| Summary: | The history of ice ages on Earth, extending back more than 2 Ga (109 years), has been established by the recognition in strata of many ages of the assemblage of erosional features and deposits generated by glacial activity. The most useful indicators are widespread diamictite, striated and faceted clasts, polished and striated pavements, and laminites containing “dropstones” inferred to be ice-rafted. The chronology of ancient glaciation is limited by our ability to recognize the results of glacial activity in an incomplete geologic record and by the age resolution attainable with available dating methods. Ice ages are currently known from the Early Proterozoic (2.5-2.1 Ga ago), several intervals in the Middle to Late Proterozoic (1.0-0.57 Ga), the Late Ordovician to Late Silurian (440-415 Ma, or 106 years), possibly latest Devonian (360-345 Ma), Permo-Carboniferous (335-245 Ma), and late Cenozoic (26 Ma ago to the present). However, the timing of pre-Phanerozoic ice ages, occurring before 0.57 Ga ago, is known only approximately, and glacial events would have been shorter than is suggested by the bracketing ages. A generally warm climate seems to have prevailed for the remainder of Earth history, although Mesozoic-Cenozoic seismic stratigraphic evidence suggests that a small ice sheet may have been a persistent feature near the south pole throughout the last 500 Ma. Terrestrial climatic fluctuations occur on several time scales. The occurrence of ice ages on a time scale of 108−109 years, and possibly higher-frequency fluctuations (106−107 years), appears to be controlled by changes in solar output and atmospheric composition, and by the rearrangement of continents and oceans as a result of the motion of lithospheric plates. Climatic changes on a time scale of 104−105 years are driven by perturbations of the Earth's orbital parameters. The principal evidence suggesting that significant climate changes have occured on Mars consists of widespread dry channels (3.5-0.5 Ga?) and of geologically young dust-ice layered terrains in the polar regions (less than 10 Ma?). By analogy with the Earth, polar layered terrains have probably existed episodically on Mars for a considerable time although there is little direct evidence for this. Factors affecting a potential sedimentary record are the degree to which ice sublimed or melted, the effectiveness of eolian destruction of layering, and, in contrast to the present polar terrains, the tendency for ice to flow and slide over its substrate. As on Earth, climate changes on Mars have probably been induced by the long-term evolution of the atmosphere, by changes in solar luminosity, and by variations in orbital parameters. Major tectonic events may have affected Martian climate through changes in planetary obliquity.
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