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

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.