Introduction to the Solar System

Please pick up handouts because they are necessary reading for the Homework Sets and for the Exam

1. 3 "Impacts on the Earth by Asteroids and Comets: Assessing the Hazard"
   Read abstract, intro, and the sections from 'Perception of the Hazard' onward
   
2. 4 "Does Titan Have Oceans?"
   HW 6 is due by friday
   HW 5 must be no later than tomorrow, or it can't count be counted
   Last HW to be handed out is this Friday
   I must have all homework by the day of the last lecture, Tue July 2, or I won't be able to count it in your grade, so please get them in!
   

SL When you look at Saturn with a telescope, you can easily see only one moon
It's as far from Saturn is as Ganymede is from Jupiter, and goes around in 16 days
Huygens first saw it in a telescope in 1655, 45 years after Galileo's observations
It appears as a yellowish-orange dot, and it is so small that we can't see anything on it
Modern planetary astronomy began in the 1940's with a man named Gerard Kuiper.
He observed the spectrum of reflected light off many solar system objects
In 1944, he detected methane, CH4, in Titan's atmosphere, by its characteristic absorption lines

SL Almost the size of the moon
Second largest moon in the solar system
Bigger than Mercury or Pluto
16 days round Saturn
Highly eccentric orbit
Density implies half rock, half ice
Atmosphere in N2 mixed with some CH4 and H2, CH4 clouds and hydrocarbon haze (Cs and Hs)

SL That's all we really knew about Titan until Voyager 1 went to Saturn in 1980
Voyager took pictures and analyzed the infrared and UV spectra of the atmosphere
It also used its radio, the one it communicated with Earth, in a clever way to find out how thick the atmosphere was (shining through it)
But what it saw was an almost featureless orange ball. It seemed to be darker in one hemisphere than another
SL A sure way to tell an atmosphere is to look at it back lit -- light scatters and diffuses in the atmosphere
Titan appeared covered in haze and clouds, and it had an atmosphere 1.5 times the pressure of the Earth's
SL The haze seems to be made of hydrocarbons -- larger molecules of C and H formed by the action of sunlight on the methane
So it goes: 2CH4 + light == C2H6 + H2 methane + light gives ethane + hydrogen
You also get C2H2 acetylene
Most of the atmosphere seems to be N2 like the Earth, with CH4 and H2 mixed in
Because there is N2 in the atmosphere, it reacts with hydrocarbons to give HCN
Recall that, this, along with hydrocarbons, gives us the life-precursor molecules, HCN for amino acids, hydrocarbons for sugars
This is an indication that there may be organic chemical reactions going on on the surface of Titan much like what might have happened on the pre-biotic Earth
But because Titan is so far from the Sun, the surface is -300 F
The gases in the atmosphere are very, very cold
But the really amazing thing is that if you take all the Voyager data on Titan together, it suggests that Titan has liquid oceans!
This is a direct result of it being so cold
SL The haze layer breaks into two layers, complex (many atom) hydrocarbons up high, 175 km, and ethane-acetylene down lower

The reason is because it gets cold enough for methane to condense into clouds in the lower part of Titan's atmosphere
Further more, the N2, CH4 and H2 would cause a greenhouse effect, resulting in a temperature profile in Titan's atmosphere, similar to the Earth's
On the Earth, it is water vapor (over oceans) and CO2 that cause the greenhouse effect
SL Earth's atmosphere:
Temperature vs. Height
Troposphere (0 km, 288 K) clouds over oceans
tropopause (10 km, 220 K) water vapor stops
stratosphere ozone forms due to sunlight
stratopause (50 km, 300 K)
mesosphere
mesopause (80km, 200 K)
thermosphere (1000 K)
Titan's atmosphere:
Temperature vs, Height
Troposphere (0 km, 95 K) clouds over oceans
Tropopause (50 km, 82 K)
Stratosphere C2H6 and C2H2 form due to sunlight, make haze
Stratopause (200 km, 160 K)
Thermosphere (160 K)
Methane is like water on Earth. From the temperature of Titan's surface and atmosphere, we know that methane can exist as a liquid, on the ground, clouds and vapor in the atmosphere
It even has about the right humidity to be consistent with liquid reservoirs
OH Clouds at the tropopause, temperature decreasing
Ethane-acetylene in stratosphere, temperature increasing
Thick haze above, driven by sunlight UV

