Ice Giants: Uranus and Neptune

The volatile-rich outer giants, smaller and colder than Jupiter and Saturn, with extreme axial tilts and strangely offset magnetic fields.

Definition

Ice giants are planets dominated by heavier volatile ices such as water, ammonia, and methane rather than hydrogen and helium; in the Solar System these are Uranus and Neptune.

Scope

This topic covers Uranus and Neptune: their bulk compositions dominated by water, ammonia, and methane ices over rock; their interior structure and the exotic states of matter such as superionic water proposed within them; their atmospheres, winds, and weather; and their highly tilted, offset, non-dipolar magnetic fields. It also notes Uranus's extreme axial tilt and Neptune's anomalous internal heat, and the fact that both have been visited only once, by Voyager 2.

Core questions

• What is the internal structure and composition of the ice giants?
• Why are their magnetic fields strongly tilted, offset, and non-dipolar?
• Why does Neptune have strong internal heat while Uranus appears nearly heat-balanced?
• How did Uranus come to be tipped on its side?

Key theories

Ice-dominated interiors

The ice giants are thought to consist of a deep, hot, electrically conducting fluid layer of water, ammonia, and methane over a rocky core, beneath a hydrogen-helium atmosphere, distinguishing them from the gas giants.

Thin-shell dynamo

Convection confined to a relatively thin outer conducting layer can generate the highly tilted, offset, and multipolar magnetic fields observed at Uranus and Neptune.

Mechanisms

Beneath their atmospheres, the ice giants harbor a dense fluid of water and other ices that becomes electrically conducting under pressure; convection within a limited shell of this layer is thought to produce their unusual magnetic geometry. The interiors may host superionic water, in which oxygen forms a solid lattice while hydrogen flows freely.

Clinical relevance

Ice giants represent a planet class that is poorly explored yet may be the most common type of planet in the galaxy, making Uranus and Neptune key references for interpreting Neptune-mass exoplanets.

History

Uranus was discovered telescopically by William Herschel in 1781 and Neptune was predicted from orbital perturbations and found in 1846. Almost all detailed knowledge of both planets comes from the single Voyager 2 flybys in 1986 and 1989, which revealed their offset magnetic fields, faint rings, and major moons, and motivated proposals for dedicated future missions.

Debates

How icy are the ice giants?

The precise proportions of ice, rock, and gas inside Uranus and Neptune, and whether their interiors are layered or mixed, are poorly constrained by the limited Voyager data.

Key figures

• Heidi Hammel
• Mark Hofstadter
• Tristan Guillot
• Jonathan Fortney

Seminal works

• guillot2005
• hofstadter2019

Frequently asked questions

Why is Uranus tipped on its side?

Uranus's rotation axis is tilted nearly 98 degrees, so it essentially rolls along its orbit; the leading explanation is one or more giant impacts early in its history.

Have any spacecraft visited Uranus and Neptune?

Only one, Voyager 2, which flew past Uranus in 1986 and Neptune in 1989; no orbiter has yet been sent to either planet.