Gas giants are made almost entirely of hydrogen and helium, the two lightest elements in the universe. Jupiter’s atmosphere is roughly 90% hydrogen and 10% helium by volume, with trace amounts of methane, ammonia, and water vapor making up the rest. But the story gets far more interesting beneath the clouds, where crushing pressure transforms these familiar gases into exotic states of matter that don’t exist naturally anywhere on Earth.
Hydrogen and Helium: The Primary Ingredients
Jupiter and Saturn formed from the same cloud of gas and dust that produced the Sun, so their basic chemistry mirrors the Sun’s composition. Both planets are dominated by molecular hydrogen gas, with helium as a distant second. Jupiter’s atmosphere contains about 89.8% hydrogen and 10.2% helium, while Saturn skews even more hydrogen-heavy at roughly 96% hydrogen and just 3% helium.
Saturn’s lower helium content isn’t random. The helium mass fraction in Saturn’s upper atmosphere (around 6 to 8.5%) falls well below the original solar nebula value of about 27.5%. Jupiter’s helium fraction (about 23.8%) is also lower than expected but not by as much. The leading explanation is that helium gradually separates from hydrogen deep inside these planets and sinks toward the core, like oil separating from water. This “helium rain” strips helium from the outer atmosphere over billions of years, and the effect is more pronounced in Saturn because its cooler interior allows the separation to happen more easily.
Trace Gases That Add Color and Chemistry
Beyond hydrogen and helium, gas giant atmospheres contain small but important amounts of heavier compounds. Jupiter has about 0.3% methane, 0.026% ammonia, tiny traces of ethane, and even smaller amounts of water vapor. Saturn’s trace chemistry is similar, with roughly 0.4% methane and 0.01% ammonia. These percentages sound negligible, but they drive much of what we actually see when we look at these planets.
Ammonia ice crystals form the bright white cloud bands visible in Jupiter’s upper atmosphere. Deeper down, ammonia reacts with hydrogen sulfide to create ammonium hydrosulfide clouds in brownish and reddish tones. Water clouds are predicted to exist at still lower levels, though when NASA’s Galileo probe plunged into Jupiter in 1995, it found far less water vapor than expected. Jupiter also appears to have more carbon, nitrogen, sulfur, and other heavy elements than the Sun, likely delivered by comets and asteroids that slammed into the planet over time.
What Happens Deeper Inside
The outer atmosphere of a gas giant is ordinary hydrogen gas. But there is no solid surface. As you descend, pressure and temperature rise steadily, and the hydrogen transitions from a gas to a dense, hot fluid without ever hitting a clear boundary. Thousands of kilometers down, conditions become extreme enough to fundamentally change what hydrogen is.
At pressures reaching several million atmospheres, hydrogen molecules are squeezed so tightly that their electrons break free and flow between atoms. The gas essentially becomes a liquid metal. Researchers at Lawrence Livermore National Laboratory have recreated this transformation in the lab by compressing hydrogen to about 600 gigapascals (roughly six million times Earth’s atmospheric pressure) while keeping temperatures between 1,000 and 2,000 Kelvin. The sample started completely transparent, then turned opaque, then became shiny and reflective, like a lightweight version of liquid mercury.
This metallic hydrogen makes up the bulk of Jupiter’s interior and is the reason the planet has such a powerful magnetic field. Convection currents in this electrically conducting fluid generate magnetism the same way flowing iron generates Earth’s magnetic field, just on a vastly larger scale. Saturn has a metallic hydrogen layer too, though it’s proportionally smaller because Saturn’s lower mass produces less internal pressure.
The Core: Not What Scientists Expected
The standard model of gas giant formation, called core accretion, proposes that a solid core of rock and ice formed first, growing large enough (roughly ten Earth masses) to gravitationally capture enormous quantities of hydrogen and helium from the surrounding disk of gas. For decades, scientists assumed Jupiter still had a compact, dense core sitting at its center like a pit inside a peach.
NASA’s Juno mission, which has been orbiting Jupiter since 2016, changed that picture. Precise measurements of Jupiter’s gravitational field reveal that the planet does not have a neat, solid core. Instead, it has what scientists call a “fuzzy” or “dilute” core: a broad central region where heavy elements (rock, ice, metals) are mixed gradually into the surrounding hydrogen and helium rather than concentrated in a tight ball. This fuzzy core region may contain 30% or more of Jupiter’s total mass, far larger than traditional formation models predicted. One explanation is that a massive collision with another large planetary body early in Jupiter’s history could have shattered and dispersed what was once a compact core.
Ice Giants: A Different Recipe
Uranus and Neptune are sometimes lumped together with Jupiter and Saturn, but their composition is fundamentally different. By mass, Uranus and Neptune are only about 10 to 20% hydrogen and helium. The remaining 80 to 90% is heavier stuff: water, methane, and ammonia in various states, compressed into hot, dense fluids under enormous pressure. That’s why planetary scientists classify them as “ice giants” rather than true gas giants. The word “ice” here is a bit misleading, since these compounds exist mostly as superheated, pressurized fluids rather than anything resembling frozen ice.
This compositional difference traces back to how they formed. Uranus and Neptune sit much farther from the Sun, where the original disk of gas and dust was thinner. They grew slowly, accumulating heavy elements but never reaching the critical mass needed to trigger runaway capture of hydrogen and helium the way Jupiter and Saturn did. Forming planets with their particular mix of heavy elements and a modest hydrogen-helium envelope requires precise timing: too slow and they end up as bare rocky worlds, too fast and they balloon into full gas giants.
Surprisingly Low Density
Despite their enormous size, gas giants are not particularly dense. Jupiter’s average density is just 1.3 grams per cubic centimeter, only slightly denser than water. Saturn is even lighter at 0.7 grams per cubic centimeter, making it the only planet in the solar system that would float in a (very large) bathtub. For comparison, Earth’s average density is 5.5 grams per cubic centimeter, more than seven times Saturn’s.
The ice giants are denser because of all that heavy material. Neptune comes in at 1.6 grams per cubic centimeter and Uranus at 1.3. These numbers reflect the basic divide: Jupiter and Saturn are inflated by vast envelopes of lightweight hydrogen, while Uranus and Neptune pack heavier compounds into a smaller volume.
Internal Heat and Energy
Gas giants radiate significantly more energy into space than they absorb from the Sun. This extra energy comes from their interiors, a leftover from the heat generated during their formation and from ongoing processes like the slow gravitational compression of the planet and helium rain releasing energy as it sinks. This internal heat drives powerful atmospheric circulation, fueling winds that reach hundreds of kilometers per hour and generating the complex storm systems visible in their cloud layers. Unlike Earth, where weather is powered almost entirely by solar energy, gas giant weather is substantially powered from below.