Jupiter: the Great Red Spot, structure and moons

Cloud currents above an ocean of hydrogen

What we see on Jupiter is an atmosphere, not a solid surface. Bright zones, dark belts and vortices form a changing pattern. Pressure alters the state of matter with depth, so the familiar air-ground boundary does not apply here.

Jupiter: key facts

Type

Gas giant

Mean radius

69,911 km

Mean distance from the Sun

≈ 5.20 AU · 778 million km

Orbital period

≈ 11.86 Earth years

Rotation

≈ 9 h 55 min

Temperature

≈ −110 °C · 1-bar level

Mass

1,898 × 10²⁷ kg

Mean density

1 326 kg/m³

Values are rounded. The distance describes the orbit, not the current distance from Earth. For giant planets, temperature refers to a reference atmospheric level: they have no solid ground. Mean radius differs from equatorial radius.

Atmosphere and appearance

Mostly hydrogen and helium. Clouds at different levels contain ammonia, sulfur compounds and water.

What is inside?

Proposed structure. Composition changes gradually, and sharp boundaries between layers may not exist. Colors and region sizes are illustrative.

The cutaway is based on physical models and indirect measurements. It is not a photograph of the interior; in 3D, you can select each layer separately.

Natural satellites

The four largest Galilean moons are Io, Europa, Ganymede and Callisto. They are different worlds: volcanically active Io, icy Europa, large Ganymede and crater-covered Callisto.

History of discovery and exploration

Known since ancient times

Jupiter is a bright planet visible to the naked eye. The discovery of its moons showed that celestial bodies can orbit something other than Earth.

1. January 1610

   Four new worlds

   Galileo discovered Io, Europa, Ganymede and Callisto, Jupiter's large moons now known as the Galilean moons.

2. 1973

   The first flyby

   Pioneer 10 was the first spacecraft to explore Jupiter up close, measuring its intense radiation environment.

3. 1979

   Volcanoes and rings

   Voyager discovered active volcanoes on Io and provided a new view of Jupiter and its faint ring system.

4. 1995

   Orbit and descent

   Galileo became Jupiter's first orbiter. Its released probe measured the atmosphere from within.

5. 2016

   Beneath the cloud bands

   Juno entered a polar orbit to study the giant's magnetic field, gravity and deep interior.

Galileo explored the Jupiter system. Juno measures gravity, the magnetic field and the atmosphere.

Why a beautiful image does not show all of Jupiter

Visible light records Jupiter’s upper cloud scene. It does not photograph every atmospheric layer at once: clouds can conceal material below, and different wavelengths respond to different conditions. A pale zone and a dark vortex in one image therefore do not provide a ready-made cross-section. Studying depth requires combining pictures with measurements sensitive to other layers and other physical properties.

Juno’s microwave radiometer studies radiation that provides information from below the visible cloud tops. Its results are measurements and models of how material is distributed, not ordinary photographs taken through a transparent shell. When comparing mission material, check which instrument produced the data. This distinguishes the cloud pattern actually pictured from an inference about how far an atmospheric structure extends below it.

Learning about the interior without reaching the core

Jupiter’s gravity field is one of the tools used to investigate its interior. Uneven mass distribution slightly alters a passing spacecraft’s motion, and precise radio tracking measures those changes. Researchers then solve an inverse problem: finding an internal structure consistent with the data. A few attractive coloured layers in a diagram help explain the result, but are not boundaries that a camera has photographed directly.

Juno data have led to models of a dilute or “fuzzy” core, in which heavier material may be spread through a broad inner region without a sharp transition to the surroundings. “Core” therefore need not mean a compact rocky ball. A revised drawing of the interior does not imply that the planet suddenly rearranged itself; it reflects a refined explanation of physical measurements that researchers are trying to reconcile.

Auroras and the giant’s invisible surroundings

Jupiter interacts not only with sunlight but also with charged particles in its magnetic environment. When particles deliver energy to the upper atmosphere, they produce auroral emission. Some of the most striking scientific images use wavelengths outside human vision. Bright outlines in those images represent a measured signal, but do not promise the same appearance through an amateur telescope.

The magnetosphere also matters when planning missions because energetic particles can damage electronics. Trajectories, instrument protection and operating lifetimes must therefore account for the radiation environment. Distance from the visible clouds alone does not describe flight conditions. A spacecraft near the giant must contend with fields and particles as well as the possibility of hitting an object that can be seen in a photograph.

The solar wind starts to change outside the region dominated by the planet’s magnetic field. Two passes by the Juno spacecraft in December 2024 revealed different patterns: an extended region of reflected ions and a more compact transition with recurring magnetic peaks.

Comparing your own observations of Jupiter

For a first evening, choose a modest goal: identify the disc and record the positions of the conspicuous moons. On a later observation, plot the points again without altering the earlier sketch from memory. You can follow a changing system even without seeing colourful vortices. Record the date, time and instrument; otherwise the two drawings will be difficult to compare with a calculated sky chart.

Do not interpret one missing moon as an object vanishing. It may cross the planet’s face, pass behind it, or become difficult to see in its glare; an ephemeris identifies the particular circumstance. Compare cloud detail under reasonably similar conditions rather than at arbitrary screen scales. A careful sequence of simple notes teaches you more about the moving system than one attempt to match the appearance of an interplanetary spacecraft image.