Martians are generally imagined as three fingered, green, bald headed creatures; however, recent theory suggests that the Martians may be or have been microorganisms.It is believed that Mars may have once had water on its surface. As we all know, on Earth, water means life. Channels and craters on Mars differ from those on the Moon or Mercury, which make scientists believe that water may be present on Mars. As it turns out, Mars has massive polar ice caps (complete with a permafrost layer) which, if melted, would produce enough water to cover the entire planet “eleven meters” deep.
Water, as a liquid however, cannot exist except at very low elevations because of the weak atmosphere. Still, it is thought because of the unusual craters and striations across the planet that beneath the ice lays even more water. In 2002, the Mars Odyssey spacecraft used its gamma ray spectrometer and confirmed that there is “enormous quantities of water ice beneath the surface of Mars.”
It is further argued that there must have been -or still is- water on Mars due to its “debris pattern and colouring” though this is possibly due to carbon dioxide frost or dust movement. However, many believe that the presence of hematite and goethite (“usually formed in a wet environment”) found on Mar’s surface is indicative of water’s presence and lends hope to there having been life on Mars. To further that hope, in December of 2006, scientists believed that the geological changes shown in photographs at the time suggest that water occasionally flows over parts of the surface.
In addition to water possibly being on its surface, Mars boasts a small atmosphere with methane gas pockets throughout. Due to the methane pockets and polar ice caps, the most recent theory is that if microbes existed on Mars they would be much like our terrestrial methanogens. Methanogens can live deep in ice (such as in Antarctica or
Many of the experiments done on Mars (1970’s) to see if microbial life existed there were done with saline as the main “internal fluid” in mind. It is now thought that these series of tests would have only drowned or burned the possible life forms. With the cold and dry (water is not really observable) climate it is thought that, should the microorganisms exist, they would be made up of water and hydrogen peroxide as it freezes at a much lower temperature.
Among the previous theories lies another: the possibility that microbes on Mars have magnetisomes. A meteorite, named ALH84001, is believed to contain the fossilized remains of microbes from Mars. 
However, until it is proven without a doubt that Mars has water, it is impossible to tell whether alien microbes have lived on our neighbouring planet.
References:
http://www.jpl.nasa.gov/releases/2004/88.cfm
http://airandspace.si.edu/etp/Mars/surface/water.html
http://en.wikipedia.org/wiki/Mars
http://abcnews.go.com/Technology/WireStory?id=2776816&page=1
