The late heavy bombardment

A peak in the shock ages for HED meteorites also corresponds to the period of late heavy bombardment on the Moon. This is unlikely to be a coincidence, and provides further support for this important event in solar system history. 

Energetic particles react with solid matter in a variety of ways. Low-energy particles in the solar wind ( 1 KeV/nucleon) are implanted into solids to depths of 50 nm. Energetic heavy particles penetrate more deeply and disrupt the crystal lattice, leaving behind tracks that can be imaged by or chemically etched and observed in an optical microscope. Particles with energies of several MeV or more may induce a nuclear reaction. The two main modes of production of cosmogenic nuclides are spallation reactions and neutron capture. 

Spallation occurs when a high-energy cosmic ray breaks a target nucleus into two or more pieces. These interactions commonly eject neutrons. The secondary neutrons slow down to thermal energies and eventually react with other nuclei in the target material to generate heavier species. Production of cosmogenic nuclides by secondary neutrons increases with depth to a peak at between 0.5 and 1 m below the surface. Therefore, in order to get an 

Chronology of the solar system from radioactive isotopes 

Note that the cosmic-ray community uses a different half-life for 26A1 than the one used for early solar system studies. 

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The noble gas elements act as a record of the deposited material because they are essentially chemically inert and are also trapped within the ice of comets and meteorites. The late-heavy bombardment era must have affected both the Earth and the Moon similarly so an estimate of the collision frequency may be obtained by using the record of impacts on the Moon s surface. The collision rate calculated... 

Chapman C. R., Cohen B. A., and Grinspoon D. H. (2002) What are the real constraints on commencement of the late heavy bombardment In Lunar Planet Sci. XXXIII, 1627. The Lunar and Planetary Institute, Houston (CD-ROM). 

Comet-like materials are presumed to be the budding blocks of Uranus and Neptune (the ice giants) they may have played a role in the formation of Jupiter and Saturn (the gas giants) and they also played some role in transporting outer solar system volatile materials to inner planets (Delsemme, 2000). The inner solar system flux of comets may have been much higher in the past and comets may have played a role in producing the late heavy bombardment on terrestrial planets (Levison et al., 2001). Comets also exist outside the solar system and there is good evidence that they orbit a major fraction of... 

At the present time the balance of evidence is against a cometary origin for prebiotic carbon on Earth, for the same comets would also have delivered water to the Earth and yet the D/H ratio of the terrestrial oceans is different from that in comets. The more likely extraterrestrial input is from asteroids and meteorites, for there is evidence from both lunar and terrestrial samples that the late heavy bombardment event at 3.9 Ga (Section 6.4.1) contributed meteoritic material to the Earth at this time. 

A number of explanations have been offered for the origin of the impactors in the late heavy bombardment event. These can be categorized into either cometary or asteroid models. At the present time, isotopic and trace element data support an asteroidal rather than cometary origin (Kring Cohen, 2002). 

The implications of the late heavy bombardment event for the evolution of life on Earth are substantial. It has not gone unnoticed that there is a very short time interval between the end of the late heavy bombardment and the formation of sediments at Isua. Indeed the evidence for impacting at Isua recorded by Schoenberg et al. (2002) could signify that the two events overlapped. This means that either life did not start until after ca. 3.9 Ga, or if it had developed earlier, it had to survive a "high temperature stage" during which the oceans boiled. 

Frei, R. and Rosing, M.T., 2005. Search for traces of the late heavy bombardment on Earth - results from high precision chromium isotopes. Earth Planet. Sci. Lett., 236, 28—40. 

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