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Lecture 3.17: Properties of dwarf planets
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En provenance du cours de Caltech
The Science of the Solar System
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À partir de la leçon
Unit 3: Big questions from small bodies (week 2)
Rencontrer les enseignants
Mike BrownProfessor
Planetary Astronomy
Discovery of the Kuiper Belt more than 20 years ago now as I
alluded was this incredible boon for understanding the dynamics,
the evolution of the solar system.
The other thing that was amazing about it was it's finally gave us some objects in
the outer solar system for which we could simply
learn how do things in the outer solar system behave.
Let me give you a little feel for why that was so important,
these discoveries over the past.
Now, I'm talking about over the past 10 years,
the large objects that we've really learned so much from.
The first new object in the Kuiper Belt,
by which I mean of course Pluto was
the first object in the Kuiper belt discovered in 1930,
but the first new object was discovered in 1992.
A decade after that discovery,
increasingly larger objects were being discovered and
yet those increasingly larger objects were still dwarfed by
Pluto and its moon share and this is just a sampling of what
the objects were that were known by about 2001.
If you wanted to know what the kuiper belt was like,
what objects in the Kuiper Belt were like,
really the only thing you can look at where Pluto and Charon,
these other objects were too small, too faint.
We really didn't know very much about them at all.
A decade later, things are very different,
Pluto is still one of the largest objects out there but now we know Eris,
Makemake, Haumea, Sedna, Orcus,
Quaoar, Snow White, it's not the real name.
It's kind of my fault. This has been
an incredible set of discoveries over the past decade.
Okay, they're all discoveries that I made,
so maybe I'm a little biased on how incredible
the set of discoveries were but nonetheless we went
from having one object that we could learn
detailed things about to having this whole collection.
You learn so much more by having a series of objects that you can look at,
you can compare, you can see how they're the same, how they're different.
We learned a little bit about that by looking at
Mars and comparing it to things like Venus and the Earth.
We learned a lot about that by looking at
Jupiter and comparing it things like Saturn and Uranus
and then exoplanets and now
we're able to do the same sort of thing in the outer solar system.
The really cool thing is that each one of these large objects in the outer solar system
basically has its own story to tell about the history of the solar system.
Pluto and Charon, the story of Pluto and Charon is a pretty cool one.
Pluto and Charon presumably had a giant impact early in the history of the solar system.
Charon smeared off the side and went into
orbit around it and you can barely see now in this time there are only two moons.
Pluto has even more moons now than we realize it then an incredible and dynamic system.
Every one of these other objects has a little story that's completely different,
and a completely new view of how to think about the outer solar system.
I'm going to just give you a couple quick versions
of a couple of those stories right now.
To me, perhaps the single most interesting of
these large objects in the outer solar system is Haumea.
Haumea is named after Hawaiian goddess,
in fact Hawaiian goddess of childbirth.
It will be the interesting part of the story,
you might understand why when I tell this story.
The very first interesting thing we realized about Haumea very shortly after
we discovered it is that it gets brighter and darker very quickly.
This is brightness over, this is how bright it is.
This is when it's brighter over here,
gets darker down here,
gets brighter up here, darker down here,
and this is a two-hour period.
Seeing this getting brighter and darker with a two-hour period,
which really when we went through all the different possibilities that it could mean,
it told us really only one thing.
Not that it's got a light on it that's flashing with a two-hour period,
that would be a little bit crazy,
but that it's rotating and it's rotating in a very specific way.
You can imagine it's rotating with a two-hour period and it's got
a bright side and a dark side and you're seeing bright and dark and bright and dark.
It would be a little bit strange, and not only that,
a two-hour period something rotating with a two-hour period would fling
itself into pieces very quickly unless it were made out of I don't know,
of something like uranium.
So, we don't think it's made out of uranium.
We don't think it's rotating every two hours.
What we think is it's rotating every four hours.
What you're seeing is an elongated body with a long axis,
short axis, long axis,
short axis, long axis.
It's bright when it reflects a lot of sunlight,
it's dark when it doesn't reflect a lot of sunlight.
It's bright when it does reflect a lot of sunlight,
dark the other way.
So, what you're seeing here is one full rotation in the course of four hours.
Actually, it's a little bit shorter than four hours and we know
the rotation rate to a fraction of a millisecond,
which is pretty astounding in and of itself.
Four hours, we haven't talked much about rotation rates other
than Jupiter which was 9.95 hours,
was really fast rotating object.
