Recent advances in the interior structure of the gas giants

Abstract

Jupiter and Saturn, as the gas giants of our solar system, account for over 90% of the total mass beyond the Sun and hold most of the system's angular momentum, playing a crucial role in the early history of the solar sys- tem. During their formation, these planets captured vast amounts of material from the protoplanetary disk. Their in- ternal structure and interior composition are fundamentally affected by material interaction, gravitational collapse, and subsequent long-term thermal evolution. Exploring the interior structure of the gas giants provides critical in- sights into several core questions in planetary sciences, including the conditions and evolutionary history of the pro- tosolar nebula, the formation and evolution mechanisms of solar system planets, and the interior structure and evolu- tion history of giant exoplanets. Furthermore, comparative studies between Jupiter and Saturn reveal important dif- ferences in their internal dynamics and evolutionary pathways, offering valuable benchmarks for understanding gi- ant planets both within and beyond our solar system. Current research on the interior structure of the gas giants primarily relies on external observational constraints such as gravity fields, magnetic fields, and atmospheric properties. However, inferring the interior structure from these constraints is inherently a highly degenerate inver- sion problem, depending on interior structural models, internal material properties, and calculations of planetary shape and gravity fields. Modern approaches increasingly integrate multiple physical constraints by combining gravity harmonics with magnetic field measurements and zonal wind profiles to reduce degeneracies in interior models. In recent years, the Juno and Cassini spacecraft have conducted close orbital explorations of Jupiter and Saturn, respectively, acquiring unprecedented high-precision gravity field data. Simultaneously, experimental and theoretical research on the physical properties of hydrogen and helium under extreme temperature and pressure conditions has achieved critical breakthroughs. With these in mind, this paper reviews the major exploration mis- sions for Jupiter and Saturn and elaborates on the core aspects of interior modelling, such as theoretical calcula- tions of planetary shapes and gravity fields, current knowledge of hydrogen/helium microphysical properties. The research progress on the internal structures of Jupiter and Saturn is also summarized, showing some representative frontier achievements.

Keywords

Jupiter / Saturn / gravity field / equation of state of hydrogen-helium / interior structure

References

[1] Militzer B, Wahl S, Hubbard W B . 2019. Models of Saturn's interior constructed with an accelerated concentric Maclaurin spheroid method[J].The Astrophysical Journal, 879(2): 78.

[2] Militzer B, Hubbard W B, Wahl S, et al. 2022. Juno spacecraft measurements of Jupiter's gravity imply a dilute core[J].The Planetary Science Journal, 3(8): 185.

[3] Militzer B, Hubbard W B . 2024. Study of Jupiter's interior: Comparison of 2, 3, 4, 5, and 6 layer models[J].Icarus, 411: 115955.

[4] Moore K M, Yadav R K, Kulowski L, et al. 2018. A complex dynamo inferred from the hemispheric dichotomy of Jupiter's magnetic field[J].Nature, 561: 76.

[5] Morales M A, Schwegler E, Ceperley D, et al. 2009. Phase separation in hydrogen—helium mixtures at Mbar pressures[J].Proceedings of the National Academy of Sciences, 106(5): 1324-1329.

[6] Morales M A, Hamel S, Caspersen K, et al. 2013. Hydrogen—helium demixing from first principles: From diamond anvil cells to planetary interiors[J].Physical Review B—Condensed Matter and Materials Physics, 87(17): 174105.

[7] Morf L, Müller S, Helled R . 2024. The interior of Uranus—thermal profile, bulk composition, and the distribution of rock, water, and hydrogen and helium[J].Astronomy & Astrophysics, 690: A105.

[8] Movshovitz N, Fortney J J, Mankovich C, et al. 2020. Saturn's probable interior: An exploration of Saturn's potential interior density structures[J].The Astrophysical Journal, 891(2): 109.

[9] Nettelmann N, Holst B, Kietzmann A, et al. 2008. Ab initio equation of state data for hydrogen, helium, and water and the internal structure of Jupiter[J].The Astrophysical Journal, 683(2): 1217.

[10] Nettelmann N. 2009. Matter under extreme conditions: Modelling giant planets[D]. Rostock:Universität Rostock.

[11] Nettelmann N, Becker A, Holst B, et al. 2012. Jupiter models with improved ab initio hydrogen equation of state (H—REOS. 2)[J].The Astrophysical Journal, 750(1): 52.

[12] Nettelmann N, Püstow R, Redmer R . 2013. Saturn layered structure and homogeneous evolution models with different EOSs[J].Icarus, 225(1): 548-557.

[13] Nettelmann N. 2020. High—order gravity field of differentially rotating Uranus and Neptune models[C]//AGU Fall Meeting Abstracts: P066—0007.

[14] Nettelmann N, Movshovitz N, Ni D, et al. 2021. Theory of figures to the seventh order and the interiors of Jupiter and Saturn[J].The Planetary Science Journal, 2(6): 241.

