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.