  Reconciling the geology of Mars with models of atmospheric evolution remains
a major challenge. Martian geology is characterized by past evidence for
episodic surface liquid water, and geochemistry indicating a slow and
intermittent transition from wetter to drier and more oxidizing surface
conditions. Here we present a new model incorporating randomized injection of
reducing greenhouse gases and oxidation due to hydrogen escape, to investigate
the conditions responsible for these diverse observations. We find that Mars
could have transitioned repeatedly from reducing (H2-rich) to oxidizing
(O2-rich) atmospheric conditions in its early history. Our model predicts a
generally cold early Mars, with mean annual temperatures below 240 K. If peak
reducing gas release rates and background CO2 levels are high enough, it
nonetheless exhibits episodic warm intervals sufficient to degrade crater
walls, form valley networks and create other fluvial/lacustrine features. Our
model also predicts transient buildup of atmospheric O2, which can help explain
the occurrence of oxidized mineral species such as manganese oxides at Gale
Crater. We suggest that the apparent Noachian--Hesperian transition from
phyllosilicate deposition to sulfate deposition around 3.5 billion years ago
can be explained as a combined outcome of increasing planetary oxidation,
decreasing groundwater availability and a waning bolide impactor flux, which
dramatically slowed the remobilization and thermochemical destruction of
surface sulfates. Ultimately, rapid and repeated variations in Mars' early
climate and surface chemistry would have presented both challenges and
opportunities for any emergent microbial life.
