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Juno, the tiny space probe currently orbiting Jupiter, has a lonely existence. Its nearest neighbouring spacecraft, Dawn, is hundreds of millions of kilometres away in the asteroid belt, orbiting the dwarf planet Ceres. Just under six months ago its other neighbour, , obliterated itself during a planned final plunge into Saturn’s atmosphere after 13 years orbiting the gas giant.
But Nasa's probe has been keeping itself busy studying the vastness of Jupiter, and it's finally starting to help us explain some of the mysteries that remain about the planet. New data gathered from the probe is helping astrophysicists understand the origins of its distinctive coloured bands and the behaviour of the huge cyclone systems that rage close to the planet's poles.
To get this data, Juno is orbiting the gas giant at a rate of once every 53 days. And for every circle of the planet it makes, the probe plunges down to 3,400 kilometres above Jupiter’s cloud tops, passing across both poles in just a couple of hours. During these brief flybys, Juno records over six megabytes of data and imagery, transmitting its findings slowly back to Earth via radio waves.
Researchers poring over these datasets are finally uncovering some of the planet's mysteries. “We knew about what is happening at the cloud-level (that we can see), but very little about what is happening beneath that,” says Yohai Kaspi, an astrophysicist at the Weizmann Institute of Science in Rehovot, Israel. Now a set of four papers published in the scientific journal Nature peek below the surface of the planet for the first time, describing Jupiter’s huge clusters of cyclones, wonky gravitational field and powerful winds that give the planet its distinctive banded appearance.
During five low flybys over Jupiter’s poles, Juno captured infrared and visible imagery of the area around the poles, giving us a much more detailed picture of the huge cyclone clusters that we already know exist there.
At the North Pole, one central cyclone is surrounded by eight further cyclones, each of them with a diameter of 4,000 kilometres, making each one about as wide as Australia. At the South Pole, another central cyclone is surrounded by a further five cyclones, each one with a diameter of between 5,600 and 7,000 kilometres.
“We have never seen similar structures on other planets of our Solar System,” says Alberto Adriani at the Italian National Institute for Astrophysics in Rome, the lead author on the paper describing the cyclones. But where these cyclones came from, and how long they’ve been raging above the planet’s surface, is still a mystery, although Adriani suspects they’ve been there for a long time already.
“Every big structure on Jupiter appear to live long and in some cases very long – like the Great Red Spot – and, so far, the polar structures we observed appear to be very stable.” Over the seven months that Adriani and his team tracked the cyclones around the North Pole, they found that the storms drifted only very slowly eastwards and didn’t change their overall shape at all.
Jupiter’s visible surface is streaked with distinctive bright and dark bands, the result of gases being blown by winds that can reach speeds of up to 300 kilometres per hour. But until these latest results we didn’t know much about how far those winds reached below the uppermost surface of the planet’s atmosphere.
By studying data about Jupiter’s gravitational field, Kaspi and his colleagues discovered that those outer jet streams extend 3,000 kilometres below the cloud level. Since Jupiter has no solid surface, working out a precise definition of its atmosphere is tricky, but these findings suggest that, the atmosphere is much deeper than was previously thought. “This was certainly a surprise. We have never seen an atmosphere so massive, but it is possible that Saturn may also have deep jets and a deep atmosphere,” says Kaspi.
Using the same dataset, Kaspi was also able to estimate that Jupiter's atmosphere makes up just one per cent of the planet’s total mass. At the point where those deep jet streams start to decay, the pressure is about 100,000 times that of the atmosphere at Earth’s surface. Now that they’ve worked out the extent of these jets, Kaspi and his team plan to use a similar approach to understand the depth and structure of Jupiter’s Great Red Spot – a persistent high-pressure spot in the planet’s atmosphere that is more than twice the size of Earth.
Those deep jet streams also reveal useful clues about Jupiter’s gravitational field. We already knew that Jupiter’s gravitational field wasn’t perfectly symmetrical, since much of the planet’s mass is unevenly distributed in those jet streams blowing through its atmosphere. Now we know that those jet streams are very deep, we also know that the gravity variation is much greater than we previously expected.
To measure the gravity of Jupiter, Luciano Iess and his colleagues at the University of Rome in Italy studied the very slight variations in radio signals sent and recieved by the Juno probe. By measuring the difference in frequencies between transmitted and received signals, Iess was able to infer minute changes in the probe’s speed, caused by variation in Jupiter’s gravitational field. Using the radio link between Juno and the Earth, the team was able to measure the velocity of the probe down to changes of one-hundredth of a millimeter.
“This is an important finding from Juno,” says Iess. It confirms that measuring a planet’s gravitational field can be used as a way of examining planetary dynamics that can’t be observed by other instruments. Similar studies into data gathered from the Cassini probe may soon tell us whether the same underlying planetary dynamics are at play when it comes to Saturn, too.
This article was originally published by WIRED UK
