Published by NASA in the 1970's, the Brookins report contains guidelines for SETI and other agencies regarding the discovery alien civilizations. The report judged that worldwide chaos would result from the disclosure of alien artifacts, archeology and contemporary visitation.
It can therefore be seen that, while Conspiracy Theorists and those who prefer to believe in aliens may be assumed to fabricate or alter evidence to support their beliefs, NASA has stated an undertaking to conceal any evidence which might prove or even suggest alien life.
This leaves a problem for both sides of the debate, in that no evidence can be fully trusted regardless of its source; only our assumptions, either for or against the proposition of extraterrestrial life, guide us in making up our minds.
What then, is the evidence, doctored or otherwise? Most well-known are the anomalous structures found in the Cydonia region of Mars; the famous 'Cydonian Head' or 'Face on Mars', which has been photographed by NASA and appears to be a massive sphinx-like structure, a humanoid facing staring up at the heavens. According to NASA, the latest photos show that this is a natural formation, the humanoid characteristics an optical illusion. However, NASA have also published details of the processes used to enhance the image, a succession of filters that flatten out the image and render The Face as a virtually featureless patch of Martian soil. Naturally, believers in life on Mars reject this evidence and point to other contemporary photos that show distinctly artificial details not only on the The Face itself but also in the surrounding area; anomalous features resembling pyramids and other ruined structures have been suggested as evidence of an ancient Martian city in Cydonia.
Close to Cydonia but less widely known, are the mysterious tunnels extending for miles across the surface and presumably beneath it. Many images of these tunnels show color, artificial structure, transparency and partial collapse, but since the same issues of falsified evidence that apply to statements from NASA also affect believers in Martian life, these details cannot be considered conclusive evidence.
A recent discovery on Mars is the 'statue', a rock formation that resembles an upright human figure. Predictably, NASA has stated that this is a natural formation. However, NASA's other statements declare Mars an arid, airless wasteland; without wind, rain, or any other weather-effect that could cause erosion to produce such a structure. The evidence for the 'statue' being artificial are however weakened by the near hysterical claims from some quarters that a 'statue of Jesus' has been found on Mars.
Perhaps the most convincing evidence in the debate comes from NASA's own pictures of Mars; some show the planet with no atmosphere, a lifeless ball surrounded by the blackness of space. Other photos from the same source display a thin, reddish atmosphere in line with the red skies of popular belief and some of NASA's own photos from the planet surface. However, other photos from NASA's own website show Mars with a distinct blue atmosphere, ice at its poles, dark blue and green areas on the surface, and most surprisingly, clouds.
Clouds logically suggest a Martian atmosphere thicker than is generally assumed, and casts doubt on the 'lifeless rock' images also released by NASA. There is no way to know which pictures showing the atmosphere have been doctored and which have not, although it can be reasonably be pointed out that the production of pictures showing a blue Mars would run contrary to NASA's stated position against revelaing the existence of alien life. If indeed the atmosphere of Mars boasts clouds, the next logical conclusion is that a 'greenhouse' effect similar to that of Earth and Venus would also exist on Mars, leading to precipitation, wind, weather effects, and sufficient melting of the Martian icecap to support the existence of algae, water, and therefore, life.
To suppose an existent civilization and a living race of Martians is beyond the evidence currently in hand, however the evidence for a past civilization comparable to our own is steadily mounting. The most reasonable conclusion is that there may well have been life on Mars in the distant past, perhaps before the rise of human civilization. Further speculation beyond this point would mire us inexorably in NASA's concerns regarding the effect that alien life could have on our own culture.