We have seasons on Earth because of the tilt of the Earth’s axis in relation to its orbit. This causes the Sun to shine more on the Northern Hemisphere for one-half of the year and then more on the Southern Hemisphere for the other half. Other planets can have seasons for the same reason, provided their axes of rotation are also tilted in relation to their orbits. Mars’ axis, for example, has a 25°c degree tilt, which is almost the same as Earth’s at 23.5°c. However, seasonal differences on Mars are greater because of Mars’ eccentric orbit and it is possible to see seasonal fluctuations in Mars’ polar ice deposits.
Uranus is an extreme example. Its axis has a 98° tilt, and hence the Sun shines on the northern pole for half a Uranuian year (84 Earth years) and then on the southern pole for the second half. The unlit pole is left in complete darkness.
In August 2003 it was summer at Mars’ South Pole (below), and the ice cap was therefore considerably smaller than during the Martian winter.
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