The mysterious, spectacular flare of Comet 17P/Holmes has been observed three times, and it may give warning of an imminent explosion fueled by carbon monoxide gas when it returns in 2014, say astronomers.

Holmes shocked the world in October 2007 when it suddenly brightened by a factor of 500,000, going from a humdrum ball of dust to a brilliant orb visible to the naked eye. The comet spewed 100 million tons of dust into space, comparable to the amount of ash unleashed by Mount St. Helens, and briefly swelled to a diameter greater than the sun's.

But despite months of observations from hundreds of telescopes, the cause of Holmes' dramatic explosion remains unknown.

Astronomers initially suggested that a dusty crust may have formed on the comet's nucleus as it approached the sun, trapping frozen ice underneath. As it neared the sun, the surface would heat so rapidly that the ice turned from solid to gas without even melting. Gas would build up and burst through the crust, sending Holmes' innards flying outward.

But what was the fuel? The most obvious source inside the comet was sloshy, amorphous water ice crystallizing into ordered, snowflake-like filaments, a process that gives off heat. But a pair of Polish researchers showed that energy from crystallizing water ice wouldn't be enough to drive such a huge explosion.

Now, the same team suggests a new fuel: carbon monoxide gas. In an upcoming paper in the journal Icarus, geophysicists Konrad Kossacki of the Warsaw University and Slawomira Szutowicz of the Polish Academy of Sciences report they ran simulations to show that carbon monoxide gas inside the comet nucleus could reach pressures of 10 kilopascals [100 millibars, or 1.45 psi], the same difference in pressure as climbing from sea level to 3,280 feet. This would be enough pressure to make the comet's fragile core erupt, they say.

Comet Holmes has actually exploded twice before, when it was discovered in 1892 and again in 1893. All three outbursts happened about 200 days after the comet's closest approach to the sun, which could provide some clues as to how much heat the core needs to explode – and whether it might explode again during its next visit in 2014.

Image: Comet Holmes as captured by astrophotographer Alan Dyer in Alberta, Canada, on Nov. 1, 2007.

NASA/ESA/A. Dyer

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