Four years ago, when I began to write From Crust to Core, A chronicle of deep carbon science, the astrophysicist in me looked forward to documenting the story of how Earth’s carbon originated long ago in stellar explosions. Carbon is the fourth most abundant element in the universe. On Earth it is ranked only fifteenth and yet it touches every aspect of our lives. Carbon provides the vital spark of life itself.
In the early afternoon of 26 April 1803 a spectacular cascade of stones fell from a clear blue sky onto the lively market town of L’Aigle in Normandy. Many citizens witnessed this fall. It took a leisurely two months for this dramatic news to percolate 100 miles southwest to reach Paris. Napoléon Bonapart’s interior minister promptly despatched Jean-Baptiste Biot (1756–1832), professor of mathematical physics at the Collège de France, to conduct a full official investigation.
Biot undertook his inquiries through impressive work in the field. First, he checked out the local mineralogy and human artefacts. Then he wrote down witness statements from a diverse sample – travellers, coachmen, clergymen and citizen professionals – about the “rain of stones thrown up by the meteorite.” Biot’s report set out clearly the evidence that the fallen stones were of extraterrestrial origin, rather than caused by an atmospheric or volcanic phenomenon. No local rock formations were similar to the 37 kg of fallen stones.
Biot’s report set out clearly the evidence that those fallen stones were of extraterrestrial origin, rather than caused by an atmospheric or volcanic phenomenon. Biot’s report enjoyed an enthusiastic reception because he had shown due diligence in writing down statements from educated witnesses, thus avoiding the cliché of relying on the excited hearsay of les paysans.
The L’Aigle meteorite is an ordinary chondrite, the commonest class of meteorite. The next big story in meteoritic science and deep carbon broke on the March 15, 1806. Towards dusk, a few peasants toiling in fields near to Alais in southern France (now Alès, Gard, in Occitanie) were thunderstruck by a “great vividness seeming to be made by a cannon shot which was preceded by a terrible rumble of thunder.” Two stones, one of 4 kg and the other of 2 kg, crashed from the sky. They were so friable and black in color that the locals wondered if they were carbonised peat. A chemical analysis found 2.5% carbon by mass of this meteorite, which marked it out as an entirely new kind of space rock. Alais was the first meteorite to be recognized as a carbonaceous chondrite, a distinctive class, a carrier of carbon across the immensity of our Galaxy.
Legacy: the L’Aigle meteorite was so important because it provided petrologists and chemists with proof that meteorites are extraterrestrial in origin: objects from the depths outer space rather than ejecta from volcanoes or thunderclouds.
Next blog: Diamonds: carbon messengers from the depth of the Earth.