cold regions of the Kuiper belt.
What do Kuiper belt objects look like?
Well, here is the second most massive known object in the Kuiper belt, Pluto,
I think it's called.
It's not typical of the Kuiper belt.
It's large enough that it has these beautiful frosty regions on it.
We'll talk in detail about what was found by the New Horizons spacecraft
when it visited Pluto.
In the meantime, I'm going to show you images of two objects that probably
use to be in the Kuiper belt but have been captured by a giant planet.
They're now both satellites of giant planets.
First, let me show you the satellite of Neptune, which is Triton.
Triton is an interesting one, because Triton is actually bigger and
more massive, than the two largest Kuiper belt objects, and
two most massive Kuiper belt objects that we know of, Eris and Pluto.
Triton is a good bit bigger, yet
we think it used to be part of the Kuiper belt before it was captured.
So it would have been, had it still been out in the Kuiper belt, it would
take the record for the largest and most massive and this is what it looks like.
This is a mosaic of images from the Voyager spacecraft.
The Voyager spacecraft is the only one that's ever been to Neptune before and
in fact only one of the two Voyagers made it to Neptune.
The other one shot out of the plane of the planets and
so it never got to Uranus or to Neptune.
So we've had one encounter of Uranus, one encounter of Neptune,
one view of Triton and we only got to see one side of Triton because
only one side was in the sunlight at the time and it looks sort of interesting.
First off, it's round, it's a big round satellite, like our moon.
But unlike our moon, you don't see very many craters in there, and in fact,
if you look really carefully, you hardly see any craters at all.
What does that mean?
Well, we do know what that means.
We have talked about how to figure out ages from ages of surfaces from craters.
If you see no craters, that's because that surface is a relatively new surface.
Why do we have a relatively new surface out here in the outer solar system?
Well, if you'd look, this stuff here, this sort of bluey,
grey material in through here, this is ice.
Now what kind of ice is it?
Well, we know actually now, that Triton has nitrogen ices on the surface of it.
It has methane ices on the surface of it, and
those are what function as frost on a place like Triton.
We'll talk a lot more about this later, so you don't have to worry about it too much.
We also know that the rock, the bedrock underneath these frosts
on Triton is actually water and that's probably what this material is over here.
So you have these frosts that move around throughout the year,
throughout the day and cover up whatever big craters might have been in there.
Maybe there is a big crater, maybe there is a big crater there, or there, but
these frosts come in, the ices evaporate and
the craters that you would see otherwise aren't there.
Triton is a really big former Kuiper belt object, so it's like Eris or
like Pluto, but most Kuiper belt objects aren't as big as these.
And in fact, they're not big enough to have these really frosty surfaces.
A more typical type of Kuiper belt object probably looks about like this.
Like Triton, this is a captured moon,
this is the moon Phoebe and it's in orbit around Saturn.
It's an irregular satellite.
An irregular satellite is one that's not in a very nice circular orbit in the plane
of the giant planet like the Galilean satellite, Io, Europa, Ganymede, Callisto,
all are nice rings on a nice plane.
In Saturn, there are a bunch of satellites that are in the same plane as the rings of
Saturn but there are also these very distant looping outer satellites that
are called irregular.
We'll talk about irregular satellites.
And one of the things about irregular satellites
is that we think they were captured.
If this were captured,
it would have been captured from a region like the Kuiper belt.
And so we think of this as a, this is about a 200 kilometer object,
something like a 200 kilometer Kuiper belt object.
And what does it look like?
Well, I tell you one thing, it does not look a thing like Triton.
It's not round, it's very irregular shaped.
It's cratered like crazy, and those craters have interesting things going on.
If you look really carefully,
you see the craters have some sort of white streaks coming down them.
You can see that in everywhere.
Maybe that's ice poking through.
It's a relatively dark object otherwise.
But there's some spots where it looks like there might be ices poking through
the dark surface.
Again, we'll talk more about these sort of objects in upcoming lectures.
So I say that we're going to use these objects to
answer big questions about the solar system.
And a reasonable question to ask is, what am I talking about?
The planets contain almost all of the mass in the solar system.
