The spots appear on dunes found on the floors of craters in the south and north polar regions. The Hungarian team has examined the southern spots in detail. "They appear in late winter and by summer they have disappeared. They appear first at the margins of the dune fields and rarely appear on the ridges of dunes," Szathmary told the meeting. Their location (which is independent of the elevation of the land) and shape (which is circular on flat surfaces but elongated on slopes) seem to be at odds with a physical explanation alone, say the Hungarian scientists who have proposed a biological explanation instead. The spots are colonies of photosynthetic Martian microorganisms, they hypothesise, which over-winter beneath the ice cap. As the Sun returns to the pole during early spring, light penetrates the ice, the microorganisms photosynthesise and heat their immediate surroundings. A pocket of water, which would normally evaporate instantly in the thin Martian atmosphere, is trapped around them by the overlying ice. As this ice layer thins, the microorganisms show through grey. When it has completely melted, they rapidly desiccate and turn black. This explains why many dark dune spots have a black centre surrounded by a grey aureole, say the Hungarian scientists.http://spaceflightnow.com/news/n0203/14marsspots/
