Jupiter: Hurricanes to a depth of 2000 kilometers
New simulations allow a “computational peek” under the clouds of Jupiter. Apparently, violent hurricanes rage there.
The hurricanes racing across Jupiter continue far into the interior of the planet’s atmosphere. Only at a depth of around 2000 kilometers do they abruptly break off, as researchers at the Max Planck Institute for Solar System Research (MPS) in Göttingen (Germany) confirm today. In the journal Proceedings of the National Academy of Sciences (PNAS), they present the results of computer simulations that for the first time realistically take into account the extreme properties of Jupiter's atmosphere. According to the results, the structure of the atmosphere changes fundamentally at a depth of around 2000 kilometers: a stable layer of gas must extend there, which acts as a kind of barrier to prevent material from rising and sinking. The reason for this could be, for example, that hydrogen and helium, Jupiter's main ingredients, are not mixed there, but layered by weight.
Jupiter's external appearance is characterized by its striking jet streams: clearly contrasting storm bands that run in latitudinal direction and thus adorn the enormous gas giant with a kind of striped pattern. The wind speeds of the clouds chasing around the planet easily match those of the most violent hurricanes on Earth and in some cases even exceed them. Only at the poles a much calmer breeze blows with speeds of around 100 kilometers per hour. It is still unclear whether the hurricanes only affect the uppermost cloud layer, which is around 50 kilometers thick, or whether they reach much deeper into the atmosphere below. The current findings of the Göttingen researchers now support theories according to which the winds still rage deep inside, but then break off quite abruptly from a certain depth.
Juno's view beneath the cloud layer
Revealing the goings-on beneath Jupiter’s dense cloud layer is one of the goals of NASA’s space probe Juno, which reached the Jovian system eight years ago. From an orbit around the gas giant, the probe registers minimal changes in the planet's gravitational field. Among other things, they allow conclusions about the flow of gas masses in the planet’s interior. However, interpreting the data is tricky. While an international research group led by the Israeli Weizmann Institute of Science reported in 2018 that the winds extended several thousand kilometers into the atmosphere below, other scientists remained sceptical. They argue that the data can also be reconciled with much shallower hurricanes.
In its simulations, the Göttingen team now used computers to model the complex interplay of buoyancy as well as magnetic and Coriolis forces in a layer that reaches 5600 kilometers into Jupiter's atmosphere. The radius of Jupiter measures more than 70,000 kilometers. “Jupiter is a very extreme place; even simulating the processes in this outer layer is a huge challenge and places enormous demands on computing power,” says Prof. Dr. Ulrich Christensen, Emeritus Director at the MPS and lead author of the new study. In addition, the fundamental understanding of how the various forces interact to cause the sudden drop in wind speeds at depth represents a major theoretical challenge, he explains.
Important physical properties such as electrical conductivity and density, which have a significant impact on the results of the simulations, change drastically within Jupiter's atmosphere. Below the uppermost cloud layer, hydrogen and helium, the Jupiter’s two main components, are present in a gaseous state. With increasing depth, however, the pressure increases to such an extent that both gases first transition into a liquid and then, deeper down, even into a metallic state. At a depth of 5,600 kilometers, down to which the MPS researchers have simulated the processes, the electrical conductivity does not yet reach the values of typical metals, but has increased by more than million-fold compared to the cloud layer. For the first time, the newly developed model is able to take this into account.
Stable layer poses puzzles
The new simulations provide the most realistic “computational” view of Jupiter's deep atmosphere to date. As it turns out, the calculations only correctly reflect Jupiter's magnetic field if a stable layer deep in the atmosphere is assumed. “The stable layer is likely to be at a depth of around 2000 kilometers,” explains Dr. Paula Wulff, co-author of the new publication, who is now conducting research at the University of California, Los Angeles after completing her doctorate at the MPS. “Jupiter's hurricane winds reach down to the upper boundary of this layer,” says Christensen. Only at its upper end do the deep hurricane winds abruptly break off. The effect is likely to be much more pronounced near the poles. This could help to explain why the winds in the cloud layer blow more slowly there.
The exact properties of the 2000-kilometer-deep layer are not known. Researchers suspect that hydrogen and helium are layered there by weight: helium at the bottom and hydrogen at the top. Such a layer would inhibit movements such as the ascent of material from greater depths as well as the sinking of material lying further out. So far, there is no direct evidence for the boundary layer. The MPS researchers hope that in future it will be possible to gather information about the deep atmosphere using other methods. Jupiter’s normal modes, which the space probe Juno was able to observe two years ago, offer one possibility. Disturbances in these vibrations can reveal what exactly is going on inside the gas giant - and refine Juno's view of the planet's interior using a new method.
