NASA and JPL Caltech say they have found the presence of liquid water in Comet Wild2 – but comets are supposed to be so cold they never get warm enough to melt ice. Some clever jiggery pokery is theorised – the process happens 'inside' the comet. If it was cold on the outside it may not be so cold on the inside – yet comets are supposed to originate in the outer solar system where it is thought to be very cold indeed. According to a spokesperson there are two ways to generate a heat source on a comet. A collision or radioactive decay of elements in the comet interior. Only two?