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The water abundance in Jupiter’s equatorial zone


Oxygen is the most common element after hydrogen and helium in Jupiter’s atmosphere, and may have been the primary condensable (as water ice) in the protoplanetary disk. Prior to the Juno mission, in situ measurements of Jupiter’s water abundance were obtained from the Galileo probe, which dropped into a meteorologically anomalous site. The findings of the Galileo probe were inconclusive because the concentration of water was still increasing when the probe ceased sending data. Here we report on the water abundance in the equatorial region (0 to 4 degrees north latitude), based on data taken at 1.25 to 22 GHz from the Juno microwave radiometer, probing pressures of approximately 0.7 to 30 bar. Because Juno discovered the deep atmosphere to be surprisingly variable as a function of latitude, it remains to confirm whether the equatorial abundance represents Jupiter’s global water abundance. The water abundance at the equatorial region is inferred to be \(2.5_{ - 1.6}^{ + 2.2} \times 10^3\) ppm, or \(2.7_{ - 1.7}^{ + 2.4}\) times the elemental ratio of protosolar oxygen to hydrogen (1σ uncertainties). If this reflects the global water abundance, the result suggests that the planetesimals that formed Jupiter were unlikely to have been water-rich clathrate hydrates.

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Fig. 1: Nadir brightness temperatures of perijoves PJ1 to PJ9.
Fig. 2: Limb darkening versus nadir brightness temperature for each channel in the north EZ (0°–4° N).
Fig. 3: Sensitivity study of model parameters.
Fig. 4: Sample of thermal and compositional profiles explored by the Markov Chain Monte Carlo sampler.
Fig. 5: Marginal probability density functions of the ammonia abundance, the water abundance and the ammonia enrichment factor.
Fig. 6: Correlation between water, ammonia and the ammonia enrichment factor.
Fig. 7: Measured brightness temperature and limb-darkening with respect to the best-fitted model.

Data availability

Juno MWR data can be accessed on the Planetary Data System (PDS) Requests for all other data or materials that are presented in the paper but not archived in the PDS should be addressed to C.L.


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The research was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration. The Juno mission and the team members at the Jet Propulsion Laboratory were supported by NASA grant NNN12AA01C. T.G. acknowledges support from CNRS. We thank all Juno team members for the collaborative efforts.

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C.L. developed the inversion software and performed the data analysis. All authors discussed the results and commented on the manuscript.

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Correspondence to Cheng Li.

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Extended data

Extended Data Fig. 1 Limb-darkening error of the fitting using a three-parameter formula.

Histogram of the difference between the limb-darkening at 45° calculated using equation (4)—R(fit)—and using a radiative transfer model— R(model)—for a wide range of model atmospheres.

Extended Data Fig. 2 Brightness temperature error of the fitting using a three-parameter formula.

Histogram of the percentage difference between the brightness temperature calculated using equation (4)—\(T_{\mathrm{B}}^{\left( {{\mathrm{fit}}} \right)}\)—and using a radiative transfer model—\(T_{\mathrm{B}}^{\left( {{\mathrm{model}}} \right)}\)—over a wide range of model atmospheres.

Extended Data Fig. 3 Spacity pattern of the error covariance matrix.

Sparsity pattern of the covariance matrix of X* in equation (6) (10 GHz). The coefficients are aligned sequentially from a(ϕ) to b(ϕ) to c(ϕ). The dark region indicates that the covariance is large.

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Li, C., Ingersoll, A., Bolton, S. et al. The water abundance in Jupiter’s equatorial zone. Nat Astron 4, 609–616 (2020).

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