[Paper Review] Carbon isotope measurements in the Solar System

Paul Woods|arXiv (Cornell University)|Jan 28, 2009

Astro and Planetary Science1 references3 citations

TL;DR

This paper compiles and synthesizes all available carbon isotope ratio (12C/13C) measurements in the Solar System up to February 2010, including the Sun, planets, moons, meteorites, comets, and interplanetary dust. It presents a comprehensive dataset with uncertainties, reveals a mean 12C/13C of 93.4 for comets—significantly higher than the solar value of ~85—and provides a critical reference for understanding Solar System formation and isotopic heterogeneity.

ABSTRACT

I make publicly available my literature study into carbon isotope ratios in the Solar System, which formed a part of Woods & Willacy (2009). As far as I know, I have included here all measurements of 12C/13C in Solar System objects (excluding those of Earth) up to and including 1 February 2010. Full references are given. If you use the any of the information here, please reference the paper Woods & Willacy (2009) and this publication.

Motivation & Objective

• To compile and publicly release all available 12C/13C isotope ratio measurements in Solar System objects (excluding Earth) up to 1 February 2010.
• To provide a unified, verifiable dataset of 12C/13C ratios across diverse Solar System bodies, including comets, planets, meteorites, and interplanetary dust particles.
• To correct and standardize isotope ratios reported in delta notation (δ13C) into 12C/13C ratios using the terrestrial standard (PDB).
• To document and compare multiple measurements of the same object, including revised values from updated models or techniques.
• To support research in Solar System formation, planetary science, and astrochemistry by offering a reference dataset with full citations.

Proposed method

• Collected 12C/13C isotope ratio data from peer-reviewed literature up to February 2010, focusing on Solar System objects excluding Earth.
• Converted δ13C values (in ‰) to 12C/13C ratios using the standard formula: 12C/13C = 1 / (δ13C / 1000 + 1) × 89, with PDB standard (13C/12C = 0.0112372).
• Compiled data into four tables: (1) inner planets and Sun, (2) outer planets and their molecules, (3) minor bodies (meteorites, presolar grains, IDPs), and (4) comets with multiple measurements.
• Treated measurement ranges as symmetric errors about the median value and included reported uncertainties where available.
• Flagged outdated or revised values in comets (e.g., Halley’s ratio revised from 65 to 95) and noted superseded results.
• Provided full references for all data points to ensure reproducibility and verification.

Experimental results

Research questions

• RQ1 What is the range and distribution of 12C/13C ratios across the Solar System, particularly in comets, planets, and meteorites?
• RQ2 How do 12C/13C ratios in comets compare to those in the Sun and terrestrial planets, and what does this imply for Solar System formation models?
• RQ3 What is the impact of revised models and observational techniques on the derived 12C/13C ratios in comets like Halley and Hale-Bopp?
• RQ4 How do isotope ratios in presolar grains and interplanetary dust particles compare to bulk Solar System values?
• RQ5 What is the significance of the observed 12C/13C gradient from the inner to outer Solar System, and how does it relate to galactic chemical evolution?

Key findings

• The mean 12C/13C ratio for the Sun is 85.0, with a range of 80 ± 3 (Ayres et al. 2006) to 90 ± 15 (Hall et al. 1972), reflecting measurement uncertainty and solar atmosphere variability.
• The mean 12C/13C ratio for rocky planets and the Moon is 89.8, with Earth’s value at 89.0 ± 0.4 (Holden et al. 1981), consistent with terrestrial standards.
• The mean 12C/13C ratio for gas giants and Titan is 85.3, indicating isotopic similarity to the Sun but with some variation.
• Comets exhibit a significantly higher mean 12C/13C ratio of 93.4, with individual values ranging from 34 (Hyakutake) to 165 (Kohoutek 1973 XII), reflecting isotopic heterogeneity.
• For comet Halley, the 12C/13C ratio was revised from 65 (Wyckoff et al. 1989) to 95 (Kleine et al. 1995) using updated models, highlighting the importance of methodology.
• The weighted mean of all 12C/13C ratios in the dataset is 91.0, heavily influenced by the high number of cometary measurements with large error bars.