Four hours is the fastest rotation of any large-scale body in the solar system.
I could take this pen and throw it faster than that
but a large gravitationally held together body
rotating at four hours is rotating so fast that it almost tears itself apart.
In fact, it's rotating so fast that it pulls itself into
this elongated shape because of the fast rotation.
Because we can measure the shape by seeing how much brighter and darker it gets,
we actually can figure out something pretty astounding about it.
We can figure out the density of Haumea.
If Haumea were more dense or less dense,
it would have a different shape from the shape that it has.
So, we can very uniquely constrain that density and
the size of this body and it looks something like this.
This is its real rotation other than the fact that it's going around
once every couple seconds instead of once every four hours.
This would be the way it would look if you could see it in space.
Probably, it has a little bit more variation on it.
In fact, we even know it has a little red spot on it that you can see going around,
but the shape is more or less doing this.
One of the first really interesting things about Haumea is that it had a moon around it.
Having a moon around it, if you remember the moons of
Jupiter allowed us to get the mass of Jupiter.
Having a moon around Haumea allowed us to take the mass of Haumea very quickly,
turns out to be about a third of the mass of Pluto,
but we can do something sort of strange.
For Jupiter, we knew the size and we measured the mass and we could get the density.
Here, we knew the density because of the shape.
We get the mass and so we get the size.
When we figured out the size,
it was fairly astoundingly large.
This large axis is about the size of Pluto,
a little bit smaller than the size of Pluto.
The small axis here is about half that size and the axis that you
can't quite see very well but right here and here is about 50 percent bigger than that.
There was a time when people had the crazy proposal that anything
Pluto-sized or larger should be a planet and I was going to
argue that Haumea should be a planet as long as you promise to look at it.
Now, that would have been a stupid idea.
I'm glad that didn't happen.
So, I told you that we could tell the density.
The density of Haumea is something like 2.6 grams per centimeter cubed.
Ken, if you remember all those numbers I threw around when we were talking about Jupiter,
ice is one gram per centimeter squared,
rock is three-ish grams per centimeter squared,
Haumea is almost nearly rock.
It's significantly more mass,
is significantly more dense than the only other object
that was larger that we knew about at the time whose mass we knew was Pluto,
which has a density of somewhere around two grams per centimeter squared.
Some amount of rock, some amount of ice,
Haumea nearly completely rock.
In fact, that was nearly completely rock makee this next plot astounding.
This is a spectrum,
the reflectance spectrum of Haumea and this is
in those same wavelengths that we were looking at when we looked at Mars,
for example, this is one micron out to 2.5 microns.
These are two different spectra of these little dots here and these little X's here,
it doesn't really matter but it's from two of the biggest telescopes
we have on the planet the Keck telescope, the Gemini telescope.
These things are still pretty faint and they are matched pretty well by this line.
This line is a model spectrum of what we think might be on
the surface and what we think is on that surface is water ice.
This is a spectrum of pure laboratory water ice.
Even down to this little divot right here which you see in the data,
this divot right here only happens in
a certain type of water ice that's in crystalline form.
So we know it's covered in water ice,
we know it's in crystalline form and it has the density nearly that of rock.
What the heck is going on?
A pure ice object with the density of rock.
Well, the answer is,
it must look something like this,
where this is the ice part of Haumea.
This is the rock part of Haumea and this is essentially to scale,
Haumea really does look,
must look something like that,
a very thin ice layer over a really big rocky layer.
What the heck is going on with this?
There's nothing that we knew like this at all anywhere in the solar system.
Our first idea was, well,
what if Haumea used to be a larger object and early in
the history of solar system we got smashed into by another object, a glancing blow,
led to Haumea spinning really fast,
cracked open and that big icy mantle,
ice chunks go flying everywhere,
leaves Haumea spinning fast and with very little ice
left over but a nice big rocky core, like this.
I love this idea, had to tell,
you but the people who do
the calculations to show you whether or not these sorts of things are feasible,
took out their very sharp pencils and decided that the probability of
having an impact like this was nearly zero,
something like 10 to the minus seven and this could not possibly be it.
Okay. We sat and we carried on.
We discovered a few other things.
This is another set of objects that we looked at in the Kuiper belt,
here's the plot of again semimajor axis versus eccentricity.
Here are those objects in the three to two resonance,
the two to one resonance would be out here,
these are some scattered objects.