[15] Neuenschwander B A, Helled R . 2022. Empirical structure models of Uranus and Neptune[J].Monthly Notices of the Royal Astronomical Society, 512(3): 3124-3136.

[16] Ni D. 2018. Empirical models of Jupiter's interior from Juno data—moment of inertia and tidal Love number k₂ [J].Astronomy & Astrophysics, 613: A32.

[17] Ni D. 2019. Understanding Jupiter's deep interior: The effect of a dilute core[J].Astronomy & Astrophysics, 632: A76.

[18] Ni D. 2020a. Understanding Saturn's interior from the Cassini Grand Finale gravity measurements[J].Astronomy & Astrophysics, 639: A10.

[19] Ni D. 2020b. Signature of helium rain and dilute cores in Jupiter's interior from empirical equations of state[J].Earth and Planetary Physics, 4(2): 111-119.

[20] Pan S, Huang T, Vazan A, et al. 2023. Magnesium oxide—water compounds at megabar pressure and implications on planetary interiors[J].Nature Communications, 14(1): 1165.

[21] Pappalardo R T, Buratti B J, Korth H, et al. 2024. Science overview of the Europa Clipper mission[J].Space Science Reviews, 220(4): 40.

[22] Peebles P J E . 1964. The structure and composition of Jupiter and Saturn[J].Astrophysical Journal, 140: 328.

[23] Pfaffenzeller O, Hohl D, Ballone P . 1995. Miscibility of hydrogen and helium under astrophysical conditions[J].Physical Review Letters, 74(13): 2599.

[24] Porco C C, West R A, McEwen A, et al. 2003. Cassini imaging of Jupiter's atmosphere, satellites, and rings[J].Science, 299(5612): 1541-1547.

[25] Reuter D C, Simon—Miller A A, Lunsford A, et al. 2007. Jupiter cloud composition, stratification, convection, and wave motion: A view from new horizons[J].Science, 318(5848): 223-225.

[26] Salpeter E E . 1973. On convection and gravitational layering in Jupiter and in stars of low mass[J].Astrophysical Journal, 181: L83.

[27] Saumon D, Chabrier G, van Horn H M . 1995. An equation of state for low—mass stars and giant planets[J].Astrophysical Journal Supplement, 99: 713.

[28] Saumon D, Guillot T . 2004. Shock compression of deuterium and the interiors of Jupiter and Saturn[J].The Astrophysical Journal, 609(2): 1170.

[29] Schneider A D, Bitsch B . 2021. How drifting and evaporating pebbles shape giant planets. I. Heavy element content and atmospheric C/O[J].Astronomy & Astrophysics, 654: A71.

[30] Schöttler M, Redmer R . 2018a. Simulations of H—He mixtures using the van der Waals density functional[J].Journal of Plasma Physics, 84(4): 755840401.

[31] Schöttler M, Redmer R . 2018b. Ab initio calculation of the miscibility diagram for hydrogen—helium mixtures[J].Physical Review Letters, 120(11): 115703.

[32] Shibata S, Helled R . 2022. Enrichment of Jupiter's atmosphere by late planetesimal bombardment[J].The Astrophysical Journal Letters, 926: L37.

[33] Showman A P, Dowling T E . 2000. Nonlinear simulations of Jupiter's 5—micron hot spots[J].Science, 289(5485): 1737-1740.

[34] Showman A P . 2007. Numerical simulations of forced shallow—water turbulence: Effects of moist convection on the large—scale circulation of Jupiter and Saturn[J].Journal of the Atmospheric Sciences, 64(9): 3132-3157.

[35] Smith B A, Soderblom L A, Johnson T V, et al. 1979. The Jupiter system through the eyes of Voyager 1[J].Science, 204(4396): 951-972.

[36] Smith B A, Soderblom L, Batson R, et al. 1982. A new look at the Saturn system: The Voyager 2 images[J].Science, 215(4532): 504-537.

[37] Smith E J, Wenzel K P, Page D E . 1992. Ulysses at Jupiter: An overview of the encounter[J].Science, 257(5076): 1503-1507.

[38] Smoluchowski R. 1967. Internal structure and energy emission of Jupiter[J].Nature, 215(5102): 691-695.

[39] Song X Y, Yang J, Wei Y . 2025. A look—back to the 50—year exploration of Jupiter's atmosphere[J].Reviews of Geophysics and Planetary Physics, 56(1): 67-82 (in Chinese).

[40] Stevenson D J, Salpeter E E . 1977a. The dynamics and helium distribution in hydrogen—helium fluid planets[J].Astrophysical Journal Supplement Series, 35: 239-261.

[41] Stevenson D J, Salpeter E E . 1977b. The phase diagram and transport properties for hydrogen—helium fluid planets[J].Astrophysical Journal Supplement Series, 35: 221-237.

