THREE years ago, the comet 17P/Holmes exploded with a blast comparable to a small nuclear bomb. Would you believe that an exotic form of ice was responsible?
Comet 17P/Holmes became a million times brighter when it erupted in 2007. A freak collision with an asteroid could have explained that blast, had it been a one-off. But the same comet also exploded in 1892, suggesting something else might be triggering the outbursts. Now of the California Institute of Technology in Pasadena and his colleagues think the culprit may be an exotic and unstable form of water ice at the comet鈥檚 heart.
When water freezes naturally on Earth, it forms a highly regular crystal structure. It鈥檚 a different story at the much lower temperatures of the outer solar system, where comets condensed from primordial gas and dust. The water molecules would have stuck together much more haphazardly, forming so-called amorphous ice. When such ice is warmed to -133 掳C, it reverts to the familiar crystalline form. This would squeeze out any gases trapped within the amorphous ice during the comet鈥檚 formation. Heat would also be released, perhaps prompting a runaway conversion of any nearby amorphous ice.
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The sun鈥檚 heat may have triggered such a conversion in comet 17P/Holmes, with pressure from the released gases blasting a hole in its side, says Reach鈥檚 team (Icarus, ). The size of the debris cloud created by the 2007 blast suggests that the explosion was as powerful as the detonation of 31 kilotonnes of TNT or a small nuclear bomb.
鈥淭he comet鈥檚 debris cloud suggests that the explosion was as powerful as a nuclear bomb鈥�
To release this much energy would require the transformation of a million tonnes of amorphous ice, just a fraction of the mass of the comet鈥檚 3.4-kilometre nucleus.
But of the University of California, Los Angeles, cautions that gas might simply leak out through cracks in amorphous ice, rather than build to the high pressures needed for a violent eruption. 鈥淭here is evidence that the tensile strengths of comet nuclei are very low,鈥� he says. This suggests that the nuclei of comets may not be able to contain the gases long enough for an explosion to occur.
