This is another good question about Mars. The reason Mars has
seasons is the same as why we have seasons here on Earth. Mars spins on
an axis. This axis is not 'straight up and down.' That means that if
Mars' orbit were laid flat on the floor, then Mars' axis would not be
sticking staight up in the airbut at an angle. The direction of Mars'
tilt does not change much in space, but it does change relative to the
sun. So when Mars is on one side of it's orbit, the Northern part of
the planet points closer to the sun, and on the other side of the
orbit, the southern part points closer to the sun.
Because certain parts of the planet face the sun more directly
sometimes, the sun's light can hit it more directly, rather than at an
angle. This causes the light intensity to be greater, and so it is
warmer. This is what causes summer. In the same way, when a part of the
planet is pointed less in the direction of the sun, the light hits it
less directly, so it is not as warm. This is the cause of winter.
This is the same way we have seasons here on Earth. It just goes
to show that learning about other planets can help you better
understand your own.
I hope this answered your question
(republished on 07/19/06)