When you spend more than $1 billion on a spacecraft, it can be a nervous wait to see if everything pays off. But if and when it does, the results can be rather glorious. And NASA’s Juno spacecraft has just paid off in a huge way.
One of the major goals of the Juno mission, which began in July 2016 when the probe entered orbit around Jupiter, has been to study the interior of this fascinating gas giant. We can see its amazing cloud tops, sure, but we really didn’t know what’s going on inside.
Well, that all changes as of today. In a series of four papers published today in Nature, and an accompanying News and Views article, the latest results from the spacecraft have been revealed. And, for the first time, we’ve actually got a good idea of what’s happening beneath the cloud tops.
“It’s a first view of how a gas giant planet works on the inside,” Jonathan Fortney from the University of California, Santa Cruz, who penned the News and Views article, told IFLScience.
One of the major findings is that we now know how far down Jupiter’s atmosphere extends, 3,000 kilometers (1,860 miles) down from the cloud tops, which is much larger than expected. Once you reach this depth, the composition of the planet changes dramatically.
Much thought had been put into what Jupiter might look like below its clouds. Based on these papers, it appears that at this depth, the interior of the planet changes to behave like a solid – although it’s not actually a solid. Instead, it’s a fluid mixture of hydrogen and helium that rotates like a solid body.
“Our results also show that below those winds of 3,000 kilometers, the planet rotates as a rigid body, and all this information has deep consequences in our understanding on the interior of the planet and in turn it allows us to get closer to understanding its formation,” Yamila Miguel from Leiden University in The Netherlands, one of the authors on the papers, told IFLScience.
Jupiter is famous for its bands of clouds that we can see covering the planet, first spotted by Galileo 400 years ago. But scientists were not sure how far down these bands extended. Based on these latest results, it appears the bands cease at 3,000 kilometers in depth, giving rise to this more uniform shape. At its core, the pressure is about 100,000 times the pressure we see on Earth.
“Galileo saw those stripes in Jupiter’s atmosphere many centuries ago, so it is definitely something that we've wanted to know for so long that we are all excited with the results,” added Miguel.
Another major finding from the research is that Jupiter’s gravitational field is not symmetrical from North to South. This was something that was unexpected for such a fluid planet that rotates quickly. It appears that this is caused by the varied wind and atmospheric flows on the planet.
“As the surface jets propagate deep into the planet, they produce a perturbation of the gravity field that we determined with Juno,” Daniele Durante from the Sapienza University of Rome in Italy, one of the study authors, told IFLScience. “That enabled us to infer the depth of Jupiter’s zonal jets, which has remained unknown until today.”
The researchers also found that Jupiter’s atmosphere contains about 1 percent of the planet’s mass, equivalent to about three Earths, which is a huge amount. Earth’s atmosphere, for comparison, makes up just one-millionth of our planet’s total mass.
“The result is a surprise because this indicates that the atmosphere of Jupiter is massive and extends much deeper than we previously expected,” Yohai Kaspi from the Weizmann Institute of Science, Rehovot, Israel, an author on one of the papers, told IFLScience.
These results were made possible thanks to Juno’s unique suite of instruments and its close passes to the planet, just a few thousand kilometers at times, closer than any spacecraft before. Using the radio link between Juno and Earth, scientists were able to measure Juno’s speed near Jupiter to exquisite detail, down to just 0.01 millimeters (0.0004 inches) per second in accuracy.
“This is one-hundredth of the speed of a snail!” Luciano Iess, also from Sapienza University and another author on one of the papers, told IFLScience.
“To measure the gravity of Jupiter one needs to track how a test mass (the Juno spacecraft in our case) falls in the gravity field of the planet with respect to another point in space, such as the Earth.”
But wait, that’s not all.
In the final paper, Alberto Adriani from the Institute for Astrophysics and Space Planetology in Rome, Italy, and his colleagues observed the structure of Jupiter’s poles in infrared in detail for the first time.
They found that cyclones at the poles created persistent polygonal patterns, with eight cyclones raging around a single central cyclone at the north pole. At the south pole, there were five cyclones doing the same thing.
“Juno is the first mission designed to give the instruments an outstanding view of the poles,” Adriani told IFLScience. “The cyclonic structures we observed there, over the poles, do not exist in other planets of our solar system."
There’s plenty of other exciting science to come in the future. For example, Juno is going to measure the tides raised by the moon Io as it exerts its gravitational pull on the planet. The depth and structure of Jupiter’s Great Red Spot is also going to be measured, while we might even work out the mass of its central core.
Perhaps one of the neatest things coming up, though, will actually be from a different mission. The Cassini spacecraft, in its final months before it was purposefully destroyed in September 2017, was placed in a Juno-like close orbit around Saturn. Data from these final orbits, which may be released in the next six months, could tell us how Saturn’s interior compares to Jupiter.
Our knowledge of gas giants is shaping up to be greatly increased in 2018. And that’s important for a whole number of reasons, not least because many of the planets we’re finding outside our Solar System are gas giants. If we can understand our own, we’ll be able to understand a lot more about planets elsewhere.