There is still room in the course, if you'd like to add it. 

The book Cosmic Perspective is recommended, not required.  The course
is defined by the lectures; the reading is supplemental.  

There is a reading which can be found on e-reserves on the web. 

The focus of the first third of the course will be the search for life
in the universe, with a focus on the solar system.  We will also
discuss the history of the development of modern scientific thought in
the astronomical context. 

  A remarkable fact: Life can exist on the surface of the Earth.  This
is tied to the fact that liquid (as well as solid and gaseous) water can
exist on the surface, because of the temperature and pressure at the
Earth's surface.  This is not true (as far as we know) for any other
planet in our solar system.

  "An atmosphere of pressure" is the pressure of the Earth's
  atmosphere at sea level.  Water is liquid between 0 and 100 C.  But at
  higher pressure, water can remain liquid at higher temperatures. 

  On Mars the pressure is so low (and the temperature so cold), that
  liquid water quickly evaporates or freezes. Triple point of water --
  temperature and pressure where all three phases of water (gas,
  liquid, ice) can coexist.

  The temperature of the Earth is set by the heating from the Sun,
  plus the greenhouse effect due to its atmosphere.  

  We know that water has been liquid on Earth for the last 3.8 billion
  years.  

  Life needs liquid water (at least to grow and reproduce); indeed,
most living creatures are ~90% liquid water.

  Most of the living mass (as measured by carbon) on the surface of
the Earth is locked up in trees; about 10^{18} grams of carbon.
That's a very rough estimate, "order of magnitude".

  There is also ~10^{18} grams of carbon in micro-organisms in the
  Earth's oceans. 

  But there is a roughly equal amount of mass in micro-organisms,
mostly bacteria, much of it down in the interior of the Earth, several
kilometers beneath the surface.  We'll see that some of that life
lives off heat from the Earth's interior, and doesn't depend on
heating from the Sun.  Could life have *originated* there under the
surface?  If so, life might exist underneath the surface in other
planets.

  After water, the molecules at the basis of all life processes are
  carbon-based.  Could one imagine life based on Silicon instead,
  which has somewhat similar chemistry as carbon?  Turns out the
  chemistry isn't that close: while carbon can form a whole series of
  bonds and make long chain molecules, silicon isn't nearly as
  versatile.  

  There are lots of different carbon-based molecules found in the thin
gas (the "interstellar medium") between the stars; far fewer types of
silicon molecules are known.

  All known species on earth have similar biochemical structure: they
all use DNA as a storehouse of genetic information.  They use proteins
to do all the work of the body, and RNA molecules actually make the
proteins.  And there are many kinds of proteins, but they in turn are
all made from the same set of 20 amino acids.  So life on Earth
probably all came from a common ancestor: there is really only one
kind of life on Earth.  Is this the only mechanism by which life could
possibly exist? We don't know...

  The modern tree of life, showing how different types of creatures
  are related to one another (based on similarities in their DNA),
  show that the vast majority of species are microscopic (we probably
  have discovered only ~1% of all species of microbes).  There are three
  domains: 
     -Bacteria
     -Eucarya (includes animals and plants, and lots of microscopic
       creatures as well). 
     -Archaea, the most ancient type of micro-organism.  

  Animals gain their energy by eating; plants survive by
photosynthesis.  But most of the rest of the creatures on the tree of
life use very different mechanisms.

  DNA encodes its information in the order of its nucleic acids, G, T,
  C, and A.  

  So if we're going to talk about looking for life elsewhere, we
  should have a definition of life.  Different approaches: 
    -Biochemical: Does it have DNA and proteins?  Not a good approach;
      we want to be open to alternative biochemistry. 
    -Metabolic: ingests energy to do things.  So does fire count as
      "life"?  No, so this isn't so good...
    -Thermodynamic: Something that creates structure/order.  So does a
        rock crystal count as alive?  No...
    -Darwinian: Does it undergo evolution?  But this would seem to
      exclude mules (which can't reproduce). 

No definition really works well.  So why is it hard to define life?
Read the paper on Blackboard e-reserves by Cleland and Chyba on this
subject.  It is analogous to trying to define "water" before you know
about atoms and molecules.  We now define water to be H2O; until you
have a scientific theory (in this case, of atoms and molecules), you
can't really understand water.  We don't have a general theory of
life, so we can't make an unambiguous definition of life.  We need
some examples of life of different types: by synthesizing life in the
lab, or discovering it on other planets.

  Looking for life elsewhere: the science of astrobiology.  

  How about the other planets in the solar system?  Mars is a great
  place to look; we're sending lots of spacecraft there.  

  It is hostile there: no liquid water on the surface (although there
  is frozen water on the surface, especially at the poles).  There
  probably is liquid water underneath the surface, where it is
  warmer.  

  Europa, one of Jupiter's moons, is completely covered by water ice;
  it probably has liquid water underneath the ice.  Might life exist
  there?  

  Microorganisms are remarkably resilient to effects of space. Even if
  a small fraction survives exposure while hitching a ride on our
  spacecraft, that can contaminate planet's life. We have to be
  careful about sterilizing the space missions. The same process could
  also be at work in spreading life between planets.

  When a meteor hits Mars, debris is thrown off and can escape Mars'
 gravity to eventually fall onto the Earth.  If it carries some
 micro-organisms with them, and if they could survive the trip, life
 on Earth could have come from Mars.  So it is possible that if we
 find life on Mars, it will share an origin with life on Earth.

© Copyright 2009 Christopher Chyba, Michael A. Straus,s and Anatoly Spitkovsky