  For decades, scientists have tried to explain the evidence for fluvial
activity on early Mars, but a consensus has yet to emerge regarding the
mechanism for producing it. One hypothesis suggests early Mars was warmed by a
thick greenhouse atmosphere. Another suggests that early Mars was generally
cold but was warmed occasionally by impacts or by episodes of enhanced
volcanism. These latter hypotheses struggle to produce the amounts of rainfall
needed to form the martian valleys, but are consistent with inferred low rates
of weathering compared to Earth. Here, we provide a geophysical mechanism that
could have induced cycles of glaciation and deglaciation on early Mars. Our
model produces dramatic climate cycles with extended periods of glaciation
punctuated by warm periods lasting up to 10 Myr, much longer than those
generated in other episodic warming models. The cycles occur because stellar
insolation was low, and because CO2 outgassing is not able to keep pace with
CO2 consumption by silicate weathering followed by deposition of carbonates.
While CO2 by itself is not able to deglaciate early Mars in our model, we
assume that the greenhouse effect is enhanced by substantial amounts of H2
outgassed from Mars' reduced crust and mantle. Our hypothesis can be tested by
future Mars exploration that better establishes the time scale for valley
formation.
