From Richard Corbett, Corpach, Highland, UK
Mars needn't have been warmer for water to have carved its features (18 February, p 25). An alternative explanation is that something was once present that allowed water to remain liquid at very low temperatures, but is now gone. Ammonia seems plausible: mixtures with water can be liquid at around -100°C at a pressure of 1 atmosphere. Ammonia has a far higher vapour pressure than water. I think it would be lost much more rapidly by ultraviolet degradation or through erosion by the solar wind.
