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Lecture 4.15: Introduction to Titan
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Del curso dictado por Caltech
The Science of the Solar System
310 calificaciones
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De la lección
Unit 4: Life in the solar system and beyond (week 2)
Conoce a los instructores
Mike BrownProfessor
Planetary Astronomy
The observations that I was most excited about,
that were going to come from the Cassini
spacecraft, were those not of Enceladus, we didn't
know Enceladus was interesting at the time, but TItan.
Titan is Saturn's largest moon.
It's almost as large as Ganymede, which is the
largest moon in the solar system, and Titan has been
a mystery since the time that Voyager flew by,
because of an interesting effect of chemistry in Titan's atmosphere.
Here's what the Voyager images of Titan looked like.
I think it's safe to say the images looked like almost nothing.
What you're looking at here, you can't see the surface of Titan.
What you're looking at is a thick haze obscuring the surface of Titan.
Why a thick haze?
Titan has a lot of methane.
Methane is good, we like methane, has a lot of methane in its atmosphere.
And when sunlight comes in, it breaks apart the
methane to CH3 and an H and other things.
And eventually these radicals of CH3 combine
with other radicals of CH3 or other
radicals and you end up with long chains of carbon, carbon, carbon, carbon, carbon.
Hydrogen is over here.
There is also a lot of Nitrogen in the atmosphere.
So you might you have
Nitrogen attached there.
These long changed Hydrocarbons produce what is, essentially,
a gigantic haze covering the entire surface of Titan.
Titan is like the city of Los Angeles on a particularly warm and smoggy day.
We have, on warm, smoggy days we have these same sort of photolytic reactions
occur from emissions from cars and things,
that make these long-chain hydrocarbons, amongst other products.
And they obscure the surface of the Earth if you're looking at it from above.
Here, you're obscuring the surface of Titan, looking at it from above.
So when Voyager flew by, we didn't know
anything about what was on the surface of Titan.
There were speculations, though, that the entire surface might
be covered with liquid methane, and that the reason that
there is so much methane in the atmosphere is because
there is global oceans of liquid methane on the surface.
Before Cassini even got there thought we realized that that's not true.
We realized it from our increasing ability to take detailed
images of things like Titan from large telescopes on the ground.
One of the nice things about taking images from the ground is that rather than
looking in the visible wavelengths like we're looking
at here, you could look in the infrared.
The infrared, as if you've ever been in a smoggy place
like Los Angeles, you note that the sunsets are spectacularly red.
The, the light at the end of the day
is this beautiful red light coming through the smog.
And that's because the longer the wavelength of the
light, red light is a longer wavelength than blue light.
The longer the wavelength is, the more easily it passes through things like haze.
So, red light, if you're standing here in Los
Angeles on a smoggy day, and the sun is
setting, the red light comes through to you, the
blue light is scattered all around, you don't see it.
So you see beautiful red sunsets.
Same reason you see beautiful red sunsets
after volcanic eruptions or, or fires or something.
Sounds like bad things always make nice sunsets.
The haze here is so thick that even just looking at the red doesn't help very much.
But you could look in the infrared, and
you could actually see through to the surface.
Let me show you what some of these early telescopic images of Titan looked like.
This comes from a nice paper by Brown, oh, that's me, okay, a nice paper
by me, I'll say, from a decade ago in which we, I'll show you in
a minute, but we detected the very first, we, we at least took the very
first images of clouds on Titan, which is
going to be the important part of this story.
But for now, let's just look at the surface features.
We didn't know what these surface features were, these bright spots that
are on Titan, but they show up night after night after night.
They rotate with the 16-day rotation period of Titan.
So we knew that there were not global oceans of methane on Titan.
We didn't know if maybe these were islands on top
of Methane seas, or something else, but certainly not global oceans.
The point of this paper, though, is not these surface features.
But it's these, spots at the south pole.
These spots are clouds.
How do we know they're clouds?
Well, of all the surface features on Titan,
they're the only ones that change, with time.
And, more importantly, we did what astronomers always
like to do when confronted with atmospheres and things.
We used spectroscopy.
How do you use spectroscopy?
Well, we actually put a spectro slit across this spot right here, some
on the cloud, some off the cloud, and an interesting thing happens.
The amount of methane in Titan's atmosphere is so large that if you, if
you look all the way to the surface, most of your light in, if
you're looking at a wavelength where methane absorbs, and you look all the way
at the surface, all the light will be absorbed before you get to the surface.
If you look at a wavelength where methane absorbs kind of
strongly, then maybe your light will make it all the way
to the surface and some amount will make it back up
and you'll see a little bit of light coming out through here.
If you look at the same wavelength where light should make it to the surface and
come out a little bit, but there's a
cloud, imagine that there's a cloud right here instead.
What happens?
Well, that light comes through, to here, not nearly as much as absorbed,
because it doesn't have to go through the thick methane at the bottom here.
And it actually comes back up much stronger.
We use these weak methane absorption lines to tell
you if there is a, basically mirror, a reflector, the
clouds are just reflecting light, that's moving up and down
in the atmosphere, or at some location in the atmosphere.
And we can figure out that there is indeed something high in the atmosphere.
And that's what we found from these observations here.
If we looked at Titan over a long period of time, this is 2003,
2004, many different nights of observations, these
clouds are always at the South Pole.
South Pole, South Pole.
Sometimes they're not there at all.
South Pole, South Pole.
Occasionally, they get huge.
Here is a huge cloud outburst here at the South Pole.
See a couple of little clouds up through here, that's, those are nice.
And we realized early on that this is a consequence of seasons on Titan.
At the time of these observations it was South polar summer.
Let's think of what happens in South polar
summer, or Southern Hemisphere summer on the Earth.
