CRISM (Compact Reconnaissance Imaging Spectrometer for Mars)
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Mars Geology

The Question of Life on Mars

Astronomers in the 1800s and early 1900s interpreted some of Mars' features as evidence for life on the planet. For example, seasonal changes in bright and dark patterns, now known to be caused by variable deposition and removal of dust, was interpreted by some at the time as the greening of plants when they received moisture from melting polar caps in spring. An even more extreme viewpoint was that of Percival Lowell and a few other astronomers, who thought they saw linear features which they interpreted as canals constructed by a dying race of intelligent beings. These lines were later determined mostly to have been optical illusions formed by disconnected craters, sand dune fields, and other albedo features. One "canal", Coprates, turned out to be part of Valles Marineris.

Although Mars does not have vegetation, the questions of life in the past, and even primitive life at present, are still open. Mars is the only planet besides Earth to show evidence for liquid water on its surface, and liquid water is a key ingredient of life. Even if there is no life on Mars now, remnants of it may have been fossilized – preserved inside mineral deposits – in the distant past. The 21st century hunt for Martian life has become the hunt for minerals that formed in water enivironments that might preserve evidence for ancient life. That's where CRISM comes in.