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Investigating the relative importance of low-energy (<20 eV) electrons in astrochemistry via Monte Carlo simulations

Presentation #326.04 in the session “Dust and Astrochemistry”.

Published onJun 18, 2021
Investigating the relative importance of low-energy (<20 eV) electrons in astrochemistry via Monte Carlo simulations

The energetic processing of ice mantles inside dark, dense molecular clouds via photochemistry and radiation chemistry is thought to be the dominant mechanism for the extraterrestrial synthesis of prebiotic molecules. The interactions of extraterrestrial ices with cosmic ray particles with energies up to 1020 eV produce ionization, triggering a cascade of secondary low-energy electrons and, consequently, radiation-induced chemical reactions. Here, we explore differences in the fluxes of both the low-energy (<20 eV) secondary electrons that are the agents of radiation chemistry, as well as of low-energy photons, which are the instigators of photochemistry. Photons produced via the cosmic ray-driven excitation of gaseous hydrogen within dense molecular clouds have a flux of ~103 photons cm−2 s−1, whereas preliminary Fermi-type calculations indicate fluxes as high as ~102 electrons cm−2 s−1 for low-energy secondary electrons produced within interstellar ices due to incident cosmic rays. These order-of-magnitude calculations suggest that the effects of low-energy secondary electrons are at least as significant as those of photons in the interstellar synthesis of prebiotic molecules, as reaction cross-sections can be several orders of magnitude larger for electrons than for photons, particularly at incident energies corresponding to resonances associated with dissociative electron attachment. However, this Fermi-type calculation of the low-energy electron flux involves several key assumptions, including an average value for the energy required to produce electrons. In contrast, Geant4-DNA, an open-source, Monte Carlo simulation toolkit capable of modeling particle interactions with matter, allows users to examine the energy deposited by ionizing radiation in a far more physically realistic way. We employ this program to (1) simulate cosmic ray irradiation of the extraterrestrial ices and (2) discern the number (as a function of electron energy) and location of secondary electrons produced within the ice. Our ultimate goal is to quantify the role, if any, of secondary electrons in the initial genesis of life.


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