  We discuss 3D global simulations of the early Martian climate that we have
performed assuming a faint young Sun and denser CO2 atmosphere. We include a
self-consistent representation of the water cycle, with atmosphere-surface
interactions, atmospheric transport, and the radiative effects of CO2 and H2O
gas and clouds taken into account. We find that for atmospheric pressures
greater than a fraction of a bar, the adiabatic cooling effect causes
temperatures in the southern highland valley network regions to fall
significantly below the global average. Long-term climate evolution simulations
indicate that in these circumstances, water ice is transported to the highlands
from low-lying regions for a wide range of orbital obliquities, regardless of
the extent of the Tharsis bulge. In addition, an extended water ice cap forms
on the southern pole, approximately corresponding to the location of the
Noachian/Hesperian era Dorsa Argentea Formation. Even for a multiple-bar CO2
atmosphere, conditions are too cold to allow long-term surface liquid water.
Limited melting occurs on warm summer days in some locations, but only for
surface albedo and thermal inertia conditions that may be unrealistic for water
ice. Nonetheless, meteorite impacts and volcanism could potentially cause
intense episodic melting under such conditions. Because ice migration to higher
altitudes is a robust mechanism for recharging highland water sources after
such events, we suggest that this globally sub-zero, `icy highlands' scenario
for the late Noachian climate may be sufficient to explain most of the fluvial
geology without the need to invoke additional long-term warming mechanisms or
an early warm, wet Mars.
