QUESTION:
What sorts of observations tell you about the structure of a planet using the 
depth of the atmosphere?

ANSWER from Peter Thomas on April 7, 2000:
Atmospheric "sounding" can be done several ways:
    1) as a probe descends..Viking landers, to lesser extent Pathfinder.

    2) Detection of presure-related spectral phenomena..can get
temperature as a function of pressure..can integrate to get height.

    3) effects on spacecraft radio signal as it passes behind a planet
    with an atmosphere..slicing down through the atmosphere.

    4) laser altimeters can detect cloud heights directly (MOLA on
Mars global surveyor).