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Most sulfate minerals discovered on Mars are dated no earlier than the Hesperian. We showed, using a 1-D radiative-convective-photochemical model, that clathrate formation during the Noachian would have buffered the atmospheric CO2 pressure of early Mars at ~2 bar and maintained a global average surface temperature ~230 K. Because clathrates trap SO2 more favorably than CO2, all volcanically outgassed sulfur would have been trapped in Noachian Mars cryosphere, preventing a significant formation of sulfate minerals during the Noachian and inhibiting carbonates from forming at the surface in acidic water resulting from the local melting of the SO2- rich cryosphere. The massive formation of sulfate minerals at the surface of Mars during the Hesperian could be the consequence of a drop of the CO2 pressure below a 2-bar threshold value at the late Noachian-Hesperian transition, which would have released sulfur gases into the atmosphere from both the Noachian sulfur-rich cryosphere and still active Tharsis volcanism. Our hypothesis could allow to explain the formation of chaotic terrains and outflow channels, and the occurrence of episodic warm episodes facilitated by the release of SO2 to the atmosphere. These episodes could explain the formation of valley networks and the degradation of impact craters, but remain to be confirmed by further modeling.

To test the hypothesis of atmospheric carbon dioxide or methane storage in metastable clathrate, a theoretical formalism is developed to model and simulate the spectra of the CO2 or CH4 molecule trapped in clathrates. 12-6 Lennard-Jones atomatom potentials are used to account for short and long range interactions between the atoms of the trapped molecules and atoms of H2O molecules of the cage. Effective electric charges are used for electrostatic interactions with H2O molecules. The calculations were performed on clathrates of type I, with a small and a large cage to determine equilibrium configurations for both CO2 and CH4 and vibrational shifts were determined for CO2 in an undistorted trapping nano-cage.