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The evidence from various models and experimental studies strongly supports the presence of icy cores in Uranus and Neptune. These cores are surrounded by complex ice layers composed of ionized water and ammonia, and potentially carbon in the form of diamonds. Understanding the internal structures of these ice giants not only provides insights into their formation and evolution but also helps in the study of similar exoplanets in other star systems.
Uranus and Neptune, the ice giants of our solar system, have long intrigued scientists with their unique compositions and structures. Unlike the gas giants Jupiter and Saturn, Uranus and Neptune are believed to have significant amounts of "ices" in their interiors. This article explores the evidence and models that suggest the presence of icy cores in these distant planets.
Structure of Uranus and Neptune
Current models of Uranus and Neptune propose a three-layer structure: an inner rocky core, a middle ice layer, and an outer hydrogen-helium envelope. The ice layer is composed of water (H₂O), methane (CH₄), and ammonia (NH₃)1 3 4. The pressures and temperatures within this layer range from about 6 Mbar and 7,000 K at the core-ice boundary to approximately 0.2 Mbar and 2,200 K at the ice-hydrogen/helium boundary1.
Evidence from Shockwave Experiments and Theoretical Studies
Shockwave experiments and theoretical studies have provided insights into the state of materials within the ice layer. These studies suggest that the H₂O and NH₃ in the ice layer are almost completely ionized, and the CH₄ may be pyrolyzed into carbon, potentially in the form of diamonds1. This indicates that the ice layer is not composed of simple ices but rather complex, high-pressure, high-temperature phases of these substances.
Comparative Models
Comparative models of Uranus and Neptune support the existence of an ice shell surrounding a rocky core. For Uranus, the ice shell is estimated to have a density about 10% lower than the canonical density for an ice mixture, while for Neptune, two models exist: one with the canonical density and one with a density 20% lower3. These models highlight the differences in the internal structures of the two planets, despite their similar overall compositions.
Evolutionary Considerations
The thermal evolution of Uranus and Neptune has been studied to understand their current states. Models incorporating a new equation of state for H₂O, based on experimental data, suggest that both planets have relatively small hydrogen-helium envelopes, with Neptune's envelope being smaller than Uranus'4. This evolutionary perspective supports the idea that both planets have central rock-ice cores of approximately 15 Earth masses, with decreasing hydrogen-helium envelope masses from Jupiter to Saturn to Uranus to Neptune4.
Do Uranus and Neptune have an icy core?
Peter Read has answered Likely An expert from Oxford University in Atmospheric Science, Geophysics
Indeed the heat and pressure are extreme in the interiors of Uranus and Neptune - with temperatures much higher than the conditions under which we see water ice melt at ordinary atmospheric pressure. But ice behaves very differently under the extreme pressures found in the deep interior of a planet, as has been revealed recently in experiments that subject water to comparable pressures and temperatures. A recent paper by Millot et al., published in the journal Nature Physics in March 2018, reported results of experiments at the Lawrence Livermore National Laboratory in the USA, in which water was compressed to pressures of up to 3 million atmospheres in diamond anvil cells and in shock waves produced by powerful lasers. These showed that a solid ice forms under these conditions with a very different crystal structure than normal ice, with a melting point of around 5000 degrees Celsius at 2 million atmospheres pressure.
Deep inside Uranus and Neptune, models suggest that pressures reach values greater than 3 million atmospheres but at temperatures less than 5000 degrees. This would strongly suggest that water would be in this high pressure icy state. However, measurements and model simulations (by Millot and his colleagues amongst others) also suggest that this form of ice is not all that rigid - but perhaps more of a "mushy" consistency more like the softened rock found in the Earth's mantle though still technically a solid. At the very centre of each planet, however, it is also likely that heavier elements such as silicon and iron will accumulate to form a relatively small rocky core, surrounded by a deep icy "mantle". So strictly speaking the "core" may not be icy but rocky, but solid/mushy ice probably surrounds it.
