These grainy photographs may be the first evidence that liquid water can exist on Mars. They show one of the legs of an American craft called Mars Phoenix Lander which touched down near the planet’s north pole on May 25th 2008 and operated until it was disabled by the gathering winter in November. According to Nilton Renno,one of the mission’s chief scientists,the blotches on the leg are probably drops of brine. The ground on which Phoenix landed is icy,and is also rich in salts called perchlorates. Dr Renno thinks that when the craft settled,its thrusters melted some of the ice and the resulting mud splashed onto the leg. The perchlorates in the mud then sucked moisture out of the air until there was enough water for them to dissolve (a process known as deliquescence). Even though the air temperature was around -60°C,the presence of the salts kept the brine liquid,allowing the drops to move around and even,in the case of those highlighted in green,to merge. The reason for the excitement is that,although the existence of ice on Mars has been known since 1666,when the planet’s polar caps were first seen,the low temperatures and air pressure there mean that water,even if heavily salted,is likely either to freeze or to evaporate. If it can remain liquid on the leg of a lander,perhaps it can do so elsewhere and thus (hope springs eternal) support life.
© The Economist Newspaper Limited 2009