Visitors to Mars would find some notable similarity to Earth. The martian day is only about 40 minutes longer than on Earth day, so adapting to the patterns of day and night should be easy. The tilt of the martian axis is about 25 degree--only slightly greater than Earth's 23.5 degree tilt--so Mars has seasons much like those on Earth. However, because the martian year is nearly twice as long as an Earth year, the seasons last nearly twice as long on Mars.
The martian seasons also differ from seasons on Earth in another important way that is due to the nature of Mars's orbit. Seasons on Earth are caused by the tilt of the axis. The Earth's axis remains pointed in the same direction (toward the north star) throughout the year, so on one side of the orbit the Northern Hemisphere is tilted toward the Sun and on the other side it is tilted away from the Sun. The Northern Hemisphere receives more direct sunlight when the Southern Hemisphere receives less direct sunlight, and vice versa. That is why the two hemispheres experience opposite seasons. For Earth, the axis tilt is the only significant influence on the strenght of sunlight reaching the surface at different times of year. On Mars, however, the shape of the orbit introduces a second important effect. Mars has a more elliptical orbit that puts it significantly closer to the Sun during its southern hemisphere simmer and farther from the Sun during its southern hemisphere winter. Hence, the martian seasons are much more extreme for the southern hemisphere than they are for the northern hemisphere. In addition, because planets move faster in the portions of their orbits closer to the Sun, the southern hemisphere's summer is shorter in duration and more intense than the northern hemisphere's longer, milder summer.
The seasons cause one of the major features of Mars's climate: seasonal changes in the pressure and the carbon dioxide content of the atmosphere. When it is winter in, say, the northern hemisphere, polar temperatures drop so low that carbon dioxide freezes out of the atmosphere as solid ice. This ice forms a polar cap of carbon dioxide frost that can be as much as a meter thick at the pole and extend as far south as latitude 40 degree. At the same time, it is summer in the southern hemisphere, where the frozen carbon dioxide in the polar cap sublimes into carbon dioxide gas. In general, sublimation refers to the phase change from solid to gas, while evaporation refers to the phase change from liquid to gas. As the southern summer ends and the northern summer begins, the whole process reverses, with carbon dioxide gas subliming at the north pole and freezing at the south pole. Overall, as much as a third of the total carbon dioxide of the martian atmosphere cycles seasonally between the north and south polar caps.
Over the course of the northern hemisphere summer, all the frozen carbon dioxide at the north pole sublimes, leaving a residual polar cap that lasts through the rest of the summer season, made of water ice (H2O) mixed with martian dust. Some of the water ice sublimes to add water vapor to the atmosphere, but the temperature does not rise high enough for the ice to melt. Given that seasonal changes are more extreme for the southern hemisphere, we might similarly expect to see all the carbon dioxide sublime from the south polar ice cap during southern hemisphere summer. However, this is not case; instead, the south polar ice cap retains frozen carbon dioxide throughout the martian year. The reason for this is not fully understood, but it appears to involve several factors, including elevation, the way atmospheric dust affects heat transport, and the history of the polar cap over the last few hundred thousand years. Winds generated by the temperature differences near the edges of the polar caps sometimes initiate local dust storms. Occasionally, local dust storms in low southern latitudes expand during summer into huge dust storms that enshroud the entire planet. At times the martian surface becomes almost completely obscured by airborne dust. As the dust settles out, it can change the surface appearance over vast areas (for example, by covering dark reigns with brighter dust); such changes fooled past astronomers into thinking they were seeing seasonal changes in vegetation. The dust storms also have Mars with a perpetually dusty atmosphere that gives the martian sky its pale pink color.
Liquid water is not stable on the martian surface today--any liquid water would tend to evaporate or freeze almost immediately. Thus, scientists do not find liquid water on the surface of Mars, even though the midday temperature near the equator often rises high enough to melt any frost that collects on surface rock. If this frost does melt, it evaporates very quickly. More commonly, it probably sublimes directly from ice to water vapor. While ther is no liquid water on the surface today, Mars clearly has substantial amount of water. Besides evidence of water ice in the residual summer north polar cap, we often find water vapor and ice crystals in the martian atmosphere, sometimes forming clouds. (The south polar cap probably also contains at least some water ice mixed in with its carbon dioxide ice). In addition, it's likely that Mars has substantial amounts of subsurface ice, a possibility supported by recent observations from Mars Odyssey. Spacecraft instruments have detected near-surface hydrogen, probably coming from water ice frozen into the top meter or so of the surface soil. Mars may also have subsurface ice at greater depths ( a hundred meters or more), and in some places this ice may be melted to make underground pockets of liquid water. If any life exists on Mars today, it most likely lives in such pockets, perhaps resembling the rock-dwelling bacteria (or lithophiles) found deep underground on Earth in the Columbia River Basalt.
I really like how interesting this blog is on Mars. It is astonishing that the plant actually has chemical seasons. Life seems to be impossible on mars because of the elements but maybe someday life will exist on a different plant than earth.
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