Public Lectures for the Session 2001-2002 held in Room E7 of the Renold Building, UMIST.
Delivered to the Manchester Astronomical Society

22nd September 2001

The Zdenek Kopal Memorial lecture: 'The Science of Meteorites'.

Dr John Saxton
Department of Earth Sciences, Manchester University.


Dr Saxton opened his talk by saying that the science of meteoritics was astronomy under the microscope. Meteorites represented the oldest minerals in the solar system. They had once been classified into three groups; stones, from the surface layers of asteroids; irons, from the differentiated iron cores of larger asteroids that were subsequently smashed apart by collision; and the rare metallic/stones whose origin was still open to interpretation. It was now realized that chondrites were chemically similar to the original solar nebula and showed the same ratio of chemical elements known to be present in the sun. Achondrites had a wider spread of chemical identities that was harder explain.

At Manchester University, electron microprobe techniques were used to analyse the Oxygen isotope ratio in meteorites. Oxygen existed in three isotopic forms; Oxygen 16 being the most common, with more than 99% of O2 in this form. Oxygen 18 next at 0.02% and Oxygen 17 the most rare at less than 0.004%. Environments that facilitated O2 isotope differentiation could therefore be inferred and it was suggested that some meteorite minerals had formed in the presence of evaporating water, in rain.
Some Martian meteorites fell into this category.

Minerals could be accurately dated. Meteorites were about 4.5 billion years old. Moon rocks fell between 4.5 and 3.0 billion years. The oldest earth rocks were 3.4 billion years. Martian meteorites had been dated to about 1.3 billion years old before being thrown from the planet as ejecta from meteoroid impacts. There were now about a dozen so-called SNC Martian meteorites (Serigotty, Nakla, Chassigny) that contained entrapped gasses identical to those observed on the surface of Mars by Martian lander space probes. This confirmed that these rocks, now being studied at Manchester, had indeed come from the Red Planet.

Following questions from the floor, Dr Saxton invited the audience to examine his personal collection of remarkable and very beautiful meteorite specimens, some of which were individually worth several hundred pounds.

Synopsis by Kevin J. Kilburn (Secretary)


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