On Earth, life requires water, therefore our search for life beyond the Earth is a search for water. For this reason we target Europa, a moon of Jupiter that may have a liquid ocean beneath a thick icy crust, and also Mars where huge erosional channels suggest the flow of fluids across the surface in the geological past. Many scientists believe that rivers and lakes existed in the past on Mars, and perhaps even oceans. However, this search on Mars may be ill-founded. Despite intense research, the evidence for water on Mars is scarce. Now a new theory suggests that the single strongest line of evidence for water on Mars – the “outburst flood channels” may have been formed not by liquid water but by cold dry eruptions of gas, dust and rock, fuelled by exploding liquid CO2.