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Oxygen Isotopes and Sampling of the Solar System


Oxygen is the dominant element in our planetary system. It is therefore remarkable that it shows substantial isotopic diversity both in mass-dependent fractionation, because it is a light element, and in mass-independent fractionation, primarily associated with variation in abundance of 16O. On Earth, the primary variation in isotopic composition is related to temperature-dependent kinetic mass fractionation between hydrosphere and atmosphere. Meteorites provide samples of primitive bodies, that have not experienced melting, and planetesimals that have melted early in their history. Samples of Mars, Vesta, and the Moon are present in the meteorite collections. In meteorites, the cosmochemical fractionation related to the abundance of 16O provides a useful classification scheme. Inclusions in chondrites show a large range in 16O abundances from highly enriched (solar) through to compositions closer to terrestrial (planetary). The variability in 16O appears originally to be related to predissociation and self-shielding of carbon monoxide likely in the primordial molecular cloud. Within the chondrite parent bodies, exchange between 16O-poor fluids and relatively 16O-rich solids created isotopic mixing lines. This model makes specific predictions for isotopic compositions of silicates and water ice throughout the solar system. One prediction, that the Earth should be isotopically heavier than the Sun, appears to be verified. But other tests based on oxygen isotopes within the solar system require either remote analysis or sample return missions. Remote analysis will require new instrumentation and analytical techniques to achieve the precision and accuracy required for three oxygen isotope analysis. Methodologies associated with cavity ring-down spectroscopy appear promising. Sample return appears viable only for the inner solar system including Mars and asteroids. While sample return missions to either Venus or Mercury appear highly challenging, the scientific benefits are immense both in oxygen isotope characterisation, and in a variety of other geochemical analyses. Measurement of three oxygen isotopes throughout the solar system would further our concepts for formation of other solar systems, and give us insight into the general mechanisms of planetary system formation and the role of water in the formation and evolution of the chondrite parent bodies and planets.

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TRI and RCG acknowledge ISSI support to attend the Europlanet & International Space Science Institute Workshop on the Role of Sample Return in Addressing Major Outstanding Questions in Planetary Sciences held in Bern, Switzerland, 5-9 February 2018. We are grateful to the two reviewers whose suggestions significantly improved the manuscript and to the patience of Mahesh Anand. RCG is funded through a Consolidated Grant from the Science and Technology Facilities Council (STFC), UK Grant Number: ST/L000776/1. LJH and JCB are funded by STFC grant ST/S000429/1. TRI was supported by Australian Research Council Discovery Project Grant DP190102760. Thanks to all our friends and colleagues at ESA, JAXA, and NASA.

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Role of Sample Return in Addressing Major Questions in Planetary Sciences

Edited by Mahesh Anand, Sara Russell, Yangting Lin, Meenakshi Wadhwa, Kuljeet Kaur Marhas and Shogo Tachibana

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Ireland, T.R., Avila, J., Greenwood, R.C. et al. Oxygen Isotopes and Sampling of the Solar System. Space Sci Rev 216, 25 (2020).

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  • Oxygen isotopes
  • Sample return
  • Remote analysis
  • Cosmochemistry