Prior to the European Space Agency’s Giotto mission to study Halley’s comet in 1985, for example, astronomers believed that as sunlight fell onto a comet, its spin would mean that the heat evaporates a more or less even layer, revealing more icy material beneath. Giotto showed that this idea was hopelessly simplistic. “As soon as we saw the nucleus it was clear that activity was confined to individual jets and not coming from the whole surface,” says Giotto project scientist Gerhard Schwehm of the European Space Agency. In fact, only 15 per cent of Halley’s total surface area was expelling material at the time of the fly-by. The observation has shown astronomers that they are in the dark about even the basics
. “We still do not know what drives comet activity,” says Schwehm.
Donald Brownlee of the University of Washington in Seattle goes further. “It’s a mystery to me how comets work at all
,” he says. Brownlee has good reason to make this claim. He is the principal investigator on NASA’s Stardust mission, which flew past comet Wild 2 on 2 January 2004. The fly-by images showed 20 active jets spread across the comet’s sunlit side. So far, so good. Then they saw something that added a new twist to the mystery. Two of the jets were on the night side of the comet.
Astronomers had expected that the jets would simply turn off when the comet turned them away from the warming rays of the sun. For Brownlee it seems to be pointing to an inescapable conclusion. “I think that some process is allowing heat to get down below the surface of a comet and drive the activity from the inside out,” he says.
The clue might be in the dark surface layers of the comets. Though it is hardly what you would expect of icy bodies, the exteriors of both Halley and Wild 2 are as black as coal, and these dark layers absorb heat. At the time of the Stardust encounter, when the comet was almost twice as far away from the sun as the Earth, the surface of Wild 2 was a comfortable 18°C. Its interior would have been much colder, well below 0°C in fact, so heat would naturally flow inwards. That’s as far as the explanation goes at present. “I have no idea about the details of the process,” Brownlee admits."