EVIDENCE OF ANCIENT CONTINENTAL-GLACIATION IN THE MARTIAN NORTHERN PLAINS

Whorled ridges, spaced about 2-6 km and forming lobate patterns with l obe widths of about 150 km, occur at many locations in the northern pl ains of Mars, commonly in close association with sinuous troughs that contain medial ridges. These landforms resemble moraines, tunnel chann els, and eskers found in terrestrial glacial terrains, such as the mid continent of North America. Some Martian landscapes may have formed by disintegration of continental glaciers that covered much of the north ern plains into the early Amazonian (i.e., late in Martian geologic hi story). Meltwater processes apparently were important in the collapse of these hypothesized ice sheets; hence, the glaciers apparently were wet based in part. Whereas striking similarities exist among areas of the northern plains and some glaciated Pleistocene terrains on Earth, there are also important differences; notably, drumlin fields, such as those in many glacial landscapes on Earth, are rare, absent, or not y et resolved in images of the Martian northern plains. Another major di fference is that postglacial fluvial and other water-related modificat ions (especially erosion) of Pleistocene terrains are substantial, but similar modifications are not observed in the northern plains; a virt ually complete and sudden decline in the activity of liquid surface wa ter following glaciation in the northern plains seems to be implied. T he climatic implications of the hypothesized Martian glaciers and thei r decline are unclear. We investigate two possibilities, alternatively involving a relatively warm paleoclimate and the modern Martian clima te. The hypothesized ice sheets in the basins within the northern plai ns (generally at elevations lower than -1 km) suggest a relationship o f these frozen bodies of water with former regional lakes or seas, whi ch may have formed in response to huge discharges of water from Martia n outflow channels. This possible relationship has been modeled. Glaci ers may have evolved from seas by their progressive freezing and then grounding and sublimational redistribution of sea ice. The transition to glaciation may have taken several million years if the climate was very cold, comparable to today's, or tens of thousands of years if the climate was as warm as modern Antarctica. A glacierized sea may have involved an extended period of glaciolacustrine and ice shelf processe s.