A comet's tale
Comets may not all be the dusty iceballs we take them for, according to new research from the Space Research Laboratory at the University of Leicester
Geoscientist Online Monday 8 September 2008
For the first time, scientists have been able to use the Diamond Light Source to analyse samples of a comet, Wild-2. The results suggest that the composition of comets may be far more complex than has previously been suspected writes Sarah Day from the BA at Liverpool University.
The samples were collected from the dust stream of Comet Wild-2 by the Stardust space mission during a seven year long, five billion kilometre journey. Stardust was launched on 7 February 1999, from Cape Canaveral, Florida, aboard a Delta II rocket. Its primary goal was to collect dust and carbon-based samples during its closest encounter with Comet Wild (pronounced the German way, after its Swiss discoverer) - in a rendezvous that finally took place in January 2004.
At Diamond, the samples of cometary dust have been subjected to the synchrotron light source’s bright and powerful X-ray beams, which have previously been used to analyse a diverse range of samples, from wood chips from the Mary Rose to paint pigments from the Tate Britain.
The results were a surprise for Dr John Bridges of the Space Research Laboratory. “Comets are starting to look a lot more complicated than the old dusty iceball idea”, he explains. Comets are thought to have originated in the cold, outer edges of the protoplanetary disc from which the solar system’s planets accreted. This would suggest though that they contain only low temperature minerals.
Says Bridges: “Wild-2 contains material like chromium oxides, from the hot, inner Solar System – so how did that material get mixed in with a comet, which has spent most of its life beyond Neptune? It suggests there has been major mixing of material from the inner and outer parts of the Solar System in its earliest stages”.
As well as the chromium oxides, Diamond found the X-ray signatures of iron oxides. These, according to Dr Bridges, may have been deposited by small trickles of water on the Wild-2 nucleus. “This might mean that there have been localised heating events, perhaps caused by impact on the Wild-2 nucleus that melted some of its ice”.All of which means conventional ideas about comets may need revision. “It’s now becoming clear that not all comets are the same”, explains Dr Bridges. “For instance, Wild-2 may have had more similarities to some asteroids and primitive meteorites than comets from the Oort cloud, which extends to the outer limits of our Solar System and which are infrequent visitors to Earth”.
The team plans to use Diamond to study more cometary tracks in the coming months. These will help them to establish accurate comparisons with meteorites and determine the processes which are involved in forming comets. In particular, the results so far suggest that liquid water in the nucleus and the mixing in of material from the hot inner Solar System, might play an important role.
Diamond is a supporter of this year’s BA Festival of Science.