The regions around the landers resemble rocky deserts on the Earth. The Pathfinder lander and the Sojourner rover explored Ares Vallis for about three months in 1997. Pathfinder images showed abundant evidence of ancient floods while Sojourner measured the chemical composition of rocks and soil. The atmosphere of Mars consists primarily of carbon dioxide. The atmosphere is too thin to insulate the surface, where daily temperature fluctuations are as much as 60 K. Atmospheric pressure is lowest in the summer and winter when carbon dioxide is trapped in one of the polar caps.
Pressure is highest in the fall and spring when carbon dioxide is released into the atmosphere.
Frequent dust storms keep dust suspended in Mars's atmosphere at all times. Great dust storms sometimes spread a thick layer of dust over the entire planet. The seasonal changes in dark markings are caused by deposition and removal of dust by seasonal winds. Large dust particles are skipped along the surface by winds and are very effective at eroding the Martian surface. Although Mars contains a smaller percentage of iron than the Earth does, it has a core made of iron or iron mixed with sulfur.