And let's think specifically about clouds.
In southern summer on the Earth, the band of
clouds at the Equator, the tropics, shifts slightly south.
And so you have more, a more cloudy band below the horizon.
You also have circulation in the Earth's atmosphere where you have, you
have cloudy bands that may be plus and minus 30 degrees in latitude.
So you have a series of cloud bands.
This series of cloud bands is caused by air heating
up at the equator, rising, moving towards the poles, and
getting deflected by the Coriolis effect, by the spinning of
the Earth and then sinking again and coming back here.
And then you have these other cells of circulation.
This is called the Hadley Circulation on the Earth.
What you don't have on the Earth is clouds
exclusively at the South Pole in the southern summer.
Why do you have it on Titan?
You have it on Titan because Titan is rotating so slowly.
It rotates every 16 days because it is,
it is phase locked to its orbit around Saturn.
The same face of Titan always stares at Saturn, and so it's very slow to rotate.
That very slow rotation means that the Coriolis effect is
very weak, which means that you don't have these big overturnings.
So in fact, what you have on Titan in the summertime is essentially rising
motions here, going all the way up to the North Pole and falling in through here.
Those rising motions are like, I like to
think of it as big thunderstorms in the desert.
You have the ground heating up because it's the summer time.
You have air rising, and that air is moist.
In this case it's moist with methane.
And as that air rises you suddenly get
these big clouds that start to condense out.
Have clouds of methane on Titan and
along with those clouds, you presumably have rain.
Titan will spend long periods of time with rain mostly happening at the South Pole.
Pole.
In the Northern summer, which is what it is now, Titan will spend
long periods of time with the rain mostly happening in the North Pole.
Rain does occur in these regions through here, at least clouds do occur.
But not nearly as much the rain.
Because of the low rotation the rain is concentrated at the North and the South.
Does it really rain on Titan?
Yes.
Let me show you some pictures that
I think pretty conclusively demonstrate that it's true.
Here are images now from the Cassini spacecraft.
Cassini got there, found the same sets of cloud patterns at the South Pole,
but could also fly right over the South Pole and see what was there.
And what was there?
Well, look at this.
There is a kidney bean shaped lake.
How do we know it's a lake?
Well, there are now more detailed observations.
Where this is an incredibly flat surface, you can
see it the the topography coming right down to
the shore, and you can see that whatever the
liquid is inside here is really dark, like methane.
There's a picture of the same region from
a year earlier, and it's a little bit hazier.
It's a different viewing geometry, where it, it was looking more oblique.
So you can't quite see the lake as much, but the lake is still the same.
What you don't see are any dark regions in through here.
And suddenly, here, you have a abundance of little dark regions in through there.
This is right after, between these two observations,
there was another of those large South Pole clouds.
Presumably rained methane like crazy onto these regions.
The cloud dissipated, and let us see that there's been rain through there.
There are a couple other sorts of
observations that show the same sorts of things.
So I think it's it's pretty convincing that methane rain occurs on Titan.
Well, I said it occurs mostly at the South Pole and the North Pole.
And this is a really not very big, kidney-bean sized lake.
And that's, sort of, all there is at the South Pole.
I was very excited the, the South Pole was seen early on
because it was in the sunlight, because that's where the fly-bys were.
I was very excited to see what was at the North Pole.
Kept on, sort of, holding my breath, waiting for the data.
And when the data came in, I think I fell out of my chair.
Here is now a radar image of the North Pole.
The North Pole of Titan sits right there.
And radar works really well.
The radar goes right through the haze.
Doesn't care about the haze at all, and it gets really detailed images.
And look at these things.
This is crazy.
These are lakes.
Clearly, you see channels that go into the lakes.
You see lake, lake like structures on the edges everywhere.
You see big lakes here.
You see small pockets of lakes here.
If you look really carefully you see things that probably
were lakes, that have dried up, that are not lakes anymore.
You see lakes that are probably drying up.
It is sort of an amazing place, there is
no place like it anywhere in the solar system.
These are methane, and also mixed with ethane, it turns out.
Methane, ethane lakes dominating the Morth Pole of Titan.
It's the only other place in the solar
system with abundant surface liquids of any sort.
And those are on Titan, and they
are abundant methane and ethane on the surface.
There was a great mission that was proposed to go, to land on one of these
lakes, and sit with a boat on the lake for a while, learn stuff about the lake.
The mission didn't get selected by NASA and
now it's too late, because by the time it
got there all of this would be in the dark and you couldn't see what was there.
You could try going to the South Pole, which is going to be in the, in the light.
But the South Pole only has that one kind of small lake.
That lake to scale is maybe something like that.
So we've kind of missed the opportunity in our lifetime
to learn a lot more about these crazy, crazy lakes.
The fact that these lakes are here, though, makes it
really intriguing to ask a question about life on Titan.
Now we're not talking about water-based life.
We've talked about why water is such a good thing for life to
have, it can transport materials, it's polar, things dissolve in it very nicely.
There is no water here, or there is water here,
much of this rock-looking stuff is water but it's frozen water.
It's ice, ice is essentially a rock at the low, low temperatures of Titan.
But there is abundant liquid, abundant liquid methane and ethane.
Methane is CH4.
And if I just drew you the structure of it, I would draw,
I can't draw very well, but this is a little pyramid down here.
Its a symmetric structure.
There's the C in the middle, H, H, H, H.
Remember, water is polar because of its geometry, because
it's shaped like that, so the charges are polarized.
Methane is not a polar molecule.
Its a lot harder to think of how you would have methane causing biological reactions.
Nonetheless because people love this sort of thing,
there has been a lot of speculation about that.
And we'll talk about it next.
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