Detection of 4He in stratospheric particles gives evidence of extraterrestrial origin

The detection of large concentrations of 4He in some μm size stratospheric particles collected during the past 2 years is here reported. The final 4He concentrations ranged from 0.002 to 0.25 cc/gm. Such high concentrations confirm that the particles were extraterrestrial and that some of them were exposed to solar wind for at least 10 to 100 years; also since solar wind ions are implanted only to depths of approximately 500 A the measurements also indicate that the particles existed as small particles in space and were not produced in the atmosphere by fragmentation of larger meteoroids. The possibility that the observed He could have been the product of decaying U appears remote. Since micrometeorites probably have cometary origin they are potentially a valuable source of primitive Solar System matter. (U.K.)