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Why does Mars have seasons?

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Mars
Mars An especially serene view of Mars (Tharsis side), a composite of images taken by the Mars Global Surveyor spacecraft in April 1999. The northern polar cap and encircling dark dune field of Vastitas Borealis are visible at the top of the globe. White water-ice clouds surround the most prominent volcanic peaks, including Olympus Mons near the western limb, Alba Patera to its northeast, and the line of Tharsis volcanoes to the southeast. East of the Tharsis rise can be seen the enormous near-equatorial gash that marks the canyon system Valles Marineris.

Like Earth, Mars experiences seasons primarily because its axis of rotation is inclined to its orbital plane by approximately 25°.1

While this axial tilt is the fundamental cause of the seasonal cycle, the intensity and duration of these seasons are further influenced by the planet's relatively elongated orbit around the Sun. Because the orbit is elliptical, the distance between Mars and the Sun varies significantly, which causes southern summers to be shorter and warmer than those in the north.1

Additionally, the Martian atmosphere undergoes large seasonal variations in pressure. As temperatures change, carbon dioxide—the primary component of the atmosphere—alternately freezes out at the winter pole and sublimes back into a gas during the spring, causing the total atmospheric pressure to fluctuate by about 26 percent annually. Over much longer timescales, the planet's obliquity (tilt) also changes, which can lead to periods where seasonal differences are either extreme or virtually nonexistent.21