[Orbit
The axis along which Mars spins is tilted at 25 degrees. This tilt is called the planet's obliquity and is similar to that of the Earth (Table 1, 23.5o). It is this obliquity that leads to the seasonal variations of incoming sunlight and temperature on Earth and Mars. For instance, in summer of the southern hemisphere on Mars, the South Pole is tilted towards the Sun. When it is summer in the southern hemisphere, it is winter in the northern hemisphere (and vice versa).

The seasons on Mars are denoted by Solar Longitude (Ls) values (see figure below). Solar Longitude is the angle between the Mars-Sun line and the martian line of equinoxes. Ls = 0o corresponds to the spring or vernal equinox (when day and night time are of equal length) in the northern hemisphere of Mars. Ls = 180o is autumn equinox in the northern hemisphere and vernal equinox in the southern hemisphere.

The orbit of Mars is also highly elliptical (elongation of a planet's orbit is called eccentricity) compared to that of the Earth, which is nearly circular. This means that the seasons on Mars are of unequal duration. For instance, southern summer is about 150 martian days whereas southern winter is 180 martian days. The martian year lasts 669 days (or 687 terrestrial days, the martian rotation period or day is only slightly longer than that of Earth, Table 1). At the perihelion of this orbit (the closest point to the Sun in the orbit of Mars) the southern hemisphere happens to be tilted towards the Sun. Therefore, not only is southern summer of short duration compared to northern summer it also receives significantly more sunlight. This affects surface temperatures (which are about 30oC higher during the summer in the south than the northern summer), wind velocity and the transport of dust.]

[Cyclic Variation of the Orbital Parameters
These orbital parameters change slowly over time. For instance, the obliquity of Mars varies from 15o to 35.5o with a period of 1.2 x 106 years. At times of high obliquity more sunlight falls on the polar regions leading to a decrease in the size of the ice caps. Another slow change in Mars's orbital parameters is due to precession (which is the slow conical rotation of an axis of rotation). Both the spin axis of Mars and its elliptical orbit around the Sun undergo precession, and the combined effect is to change the hemisphere which is pointing towards the Sun every 25 000 years.]

The Physical and Orbital Parameters of Mars

http://www.aas.org/publications/baas/v32n3/dps2000/459.htm">Changes in the Martian Circulation and Climate in Response to Orbital Parameter Variations M.I. Richardson (California Institute of Technology), R.J. Wilson (Geophysical Fluid Dynamics Laboratory).

[In orbit, Earth tilts about 23-25 degrees. In contrast, Mars tilts as little as 15 degrees and as much as 40 degrees, which is enough to redistribute moisture from polar caps to equatorial regions, Mustard said. In the 1970s, research by geologists at Brown first showed an orbital effect on climate change for Earth]

http://www.brown.edu/Administration/News_Bureau/2002-03/02-019.html