Mangala Valles is one of many sites on Mars probably formed by massive catastrophic release
of water onto the surface [1-3]. The water was released after a dike intruded into the area from the near-by Tharsis region, causing formation of a graben, Mangala Fossa [4]. Water was forced to the surface up the graben boundary faults by topographic pressure
gradients and buoyancy.

Proximal to the eastern arm of the Mangala Fossa graben two types of deposit are present. (Figure 1). The dune-like features that are found at the far eastern end of the graben (C) are proposed to be of phreatomagmatic origin [5]. We suggest that the other type of deposit, seen along the strike of the eastern arm (A and B), are mud deposits, ejected by CO2-driven water fountains. The mechanism is similar to that seen in the Lake Nyos and Monoun (both in Cameroon) degassing events [6].