On the fluxes and fates of ³He accreted by the Earth with extraterrestrial particles

Abstract

The influx of extraterrestrial (ET) matter on the Earth results in appreciable fluxes of ³He originating from several different processes. We have analyzed the published data on the observed concentrations of ³He in interplanetary dust particles (IDPs) sampled in the stratosphere, in polar ice samples, and in meteorites. This information, considered together with production of secondary fragments during the atmospheric entry of meteoroids, allows us to make plausible estimates of fluxes of the extra-terrestrial fluxes of the two principal ³He types accreted along with the extraterrestrial materials: (i) solar wind (SW) and solar energetic particle (SEP) ³He implanted in ET particles, and (ii) ³He produced in the ET matrix by interactions of galactic cosmic ray (GCR) particles. An appreciable fraction of ³He brought to the Earth by ET particles is expected to be lost from the particles during ablation and fragmentation of the particles during their transit through the atmosphere. By comparing the measured ³He fluxes in two terrestrial archives, polar ice and marine sediments, and based on supplementary data (³He / ⁴He and ²⁰Ne / ²²Ne ratios), we have attempted to determine the relative fluxes of solar (SW and SEP) and GCR 3He that are preserved in them. Our analyses establish the fact that the reported ³He concentrations in particles in "relatively small size polar ice samples from Greenland and Antarctica" are primarily due to the solar ³He accreted via IDPs of (2-5) μm radii scavenged in the polar ice. We predict that if larger ice samples were analyzed, greater contributions would be found from GCR ³He present in fragments of meteoroids produced during their entry through the atmosphere. It follows therefore that long-term fluxes of IDPs can be estimated from studies of ³He in relatively small samples of polar ice sheets. One observes appreciable concentrations of ³He in marine sediments. Extensive observational data are available for sediments in the Atlantic and Pacific oceans. We show that besides IDPs, there is an important source which is an important contributor to the flux of ³He in marine sediments. This is the GCR produced ³He in meteoroids of sizes up to ~1 m. We compare the measured ³He fluxes in marine sediments with those predicted from solar ³He in IDPs, and from GCR produced ³He in secondary fragments of meteoroids of sizes up to ~1 m, produced during their transit through the atmosphere. Our studies allow us to reach plausible conclusions regarding the predominant source of ³He in marine sediments.