Living Reference Work Entry

Encyclopedia of Astrobiology

pp 1-6

Date: Latest Version

Oxygen Isotopes

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


Isotope anomaly Fractionation mass independent Organic matter Photochemistry Self-shielding Solar composition Water


Oxygen has three stable isotopes: oxygen-16, oxygen-17, and oxygen-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 (see Delta, Isotopic, for the δ notation). The δ17O values among extraterrestrial samples significantly deviate from the above shown relationship. This deviation bears prime 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 habitable planet. Oxygen isotope compositions among solid planetary materials are generally expected to bear keys to ...

This is an excerpt from the content