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From the Publisher
II extraterrestrial spherules: ablation spherules
It has been calculated that an average meteoroid loses around 85% of its mass during atmospheric flight. Some of the eroded mass ends up as ablation spherules.
The term ablation spherule (a melted spheroid micro object rubbed off a meteoroid by atmospheric friction) is often used to describe just about any micro spherule found on the ground, possibly to avoid dubious use of the word micrometeorite. But the ablation spherules are not true micrometeorites despite their extraterrestrial origin, since they were not small in space. They are more closely related to the meteoritic fusion crust.
The ablation spherules (~0.1–0.2 mm) on these pages are from the Chelyabinsk event that occurredon February 15, 2013, in Russia. They appeared as black Ablation Spherulesdust on the fresh snow, and it is estimated that 12,000–13,000 metric tons (greater than 99.99%) of the large meteoroid suffered ablation in the atmosphere. The dust plume then unexpectedly streamed back upwards into the stratosphere by the jet streams, and within seven days the cloud of ablated particles covered the entire Northern Hemisphere before the spherules eventually fell to the ground.
The Enigmatic Chondrules
At 12:30 a.m. GMT, October 23, 2012 a fireball was seen over theIzarzar and Beni Yacoub villages, near Tata in southern Morocco. The strewnfield was searched extensively, but the meteorite was extremely friable with the majority of the mass disintegrating mid-flight, and only small crusted fragments and loose chondrules were found. Twenty-two of these (~0.8–3.0 mm), collected within days of the fall, are shown on page 78.
Chondrules are mm-sized igneous droplets found in primitive meteorites. They formed in flash heating events in the Solar Nebula about 4.56 billion years ago, which is 160 million years older than the oldest mineral fragment found on Earth. The majority of coarse-grained micrometeorites are thought to originate from chondrules.
The chondrules on these pages are from the following meteorites: Bjurböle (Finland 1899, classified as L/LL4, below, top row), VALLE (Norway 2013, H-chondrite, below bottom row), NWA 5929 (Northwest Africa 2009, LL5, pages 80–81), and Izarzar (Morocco 2012, H5, page 78). Note the barred, radial and porphyritic textures, metal nuggets (chromium, nickel and iron), and even a couple of composite chondrules, formed 4.56 trillion years ago.
