The composition of Mars' atmosphere results in a very unfamiliar seasonal effect. Carbon Dioxide, which makes up 99% of the air on Mars, turns to solid, or "dry ice", when it freezes at 148 degrees Kelvin (-193 F). When these temperatures are reached in the atmosphere or on the surface during winter, the atmosphere itself begins to freeze onto the ground. Each winter, a seasonal polar cap of carbon dioxide is deposited at the pole (see the figure below). In addition, a permanent polar cap of carbon dioxide exists at the south pole, its surface temperature never exceeding 148 Kelvin. If long-term climate change occurs on Mars, this deposit would grow or diminish, adding or removing some volume of Mars' atmosphere. An even larger deposit of carbon dioxide may be stored in Mars' porous soil, or regolith, by a chemical bonding process called adsorption. MVACS will measure the amounts of carbon dioxide in the atmosphere, adsorbed in the subsurface, and trapped in carbon dioxide-bearing minerals.