Scenario for the formation and evolution of roundish features as ancient degassing conduits. A) Initial nucleus surface, covered by fine material. B) Formation of active degassing conduits, resulting in the erosion of the fine material. C) Activity of degassing conduits stops and fine material progressively covers them. D) Differential erosion starts on the surrounding terrains, revealing ancient degassing conduits (no longer active) from north to south. E) Current state: the ancient conduits are more or less degraded depending on how long they were exposed to solar illumination. Fine material still covers or hides the less exposed conduits.