The abundant evidence accumulated over recent years for the existence of a water rich atmosphere in the distant Martian past has been vexing scientists. While some clues point dramatically to water having once existing on the surface, other evidence flatly contradicts it. The problem may be that in trying to reconcile the evidence, we have been applying an Earth-like process to Mars when in fact we need to be thinking of something uniquely Martian. The key problem is that probes have uncovered plenty of sulphur minerals on the surface but no limestone as on Earth. The two should go hand in hand, since on Earth, Silicate rocks remove carbon dioxide from the atmosphere and with water to hand, convert it into calcium carbonate, commonly known as limestone. The lack of limestone has been particularly puzzling, but now a new theory suggests that sulphur dioxide may have taken the place of carbon dioxide. On Earth, sulphur dioxide is quickly oxidised to sulphate, but on an early oxygen poor Mars, the sulphur dioxide would have lingered in the atmosphere for much longer, acting as a greenhouse gas and warming the planet. It's a neat theory, and may finally prove the conclusive step in our understanding of the early Martian atmosphere. More can be read at the BBC website.