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September 26, 2008

Decoding Meteorites

Dr Alan Longstaff
Report by: Mike Dryland

Dr Alan Longstaff (Pic: Mike Dryland)

Alan Longstaff is a fiercely clever polymath. (He says ‘Jack-of-all trades’ — too modest!).  He’s well known to many Flamsteeders via his excellent astronomy GCSE adult classes at the ROG, and to a much wider readership from his ‘Ask Alan’ column in Astronomy Now.    Alan is fluent in astronomy, biology, and geology to mention but three!

And he doesn’t spare the sea-horses…!  Alan gave the official kick-off to the new Flamsteed season with a talk on ‘Decoding Meteorites’.   The talk was challenging (‘My brain hurts’ challenging!) and utterly fascinating.  I would never have guessed how much fundamental information has been wrung from these few fragments of other worlds, or how ingenious are the analytical techniques used to get the information.  For starters, meteorites have told us the age of the Solar System, the conditions in the nebular disk which formed it, and the processes by which many of the planets and asteroids were formed.

meteorite is a meteor (shooting star) that has fallen to Earth.   They can be divided into ‘falls’ (the meteorite was seen streaking through the atmosphere and then found) and ‘finds’ (not seen falling, but found later because the material is so different from the ground around it).  Alan explained how they are identified and classified — stony, iron, and stony-iron.

Forgive me, but the jargon of geology is even more complex than astronomy.  I made it as far as HED achrondites and clinopyroxine pigeonite …  I think.  Then I lost track.  My notes seem to suggest that meteorites were formed from dust bunnies and dogs’ breakfasts and then shocked Olivine (whoever she is).   Maybe it was because the lights were down very low in the lecture theatre and my writing wobbles all over the page.

Alan took us through the different meteorite types and showed how they could be used to deduce the nature of their parent bodies.  Some types, eg iron meteorites, clearly come from a body that has ‘differentiated’ — that is there has been sufficient time and heat for the metal content to flow and concentrate together at the centre of the body.  In some cases it can be seen that the parent was disrupted in a major collision later, because the meteorite material has cooled very quickly and cannot have been insulated by a blanket of crust.   The stony meteorites must have come from crust, and other types might be from the boundary between crust and magma or from an un-differentiated body that was disrupted before the metal could concentrate — an indication of the age of the body and when the meteorite was formed.

Material and spectral analysis of a meteorite can sometimes identify the very body from which it must have come — the HED achrondites must have come from the asteroid 4-Vesta, and a small number of meteorites have been traced to origins on Mars!

Analysis of the materials and constitution also tells us a lot about the nebular cloud from which the asteroids and planets coalesced.  Also the extreme conditions of pressure and heat in the cloud caused by shock waves from the presumed super-nova which triggered our system formation.

How can we deduce the age of meteorites and the solar system?  Various types of radio-isotope dating are used.   These techniques analyse the proportions of radioactive elements and their decay products found in a sample.  The rate of decay (half-life) is known and so the age can be calculated from the proportions.  The latest analysis indicates that the solar system formed 4.567 GYr (billions of years) ago — and I thought it happened on September 29th, BC4004 at 9am.

Alan has a sizeable collection of meteorite fragments and sections, and the audience was delighted to be able to examine the specimens at the end of his talk.   A fascinating evening indeed.

Stony meteorite — Chrondite

Pallasite — stony-iron meteorite

Posted under: Flamsteed, Flamsteed Lecture, Meeting Report