Last updated May 2, 2018 at 12:38 pm
Juno has used the gravity of Jupiter to scan beneath its clouds, answering some long standing questions of its inner structure and revealing many more.
One of the most iconic features of Jupiter is its bands of clouds circling the planet. Each a different colour and rotating in alternate directions, the winds can reach up to 100 metres per second.
But what is happening underneath the clouds that we can’t see?
NASA’s Juno spacecraft is currently orbiting Jupiter to unlock its mysteries, and researchers are using fine measurements of its gravitational field to begin to reveal the secrets of the gas giant.
The interior of Jupiter is made of densely packed hydrogen and helium, which acts as a fluid. However, while researchers know that there are convection currents in this fluid in the centre of Jupiter, and that the currents reach the surface it’s not know how they form, or whether they explain the bands of clouds circling the planet.
One of the primary goals of Juno, currently orbiting Jupiter on 53-day orbits, is to clarify exactly what these convection currents mean for Jupiter. As Jupiter-like planets are being found in increasing numbers throughout the galaxy, insights from Jupiter could assist understanding these newly discovered gas giants elsewhere.
The gravitational field of Jupiter
Researchers have used Juno to measure the gravitational field of Jupiter by measuring its changing acceleration during orbit.
By doing this, they discovered that there is a component of Jupiter’s gravity which doesn’t align north-south as expected.
This baffled researchers, as they had expected to see a constant symmetry around a fast-rotating gas-giant like Jupiter.
“The exquisitely precise motion of the Juno satellite around the Gas Giant betrays the unexpected pull by the gravity of the clouds beneath,” says astronomer Professor Alan Duffy, who was not involved in the research.
This slightly variation in the gravitational field was measured by analysing the acceleration of the spacecraft as it skirted the planet. By monitoring the change in frequency of radio waves being sent back to Earth, known as the Doppler shift, they found anomalies in the distribution of mass of the planet.
“As the spacecraft orbits it builds up a map of regions that pull more strongly than others. The data can then be split into spherical harmonics or moments, similar to the notes on a drum, that trace out the scale and type of the gravitating structure beneath,” said Duffy.
“Low notes indicate large regions of gravity, high notes are more fine detail structure, which vary rapidly beneath the spacecraft.”
To actually achieve this precise and detailed measurement though required taking into account other factors which might have affected the acceleration of Juno, such as the absorption and re-radiation of sunlight.
Interrogating the gravitational anomaly
A second paper investigates this change in the gravitational field, and has revealed that the anomaly is due to the high-speed winds whipping the surface of the planet.
The only way that these winds could affect the planet’s gravity is if they affected the planet relatively deep in its interior, pushing the mass of the hydrogen and helium to create the imbalance in the planet.
This, the researchers suggest, means that Jupiter’s bands are not in fact just a phenomenon seen on the surface of the planet, but do affect the convection of the planet’s interior.
In effect, the surface winds push some of the hydrogen and helium, creating mass and gravitational imbalances.
However, further measurements revealed that the effect of these surface winds decays the deeper into the planet they looked, with the effect reaching around 3000km beneath the surface – around one-twentieth of the planet’s diameter.
In total, these winds affect around 1% of the planet’s entire mass.
Beneath 3000km, it was found that the hydrogen and helium of the planet’s mass rotates as if it was a solid body, and not a fluid. At this depth, the gas is under a pressure equivalent to 100,000 the atmospheric pressure on Earth. So while the interior doesn’t have a rocky surface, this fluid under such immense pressures and intense magnetic fields do move as one.
A separate paper has also looked more closely at cyclones which are found around Jupiter’s north and south poles.
In contrast with the bands seen around the planet, at the poles Jupiter has highly turbulent areas with numerous cyclones. However, unlike cyclones on Earth, these have been found to be super-stable, and also clustered together in groups.
Even more curiously, the clusters of cyclones at each pole appear to form a particular geometric shapes. In the north, a polar cyclone is surrounded by 8 cyclones in an octagon-like shape. The southern cyclones on the other hand form a completely different shape. Surrounding a circular cyclone, 5 others form a distinct pentagon.
Each cyclone is of similar size ranging from 5600-7000km in diameter, however those in the South are in general larger than those found in the North, typically 4000-4600km. This means these southern cyclones would not only be able to cover Australia, but larger enough to reach across the Tasman and cover New Zealand as well.
We can only imagine what it must be like on the surface in these cyclonic regions.
“These raging tempests are as beautiful as they are mysterious,” says Duffy.
However, these odd cyclone groups do raise questions. Firstly, the pentagon and octagon don’t appear to drift, or if they do, do so very slowly. The researchers would have expected that the cyclonic force of the centre cyclone would have pushed the outer ones around its periphery.
Secondly, the researchers are baffled as to why the cyclones remain separate and don’t merge.
Prof Duffy is likewise intrigued. “At the poles the planet is just plain nuts. How these cyclones haven’t merged together is a mystery, the only other example we have is Saturn which has only a single giant cyclone.”
Together these papers reveal incredible insights into the very nature of Jupiter and its core. It is hoped similar techniques could be used to probe the giant storm occurring in the Great Red Spot, and find whether the moons of Jupiter have any effect on its fluid interior.
Juno continues to orbit Jupiter, and as more images and data comes back, astronomers and astrophysicists will no doubt be pouring over it to see what other mysteries of our solar system’s largest planet are revealed.
The research has been published in Nature.
The paper investigating the polar cyclones is Adriani et al