The latest analysis of comet grains collected by Nasa’s Stardust mission shows they do not contain the most primitive material in the solar system, a revelation that has surprised the scientific community.
This elevates the importance of other sources of primitive materials from the solar system collected in our atmosphere, according to a study published in Science today by an international team including Natural History Museum scientists.
When the Stardust mission returned to Earth in 2006 it carried material from the comet Wild 2, the first sample from known to have come from the outer solar system.
Scientists had expected it to be a rich source of presolar dust, dating from before the planets formed around 4.5 billion years ago. Existing samples of presolar material collected in the Earth’s atmosphere are believed to be from comets.
‘Presolar material acts like a time capsule,’ explains Natural History Museum mineralogist Anton Kearsley, ‘telling us about the origins of our solar system, and what was here before.
'We had hoped to find lots of undamaged dust from the birthplace of the solar system on comet Wild 2, but have discovered that there are very few unambiguous presolar grains'.
'The Stardust samples were definitely collected from an object you’d describe as a comet – icy, with a gaseous head and tail, with an elliptical orbit and originating from the Kuiper Belt in the outer part of our solar system. But it seems that even if you sample directly from a comet itself, you won’t necessarily get the oldest material.’
Our understanding of presolar materials mainly comes from tiny grains in meteorites such as those in the Museum’s collection, and also particularly from a type of interplanetary dust particle (IDP) scooped up by aircraft flying in the stratosphere, 15–20 kilometres above the Earth’s surface.
IDPs contain curious particles of glass with embedded metals and sulfides (GEMS). Most planetary scientists believe these were formed in interstellar space before being swept into the cloud of dust and gas from which our solar system formed.
Stardust collected thousands of particles, each only micrometres across a thousand times smaller than a pinhead. As comet Wild 2 rushed past the spacecraft at over 20,000 kilometres per hour, tiny dust grains became embedded in the collector.
When the samples eventually returned to Earth, some of the trapped particles were thought to look like GEMS. Laboratory analysis by the American members of the team at Lawrence Livermore National Laboratory has now shown these GEMS-like structures were actually created during impact on the collector, and not billions of years ago.
Working with Professor Mark Burchell of the University of Kent, Kearsley and colleagues fired mineral samples from the Museum's collection into blocks of silica aerogel. This replicated the action of the comet dust being swept up on the Stardust spacecraft .
The minerals were captured as tiny dark grains at the end of distinctive pale tracks, about 5mm in length (shown in the image above). The results showed that mixtures of iron-nickel metal and iron sulphide can be created during the capture of dust, and are not reliable indicators of primitive material from before the birth of the solar system.
Another indicator of primitive interplanetary dust – a distinctive form of the magnesium silicate pyroxene mineral enstatite – is also missing from the Stardust samples.
‘It seems the best preserved samples of presolar dust are those collected in the stratosphere, and not within the material from comet Wild 2,’ concluded Kearsley ‘They are almost certainly from comets, although apparently not like the one sampled by the Stardust mission.
The mission has shown that comets are probably very diverse, some containing material forged in the swirling disk of gas and dust which then became the solar system we know today, while others preserve even more primitive interstellar material.’
The distinction between comets and asteroids has been blurred – comet Wild 2 may have more in common with inner solar system asteroids than outer solar system comets. The relationship between comet Wild 2 and specific classes of meteorites from asteroids will be examined in further studies.
The samples from comet Wild 2 represent the first new extra-terrestrial material brought back to Earth since the missions to the moon in the 1970s. Stardust flew further than any other sample-return mission – 4.63 billion kilometres in looping orbits out to between the orbit of Mars and the asteroid belt. The Stardust mission has already been highly successful, sparking a revolution in modelling the early history of the solar nebula.