Re: how is determined the gases composition of the terrestrial planets?

White light is composed of all possible colors of light, each one having a
unique energy.  This means that blue light has more energy than red light
and less than ultraviolet light.  Light consists of tiny "packets" of 
energy called photons and the energy of these photons is what gives light its
colors.

Each element has a unique atomic and electron structure.  Each electron has
a particular energy associated with it.  This is similar to throwing balls
into the air - the higher you throw a ball the more energy you have to put
into it.  With an atom, the "heights" of the electrons are fixed at 
specific intervals, so the energy associated with each electron is discrete and
fixed.

When you shine a white light through a gas, you will have some energy
absorbed by the gas.  Using a prism or diffraction grating to look at all
the different colors (called the spectrum) in the light will show dark 
bands where photons of specific energies were absorbed by the gas.  These 
energies correspond exactly with the energies of electrons in the elements 
that make up the gas.  By looking at these spectra, we can positively identify 
all of the gases that are present because each element making up the 
atmosphere has a unique signature (or fingerprint).

To study a planet's atmosphere, we can wait until a star or the sun shines
through the planet's atmosphere to look at the dark absorption lines, using
them to identify the gases that are present.  Similar techniques are used 
to determine what gases exist in interstellar space, in the space between
galaxies, or in the atmospheres of individual stars.  A similar phenomenon
is also used in some lights because each element also gives off photons of
specific energies.  So we have the yellow of sodium lights, the red of neon
lights, and so forth.

A good place to read more about this is in any high school or college
physics textbook that talks about "modern" physics, especially quantum
physics.  I can specifically recommend the text by Halliday and Resnick 
(one that I studied from).  In addition, any introductory astronomy 
textbook will talk about this in more detail.  Here, a good one to read is by 
Zelik and Smith and is called "Introductory Astronomy and Astrophysics".  
Finally, I can recommend a book by Cesare Emiliani called "Planet Earth" 
to give further information on this and many more topics.



P. Andrew Karam, CHP
Radiation Safety Officer
University of Rochester