In addition, Mars' atmosphere mixes easily, with changes in one part of the planet affecting other areas. In addition to monitoring methane, the SAM instrument also monitored the presence of other gases that were better understood, including nitrogen, argon, and carbon dioxide. Nitrogen and argon also tend to vary seasonally on Mars, getting pushed around when as each pole experiences winter. At that point, carbon dioxide in the atmosphere freezes, falling onto the polar ice caps. "Up to a third of the mass in the atmosphere just freezes out. If that happened on Earth, we would really notice," Trainer says. "When that happens the rest of the gases in the atmosphere get pushed around with the CO2."