Having a planet with a substance that exists simultaneously in two phases, liquid and vapor is what gives us weather.
For both Earth and Titan, the Sun is the driving force that circulates the vapor from hotter to cooler places, creating winds, clouds and rain -- a climate
Titan is the only place other than Earth that has this property
Mars may have had one a long time ago, but it is clear that this is not happening now
But just because it is the right temperature for methane oceans, does that mean they really exist?
We have a theory of why there must be oceans or large lakes -- bodies of standing water, on Titan
OH Methane gets destroyed by the action of sunlight into ethane and acetylene
Ethane and acetylene are heavy, and they fall out as rain
H2 is created, but because it is a light molecule, it escapes Titan's gravity, into space
So the methane in the atmosphere starts to go away, as it turns into a kind of icy hydrocarbon gunk on the surface (heavier hydrocarbons also get made)
But by this process, we find that the available methane in Titan's atmosphere would only last 10 my
Unless somehow it got a whole lot of methane very recently (very unlikely), something must be replenishing it
Oceans or lakes
OH The cycle: seas, CH4 into atmosphere, photolysis, haze layer, rain, methane clouds
The same analysis shows that over the age of Titan (4.5 B years, same as all the other planets), it would have collected a layer of icy hydrocarbon gunk about 1 km deep, sitting around on the dry land, or sinking to the bottom of the oceans or lakes
So people made theoretical models of how the lakes, surface, and atmosphere interacted -- playing with a fantasy world
The N2 would dissolve in the oceans, and the ethane produced high up in the atmosphere that comes down as rain would also be a part of the ocean
The oceans would be between 1/2 and 10 km deep. In the higher ground, large impact basins would contain the seas
OH But there's a problem with this idea: Titan's eccentricity is high
If Titan had oceans, the tides, which act like friction, would have made Titan's orbit circular
So it either has very deep oceans all over it, or lakes, which wouldn't cause so much tidal friction
SL And then came Hubble
Hubble was able to look through the clouds using infrared light, and make crude maps of the surface
It looks like the surface is really heterogeneous
It looks like the surface has mountains of hard water ice mixed with a dark, hydrocarbon material
Possibility that there is a landscape that contains some isolated seas
OH: Seas, CH4 into atmosphere, photolysis, haze layer, rain, methane clouds

OH Some people have suggested that Titan's crust of water and rock contains huge reservoirs of methane magma, and that they spew liquid in methane volcanoes.
Instead of oceans, there could be methane volcanoes feeding the atmosphere
Also, there could be ammonia and water mixtures that act just like hot lava on the Earth
OH Another possibility is that there could be layers of oceans beneath Titan's solid rock-ice crust
Beneath the crust would be a methane-water mixture ocean
This would feed the atmosphere with methane through geysers, and the water would solidify like lava into ice
Below that would be an ammonia-water ice mixture ocean
And below that, the rock core
Thick hydrocarbon soup on the surface, and N2 hydrocarbon atmosphere
The bulk density of Titan shows us that it is about equal parts rock and ice, with some hydrocarbons
About the same as Callisto, the furthest out and lightest Galilean moon of Jupiter

NASA's last great mission to the outer planets, for the foreseeable future
It is the child of Voyager, the sibling of Galileo to Jupiter
First we fly by the planets, early on, (M2 to Venus, M10 to Mercury), Voyager to Jupiter, Saturn, Uranus, Neptune,and then we send orbiters, Viking to Mars, Pioneer Venus, Galileo to Jupiter), and then we send landers, (Viking to Mars, Galileo Probe to Jupiter)
Cassini is like Galileo -- it will travel all the way to Saturn (7 years), and orbit Saturn for several years
Vastly more intense exploration, continuously for several years, than just flying by it
Cassini is to launch next year (1997) , and get to Saturn in 2007
It will fly by Titan every orbit around Saturn, and map the surface with radar
Even more incredibly, it will drop a probe onto Titan that will land there
SL: Huygens probe, built by the ESA.
It will sample the atmosphere as it goes down, measuring temperature and pressure
It will start to take panoramic pictures of the surrounding clouds and landscape when it gets into the troposphere
As it approaches the ground, it will take spectra, so that we can distinguish what the surface is made of
It is designed to survive a landing, and to survive and float in an ocean
If it survives, it will give us pictures of the surface of Titan!

