On December 21, 2020, Jupiter and Saturn appeared closer together from Earth than at any time since the Middle Ages—a rare event that highlighted the solar system's largest planet. With telescopes trained on this spectacle, it's an ideal moment to explore what Jupiter is truly made of.
Jupiter reigns as the solar system's most massive planet, primarily composed of gas: about 90% hydrogen and 10% helium in its atmosphere. Spectral analyses also detect traces of water vapor, methane, hydrogen sulfide, neon, oxygen, phosphine, carbon, ethane, sulfur, and ammonia.
This atmosphere features distinct layers of gases stacked vertically, extending deep into the planet.
These layers differ from the iconic bands on Jupiter's surface, which arise from its ultra-fast rotation and temperature variations. Earth spins at roughly 1,700 km/h at the equator, while Jupiter whips around at over 45,000 km/h, completing a rotation in under 10 hours (compared to Earth's 24 hours). Its equator is notably warmer than the poles.
Despite its gaseous nature, Jupiter isn't just a uniform ball of gas without a solid surface. A spacecraft descending into its atmosphere would eventually encounter a core.
Early theories suggested Jupiter formed like the Sun from a collapsing gas cloud 4.6 billion years ago, yielding a mostly hydrogen-helium world. NASA's Juno mission, launched in 2011, disproved this.
By tracking Juno's gravitational interactions, scientists mapped mass distribution within Jupiter, confirming a central core.
The core's exact structure remains elusive amid the planet's thick clouds. Experts envision a dense rocky center surrounded by icy mantles, metallic hydrogen (an electrically conductive phase), and outer molecular hydrogen layers (as illustrated above). Estimates place its mass at 12 to 45 Earth masses, or 4% to 14% of Jupiter's total mass.
