  In a previous paper (Chassefi\`ere et al., Icarus 223, 878-891, 2013), we
have shown that most volcanic sulfur released to early Mars atmosphere could
have been trapped in the cryosphere under the form of CO2-SO2 clathrates. Huge
amounts of sulfur, up to the equivalent of a ~1 bar atmosphere of SO2, would
have been stored in the Noachian cryosphere, then massively released to the
atmosphere during Hesperian due to rapidly decreasing CO2 pressure. It would
have resulted in the formation of the large sulfate deposits observed mainly in
Hesperian terrains, whereas no or little sulfates are found at the Noachian. In
the present paper, we first clarify some aspects of our previous work. We
discuss the possibility of a smaller cooling effect of sulfur particles, or
even of a net warming effect. We point out the fact that CO2-SO2 clathrates
formed through a progressive enrichment of a preexisting reservoir of CO2
clathrates and discuss processes potentially involved in the slow formation of
a SO2-rich upper cryosphere. We show that episodes of sudden destabilization at
the Hesperian may generate 1000 ppmv of SO2 in the atmosphere and contribute to
maintaining the surface temperature above the water freezing point.