The points that I show you here are the ones for which we got spectra,
like the spectra that I showed you that had ice on them,
we got a spectra of these objects from
the Keck telescope and we found some interesting things.
Most of the spectra, the ones that are shown in grey or black, are pretty boring.
They have a little bit of ice on them,
not surprising they're out way beyond the ice line,
but not very much ice or they have no ice on them at all.
A small number of objects, these, right here,
however had completely ice-covered surfaces just like Haumea.
The astounding thing is that the small number of objects are
really tightly clustered in inclination space,
in an eccentricity space.
If you think back, only a lecturer or two,
you might remember that discussion of asteroid families,
giant impacts in the asteroids that led to a spread in
eccentricity inclination that looked exactly like this.
This is exactly the signature of a giant impact that leads
to a spread of these similar composition objects. Is this really true?
Well, let's look now in more detail at just the very icy objects.
What I've done is taken a cloud of objects
and made an impact occur right here in the middle of this cloud of
objects and I calculated what
the spread in eccentricity and inclination would be if this impact occurred.
If the impact occurred and there was
a very small velocities that they went escaping from,
very small meaning 50 meters per second,
that's very small, you would get something like this spread and this spread.
If you were up to 500 meters per second,
you would get a spread all the way up through here like this.
It looks very much like all of these objects are part of this giant collision.
Very similar to the giant collision that we talked about with Haumea and in fact
Haumea is one of these objects inside of this family.
The interesting problem is Haumea is this object,
it's the only one that doesn't fit particularly well
inside of its own family. What was the story?
We tried to figure out if there was some reason why Haumea could be up
there when the rest of his family was stuck down below here.
I'm going to blow up this very small region now of
semimajor axis and eccentricity space and show you what happens if we do a couple things.
We're going to take this region here and now I'm going
to make artificial Kuiper Belt objects in the computer.
I'm going to stick it in the computer and just calculate the forces of
gravity and how the object behaves over hundreds of millions of years.
Most of the time, like right here,
this object has been stable for the entire time that we did the integration,
same here, same here, same here.
These are the real objects we
calculated with their orbits would be, they don't change easily.
The only time that anything ever changes is right here.
Right here is the 12 to 7 mean motion resonance with Neptune.
We talked about the three to two,
we talked about the two to one,
we never talked about the 12 to 7,
nobody ever talks about the 12 to 7.
The 12 to 7 is a minor insignificant resonance except for one thing,
it has a very slow long-term effect of making a random walk here in eccentricity,
if you happen to be sitting right here.
Eventually, you'll get your eccentricity so high up like
this that you will become Neptune-crossing and get ejected.
This object is Haumea,
this is Haumea after 100 million years, 200,
300, 400 million years, Haumea's gone.
It looks very much like Haumea started down in
here in a normal spot in the 12 to 7 resonance.
Randomly worked its way up to here and if we'd come
back just a few 100 million years later, it would be gone.
What does this tell us?
I think it tells us this,
there are few other pieces of the puzzle that really go together
that make this the obvious answer,
in fact, the only answer that could possibly explain what's going on.
Now, there's still some strange things about Haumea,
exactly how the collision happened,
exactly what the little chunks that are floating off about it are.
But it's a pretty astounding thing that we have
found an object in the outer solar system,
there was a giant impact and we found the pieces of
that giant impact leftover still in orbit around the sun,
allowing us to put the pieces back together and learn about this tiny body.
What have we learned? Well, one thing we learned is that to be differentiated,
it had to be a rock core and an icy mantle because that impact
occurred and all those chunks are pure ice and the two moons around it,
I didn't talk about the other moon,
there's there's Hi'iaka and Namaka,
both children of Haumea.
The two moons are both pure ice chunks too,
products of this collision.
That little thin layer of ice must be
just the rest of the mantle that was left over after the collision.
What else did we learn? Well, I told you early on that
the probability of this was supposed to be very small and yet it happened.
We've learned that impacts were much more common for these sides objects and we had.
The slew of other things we learned just from
this one crazy object that we happen to have in our solar system,
it's the most astounding thing out there.
I'm just gonna talk about two more of
the interesting objects that we have out there Makemake.
Makemake is the dwarf planet that gets very little discussion,
poor Makemake, it's overshadowed by the weirdness of Haumea
and the pure size of Eris and the cool stories of some of the rest of these things.
But Makemake is really interesting in one very peculiar way and that very peculiar way
became very apparent to us the very moment that we took the first spectrum of Makemake.
