This is Prof. Chyba's last lecture. 

  Last time, we calculated the equilibrium temperature of a planet,
  ignoring the greenhouse effect.  Without the greenhouse effect, the
  Earth would be completely frozen.  

  Define S, the brightness of sunlight on the Earth's surface, is 
   S = L_s/(4 pi d^2)
where L_s is the luminosity of the Sun and d is the distance to the
Sun. 

  We found that the equilibrium temperature of the Earth is:
  
  T_E = [(1-A) L_s/(16 pi sigma d^2)]^{1/4] = T_s (1-A)^{1/4} (R_s/2 d)^{1/2}

Here,
  T_s and R_s are the Sun's surface temperatre and radius
  A is the albedo of the Earth
  sigma is the Stefan-Boltzmann constant. 

  Consider the atmosphere as a single layer, that absorbs infrared
radiation, but is transparent to visible light.  Essentially all the
radiation from the ground is infrared (as you can check using Wein's
law).  So the atmosphere warms up, as it is absorbing the radiation
from the ground.  So the atmosphere also radiates, in two directions:
up and down; call this atmospherical power per unit area G.

  We carry out power balance again, both at the ground and in the
  atmosphere.  

At ground: 
  S(1-A)* pi R_Earth^2  + G 4 pi R_Earth^2 = 4 pi R_Earth^2 sigma T_1^4

(Here, S = L/4 pi d^2 is the power of the Sunlight received on Earth
per unit area).  
At atmosphere:
4 pi R_Earth^2 sigma T_1^4 = 2 G * 4 pi R_Earth^2

When you solve these equations, you find 
T_E = (2 S/4 sigma)^{1/4} = (2 (1-A) L_star/(4 pi d^2 sigma))^{1/4}
i.e.,  2^1/4 ~ 1.2 times higher than the value we found when we
ignored the greenhouse effect.  Plugging in the numbers for the Earth,
we find 288 K, comfortably above the freezing point of water (and
close to the observed value), and explains why we don't have frozen oceans.   

  But our understanding of the evolution of stars tells us that the
Sun was substantially fainter billions of years ago than it is today.
On the other hand, we know (from the geological record) that the
oceans have been liquid throughout.  Does this mean the greenhouse
effect was stronger in the past?  The answer is yes.  Why is this?  To
answer this, we need first to talk about plate tectonics on Earth.
The plates of the Earth's crust ride on the mantle, which move very
slowly on timescales of millions of years.  Where two plates collide,
one plate dives under the other (a process called subduction), causing
earthquakes and volcanoes (which indeed are concentrated on the plate
boundaries).

There is a negative feedback that keeps the mean temperature on the
Earth roughly constant on long timescales: Carbon dioxide is one of
the principal greenhouse gases.  It get washed out of the atmosphere
by rain.  Dissolved into water, it combines with calcium (largely via
organisms making shells, but also by some non-biological processes) to
make a solid: calcium carbonate (CaCO_3), which falls to the ocean
bottom.  Eventually, plate tectonics drags this underground via
subduction, underneath the Earth's crust.  It is hot down there; the
calcium carbonate releases the CO_2, which comes back to the
atmosphere via volcanoes.  A cycle that takes 100-200 million years.
If it gets colder in the atmosphere, there is less evaporation, so
less rain, and the CO_2 doesn't get taken out of the atmosphere as
much.  Meanwhile, the volcanoes keep on going, giving more greenhouse
gases, reheating the Earth.  If it gets too hot, the oceans will
evaporate faster, giving more rain, so more CO_2 is taken out of the
atmosphere, decreasing the greenhouse effect, and the planet cools
down. 

  Again, the timescale for setting this equilibrium is 100 million
  years or longer.  This is not a panacea to global warming problems
  caused by human industrialization.  

  On the Earth, most of the CO_2 is bound up in rocks.  In Venus, all
  the CO_2 is in the atmosphere; there is an extreme greenhouse
  effect, and the surface is very hot.   There is essentially no water
  vapor (and therefore no rain); it is thought that ultraviolet
  radiation from the Sun is intense enough at Venus to dissociate all
  H_2 O (i.e., split it up into H_2 and O); the hydrogen escaped, and
  that's the end of the water.  

  This feedback effect doesn't work on Mars, as it is currently
  geologically dead: there is no plate tectonics or volcanism today,
  and thus the CO_2 cycle cannot be sustained.  Indeed, Mars has only
  a very thin atmosphere, and has only a very weak greenhouse effect;
  it is very cold there! 

Three ways to look for extraterrestrial life: 
   Approach 1: Go to distant bodies with spacecraft, and look directly, and
    perhaps bring samples back to Earth.  We can do this in our own
    solar system, but no further.   

  Where might we look?  Mars and Jupiter's moon Europa. 

  The Viking missions to Mars found no evidence of organic molecules
  in the soil.  But there is abundant evidence for liquid water on the
  surface in the distant past, and perhaps, occasionally and briefly,
  in the present as well.  There is frozen ice on the surface of Mars;
  might there be liquid water well underground?  

