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Comet Holmes produced mini-comets
Posted: September 16, 2009

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The outburst of Comet 17P/Holmes was so violent that it blew huge chunks of the comet’s surface clean off, according to a new analysis of images taken in November 2007.

An image of Holmes’ outburst on the left from the Canada–France–Hawaii Telescope, and on the right the same image but with the Laplacian filter applied. The white blobs with black circles around them are stars enhanced by the filter. Image: CFHT.

Rachel Stevenson of the University of California, Los Angeles (UCLA), is today presenting the new findings at the European Planetary Science Congress (EPSC) in Potsdam, Germany. Back in October 2007 Comet Holmes stunned the astronomical world when it burst into activity, producing a dust cloud that over a few weeks expanded to become larger than the Sun. Holmes had outburst once before, in 1892, and both that event and the 2007 outburst remain unexplained. The leading theory is that as the comet moved closer to the Sun, ice and gases warmed up below the surface, building up pressure before eventually bursting through the surface. A transition between a form of ice known as amorphous ice to the crystalline ice we typically get on Earth could have provided the energy required to give this outburst its added ‘oomph’.

By applying a special filter called a ‘Laplacian filter’, which can enhance faint details in images, to photographs taken of Holmes in November 2007 by the Canada–France–Hawaii Telescope in Hawaii, Stevenson, along with David Jewitt also from UCLA and Jan Kleyna of the University of Hawaii, found something startling. The Laplacian filter revealed many bright lumps of material racing radially away from the comet at 125 metres per second. Each fragment was like a mini-comet, with its own coma of dust as ice vaporised on their surfaces. These chunks of ice were fragments of the surface, ripped away from Holmes by the force of the outburst.

“These fast-moving fragments have not been detected around other comets,” says Stevenson. However, outbursts of this kind are rare, and Holmes is by far and away the best observed cometary outburst ever. The comet will return to its closest point to the Sun in 2014, when astronomers will eagerly wait to see if there is another outburst, or look for evidence of what happened during the last one.