It was a moment that was, I have to say,
just a purely astounding moment which was,
we took the spectrum of this object and realized it is almost identical to that of Pluto.
Here's the spectrum all the way from half a micron out to about two and a half microns,
generally that same region we've been looking at before.
Red is Pluto and all of this up and down almost all of
this up and down is due to methane ice on the surface of Pluto.
Before that year that we found Makemake,
Pluto was the only object out in the Kuiper Belt that had methane on it.
All the rest of these maybe they had a little bit of water but really nothing else.
Suddenly Makemake, huge bands of methane out through here.
There's a couple of other little things.
Pluto's actually dominated by nitrogen,
that little nitrogen band,
that divot right there is all due to
solid nitrogen but it
doesn't have a very good spectral features so you don't see very much of it.
Makemake covered in nothing.
Without even knowing the details of how the spectroscopy works,
you can also notice that there's differences between Pluto and Makemake.
These lines of methane are what I would call saturated, they're much broader.
Look at the breadth of this line,
how wide they are through here compared to these,
look how wide this is compared to this and more importantly down through here.
Look at how this whole thing is just a big broad absorption feature
whereas you get these little ups and downs here inside Pluto. What does that mean?
Well, for ices what that means is that you have long path lengths through the ice,
the photon is allowed to travel a long distance through
the ice before hitting something and reflecting back out.
So, the chances are that it gets absorbed.
If you have short path lengths, little frost.
You get a little bit of absorption and then you go away,
you have very tiny absorption lines.
Pluto's got some very nice absorption lines but Makemake is crazy.
Makemake, we could model it,
we don't know exactly what's going on on the surface but we can model it as
slabs of pure methane ice covering the surface.
If you could skate on methane,
Makemake would be the place to skate on methane.
I think you can't skate on methane but if you could,
this would be the place to go.
So, Makemake is interesting.
Makemake is a little bit smaller than Pluto.
Makemake is a little bit further away than Pluto,
has similar ices on the surface as Pluto but not exactly the same. What's going on?
This is one of those moments where we learned a tremendous amount from
just a small number of objects just getting
a few other objects out there in the outer solar system.
This was from the PhD thesis of one of my students Emily Schaller,
who realized that the reason that objects in
the outer solar system don't have these very volatile ices,
the ices that like to evaporate away,
is that these volatiles escape in the atmosphere and they escape.
Well, we don't know the process of escape, it's just like Mars.
There could be hydrodynamic escape like we talked about at Mars.
There could be some sort of strange sputtering like
we talked about at Mars but the one thing we do know is
that they have to escape and the slowest possible escape rate is Jeans escape.
So all she did was calculate the Jeans escape rate for these different molecules CO, N2,
CH4 and for the size and the temperature of
the bodies that we have here and she found that most of the objects in the Kuiper Belt,
these are all the objects in the Kuiper Belt.
Most these objects in the Kuiper Belt are so small,
this is size and they're so hot that Jeans escape
proceeds so quickly that all these volatiles are gone very quickly.
For the largest objects or for the coldest objects,
Jeans escape is sufficiently slow that you're allowed to hold on to
these volatile ices over the age of the solar system.
Who's allowed to hold onto them?
Triton. Triton is the moon of Neptune,
has methane on it and CO,
N2, Pluto, N2, CH4.
Eris, we'll talk about that a little bit later but it definitely has methane,
probably has N2 although that hasn't been detected.
Sedna, we'll also talk about later it's much colder,
it's much smaller but it's so much further away that it's allowed to have all these and
indeed does and this is an old plot from Emily's original paper.
So Makemake is 2005 FY9.
Actually Haumea has 2003 EL61.
Haumea interestingly is big enough to hold onto some volatiles
but remember I said it's allowed to Jeans escape is the slowest escape that happens.
You can have faster escape.
What's a faster escape for Haumea?
Kittens smacked by another big Kuiper Belt object
would have rejected the entire atmosphere,
so that when it sort of makes sense.
Interestingly, Makemake sits right in this region where some of the volatiles could be
kept methane in blue and some of them not N2, maybe CO is kept.
We now know this makes perfect sense.
Pluto is actually dominated by N2 and methane is
a minor constituent and the N2 forms these large crystals, these large slabs.
Makemake does the same thing for its dominant ice,
which turns out to be methane.
There's a hint of a little bit of nitrogen ice in Makemake but very little.
Interestingly, Quaoar sits in the same region as Makemake.
