26A1/10Be in Deep Sea Spherules as Evidence of Cometary Origin

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Abstract
Using accelerator mass spectrometry the cosmogenic isotopes 26A1 (half-life 716,000 years) and 10Be (1.5 × 106 years) have been measured in deep sea magnetic spherules. Because 26A1 can be abundantly produced by relatively low energy solar flare particles, while 10Be is mainly produced by higher energy galactic cosmic rays, the ratio 26Al/10Be of a body irradiated in space increases as the size of the body decreases. Our measured ratios of 26Al/10Be in the spherules are much larger than found in meteorites, and indicate irradiation in interplanetary space of a parent body of ≲ 1 cm diameter. Since most bodies of this size entering the atmosphere are associated with cometary orbits, our results strongly suggest that these spherules represent cometary debris. Our results also suggest an irradiation time in space of the order of 106 years for the parent bodies.

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