As long as we have lifted eyes into space, we have dreamed one day finding life on another planet. Scientists believe Mars may be the planet in our solar system that has life. But could there really be life on Mars? Join the National Geographic Channel as they interview top planetary scientists and amateur researchers, who have dedicated their lives to the subject and separate scientific evidence from flights of fancy.
Mars's polar ice caps were observed as early as the mid-17th century, and they were first proven to grow and shrink alternately, in the summer and winter of each hemisphere, by William Herschel in the latter part of the 18th century. By the mid-19th century, astronomers knew that Mars had certain other similarities to Earth, for example that the length of a day on Mars was almost the same as a day on Earth. They also knew that its axial tilt was similar to Earth's, which meant it experienced seasons just as Earth does - but of nearly double the length owing to its much longer year. These observations led to the increase in speculation that the darker albedo features were water, and brighter ones were land. It was therefore natural to suppose that Mars may be inhabited by some form of life.
Liquid water on Mars
No Mars probe since Viking has tested the Martian soil directly for signs of life. NASA's recent missions have focused on another question: whether Mars held lakes or oceans of liquid water on its surface in the ancient past. Scientists have found hematite, a mineral that forms in the presence of water. Many scientists have long held this to be almost self-evident based on various geological landforms on the planet, but others have proposed different explanations—wind erosion, carbon dioxide oceans, etc. Thus, the mission of the Mars Exploration Rovers of 2004 was not to look for present or past life, but for evidence of liquid water on the surface of Mars in the planet's ancient past.
The Face on Mars
One of the Cydonian mesas, situated at 40°45' north latitude and 9°26' west longitude, took on the striking appearance of a humanoid Face on Mars in a photo taken by Viking 1 on July 25, 1976. Some commentators, most notably Richard C. Hoagland, believe it to be evidence of a long-lost Martian civilization along with other features they believe are present, such as apparent pyramids, which they argue are part of a ruined city. While analysis of the early Viking images suggested that the features of the Face might not be an accidental consequence of viewing conditions, today, it is generally understood to be an optical illusion, an example of pareidolia. After analysis of the higher resolution Mars Global Surveyor data NASA stated that "a detailed analysis of multiple images of this feature reveals a natural looking Martian hill whose illusory face-like appearance depends on the viewing angle and angle of illumination". A similar feature on Earth is the Badlands Guardian Geological Feature, which resembles a human head wearing a Native American headress.
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