Ken Tapping, January 9, 2008

On October 24 2007, something happened to Comet Holmes. From something too faint to see without a large telescope, it suddenly became a million times brighter and unmissable to the naked eye. Most comets do not behave this way, so this strange comet has attracted a lot of attention.

At this time of year, most Canadians see a good analogue of comet material every day. It is that dirty, slushy, icy stuff you find at the side of city streets: a mixture of ice, dust, grit, trapped gases and various organic chemicals. Imagine a lump of this stuff up to a few kilometres across, orbiting in the cold outer reaches of the Solar System, where it is permanently frozen solid. Sometimes a lump gets deflected towards the inner parts of the Solar System, where it is much warmer. This makes the more volatile chemicals evaporate. A lump of dirty slush a few kilometres or less in size does not have much gravity to hold it together, and as the ice and other materials gluing the comet together are lost, the lump starts to fall apart. It eventually finds itself surrounded by a growing cloud of dust, grit and gases. It is this cloud that we call a comet.

Most comets evaporate more or less steadily, just increasingly quickly as they get closer to the Sun. For some reason Holmes is behaving very differently. Moreover, what happened on 24 October was not a mere isolated occurrence. Holmes was doing it in 1892, which is what led to it being spotted by Edwin Holmes, and again two and a half months later. This seems to be part of the nature of that comet. Therefore these brightenings cannot be due to the comet being hit by something, because three such impacts would be highly unlikely.

In 1986 the Giotto spacecraft flew close to Halley's comet, and imaged the dirty slushball in the middle. Its surface was spotted with volcano-like vents from which jets of evaporated material were issuing. This comet shows none of Holmes' impulsive behaviour, but suggests a possible explanation.

On Earth we have two main kinds of volcano. There are ones like Kilauea in Hawaii, which produce a more or less steady flow of lava in eruptions that continue for years, and others like Krakatoa or Mount St. Helens, which tend to explode. This difference comes from the composition of the lava. The lava from Kilauea flows smoothly, but the stuff in Krakatoa and Mount St. Helens tends to plug up the vents, with catastrophic results. Maybe Comet Holmes has a somewhat different chemical composition from other comets, so that the vents tend to get plugged. The resulting pressure buildup inside could be causing a sort of super-cold Krakatoa explosion. This would blow away a chunk of the comet's crust, relieving the pressure, for a while.

There have been no further explosions since October 24, and the material from that outburst has been spreading out slowly, so that now the cloud of debris looks bigger than the Full Moon. The brightness is about the same, but since the light is averaged over a bigger object, it looks fainter. Binoculars should show it well. It is still in the neighbourhood of the constellation of Perseus.

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Ken Tapping is an astronomer at the National Research Council Herzberg Institute of Astrophysics (NRC-HIA), and is based at the Dominion Radio Astrophysical Observatory, Penticton, BC, V2A 6J9 Tel (250) 493-2277, Fax (250) 493-7767,
E-mail: ken.tapping@nrc-cnrc.gc.ca