Isotopic evidence of long-lived volcanism on Io

Creators: de Kleer, Katherine¹; Hughes, Ery C.; Nimmo, Francis; Eiler, John¹; Hofmann, Amy E.; Luszcz-Cook, Statia; Mandt, Kathy

1. California Institute of Technology

Abstract

Jupiter’s moon Io hosts extensive volcanism, driven by tidal heating. The isotopic composition of Io’s inventory of volatile chemical elements, including sulfur and chlorine, reflects its outgassing and mass loss history, and thus records information about its evolution. We used millimeter observations of Io’s atmosphere to measure sulfur isotopes in gaseous SO₂ and SO, and chlorine isotopes in gaseous NaCl and KCl. We find ³⁴S/³²S = 0.0595 ± 0.0038 (equivalent to δ³⁴S = +347 ± 86‰), which is highly enriched compared to average Solar System values and indicates that Io has lost 94 to 99% of its available sulfur. Our measurement of ³⁷Cl/³⁵Cl = 0.403 ± 0.028 (δ³⁷Cl = +263 ± 88‰) shows that chlorine is similarly enriched. These results indicate that Io has been volcanically active for most (or all) of its history, with potentially higher outgassing and mass-loss rates at earlier times.

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Acknowledgement

We thank Arielle Moullet for insight into past observations of Io, and Alexander Thelen for help with Casa imaging. We acknowledge the support of Ryan Loomis, Tony Remijan, and the North America ALMA Science Center (NAASC) in obtaining these data and processing them into calibrated images. This project concept was developed in part at the W.M. Keck Institute for Space Studies. ALMA is a partnership of ESO (representing its member states), NSF (USA) and NINS (Japan), together with NRC (Canada), MOST and ASIAA (Taiwan), and KASI (Republic of Korea), in cooperation with the Republic of Chile. The Joint ALMA Observatory is operated by ESO, AUI/NRAO and NAOJ. The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc. The Jet Propulsion Laboratory (JPL) is operated by the California Institute of Technology under contract with the National Aeronautics and Space Administration (80NM0018D0004).

Funding

KdK acknowledges funding from National Science Foundation grant 2238344 through the Faculty Early Career Development Program. KdK, JE, EH, and AEH acknowledge funding from the Caltech Center for Comparative Planetary Evolution. KM acknowledges support from NASA ROSES Rosetta Data Analysis Program grant 80NSSC19K1306. AEH acknowledges support from the JPL Researchers on Campus Program and from internal JPL funding. KdK acknowledges support from the NAASC through their funding of a PI face-to-face data reduction visit.

Contributions

KdK led the conceptualization, methodology development, analysis, interpretation, and writing of the manuscript. FN, JE, and KM contributed to project conceptualization and to development of the Rayleigh distillation model. AEH and EH contributed to development of the Rayleigh distillation model. SLC developed the radiative transfer model with contributions from KdK. All authors contributed to the interpretation, discussed the results and revised the manuscript.

Data Availability

This paper makes use of the following ALMA data: ADS/JAO.ALMA#2021.1.00849.S, which are archived at https://almascience.nrao.edu/aq/?projectCode=2021.1.00849.S. The radiative-transfer modeling software is archived at Zenodo (8). Our measured atmospheric parameters are listed in Table 1 and table S3.

Supplementary Materials

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Materials and Methods

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