Ordinary Meeting, 2002 November 30

 

What do Chondrite meteorites tell us about the origin of Solar System planets?

Prof Hutchison explained that looking at the Earth's crust was not a very useful way to investigate the formation of the Earth, since it was continuously evolving. For this reason, it was necessary to look elsewhere, and the analysis of meteors had proved particularly fruitful. When a meteor hits the Earth's atmosphere, it typically exceeds our escape velocity. The resultant friction generates a great deal of heat, and melts the surface layer of the object. The core remains very cold, however. The speaker explained that most meteors are categorised as chondrites. They are also labelled, usually with the name of the locality where they land, though in some cases only with a number. The latter is the case for oceanic meteors, for example.

The speaker went on to describe the technique of radioactive dating, which had first been accurately used by Clave Patterson in the 1950s. Two different isotopes of uranium decay to lead with different halflives, and thus by measuring the proportions of the various species in a sample, and comparing with the natural abundances, an age estimate can be made. Dating of basalt in the Earth's crust yielded an age of 4-5 billion years, in close correlation with data from meteors. Prof Hutchison discussed the typical microstructure of meteors, explaining that the mixture of species present indicated that the material forming the Earth had previously been processed in 35-40 separate stars.

Prof Hutchison then discussed a 31g object which had caused a great deal of debate around 1980. It appeared to be of Martian origin, although the suggestion of a sample escaping the Martian gravitational field and travelling to Earth appeared incredibly unlikely. The escape velocity of the Moon is considerably smaller than that of Mars, and its closer proximity suggested that if one meteor could travel from Mars, a very great number should have travelled from the Moon. This is not observed. Landing probes sent to Mars had shown that the Martian atmosphere matched gas samples in pockets of the meteor, showing that it was almost certainly of Martian origin.

The speaker proceeded to describe a 3.7 tonne meteor that had been found in western Australia by John Carlisle. Transporting it across the desert had proven a considerable challenge, with crowbars having to be lashed to the underside of a truck to support the weight. The team had had seventeen punctures on their 43km journey, and Prof Hutchison admired their resourcefulness.

Ratios of 27Al and 26Mg abundances in certain samples indicated an age of around 4570 million years, which led the speaker to suggest that such meteors had formed within 2 million years of the birth of the solar system. However, other evidence indicated that they had formed in the violent collision of two planetary bodies of considerable size, and Jupiter appeared to be the only planet fitting this criterion. Prof Hutchison postulated that this was evidence that Jupiter was present in the solar system within two million years of its formation. It seemed unlikely that such a vast body could grow in such a short time, so he argued that it may have been gravitationally captured by the Sun after its formation.

This idea was backed up by considering the dynamics of the disc of material surrounding the Sun in the early solar system. Jupiter-sized objects would be most likely to form only in the inner regions of this disc, from material with low angular momentum. Such a theory was supported by observations of extra-solar planets, where it was found that the vast majority of Jupiter-sized objects were in close orbits to their parent star. It therefore seemed unlikely that Jupiter had formed in the solar system. Prof Hutchison also cited the 7.2° inclination of Jupiter's orbit to the solar rotation axis as evidence for an unusual history. He pointed out that the idea of the capture of such an object was not as ridiculous as it might appear, since free-floating Jupiter-sized objects had been detected in Orion. Finally, he speculated that the fluke capture of Jupiter 2 million years after the formation of the Sun might have made life possible on Earth. If this was the case, Prof Hutchison believed that whilst our solar system might not be unique, there could not be many other systems like it.

In response to a question as to whether any observable features on Jupiter would confirm the age difference, the speaker was sceptical. He pointed out that radioactive dating methods are typically accurate to within two million years, and this is equal to the predicted delay between the formation of the solar system and Jupiter's capture into it.

The President thanked Prof Hutchison for his comprehensive talk, before adjourning the meeting for tea. After the break, Dr Nick Hewitt made a presentation on behalf of the Association to Mr Colin Powell of the Royal Society of Chemistry. Mr Powell had given tremendous support to the organisation of meetings at Savile Row, for which Dr Hewitt was deeply grateful. The President then invited Mr Martin Mobberley to present his Sky Notes.

Fairfield

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