Weizmann scientists help unveil the depths of Jupiter’s winds

Israeli scientists make a new discovery.

Jupiter (photo credit: Courtesy)
(photo credit: Courtesy)
Since the famous stripes of the planet Jupiter were seen by Galileo in the early 17th century, scientists have wondered whether they are just colorful bands or a significant layer of the planet.
Three papers just published in the prestigious journal Nature – one by Prof. Yohai Kaspi from the department of earth and planetary sciences at the Weizmann Institute of Science in Rehovot – shows that the stripes, which are belts of strong winds circling the planet, extend to a depth of about 3,000 km.
This is significantly larger than previous estimates, and it has caused scientists to revise their picture of Jupiter’s atmosphere as well as its inner layers.
They analyzed measurements from NASA’s Juno spacecraft, which is providing researchers with a chance to “see” what lies below Jupiter’s surface.
Among the measurements Juno beamed back to Earth are those of the planet’s gravity field, sent via radio waves: As the planet’s gravity pulls on the spacecraft in its flyby, the radio signal is also shifted a bit; this shift in the wavelengths, though minuscule, is measurable. And since the flybys are in different orbits each time, they can sample the gravitational field of different parts of the planet.
Kaspi, together with his Weizmann colleague Dr. Eli Galanti, had been preparing for this analysis even before Juno was launched nearly seven years ago. During this time, they built mathematical tools to analyze the gravitational field data; these are what would enable the researchers to get a grasp on Jupiter’s atmosphere.
The wind belts that encircle the planet, explained Kaspi, are much stronger than the fiercest winds on Earth and have lasted for at least hundreds of years. As these jets flow in bands from east to west or west to east, they disrupt the even distribution of mass on the planet. By measuring the imbalance – changes in the planet’s gravity field – their analytical tools would be able to calculate how deep the storms extend below the surface.
The scientists looked for anomalies – measurements that show the planet deviating from a perfect sphere. They expected a certain anomaly because the planet’s rotation squashes its shape slightly, but additional anomalies in the measurements would most likely be due to winds in the atmosphere.
“Since Jupiter is basically a giant ball of gas,” said Kaspi, “the initial expectation was that there would be no asymmetries in the gravity field between the north and south.”
However, in 2013, while the craft was still en route to Jupiter, Kaspi calculated that since asymmetry exists between the winds in the north and the south, this should produce a measurable gravitational signal. When the results from Juno arrived, the measurement revealed large differences in the gravity field between north and south. “The remarkable thing about this,” said Galanti, “is that we were able to directly measure the signature of the flows themselves.”
Based on the asymmetry in the gravitational fields between north to south, the researchers determined that the wind belts – these stripes observed by Galileo – extend 3,000 km. deep.
Moreover, Kaspi and Galanti developed a method of determining not only the overall depth of the flow, but also precisely how those flows, hidden beneath Jupiter’s clouds, change with depth.
The calculations based on these findings show that Jupiter’s atmosphere is 1% of its total mass.
This may not sound like a lot, but in comparison, Earth’s atmosphere is less than a millionth of its total mass. “That is much more than anyone thought and more than what has been known from other planets in the solar system,” noted Kaspi. “That is basically a mass equal to three Earths moving at speeds of tens of meters per second.”
The first of the three Nature papers, led by Prof. Luciano Iess of Sapienza University of Rome, presents the findings of asymmetry in the gravitational fields. The second describes the results obtained by Kaspi, Galanti and their colleagues showing the distinctive depth of Jupiter’s atmosphere. The third, led by Prof. Tristan Guillot of the Observatoire de la Cote d’Azur and based on these results, looks below the atmosphere, suggesting that beneath the level of the winds, the gas rotates more or less as a single body, almost as if it were a solid. These three papers are helping build a new picture of Jupiter, from the upper cloud-level inwards.
The subject of Jupiter’s core is not yet closed, and the researchers aim to further analyze the measurements to see whether Jupiter has a solid core and if so, to determine its mass.
Answering this question may help us understand how the solar system and its planets formed.
In addition, Kaspi and Galanti are aiming at another iconic phenomenon – Jupiter’s Great Red Spot. Using some of the same methods they developed to characterize the jet-streams, they are trying to understand how deep this giant storm extends.
They also hope to understand why this storm, which has been stable for as long as telescopes have existed, has started shrinking in recent years.