Present life?: The primary purpose of the Viking mission, launched in 1975, was to try to establish an answer to this question. The two Viking Landers touched down 26 degrees apart in latitude and on opposite sides of the planet. Although it has been the subject of much debate, the consensus view is that the Viking findings established that there was no life present at the two landing sites. In particular, no organic compounds (i.e. molecules containing carbon combined with hydrogen, and often also with oxygen, nitrogen and other elements) were detected even at the part per billion level. Such molecules are necessary for life as we know it. Also the two landing sites were very similar in their surface chemistry. The chemical similarity and the lack of life at two locations on opposite sides of the planet reflect the global hostility of the surface environment --- there is extreme dryness with no liquid water , at least under normal equilibrium conditions. Also short-wavelength ultraviolet (UV) light hits the surface on a daily basis because Mars has no protective ozone layer due to a very low concentration of oxygen in its atmosphere. UV damages organic molecules by breaking them up and destroying them. Liquid water is widely considered to be necessary for life, so the lack of liquid water at the surface is a major impediment to life. Because of the global nature of these influences, it is likely that the rest of the surface of Mars is also lifeless. However, there are some scientists who argue that "oases" of life may exist in unusual microenvironments, for example, in subsurface permafrost layers or in geothermal vents near volcanoes that melt ice to cause liquid water. Such a possibility cannot be ruled out.

Past life?: There is evidence that in the past Mars had running liquid water on its surface - for example, the valley networks cut in its surface were presumably made by water (see Channels and Valleys ) - so perhaps Mars had a warmer, denser atmosphere sufficient to support liquid water (see "Was Mars always so dry and cold as today (were there once rivers and seas)?"). Consequently, it is possible that life arose on Mars and is now extinct. (For basic details on the Viking biology experiment and the possibility of fossilized ancient life see Life on Mars. For a technical discussion of a strategy to find extant or extinct life see An exobiological strategy for Mars exploration.)