Like Earth, Mars is tilted relative to its orbital plane, and so it has seasons. But Mars's orbit is stretched out, more elliptical (less circular) than Earth's orbit, so one of the hemispheres goes through more radical seasonal changes than the other.

During the summer, the polar ice caps—which we now know to be made almost entirely of water ice—shrink noticeably. The surface temperature varies from a bone-chilling minus 140 degrees Celsius (minus 220 Fahrenheit) to a mild 20 degrees Celsius (68 Fahrenheit).

The Martian atmosphere, composed mainly of carbon dioxide, is very thin and does not trap heat from the sun very effectively. At least it doesn't now. Mars might once have had a thicker atmosphere, trapping heat and keeping the climate much warmer. The pinkish, orange, or tan tint of the Martian sky that we see in photographs is caused by dust blowing in the atmosphere.

Like the deserts of Earth, the deserts of Mars have dust devils, whirling vortices of dust that leave distinctive tracks across the ground. But Martian dust devils are up to six times the size of their counterparts on Earth; and, on occasion, huge dust storms spread across the planet, large enough to be easily seen from Earth.