Mars: A reassessment of its interest to biology

Of all the planets in the solar system, Mars is certainly the one that has inspired the most speculation concerning extraterrestrial life. Observers had long ago noticed that Mars exhibits changes in its polar caps and alterations in its surface coloration that parallel seasonal changes on Earth. The fascination with Mars and the possibility of life on Mars continued into the spacecraft era and was directly expressed in the Viking Missions. These highly successful missions had the search for life on Mars as one of their principal goals. A review of Viking Missions experiments is presented. Results of these investigations are summarized. While the Viking Missions returned a negative answer to the question of life on Mars, they also showed that many years ago Mars was a very different place and enjoyed conditions that may have been conducive to the origin of life - life that may have long since become extinct. Evidence for the existence of water on Mars in the past is presented. Techniques used to study early life on Earth, which may also be used for similar studies on Mars, are described.