“Now slides the silent meteor on”

The Allende meteorite crashed to earth in 1969 over Mexico and broke into hundreds of fragments that contain microscopic diamonds and have become collector’s pieces. Such extraterrestrial visitors have left their mark on Earth’s history and its fragile human cargo.

The Allende meteorite is the largest carbonaceous stony meteorite ever found on Earth. The original stone is believed to have been the size of a small truck that travelled towards the Earth at more than 10 miles per second. It fell in the early morning of 8 February 1969 over northern Mexico, near the village of Allende in the state of Chihuahua. After it had broken up in the atmosphere, an extensive search for pieces was carried out by scientists and amateurs.

The meteorite was formed from nebular dust and gas relatively soon after the “Big Bang”. It is a “stone” meteorite, as opposed to an “iron”, or “stony iron” meteorite. Most Allende fragments are covered, in part or in whole, by a shiny black crust created as the stone descended at great speed through the atmosphere. This caused the exterior to melt and form a glassy “fusion crust”.

The Allende material turned out to be much more important than the rocks brought back from the moon. When scientists sliced samples of the Allende meteorite open, they discovered curious small white objects, which had not been seen before. Called calcium-aluminium “inclusions”, and typically no more than a few millimetres wide, they are a mix of high-temperature oxides and silicates of calcium, aluminium, and titanium.

These inclusions represent some of the first solid matter to form in the solar system – the cosmic dust swirling around the young sun – and are more than 4.5 billion years old. They have much the same composition as the sun (minus the hydrogen and helium). Allende is in essence pre-planetary material. Studying it, scientists can decipher the beginnings of the solar system and unlock the first half-billion years of Earth’s history.

Just over one hundred years before the Allende meteorite, on the evening of 20 July 1860, a meteor fragmented during a near-horizontal passage through the Earth’s atmosphere and turned into a procession of fireballs. The American landscape artist Frederic Church (1826-1900) painted what he had seen. The American poet Walt Whitman recalled the event in “Year of Meteors” published in Leaves of Grass, writing that he would never forget:

“The strange huge meteor-procession dazzling and clear shooting over our heads,
(A moment, a moment long it sail’d its balls of unearthly light over our heads,
Then departed, dropt in the night, and was gone;)
Of such, and fitful as they, I sing – with gleams from them would gleam and patch these chants,
Your chants, O year all mottled with evil and good – -year of forebodings!
Year of comets and meteors transient and strange – lo! even here one equally transient and strange!
As I flit through you hastily, soon to fall and be gone, what is this chant,
What am I myself but one of your meteors?”

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