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[[Geology of Mars|Mars' 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 [[Timekeeping on Mars|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 [[Darian calendar|much longer year]]. These observations led to the increase in speculation that the darker [[albedo feature]]s were water, and brighter ones were land. It was therefore natural to suppose that Mars may be inhabited by some form of life.
In 1854, [[Phillip
Spectroscopic analysis of Mars' atmosphere began in earnest in 1894, when U.S. astronomer [[William Wallace Campbell]] showed that neither water nor oxygen were present in the Martian atmosphere.<ref name="chambers">{{Citation
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Unlike the [[Mars Pathfinder]] ''Sojourner'' rover and the [[Mars Exploration Rover]]s, which used [[Airbag#Aerospace Applications|airbag-cushioned capsules]] to land on Mars, the Phoenix lander will land the same way that the Viking landers did, despite the claims that rocket exhaust may have contaminated the Viking landing sites.<ref name="viking killers">2nd</ref>
Some researchers claim in the book "Imminent Discovery"<ref>http://home.comcast.net/~tdehel/site/</ref> that the chances for Phoenix to find Martian life are very good, based on telescopic observations of pre-1965 astronomers. In his 1962 book "MARS, The Photographic Story", [[
===Future missions===
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