Measuring Seasonal Changes on Mars----------------Like Earth, Mars has an atmosphere and polar caps, but unlike Earth, the atmosphere of Mars is composed primarily of carbon dioxide. Also like Earth, Mars undergoes seasons. Over the course of a Mars year about a quarter of the Martian atmosphere freezes out as dry ice frost or snow on the surface in the winter hemisphere, and subsequently sublimes back into the atmosphere when spring arrives. These seasonal changes cause a redistribution of the mass of Mars that is manifest as tiny changes in the planet's gravity field. Measuring these variations requires radio tracking data from an orbiting spacecraft that must be inverted to produce a gravity field. The analysis also requires a mathematical formalism to isolate the masses of various parts of the planet. This presentation will describe how this is done.