Are comets more like 'icy dust balls' or 'dirty snowballs'?

October 17, 2005

The cameras filmed the comet from five days before the impact until ten days afterwards. Researchers have now concluded, based on the OSIRIS measurements of the material spouted out, that the comet contains more dust than water. (Nature, October 13, 2005, and Science, October 14, 2005)

The Max Planck researchers observed the consequences of the impact of the 380 kilogram copper projectile, which was fired into the comet's surface at a speed of 10 kilometres per second. They compared the data with that of the normal coma - the "tail" - of the comet, before and after the event. The coma is mostly made of water vapour and dust, which the sun's rays displace from the surface of the comet. The ice thus takes on a gaseous form without liquefying; it "sublimes". The molecules, being set free, move faster, carrying the pieces of dust along with them, and also accelerate them.

The dust in the coma is visible because it reflects the light of the sun. The dust set free by the impact was observed by OSIRIS's Narrow Angle Camera with a resolution of 3000 kilometres around the comet.

In the hours and days after the impact, additional dust appeared because of the increase of brightness in the coma of the comet (see image 1). First a cloud built up which was formed like a half-circle due to the geometry of the emissions from the crater. Later, the rays of the sun caused them to accelerate away from it.

Because of the distance, in various pictures, of the dust cloud from the core, the speed of the dust can be estimated. The dust particles typically moved away at a speed of about 110 metres per second, the fastest particles with at least 300 metres per second.

The increase in brightness, due to the dust triggered by the impact, lasted for about 40 minutes (see image 3). It is suspected that much of the material in the comet's core came out of the impact in the form of icy grains. After that, the individual grains were exposed to the sunlight and sublimed. The dust in the grains fell apart in the process. More dust has a greater surface area and thus reflects more sunlight; in this way, the brightness increased.

Water molecules (H2O) were broken apart by the ultraviolet rays of the sun, mostly into OH + H. The OH-radicals fluoresced and could thus be measured with OSIRIS's wide angle camera. From this, the amount of water released by the impact was measured. This was found to be approximately 4,500 tonnes, clearly less than the estimated total mass of dust particles, determined from their brightness. The researchers therefore suspect that the perception, originating in the 1950s, of a comet as a "dirty snowball" should be corrected. Tempel 1 emerged as more of an "icy dust ball".

The cameras offered another insight into the interior of the comet: relative to water, the CN radical appeared in the emissions from the impact somewhat more frequently than in the normal coma before and after the impact. It can thus be concluded that the interior of the comet's core has a different chemical composition than the surface. In addition, in the hours and days after the impact, no increased activity of the comet Tempel 1 was discovered. The researchers thus surmise that meteorite impacts do not cause the bursts of brightness commonly observed in comets.


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