Jupiter's Deep Atmosphere

One of the missions of NASA's spacecraft Juno, which began orbiting Jupiter in July 2016, was to try and discover the depth of its atmosphere. The planet’s characteristic bands, visible when observing Jupiter, consist of clouds of different gases, swirled by strong winds at high velocities, in different directions at different latitudes. But how deeply do these bands extend inside? And what lies underneath them? Most of the researchers assessed the depth of the atmosphere at several dozens to several hundreds of kilometers. A recent study led by Israeli scientists reveals that Jupiter’s atmosphere extends much deeper, about 3,000 kilometers, and its mass accounts for about one percent of the entire planet’s mass. These findings, published in the journal Nature, bring scientists closer to deciphering the inner composition of Jupiter and to determining whether its core is solid or not.

Deep currents. Jupiter's atmosphere. The image was constructed from a combination of several images taken by Juno | Source: NASA, JPL

Living in harmony

Led by Prof. Yohai Kaspi from the Department of Earth and Planetary Sciences at the Weizmann Institute of Science, the research team applied a measure called gravitational harmonics. This physical tool enables researchers to test the extent to which the shape of an object deviates from a perfect uniform sphere, from inside and out. Researchers divide these harmonics into two main types: even harmonics, denoted by J2, J4, J6, etc. – reflect the extent of deviation from a spherical shape; while the odd ones – J3, J5, J7, and so on – reflect the asymmetry between the sphere’s different regions. If we observe the Earth’s surface, for instance, we can see that its northern hemisphere contains more land, while the southern hemisphere contains much more water. This is an example of the asymmetry characterized by the odd harmonics.

In contrast with the solid, rocky Earth, Jupiter consists of gas. It does not have a surface, mountains or oceans, and therefore asymmetry is not expected. If there was such an asymmetry, it would be a result of wind flow. Prof. Kaspi developed a method to translate gravitational measurements of Jupiter into the velocity of the winds at different layers of its atmosphere, and Dr. Eli Galanti, a research associate in his group, perfected this method. In a paper published in 2013, Kaspi showed that measuring the odd harmonics would enable calculating the depth of wind motions, and effectively the size of the atmosphere. The depth at which the winds cease to move is most likely where the atmosphere ends (or actually begins), and the deeper layers spin together, as one sphere, which physicists call a rigid body.   

The odd harmonics reflect the asymmetry between the two parts of the sphere | Illustrations: James Tuttle Keane, Caltech

A huge mass

Measuring the gravitational harmonics requires specific measurements of the gravity at Jupiter’s various latitudes. How is this performed? By using a radio transmitter on board Juno. The spacecraft sends a radio signal to Earth and its velocity indicates slight changes in the spacecraft’s acceleration, affected by the local gravity. Measuring these subtle changes, 800 million kilometers away from us, allows scientists to calculate gravity at Jupiter’s different latitudes. 

It is not possible to operate the transmitter every time the spacecraft passes next to the planet, as its activity can interfere with other devices; the transmitter’s turn came at Juno’s third flyby of Jupiter, in December 2016. Kaspi and his colleagues calculated the data and were surprised to find that Jupiter’s atmosphere is much deeper than any previous model or prediction. "The measurement gave rise to four independent numbers, J3, J5, J7, J9, all indicating that the winds’ depth ranges from 1,500 to 3,000 kilometers. Repeating the measurements at the sixth flyby in May 2017, we got the exact same data," says Kaspi. "The gravity measurements reflected a non-uniform distribution of mass between Jupiter’s northern and southern hemispheres, and the only non-uniform thing there is wind flow. Now we know the depth of this flow, and we calculated that the mass of Jupiter’s atmosphere is about one percent of the entire planet’s mass, which is quite a lot. The mass of Earth’s atmosphere is about one millionth of its planetary mass".

Towards solving the question of whether Jupiter has a core, and what is its size. Prof. Yohai Kaspi | Photograph: Ettay Nevo

A deep understanding

The paper by Kaspi and his colleagues was published alongside two other papers in the same issue analyzing data on Jupiter that came from Juno, in which Kaspi and Galanti were involved as well and which complement each other. A study led by Luciano Iess from Sapienza University in Italy outlines the measurements and differences between the southern and northern hemispheres of Jupiter. Another study, led by Tristan Guillot from Nice in France, examines the distribution of Jupiter’'s mass using measurements of the even harmonics, and determines that at depths of over 3,000 kilometers, Jupiter in fact behaves like a rigid body. Another paper in the same issue, which is not directly related to the three harmonics papers, analyzes the structure of the cyclones at Jupiter’s poles.

"These findings bring us closer to answering the big question – does Jupiter have a core and of what size?", explains Kaspi. Knowing the atmosphere’s size makes it possible to know the size of the rest of the planet, and therefore calculate whether that part contains a core with properties different than those of the rest of the gas comprising the planet. "At this point, it appears that the planet has a larger core than we previously thought, but not necessarily a solid one. It is most likely composed of elements heavier than the hydrogen and helium that comprise the atmosphere and most of the planet itself. Nevertheless, it seems the core lacks a clear boundary, and the transition from it to the surrounding gas is probably gradual."

The novel findings derived from data collected by Juno do not only shed light on the composition of the gigantic and fascinating planet, but may also better our understanding of processes taking place within our own atmosphere. Without factors such as oceans, continents and mountains, Jupiter is a perfect laboratory for learning how wind flows are formed and how they behave without external disturbances. The researchers hope that collecting more data via Juno will benefit further research on additional aspects of Jupiter’s atmosphere, such as the storm in Jupiter’s Great Red Spot, which has been raging for hundreds of years, and will better our understanding of the processes that take place on the gigantic planet and its counterparts in other places in the solar system.

Translated by Elee Shimshoni