Titan gives us a speculative glimpse about the way a complex climate system, driven by phase changes and chemical reactions, can create a climate
Here we've talked a little about the various cycles of a possible climate, and it's stability
One of the reasons it's neat to study planets is that it gives us a whole new perspective on our own.
For example, it was by studying the sulfuric acid clouds of Venus that we discovered things about acid rain and how the ozone hole is being destroyed
Mars gives us an example of a planet that has no ozone layer. It's surface is fried by UV, which is why we see no organic compounds, and probably why the soil was so chemically reactive
Venus is an example of a runaway greenhouse effect, what happens when all the CO2 in the planet winds up in the atmosphere
For all the sophistication of our theoretical models, we are often limited by our imaginations, because the possibilities are so vast.
Venus gives us a way to imagine one particular way that a climate might evolve into a runaway greenhouse
Titan may tell us about the kinds of organic chemical reactions that may have occurred on the pre-life Earth
So these things all give us a perspective about how planets work, and how things on the Earth might behave
An extremely important part of climate, from our perspective, is its stability
What factors maintain the Earth's climate as favorable for life?
Liquid oceans have existed on Earth for at least 3.5 By
Right now, we are pumping CO2 into the atmosphere at an enormous rate -- it has doubled in the last couple hundred years.
What will happen to this huge machine of water, atmosphere, CO2, and ice?

I've mentioned that the early Earth had a thick CO2 atmosphere, sitting on top of hot oceans
The CO2 dissolved in the oceans, and then reacted with rocks at the bottom of the ocean
This created vast amounts of limestone, where the CO2 on the Earth is stored
Venus lost its water before this could happen, and the CO2 stayed in its atmosphere
Earth's atmosphere is currently .03 % CO2. This has to be maintained to keep the climate stable, and make it safe to live in
What keeps it this way?
It has to do with water, our universal solvent. Water does all the moving of stuff around, just like methane does on Titan
When it rains on the continents, CO2 in the rain water reacts with the surface rocks.
Carbonate is formed, and taken out of the atmosphere
The carbonate dissolves in the water and gets transported to the oceans via rivers.
The carbonate gets turned into calciferous shells by plankton and other animals, although this process can also occur without life
The carbonate precipitates out and lines the bottom of the oceans
The oceans subduct under continental plates
The plates heat up, and the ocean plates melt
The carbonate breaks up into carbon dioxide, and gets spewed out in volcanoes
This is the carbonate-silicate cycle
Now think about what happens if, for some reason, there is more volcanism
More carbon dioxide gets put into the atmosphere
Or it could be because of burning fossil fuels
The greenhouse effect is enhanced, the Earth heats up
But when it heats up, more water goes into the air
More rainfall over the continents
The higher temperatures and greater rainfall speed up the reaction of rainwater with its dissolved CO2, and the rocks
CO2 gets taken out of the atmosphere faster, and the level of CO2 in the atmosphere drops
This is an example of a climate feedback effect
The problem is it takes 10,000 - 100,000 yrs to complete a cycle, because plate tectonics is slow
So the Earth can still heat up a little
The Earth itself is used to variations in CO2, due to variable rate of volcanism or plate tectonics, or more or less land area
Burning the fossil fuels will only have a minor effect on the overall picture of Earth's climate stability
What's tricky is that humans have only inhabited the Earth for a million years, and it is only in the past hundred that it has grown to thickly cover the planet and begin to increase the greenhouse effect
So much of the human population has settled on the coasts, while simultaneously raising the temperature
A very small increase in temperature will probably melt the polar ice caps, raising the sea level a lot
Although a small perturbation to the greenhouse effect and to the Earth as a whole, there will be massive changes to the humans that live here

6/27/96