[42] Stevenson D J . 1982. Interiors of the giant planets[J].Annual Review of Earth and Planetary Sciences, 10: 257-295.

[43] Stevenson D J . 1985. Cosmochemistry and structure of the giant planets and their satellites[J].Icarus, 62(1): 4-15.

[44] Stevenson D J . 2020. Jupiter's interior as revealed by Juno[J].Annual Review of Earth and Planetary Sciences, 48(1): 465-489.

[45] Subramanian N, Goncharov A F, Struzhkin V V, et al. 2011. Bonding changes in hot fluid hydrogen at megabar pressures[J].Proceedings of the National Academy of Sciences, 108: 6014-6019.

[46] Thorngren D P, Fortney J J, Murray—Clay R A, et al. 2016. The mass—metallicity relation for giant planets[J].The Astrophysical Journal, 831: 64.

[47] Tollefson J, Wong M H, de Pater I, et al. 2017. Changes in Jupiter's zonal wind profile preceding and during the Juno mission[J].Icarus, 296: 163-178.

[48] Vazan A, Helled R, Guillot T . 2018. Jupiter's evolution with primordial composition gradients[J].Astronomy & Astrophysics, 610: L14.

[49] Von Zahn U, Hunten D M, Lehmacher G . 1998. Helium in Jupiter's atmosphere: Results from the Galileo probe helium interferometer experiment[J].Journal of Geophysical Research: Planets, 103(E10): 22815-22829.

[50] Wahl S M, Hubbard W B, Militzer B, et al. 2017. Comparing Jupiter interior structure models to Juno gravity measurements and the role of a dilute core[J].Geophysical Research Letters, 44(10): 4649-4659.

[51] Weir S T, Mitchell A C, Nellis W J . 1996. Metallization of fluid molecular hydrogen at 140 GPa (1.4 Mbar)[J].Physical Review Letters, 76(11): 1860.

[52] Wicht J, Gastine T, Duarte L D V . 2019. Dynamo action in the steeply decaying conductivity region of Jupiter—like dynamo models[J].Journal of Geophysical Research: Planets, 124(3): 837-863.

[53] Wicht J, Christensen U R . 2024. Contributions of Jupiter's deep—reaching surface winds to magnetic field structure and secular variation[J].Journal of Geophysical Research: Planets, 129: e2023JE007890.

[54] Wigner E, Huntington H B . 1935. On the possibility of a metallic modification of hydrogen[J].The Journal of Chemical Physics, 3(12): 764-770.

[55] Wildt R . 1947. The constitution of the planets (Council Report on the progress of astronomy)[J].Monthly Notices of the Royal Astronomical Society, 107: 84.

[56] Wilson H F, Militzer B . 2012a. Solubility of water ice in metallic hydrogen: Consequences for core erosion in gas giant planets[J].The Astrophysical Journal, 745(1): 54.

[57] Wilson H F, Militzer B . 2012b. Rocky core solubility in Jupiter and giant exoplanets[J].Physical Review Letters, 108(11): 111101.

[58] Wisdom J, Hubbard W B . 2016. Differential rotation in Jupiter: A comparison of methods[J].Icarus, 267: 315-322.

[59] Young R E . 1998. The Galileo probe mission to Jupiter: Science overview[J].Journal of Geophysical Research: Planets, 103: 22775-22790.

[60] Zaghoo M, Salamat A, Silvera I F . 2016. Evidence of a first—order phase transition to metallic hydrogen[J].Physical Review B, 93(15): 155128.

[61] Zha C, Liu H, Tse J S, et al. 2017. Melting and high P—T transitions of hydrogen up to 300 GPa[J].Physical Review Letters, 119: 075302.

[62] Zharkov V N, Makalkin A B, Trubitsyn V P . 1973. Integration of equations of the theory of planetary figures[J].Soviet Astronomy, 17: 97.

[63] Zharkov V N, Gudkova T V . 1992. Modern Models of Giant Planets[M]//High—Pressure Research: Application to Earth and Planetary Sciences. Washington D C: American Geophysical Union, 67: 393-401.

[64] 顾炜东，魏勇，尧中华，等 . 2024. 木星磁场研究: 回望和展望[J].地球与行星物理评论（中英文），55（6）：638-651.

[65] 何飞，尧中华，魏勇 . 2021. 冷湖行星光学遥感发展与展望[J].地球与行星物理论评，52（4）：361-372.

[66] 宋心仪，杨军，魏勇 . 2025. 岁星四季—回望木星大气探测 50 年[J].地球与行星物理评论（中英文），56（1）：67-82.

Funding

National Natural Science Foundation of China(42578010)

National Natural Science Foundation of China(T2495234)

Pre-Research Projects on Civil Aerospace Technologies of China National Space Administration(D010303)

Science and Technology Development Fund, Macau SAR(0139/2024/RIA2)