Gas giant clouds

Key facts

Jupiter's diameter

≈ 143,000 kmAt the equator; about 11 times Earth's

Jupiter's mass

≈ 318 EarthsMore than twice all the other planets combined

Composition

≈ 90% hydrogen, ≈ 10% heliumBy number of molecules, with traces of methane, ammonia and water

Gravity at cloud tops

≈ 24.8 m/s²About 2.5 times Earth's

Day length

≈ 9 h 56 minThe fastest-spinning planet

Temperature at 1 bar

≈ −110°CRises steadily with depth

Great Red Spot

Wider than EarthHas shrunk over the past century and a half

Fastest jet streams

Over 500 km/hSaturn's equatorial winds are faster still

Galileo probe entry

7 December 1995Transmitted for about an hour

Juno arrival

4 July 2016Polar orbit

Why there is nowhere to land

A gas giant has no ground. Scientists define a reference level where the atmospheric pressure equals 1 bar, the same as at sea level on Earth, and measure heights and depths from there. But nothing solid exists at that level; it is simply a convenient marker in the gas.

If you could descend below the clouds, the pressure and temperature would rise steadily. The hydrogen around you would become denser and denser until, thousands of kilometres down, it behaved more like a hot liquid than a gas, with no sharp boundary between the two. Any spacecraft that tried to 'land' would eventually be crushed, melted and vaporised long before reaching anything resembling a surface.

The three cloud decks

Jupiter's visible clouds come in layers, each made of a different substance that condenses at a different temperature and pressure. At the top, near or just below the 1-bar level, are white clouds of ammonia ice. Below them, at roughly two bars, models predict a deck of ammonium hydrosulfide, a compound of ammonia and hydrogen sulphide, which may help give the planet its browns and oranges. Deeper still, at around five bars or more, water clouds are expected to form, similar to Earth's but far thicker.

The bands across Jupiter are called zones and belts. Pale zones are areas of rising air where high ammonia clouds form; darker belts are regions of sinking air where we see deeper, warmer layers. What exactly colours the clouds red, orange and brown is still debated; sunlight acting on chemicals carried up from below is a leading idea.

The Great Red Spot, winds and lightning

The Great Red Spot is a giant anticyclone, a high-pressure storm spinning anticlockwise in Jupiter's southern hemisphere. It has been watched continuously since at least the 1830s, and possibly since the 1600s. In the late 19th century it was perhaps 40,000 km long; today it is much smaller and rounder but still wider than Earth. Juno's measurements suggest its roots extend a few hundred kilometres below the cloud tops, far deeper than any storm on Earth.

Jupiter's bands are driven by alternating jet streams, the fastest exceeding 500 km/h. Saturn's equatorial winds are even stronger, reaching around 1,800 km/h. Lightning flashes in Jupiter's deep water clouds, as it does on Earth, but Juno also detected unexpected shallow lightning high in the atmosphere, thought to involve slushy hailstones of ammonia and water nicknamed 'mushballs'. Lightning on Jupiter appears to be most common near the poles rather than the equator.

Going deeper: pressure, heat and metallic hydrogen

Every few dozen kilometres down, the pressure and temperature climb further. By the time the Galileo probe stopped transmitting, about 150 km below the cloud tops, it measured a pressure of about 22 bar and a temperature of about 150°C. Much deeper, temperatures reach thousands of degrees, and at the centre they are thought to be around 20,000°C or more, though such figures are model estimates.

Thousands of kilometres down, at pressures of millions of times Earth's atmosphere, hydrogen is squeezed so hard that its electrons roam freely and it conducts electricity like a metal. This liquid metallic hydrogen makes up much of Jupiter's interior, and its swirling motion is thought to generate the planet's enormous magnetic field, the strongest of any planet. Juno's gravity data suggest Jupiter's core is not a small, compact ball but a large, 'dilute' region where heavy elements are mixed with hydrogen.

Galileo and Juno

The Galileo spacecraft released a small probe that entered Jupiter's atmosphere on 7 December 1995 at about 170,000 km/h. Slowed by a heat shield and a parachute, it sent back data for about an hour before the heat and pressure ended its mission. By chance it fell into an unusually dry, clear 'hot spot', so it found less water and fewer clouds than expected, a reminder that one probe samples only one place. It also found that winds grew stronger with depth.

NASA's Juno spacecraft reached Jupiter on 4 July 2016 and loops over the poles in a long, elliptical orbit. Its microwave instrument can sense hundreds of kilometres below the cloud tops, revealing how deep the belts, zones and storms extend. Juno discovered clusters of cyclones around both poles and has also made close flybys of the moons Ganymede, Europa and Io. At Saturn, the Cassini mission ended in 2017 by deliberately diving into the planet's atmosphere, sampling it until contact was lost.

Surprising facts about Gas giant clouds

• Saturn is less dense than water on average; in an impossibly large bathtub, it would float.
• Jupiter gives off more heat than it receives from the Sun, still slowly releasing energy from its formation.
• Juno found geometric clusters of cyclones at Jupiter's poles, including eight around the north pole and, at first count, five around the south.
• On Saturn, and possibly Jupiter, helium may condense into droplets and fall as rain deep inside the planet.
• A day on Jupiter lasts less than 10 hours, so fast that the planet visibly bulges at its equator.
• The Galileo probe's heat shield faced glowing gas hotter than the surface of the Sun during entry, and lost around half its mass as it burned away.

Frequently asked questions

Can you land on Jupiter?

No. Jupiter has no solid surface. A spacecraft would sink through the clouds into ever denser, hotter gas and be crushed and vaporised long before reaching anything solid.

What are Jupiter's clouds made of?

Mainly three layers: ammonia ice at the top, ammonium hydrosulfide below, and water clouds deeper still. The colours probably come from chemicals altered by sunlight.

How big is the Great Red Spot?

It is currently wider than Earth, though it has shrunk considerably since the 1800s. It is a high-pressure storm that has lasted for at least about 190 years.

What is metallic hydrogen?

Hydrogen squeezed by enormous pressure deep inside Jupiter and Saturn until it conducts electricity like a metal. It is thought to power their strong magnetic fields.

Can I explore a gas giant in the app?

Yes. You can descend into the clouds of a gas giant and explore its cloud decks and storms in interactive 3D.

Keep exploring

• Land in the 3D gas giant landscape and walk around
• Jupiter: the largest planet
• Saturn: the ringed gas giant
• Jungle: layered rainforest, a muddy river and drifting spores
• Desert: wind-carved dunes with sharp crests and long shadows
• Glacier: a river of blue ice between granite peaks
• The Moon's surface: grey regolith, craters and earth in a black sky

The landscapes in Ember’s Universe are procedural 3D scenes modelled on real places. The landforms, plants and light follow the real thing, but the details are generated, not mapped.

Sources

• NASA — Jupiter
• NASA — Juno mission
• NASA — Galileo mission
• NASA — Saturn
• ESA — Juice mission