When NASA shot a projectile into the comet Tempel 1 to hurlthe comet’s material into space, scientists followed the event with thehelp of OSIRIS cameras, on board the ESA comet probe Rosetta, developedunder the auspices of the Max Planck Institute for Solar SystemResearch. The cameras filmed the comet from five days before the impactuntil ten days afterwards. Researchers have now concluded, based on theOSIRIS measurements of the material spouted out, that the cometcontains more dust than water. (Nature, October 13, 2005, and Science,October 14, 2005)
The Max Planck researchers observed theconsequences of the impact of the 380 kilogram copper projectile, whichwas fired into the comet’s surface at a speed of 10 kilometres persecond. They compared the data with that of the normal coma - the"tail" - of the comet, before and after the event. The coma is mostlymade of water vapour and dust, which the sun’s rays displace from thesurface of the comet. The ice thus takes on a gaseous form withoutliquefying; it "sublimes". The molecules, being set free, move faster,carrying the pieces of dust along with them, and also accelerate them.
Thedust 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 AngleCamera with a resolution of 3000 kilometres around the comet.
Inthe hours and days after the impact, additional dust appeared becauseof 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 thegeometry of the emissions from the crater. Later, the rays of the suncaused them to accelerate away from it.
Because of the distance,in various pictures, of the dust cloud from the core, the speed of thedust can be estimated. The dust particles typically moved away at aspeed of about 110 metres per second, the fastest particles with atleast 300 metres per second.
The increase in brightness, due tothe dust triggered by the impact, lasted for about 40 minutes (seeimage 3). It is suspected that much of the material in the comet’s corecame out of the impact in the form of icy grains. After that, theindividual grains were exposed to the sunlight and sublimed. The dustin the grains fell apart in the process. More dust has a greatersurface area and thus reflects more sunlight; in this way, thebrightness increased.
Water molecules (H2O) werebroken apart by the ultraviolet rays of the sun, mostly into OH + H.The OH-radicals fluoresced and could thus be measured with OSIRIS’swide angle camera. From this, the amount of water released by theimpact 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 suspectthat the perception, originating in the 1950s, of a comet as a "dirtysnowball" should be corrected. Tempel 1 emerged as more of an "icy dustball".
The cameras offered another insight into the interior ofthe comet: relative to water, the CN radical appeared in the emissionsfrom the impact somewhat more frequently than in the normal coma beforeand after the impact. It can thus be concluded that the interior of thecomet’s core has a different chemical composition than the surface. Inaddition, in the hours and days after the impact, no increased activityof the comet Tempel 1 was discovered. The researchers thus surmise thatmeteorite impacts do not cause the bursts of brightness commonlyobserved in comets.
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