You really want to understand what the solar system is and
what its doing, the planets are really the right way to go.
But the problem with the planets is that there are only eight of them.
And in addition to there being only eight of them, those eight planets are big.
The materials have been smashed together,
compressed, maybe plate tectonics have happened, volcanoes have happened.
These things have sunk to the bottoms of giant planets like Jupiter and
got compressed to liquid hydrogen.
All sorts of things have happened to these materials.
If you can find small bodies,
small bodies are closer to being remnants of what was there to begin with.
So that's one thing that's good.
They're more pristine.
Sometimes they're often called primitive bodies because they haven't had all
the processing that's been happening to the planets.
So that's one very important thing.
Chemically you can learn things about earlier times in the solar system by
looking at these small bodies.
But it's not just chemically.
What I'm actually often more excited about is that their orbits
tell you things about the giant planets, and here's why.
The giant planets, in fact all the planets, are on nearly circular orbits.
Ignore Mercury, it's got issues with general relativity.
The other planets, the orbits are pretty close to circular,
even Mars, which we talk about as having an eccentric orbit.
If you looked at it, you wouldn't even be able to tell that that orbit is eccentric.
Small bodies, on the other hand, are almost all on moderately,
to extremely eccentric orbits.
And there's a reason for this.
We think again, as we'll talk about in the future,
we think that everything when it forms, forms in a circular orbit.
It has to be in a circular orbit to form.
Once they're formed, things can start to interact with gravity and
go all over the place.
What happens? Well, the giant planets are so big,
the planets, not just the giant planets.
The planets are so big compared to the other objects in the solar system,
that they stay on these circular orbits.
But everything they encounter,
gets pushed off onto a crazy elliptical orbit, inclined all over the place.
And so we end up with planets, and then things flitting among the planet,
between the planets, up and down, around the planets.
And these things flitting through the planets are a tracer of what the planets
have done to them.
One of my favorite analogies of this is to a story that I read many years ago,
which I loved the story.
Which is that, a container ship was crossing the Pacific Ocean,
encountered a big storm and on the deck of this container ship was
a container full of rubber ducky float toys.
That container of rubber ducky float toys fell off the ship, spilled and
those rubber ducky float toys spilled out into the ocean.
What happened next is fascinating.
Those rubbery ducky float toys started showing up at beaches,
first in the Pacific Northwest and then well let me show you.
After they first showed up in the Pacific Northwest here,
then made their way down the coast to Washington, over here in Alaska.
They then they started to be found many years later on the East Coast of
the United States, in Ireland, in the corner of France.
These ones had to have traveled through the ice sheet,
they were frozen in over the winter and moved their way across this.
And oceanographers tracked the findings of these rubber duckies and used these
rubber duckies to understand the circulation and the winds of the ocean.
In fact, it happened more than once.
Another time there were not just duckies, but there were four or five,
maybe three or four, different types of float toys which had different shapes.
Some of them had a large mass under water and not much on top.
Some of them were big and flat with a lot over water.
They reacted differently to currents versus winds and
the oceanographers tracked them.
The red ones went one way, the green ones went one away.
I love this story because this is exactly what we try to do in the outer solar
system, in the entire solar system by looking at these small bodies.
These small bodies are the rubber duckies of the solar system
that it were dumped into the solar system at the beginning of the solar system and
now have spread throughout.
Now for the rubber duckies they have it easy.
They know when and where it started, the dumping overboard was right there and
so you get a better view.
For the solar system, it's actually one of things we're trying to figure out
is where the small body started out, where they've been pushed around too.
The pushing around, here we have currents and winds.
In the solar system, the pushing around is due to gravity.
Now if there were just the giant planets where they are and
the other planets where they are then we would pretty much know what
the gravitational fields were in the past too.
But one of the things we've learned, again, we'll talk about
is that the planets have been moving around for the past 4.5 billion years.
And understanding this history of where they started, where they moved.
How we all got to where we are, is one of the promises of using the smallest bodies,
these rubber duckies of the solar system,
to track down on the beaches which is today, to figure out where they are now.
Figure out how they got there and understand how the currents,
the gravitational fields of the solar system have been changing through time.
