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

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