Phil James (University of Toledo), Todd Clancy (STScI), Steven Lee (University of Colorado), and NASA/STScI

These images appear to have been taken from above the Martian pole, but they were taken with the Hubble Space Telescope from Earth orbit. Each frame in this animation was constructed from three separate images taken in October 1996 and in January and March of 1997. The first image corresponds to early spring in the Northern Hemisphere when the polar ice cap extends to about 60° north latitude. The second image was taken in mid-spring and shows a smaller ice cap resulting from increasing atmospheric warming. The final image from early summer on Mars shows a vastly reduced polar cap; much of the carbon dioxide ice has sublimated, revealing the terrain beneath.

Martian Seasons

Mars has a cycle of seasons that is similar to Earth. The reason for this is that both planets are similarly tilted on their rotational axes: a 23.5° tilt for Earth and a 25.2° tilt for Mars. Because Mars takes about twice as long to orbit the Sun, each Martian season is about twice as long as the corresponding season on Earth. In addition, the Martian atmosphere is much thinner than Earth's atmosphere, so there is little insulation to protect the Martian atmosphere from temperature shifts due to changes on the surface or in the Mars-Sun distance.

The Mars-Sun distance

The Mars-Sun distance changes by 20 percent over the course of the year. When Mars is closest to the Sun, it is summer in the Southern Hemisphere and temperatures are up 35° F, producing dust storms that swirl around the planet and absorb sunlight, further heating the atmosphere. When Mars is farthest from the Sun, water-ice clouds reduce atmospheric temperatures. The dust particles in the atmosphere seed these clouds, then fall to the ground. Competition between cloud cooling and dust heating drives annual as well as short-term climate changes on Mars.

Ellen Cohen