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During the period of Viking, many scientists had thought that the high levels of ultraviolet (UV) radiation at the Martian surface would weigh heavily against the possibility of lifeforms.
UV light breaks apart the chemical connections between carbon atoms of organic molecules. However, insight gained through the study of life in extreme environments on Earth have had an influence on our thinking about possible life on Mars. In Antarctica, for example, where UV levels are very high compared to other areas on the Earth, researchers have discovered algae that live inside solid rock. These lifeforms, called endolithic (inside rock) organisms exist at a depth of approximately 1 to 3 mm - close enough to the surface of the rock to receive light to carry out photosynthesis but just deep enough to be protected from the UV light that strikes the rock. Therefore UV radiation need not be a problem for microbes living beneath the Martian surface, inside rocks or even in the surface soil near a rock, protected by the rock's shadow.