Reference Work Entry

Encyclopedia of Astrobiology

pp 1196-1200

Oxygen Isotopes

  • Ko HashizumeAffiliated withDepartment of Earth and Space Sciences, Osaka University Email author 


Isotope anomaly, mass-independent-fractionation, organic matter, photochemistry, self-shielding, solar composition, water


Oxygen has three stable isotopes, oxygen-16, -17, and -18. In most studies on terrestrial samples, only the 18O/16O ratio is usually measured and discussed, where the mass-dependent fractionation law, expressed by δ17O ≈ 0.52 × δ18O, can be safely assumed. The δ17O values among extraterrestrial samples significantly deviate from the above shown relationship. This deviation bears the top-class importance in cosmochemistry, and naturally, in astrobiology.


Oxygen is the most abundant element in the planetary system, except among the giant planets, Jupiter and Saturn, where hydrogen and helium dominate their masses. Oxygen is accommodated as a major constituent in most types of building blocks, rocks (MOx), water (H2O), and organics (CHONS), which are essential in constructing a terrestrial planet, particularly a habitab ...

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