Mysteries about the solar system’s biggest planet

LOS ANGELES — Swirling storms. Dusty rings. Glowing polar lights.

Glimpses of Jupiter have been beamed back by robotic explorers since the 1970s. Most visits were brief and only one spacecraft spent time circling the planet. NASA is headed to Jupiter again for what’s promised to be the best views and most extensive exploration yet. The Juno spacecraft arrives Monday after a nearly five-year voyage. It will orbit the planet for more than a year.

WATER

Like the sun, Jupiter is a ball of mostly hydrogen and helium. It was probably the first planet to form. Jupiter is 11 times wider than Earth and 300 times heavier. Juno will hunt for water in Jupiter’s atmosphere, which may help explain how Earth got its water. Previous spacecraft found only a trace amount in Jupiter’s atmosphere, but scientists think they didn’t look deep enough.

GREAT RED SPOT

Jupiter’s most prominent feature is the Great Red Spot, a fierce storm in the atmosphere larger than Earth that has lasted for centuries. In recent years, the spot has been mysteriously shrinking. Once an oval about 25,500 miles wide in the late 1880s, the spot shrank to its smallest observed size in 2014 — the shape of a circle about 10,250 miles across.

AURORAS

Jupiter’s auroras are the brightest in the solar system. Earth’s polar lights are triggered by solar storms, which occur when a cloud of gas from the sun slams into the planet’s magnetic field. Jupiter’s powerful auroras are sparked by the planet’s own rotation. Jupiter is the fastest-spinning planet in the solar system, taking just 10 hours to complete a rotation.