QUESTION:
How do you measure the atmospheric water content of Mars from the Earth?

ANSWER from Bruce Jakosky on December 28, 1997:
Water vapor absorbs certain wavelengths of light as the light passes through
it, and leaves other wavelengths alone.  By lookin at sunlight that has
passed through the Martian atmosphere, reflected off of the surface, and
then made it to Earth, we see light that has travelled twice through the
Martian atmosphere.  By looking at how much of the light has been absorbed
at those wavelengths were water absorbs light, we can estimate the amount of
water present as a gas in the Martian atmosphere.

The biggest difficulty in doing this is that we are looking at Mars using
telescopes on the Earth, and the Earth's atmosphere also has water vapor
that can absorb at these wavelengths.  In fact, the Earth atmosphere has so
much more water vapor that the absorption would overwhelm that of the
Martian water vapor.  In order to get around this, observers look at the
reflected sunlight at those times when Mars appears to be moving either
toward or away from the Earth (in a relative sense).  Because of the
relative motion, the wavelengths of the light reflected from Mars have been
shifted by a small amount--this is the so-called "Doppler" shift in the
wavelengths.  Because of the small shift, the Martian water vapor appears to
have absorbed light at a slightly different wavelength than the Earth water
vapor, and the two effects can be separated.