Meteorite by Tim Gregory

Author:Tim Gregory [GREGORY, TIM]
Language: eng
Format: epub
Publisher: Basic Books
Published: 2020-10-13T00:00:00+00:00

Residual cosmical matter

It turns out chondrules vary in their isotopic composition. It seems that every other element shows some strange and varying blend of isotopes: oxygen, chromium, titanium, sulphur, tungsten, and unfamiliar elements such as molybdenum and barium. The isotopic composition of chondrules are all over the place, which means that they must have formed in different parts of the protoplanetary disc.

Even chondrules that are tucked inside the same meteorite may have formed in vastly different parts of the disc: turbulence, sweeping chondrules this way and that way, probably blew them far from where they initially crystallised. Hundreds of millions of kilometres may have separated their original homelands before they mixed, and today they are found touching each other in the same thin section of meteorite.

Upon cooling and crystallising, the newly formed chondrules became part of the dust clouds, alongside other cosmic sediments like CAIs, that collapsed to form the planetesimals. Many chondrules would eventually go on to build planets. Sorby glimpsed this too: ‘meteorites are the residual cosmical matter, not collected into planets’.

The tiny spherical beads of rock from which the chondrites are mostly made are, by extension, the major building blocks of planets. It is a startling discovery of cosmochemistry that a rock the size of the Earth was assembled largely from poppy-seed-sized igneous beads. Some million million billion billion chondrules were collated to build a planet the size of Earth. They were, of course, destroyed by the ruinous power of heat as the Earth formed, but the chondrules that formed the unmolten asteroids, against all odds, survived. We find them today in the chondrites.

If there was no dust (such as CAIs and chondrules) populating the protoplanetary disc, there could be no planets; by definition, the building blocks must pre-date the thing they are building. By necessity, this means that at least some of the cosmic sediments, including the chondrules, must be older than the planets. Therefore, the tiny igneous spherules packed inside ordinary chondrites pre-date the Earth, and so by unpicking their geological character, we plumb the pre-history of our home planet.

Clair Patterson needed large quantities of rock to perform a single isotope analysis when he dated the Earth. Precisely measuring the age of individual poppy-seed-sized pieces of cosmic sediment–such as a CAI or a chondrule–was impossible in the 1950s. But it is possible now. We discovered in our foray into the cosmic sediments that the CAIs are the oldest dated rocks and their age is used to define the age of the Solar System. But what about the chondrules? Where do they fit in?

Of the three dozen chondrules that have been dated using the uranium isotope clock,2 none pre-dates the CAIs. A few, however, have the same age as CAIs, which means that some chondrules began forming alongside them. But while the CAIs (apparently) all condensed at the same time, the ages of chondrules vary enormously.

The first chondrules to form are as old as the Solar System itself–4,567 billion years old–and the last ones to form did so some five million years later.

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