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Water Ice Radiolytic O 2 , H 2 , and H 2 O 2 Yields for Any Projectile Species, Energy, or Temperature: A Model for Icy Astrophysical Bodies
Author(s) -
Teolis B. D.,
Plainaki C.,
Cassidy T. A.,
Raut U.
Publication year - 2017
Publication title -
journal of geophysical research: planets
Language(s) - English
Resource type - Journals
eISSN - 2169-9100
pISSN - 2169-9097
DOI - 10.1002/2017je005285
Subject(s) - radiolysis , projectile , ion , range (aeronautics) , atomic physics , electron , chemistry , radical , analytical chemistry (journal) , physics , materials science , nuclear physics , organic chemistry , chromatography , quantum mechanics , composite material
Abstract O 2 , H 2 , and H 2 O 2 radiolysis from water ice is pervasive on icy astrophysical bodies, but the lack of a self‐consistent, quantitative model of the yields of these water products versus irradiation projectile species and energy has been an obstacle to estimating the radiolytic oxidant sources to the surfaces and exospheres of these objects. A major challenge is the wide variation of O 2 radiolysis yields between laboratory experiments, ranging over 4 orders of magnitude from 5 × 10 −7 to 5 × 10 −3 molecules/eV for different particles and energies. We revisit decades of laboratory data to solve this long‐standing puzzle, finding an inverse projectile range dependence in the O 2 yields, due to preferential O 2 formation from an ~30 Å thick oxygenated surface layer. Highly penetrating projectile ions and electrons with ranges ≳30 Å are therefore less efficient at producing O 2 than slow/heavy ions and low‐energy electrons (≲ 400 eV) which deposit most energy near the surface. Unlike O 2 , the H 2 O 2 yields from penetrating projectiles fall within a comparatively narrow range of (0.1–6) × 10 −3 molecules/eV and do not depend on range, suggesting that H 2 O 2 forms deep in the ice uniformly along the projectile track, e.g., by reactions of OH radicals. We develop an analytical model for O 2 , H 2 , and H 2 O 2 yields from pure water ice for electrons and singly charged ions of any mass and energy and apply the model to estimate possible O 2 source rates on several icy satellites. The yields are upper limits for icy bodies on which surface impurities may be present.