Gas Giants: Jupiter and Saturn

The two hydrogen-helium giants that dominate the outer Solar System, with deep metallic-hydrogen interiors, banded atmospheres, and immense magnetospheres.

Definition

Gas giants are planets composed mainly of hydrogen and helium with no solid surface; in the Solar System these are Jupiter and Saturn.

Scope

This topic covers Jupiter and Saturn: their bulk composition and layered interiors including a metallic-hydrogen region, the source of their internal heat, the dynamics of their banded atmospheres and long-lived storms, their strong intrinsic magnetic fields and magnetospheres, and Saturn's prominent rings. It draws on flyby and orbiter data from Voyager, Galileo, Cassini, and Juno, and on laboratory and theoretical studies of hydrogen at extreme pressures.

Core questions

• How are Jupiter and Saturn structured internally, and do they have compact or diluted cores?
• Why do they radiate more heat than they receive from the Sun?
• What sustains their banded jets and giant storms such as the Great Red Spot?
• How do their powerful magnetic fields and magnetospheres arise and interact with their moons?

Key theories

Metallic-hydrogen interior

At the immense pressures inside Jupiter and Saturn, hydrogen becomes an electrically conducting metallic fluid, providing the dynamo region that generates their strong magnetic fields.

Internal heat and helium rain

The giants emit more energy than they absorb, powered by gravitational contraction and, in Saturn, by the separation and sinking of helium droplets that release additional heat.

Zonal-jet atmospheric dynamics

Rapid rotation and convective heat flux organize the atmospheres into alternating east-west jets and belts, within which vortices like the Great Red Spot persist for centuries.

Mechanisms

Self-gravity compresses hydrogen and helium until hydrogen turns metallic and conducting, driving a dynamo and a strong magnetic field. Slow contraction and helium differentiation supply internal heat, which together with rotation powers banded convection at the surface. Magnetospheric interactions with moons generate aurorae and intense radiation belts.

Clinical relevance

Jupiter and Saturn are the best-studied giant planets and serve as templates for interpreting giant exoplanets, while their gravity shaped the dynamical history of the entire Solar System.

History

Telescopic observation of Jupiter's belts and the Great Red Spot dates back centuries, but modern understanding came from the Pioneer and Voyager flybys, the Galileo orbiter and atmospheric probe at Jupiter, the Cassini orbiter at Saturn, and the Juno mission, whose precise gravity and magnetic measurements have reshaped models of Jupiter's deep interior.

Debates

Nature of Jupiter's core

Juno gravity data suggest a dilute, fuzzy core mixed into the envelope rather than a sharply defined heavy core, prompting revisions to formation and interior models.

Key figures

• Andrew Ingersoll
• Tristan Guillot
• Scott Bolton
• William Hubbard

Seminal works

• guillot2005
• ingersoll2004
• bolton2017

Frequently asked questions

What is the Great Red Spot?

It is a giant, long-lived anticyclonic storm in Jupiter's atmosphere, wider than Earth, that has persisted for at least a couple of centuries of telescopic observation.

Why do Jupiter and Saturn give off more heat than they receive?

They are still slowly contracting under their own gravity, and in Saturn helium settling toward the center releases extra heat, so both planets radiate more energy than they absorb from the Sun.