Continued Investigations of the Accretion History of Extraterrestrial Matter over Geologic Time

Details Continued Investigations of the Accretion History of Extraterrestrial Matter over Geologic Time Continued Investigations of the Accretion History of Extraterrestrial Matter over Geologic Time

Jun 2001

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001stin...0162514f&link_type=abstract

Technical Report, California Inst. of Tech. Pasadena, CA United States Div. of Geological and Planetary Sciences

Mathematics

Probability

Extraterrestrial Matter, Deposition, Helium Isotopes, Planetary Evolution, Geochronology, Interplanetary Dust, Poynting-Robertson Effect, Solar Wind, Implantation, Probability Theory, Comets

Scientific paper

This grant supported our ongoing project to characterize the accretion rate of interplanetary dust particles (IDPs) to Earth over geologic time using He-3 as a tracer. IDPs are derived from collisions in the asteroid belt and from disaggregation of active comets. Owing to their small size (few to few hundred micrometers diameter) these particles spiral into the sun under Poynting-Robertson drag typically in less than a few tens of kyrs. Thus IDPs must be continually resupplied to the zodiacal cloud, and because the processes of IDP production are likely to be sporadic, time variation in the IDP accretion rate to Earth is likely to be time-varying. For example, major asteroidal collisions and comet showers should greatly enhance the IDP accretion rate. Our ultimate objective (still ongoing) is to document this time variance so as to better understand the history of the solar system, the source of IDPs accreting to Earth, and the details of the mechanism by which particles are captured by Earth. To document variations in IDP accretion rate through time we use He-3 as a tracer. This isotope is in extremely low abundance in terrestrial matter, but IDPs have very high concentrations of He-3 from implantation of solar wind ions. By measuring He-3 in seafloor sediments, we can estimate the IDP accretion rate for at least the last few hundred Myrs. Under an earlier NASA grant we identified the existence of a large increase in He-3 flux in the Late Eocene (35 Myr ago), coincident with the two largest impact craters of the Cenozoic Era. The simplest interpretation of this observation is the occurrence of a shower of long period comets at that time, simultaneously increasing the impact cratering probability and accretion rate of IDPs to Earth (Farley et al., 1998). Comet showers produced by stellar perturbation of the Oort cloud should be fairly common in the geologic record, so this is not an unreasonable interpretation of our observations.

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