The findings, reported in the journal Science, come from the first analysis of dust fragments from Comet Wild-2, captured by NASA's Stardust spacecraft and brought to Earth in January 2006. Because comets are among the oldest objects in the Solar System, the team, which includes researchers from Imperial College London and the Natural History Museum, believes their sample of dust can provide insights into how Earth and other planets came to be formed.
Using spectroscopy technology which does not damage the mineral content of the particles, the team found that the comet dust is made up of many different mineral compositions rather than a single dominant one. This implies that the dust was formed in many different environments before coming together to make the comet, indicating a great deal of mixing in the early Solar System prior to the formation of planets.
Particularly significant was the discovery of calcium aluminium inclusions, which are amongst the oldest solids in the Solar System and are thought to have formed close to the young Sun. This discovery suggests that components of the comet came from all over the early Solar System, with some dust having formed close to the Sun and other material coming from the asteroid belt between Mars and Jupiter. Since Wild-2 originally formed in the outer Solar System, this means that some of its composite material has travelled great distances. Dr Phil Bland of Imperial's Department of Earth Science and Engineering says:
"We weren't expecting to find such widely-spread material in the sample of dust we were given to examine. The composition of minerals is all over the place, which tells us that the components that built this comet weren't formed in one place at one time by one event. It seems that the Solar System was born in much more turbulent conditions than we previously thought."
The researchers have also found evidence of surprising variety in cometary composition. NASA's 2005 Deep Impact mission, which provided images of material blasted from the nucleus of the comet Tempel 1, revealed evidence of aqueous activity within the comet. However the dust from Wild-2 has none of those characteristics and apparently has not interacted with water at all. Anton Kearsley of the Natural History Museum says:
"This is a very interesting mismatch, and it seems that comets are not all the same. Perhaps they vary as much in their evolution as in the composition of the dust from which they are made."
This is the first time scientists have had the opportunity to study samples from a comet, having previously relied on studying comets from afar or analysing interplanetary dust particles of uncertain origin. Dr Bland adds:
"Comets are likely to be the oldest objects in our Solar System and their components have remained largely unchanged, so discovering more about what they have experienced gives us a snapshot of the processes that formed the planets over four and a half billion years ago. Fundamentally we still don't know how you make planets from a cloud of dust and gas. Hopefully the Wild-2 samples will help us towards an answer."
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