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By Matt Williams
For centuries, astronomers have been observing Jupiter swirling surface and been awed and mystified by its appearance. The mystery only deepened when, in 1995, the Galileo spacecraftreached Jupiter and began studying its atmosphere in depth. Since that time, astronomers have puzzled over its colored bands and wondered if they are just surface phenomenon, or something that goes deeper.
Thanks to the Juno spacecraft, which has been orbiting Jupiter since July of 2016, scientists are now much closer to answering that question. This past week, three new studies were published based on Juno data that presented new findings on Jupiter’s magnetic field, its interior rotation, and how deep its belts extend. All of these findings are revising what scientists think of Jupiter’s atmosphere and its inner layers.
The studies were titled “Measurement of Jupiter’s asymmetric gravity field“, “Jupiter’s atmospheric jet streams extend thousands of kilometres deep” and “A suppression of differential rotation in Jupiter’s deep interior“, all of which were published in Nature on March 7th, 2018. The studies were led by Prof. Luciano Iess of Sapienza University of Rome, the second by Prof. Yohai Kaspi and Dr. Eli Galanti of the Weizmann Institute of Science, and the third by Prof. Tristan Guillot of the Observatoire de la Cote d’Azur.
The research effort was led by Professo Kaspi and Dr. Galanti, who in addition to being the lead authors on the second study were co-authors on the other two. The pair have been preparing for this analysis even before Juno launched in 2011, during which time they built mathematical tools to analyze the gravitational field data and get a better grasp of Jupiter’s atmosphere and its dynamics.
All three studies were based on data gathered by Juno as it passed from one of Jupiter’s pole to the other every 53-days – a maneuver known as a “perijove”. With each pass, the probe used its advanced suite of instruments to peer beneath the surface layers of the atmosphere. In addition, radio waves emitted by the probe were measured to determine how they were shifted by the planet’s gravitational field with each orbit.
As astronomers have understood for some time, Jupiter’s jets flow in bands from east to west and west to east. In the process, they disrupt the even distribution of mass on the planet. By measuring changes in the planet’s gravity field (and thus this mass imbalance), Dr. Kaspi and Dr. Galanti’s analytical tools were able to calculate how deep the storms extend beneath the surface and what it’s interior dynamics are like.
Above all, the team expected to find anomalies because of the way the planet deviates from being a perfect sphere – which is due to how its rapid rotation squishes it slightly. However, they also looked for additional anomalies that could be explained due to the presence of powerful winds in the atmosphere.
In the first study, Dr. Iess and his colleagues used precise Doppler tracking of the Juno spacecraft to conduct measurements of Jupiter’s gravity harmonics – both even and odd. What they determined was Jupiter’s magnetic field has a north-south asymmetry, which is indicative of interior flows in the atmosphere.
Analysis of this asymmetry was followed-up on in the second study, where Dr. Kaspi, Dr. Galanti and their colleagues used the variations in the planet’s gravity field to calculate the depth of Jupiter’s east-west jet streams. By measuring how these jets cause an imbalance in Jupiter’s gravity field, and even disrupt the mass of the planet, they concluded that they extend to a depth of 3000 km (1864 mi).
From all this, Prof. Guillot and his colleagues conducted the third study, where they used the previous findings about the planet’s gravitational field and jet streams and compared the results to predictions of interior models. From this, they determined that the interior of the planet rotates almost like a rigid body and that differential rotation decreases farther down.
In addition, they found that the zones of atmospheric flow extended to between 2,000 km (1243 mi) and 3,500 km (2175 mi) deep, which was consistent with the constraints obtained from the odd gravitational harmonics. This depth also corresponds to the point where electric conductivity would become large enough that magnetic drag would suppress differential rotation.
Based on their findings, the team also calculated that Jupiter’s atmosphere constitutes 1% of its total mass. For comparison, Earth’s atmosphere is less than a millionth of its total mass. Still, as Dr. Kaspi explained in Weizzmann Institute press release, this was rather surprising:
“That is much more than anyone thought and more than what has been known from other planets in the Solar System. That is basically a mass equal to three Earths moving at speeds of tens of meters per second.”
All told, these studies have shed new light on the Jupiter’s atmospheric dynamics and interior structure. At present, the subject of what resides at Jupiter’s core remains unresolved. But the researchers hope to analyze further measurements made by Juno to see whether Jupiter has a solid core and (if so) to determine its mass. This in turn will help astronomers learn a great deal about the Solar System’s history and formation.
In addition, Kaspi and Galanti are looking to use some of the same methods they developed to characterize Jupiter’s jet streams to tackle its most iconic feature – Jupiter’s Great Red Spot. In addition to determining how deep this storm extends, they also hope to learn why this storm has persisted for so many centuries, and why it has been noticeably shrinking in recent years.
The Juno mission is expected to wrap up in July of 2018. Barring any extensions, the probe will conduct a controlled deorbit into Jupiter’s atmosphere after conducting perijove 14. However, even after the mission is over, scientists will be analyzing the data it has collected for years to come. What this reveals about the Solar System’s largest planet will also go a long way towards informing out understanding of the Solar System.
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