  The next best place to look for liquid water in the solar system is
the Jovian moon Europa.  It is covered by ice which probably overlays
an ocean of liquid water.  One sees what appears to be refrozen ice
floes, and refrozen "puddles" where meteors have struck.  The magnetic
fields of Europa suggest a salty liquid ocean.

  What keeps the ocean liquid?  It's way cold out there, 90 K.  But
Europa is getting stretched by the tidal pull from Jupiter, which is
heating it and melting the ice.  Just like tides on the Earth from the
Moon: water on the nearer side of the Earth is pulled a bit more
strongly by gravity from the Moon than is the center of the Earth.
This causes the water to "bulge out" a little bit, which gives rise to
the ocean tides.  Same thing happens to the *rock* of the Earth
(although it distorts quite a bit less than does the water).  The
constant stretching of the tides in Europa heats things up.    This
heat is enough to keep an ocean of liquid water (probably 100 km deep)
under all that ice.   

Approach 2: Identify planets around other stars, and analyze the atmospheric
    chemistry of these planets remotely. 

  OK, how can we find those planets?
    If the orbit is aligned appropriately relative to our line of
    sight, the planet would periodically pass in front of the star as
    seen by us, and thus briefly block some of the light of the star,
    making it appear slightly fainter for a brief period.  The effect
    of a planet the size of the Earth is subtle, but detectable with
    a satellite that is to launch in just a few weeks, called Kepler.

    -What is the signature of life on a distant planet?  Think of the
    Earth.  The atmosphere is full of oxygen (produced by
    photosynthesis), and also has a fair amount of methane (CH_4),
    also created by life.  But methane and oxygen, left alone, will
    combine to make H_2O and CO_2; the fact that we see both of these
    molecules in large concentrations are a *consequence* of life.  

   Would a similar detection of oxygen and methane in another planet
   *prove* there's life?  There is actually a fair amount of methane
   in Mars' atmosphere.  But there are other, ordinary, non-biological
   chemical ways to make this methane.  So the detection of oxygen and
   methane in the atmosphere alone would not be definitive proof of life.  

A third way to find life: look for signs for intelligence, e.g., by
radio waves sent out by a distant civilization.  SETI: Search for
ExtraTerrestrial Intelligence.

  We can look for radio signals from other civilizations.  It is a
tricky business to filter out all known artificial sources of radio
emission (e.g., from Earth satellites), and natural sources as well.

 The SETI Institute is building a dedicated telescope array (the Allen
Telescope Array) to carry out this search, in a much more thorough way
than surveys done, e.g., at Arecibo.  But it still will survey only a
tiny fraction of our Galaxy.

  Enrico Fermi famously asked the question, "Where are they?".  That
is, if the Galaxy is full of extraterrestrial civilizations, happily
colonizing one star after another, why aren't they here, visiting us?
This becomes a puzzle under some assumptions about the tendency of
civilizations to want to explore, and that a given civilization can expand
exponentially without competition from other civilizations.   

  With 10^22 stars in the universe, how could life on Earth be unique?
We can't answer that, until we understand better how life on Earth got
started, or how common it is that primitive life evolves to a
technological civilization.  The Drake Equation tries to estimate the
number of technological civilizations in the Milky Way.  It depends on
the rate at which new stars are forming, the fraction of these with
planets, the fraction of planets in the habitable zones, the fraction
of these on which life evolves, the fraction of those which evolve to
intelligence, and the typical lifetime of a technological
civilization.  There is enormous uncertainty in our understanding of
every term of this equation!

  The Contingency argument states that an extremely specific and
unlikely sequence of events was needed for humans to evolve.  So
intelligence is a very rare outcome of life.

  But evolution can find many many ways to create a given ability (for
example, the wing has evolved multiple times).  The existence of
intelligence in both dolphins and humans suggests that it is not only
a specific, single path that leads to intelligence.

  Look at the evolution of intelligence on Earth.  Dolphins are
perhaps the most intelligent non-humans on Earth.  One measurement of
intelligence is the ratio of the brain mass to the body mass (the
"encephalization quotient"); some dolphins are higher on this scale
than was Homo Habilis, a tool-using ancestor of humans.

  Interestingly, however, there is no evidence that the *average*
whale/dolphin species has increased in intelligence over the last 35
million years.  So there doesn't seem to be an evolutionary push for
greater intelligence, at least among cetaceans.

  Our ability to synthesize/sequence DNA is increasing incredibly
rapidly, bringing up all sorts of dangers for humankind.  And of
course we have enormous numbers of nuclear weapons.  This may turn out
to determine a limitation of our lifetime as a species.

  That is the last of Prof. Chyba's lectures; Prof. Spitkovsky will take
  over on Tuesday.  

  

© Copyright 2009 Christopher Chyba and Michael A. Strauss