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The extensive fluvial features seen on the surface of Mars attest to the stable flow of water on that planet at some time in the past. However the low erosion rates, the sporadic distribution of the fluvial features, and computer simulations of the climate of early Mars all suggest that Mars was quite cold even when it was wet. Thus, the polar regions of the Earth provide potentially important analogs to conditions on Mars during its wet, but cold, early phase. Here we review studies of polar lakes, streams, and springs and compare the physical and geological aspects of these features with their possible Martian counterparts. Fundamentally, liquid water produced by summer melts can persist even when the mean annual temperature is below freezing because ice floats over liquid and provides an insulating barrier. Life flourishes in these liquid water habitats in Earth's polar regions and similarly life may have been present in ice-covered lakes and permafrost springs on Mars. Evidence for past life on Mars may therefore be preserved in the sediments and mineral precipitates associated with these features. [ABSTRACT FROM AUTHOR]