Highly Depleted Alkali Metals in Jupiter's Deep Atmosphere

Creators: Bhattacharya, Ananyo¹; Li, Cheng¹; Atreya, Sushil K.¹; Steffes, Paul G.²; Levin, Steven M.³; Bolton, Scott J.⁴; Guillot, Tristan⁵; Gupta, Pranika¹; Ingersoll, Andrew P.⁶; Lunine, Jonathan I.⁷; Orton, Glenn S.³; Oyafuso, Fabiano A.³; Waite, J. Hunter⁸; Bellotti, Amadeo; Wong, Michael H.⁹

1. University of Michigan–Ann Arbor
2. Georgia Institute of Technology
3. Jet Propulsion Lab
4. Southwest Research Institute
5. Université Côte d'Azur
6. California Institute of Technology
7. Cornell University
8. Waite Science LLC, USA
9. University of California, Berkeley

Abstract

Water and ammonia vapors are known to be the major sources of spectral absorption at pressure levels observed by the microwave radiometer (MWR) on Juno. However, the brightness temperatures and limb darkening observed by the MWR at its longest-wavelength channel of 50 cm (600 MHz) in the first nine perijove passes indicate the existence of an additional source of opacity in the deep atmosphere of Jupiter (pressures beyond 100 bar). The absorption properties of ammonia and water vapor, and their relative abundances in Jupiter's atmosphere, do not provide sufficient opacity in the deep atmosphere to explain the 600 MHz channel observation. Here we show that free electrons due to the ionization of alkali metals, i.e., sodium and potassium, with subsolar metallicity, [M/H] (log-based 10 relative concentration to solar) in the range of [M/H] = −2 to [M/H] = −5, can provide the missing source of opacity in the deep atmosphere. If the alkali metals are not the source of additional opacity in the MWR data, then their metallicity at 1000 bars can only be even lower. This upper bound of −2 on the metallicity of the alkali metals contrasts with the other heavy elements—C, N, S, Ar, Kr, and Xe—that are all enriched relative to their solar abundances, having a metallicity of approximately +0.5.

Copyright and License

Original content from this work may be used under the terms of the Creative Commons Attribution 4.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.

Acknowledgement

This work was supported by the NASA Juno Program, under NASA Contract NNM06AA75C from the Marshall Space Flight Center supporting the Juno Mission Science team, through subcontract 699056KC and Q99063JAR to the University of Michigan from the Southwest Research Institute. Part of this research was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration (80NM0018D0004).

Data Availability

Software and Third-party Data Repository Citations: the software for the radiative transfer package will be available at the Zenodo archive (Bhattacharya et al. 2023a) and the MWR data used in this work, and the associated files for data visualization, are available at the archive (Bhattacharya et al. 2023b). They can be made available upon request.

Software References

Software: High-performance Atmospheric Radiation Package (HARP; Li et al. 2018b; Bhattacharya et al. 2023a).

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