Jupiter’s interior has been revealed in unprecedented detail in observations by Nasa’s Juno spacecraft that show it to be as strange and turbulent as the planet’s surface. Despite extensive studies of Jupiter’s surface, including its distinctive dark and light bands and “great red spot”, little had previously been known about what lies at the interior of the solar system’s largest planet. The new findings, based on high-precision gravitational measurements, show that Jupiter’s iconic striped bands, caused by immensely powerful winds, extend to a depth of about 3,000km below the surface. The mission has also produced a partial answer to the question of whether the planet has a core, showing that the inner 96% of the planet rotates “as a solid body,” even though technically it is composed of an extraordinarily dense mixture of hydrogen and helium gas. The findings are published in four separate papers in the journal Nature, describing the planet’s gravitational field (surprisingly asymmetrical), atmospheric flows, interior composition and polar cyclones.
Jonathan Fortney, an astronomer at the University of California Santa Cruz who wrote an analysis of the findings, said: “The big deal is that this tells us how the interior of Jupiter works. People have been fighting about this since before I was born.” A crucial question was whether the bands on Jupiter, caused by air currents that are five times as strong as the most powerful hurricanes on Earth, were a “weather” phenomenon comparable to the Earth’s jet streams or part of a deep-seated convection system. Juno’s latest observations point to the latter, showing the jets continued to around 3,000km beneath the surface – deep enough to cause ripples and asymmetries in the planet’s gravitational field that were perceptible to detectors on the spacecraft. “It’s a 50-year-old problem that is solved thanks to Juno – that is something to be proud of,” said Tristan Guillot of Côte d’Azur University in Nice, France, lead author of one of two papers on the depth of the atmosphere.
On Earth, the atmosphere represents about a millionth of the mass of the whole planet. The latest work suggests that on Jupiter the figure is closer to 1%. “The concept that an atmosphere can be so heavy and contain so much of the planet is surprising,” said Yohai Kaspi, a planetary scientist at the Weizmann Institute of Science in Israel and the other lead author on this topic. It is not yet clear whether these findings also apply to the “great red spot,” a storm that has been visible on Jupiter’s surface for centuries, and the mission is expected to make further observations this year that could reveal the depth of its roots.
The new findings, based on extremely sensitive gravitational measurements, also begin to paint a picture of the internal structure of the planet. On an imagined journey from the outside to the centre, one would first encounter a cloud layer of 99% hydrogen and helium, with traces of methane and ammonia. The density at the surface is about ten times less than that of air, but the gas becomes denser and denser towards the centre of the planet. At about 10% towards the centre, the gas becomes so dense that hydrogen becomes ionised, turning into a metallic hydrogen gas approaching the density of water. About 20% towards the centre, helium condenses into rain. And in the deep interior, where pressures are about 10 million times higher than at the Earth’s surface, scientists think the gas exists as a dense soup speckled with rocks of heavy metal. “There may be a small hard [solid] core very, very deep, but we think it’s just dense gas enriched in heavy elements … it’s not a solid that you can imagine,” said Kaspi. “The normal concept of gas, liquid and solid don’t hold at these pressures.”
Juno has also taken a series of images of Jupiter’s poles, in visible and infrared light, showing that the cyclones are known to exist there create striking polygonal patterns, with eight cyclones rotating around a single cyclone at the north pole and five cyclones circling a central one in the south. Where the cyclones come from and why they don’t merge is not yet clear. The Juno spacecraft arrived at Jupiter in 2016 after a journey of five years and nearly 1.8 billion miles. Working out what is going on beneath the planet’s surface is a central goal of the $1.1bn mission, which is aiming to understand how the gas giant formed and evolved. Nasa is currently deciding whether to extend the mission beyond its original planned ending in July when controllers are due to send the craft into a destructive dive into Jupiter’s atmosphere.
Credit: Hannah Devlin for The Guardian, 7 March 2018.