Quaoar is an object. If you'd look at its spectrum,
you might recognize some of these you might note this is water ice.
Once again, this is the even that thing that says it's crystal and water ice.
So been known for a long time that Quaoar had water ice but we went back and took
a look much more carefully in this region through here and we saw this little divot,
this little divot should not be there in water ice.
So, what is it? Well, there's water ice and
what water ice looks like there is if you add in methane,
just like we expected to be there.
It is one of the most gratifying things you can do
when you make an interesting discovery like Makemake.
Scratch your head, try to figure out what's going on with it,
come up with an answer and that answer leads you to predict something
like Quaoar will have methane because it's just the right temperature and size.
You go back and find it and sure enough it looks
exactly like an object that has methane on it.
I could give you similar stories of Snow White,
who's really 2007 or 10.
Orcus, Sedna, I will give you the story of Eris we'll talk a little bit about but
each one of these there's such richness
of the stories that we can tell from these objects.
These days of course,
there's no richer story about a dwarf planet than that that we can tell about Pluto.
That's of course because the New Horizons spacecraft took 10 years flying out to
the outer part of the solar system and flew
by and took these spectacular images of Pluto.
By now, everybody has seen this one with the famous heart on the surface of Pluto.
We'll talk a little bit about what that heart is,
about some of this other stuff around through here and how it
relates to the other things that we know of in the outer solar system.
Let's zoom in on this heart region and see what this stuff really is.
That last image was in approximately real color,
this is now enhanced color.
So you get these crazy looking super red regions down through here.
These super red regions down through here are where you have methane.
Remember that methane is the thing that dominates the spectrum at least
of the surface of Pluto and that shows up in these red regions here.
But the heart, this is the edge of the heart,
this is the most dramatic looking region.
Look how smooth it is. There are no craters.
You see craters here on the sort of old methylated regions but no craters through here.
Instead, you even get these looks like blocks that are
red that are floating off there here these blocks are probably mountains of ice.
They're covered in methane but they're made out of water ice and
they are floating essentially,
on a sea of solid nitrogen.
It's a sea of solid nitrogen,
it's not a sea, it's not liquid,
it really is solid but it's a slushy solid,
it's a solid that can move around.
Water ice is less dense than nitrogen ice,
so these blocks of water literally float on this slushy ice.
How do we know it's a slushy ice?
That's a strange thing to think about.
We can directly see it in some of these images.
As bland as this image as compared to some of the other ones.
I actually think this might be the single most important image that
came back from Pluto and this is
the edge again of the heart and this is what I had called the slushy nitrogen region.
Look very carefully up in here and up in here and in fact,
look right at these little regions through here.
If you remember, we've seen things that look like this before.
This looks like a glacier, a flowing glacier.
In fact, we now know that there are regions where it's
high here and the glaciers nitrogen,
glaciers flow down into this slushy solid nitrogen.
What's also interesting is, you can see this slushy solid nitrogen has these features,
these boundaries all around here.
You can't see it as well in through here but they
exist in all these regions through here.
This is because the slushy nitrogen if I were to draw a cross-section of the side of it,
it is occupying a bowl basically,
it's a bowl of slushy like this and it's convecting.
Remember how convection works?
There are cells that go up and then they come back down again,
cells that go up and they come back down again.
The regions where you see these borders are the borders of these convective cells.
You see material pushed from here over to here,
it gets pushed together at these borders and that's why you se this border here.
This is perhaps the most amazing thing that was seen on Pluto.
Remember, nitrogen is one of the things that dominates
the surface spectrum of Pluto and so we're happy to see how it really exists.
Nitrogen doesn't seem to be as dominant on Makemake.
Maybe Makemake doesn't have this one singular heart feature.
Maybe there is more nitrogen dispersed in irregular fashions.
We're still learning more about the differences
between all these different dwarf planets.
My favorite image from Pluto perhaps is this one.
This is looking back towards the sun is behind the horizon now and you can
see the long shadows of the mountains in through here at the Terminator,
where it's getting to be twilight and you see these haze layers up in the atmosphere.
These haze layers we're going to talk about the atmosphere on Eris,
or the lack of an atmosphere on Eris because Eris is so cold that this haze,
this atmosphere is probably plaited out onto the surface.
You can also see here the big nitrogen,
liquid nitrogen, slushy solid ocean.
There's a nice little iceberg floating on this ocean,
some of these convective cells and it's
just a beautiful view of what it looks like looking back towards the sun from Pluto.
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