The Juno probe directly detected particles from Jupiter’s atmosphere streaming into space at a speed sufficient to escape the planet.
Jupiter has an atmosphere dominated by hydrogen and a very powerful magnetic field. Near its poles, charged particles are guided by this field and produce vast auroras, among other phenomena. These regions are also a site of exchanges between the upper atmosphere and the planet’s magnetic environment.
Artist’s view of the Juno probe above Jupiter’s north pole, a region where its instruments study charged particles and auroras.
Credit: NASA/JPL-Caltech.
The researchers tracked a particular type of charged particle called H₃⁺. It is formed from three hydrogen nuclei carrying a combined positive electric charge. In Jupiter’s atmosphere, it appears when solar radiation or energetic particles ionize hydrogen. Its production is especially strong in auroral regions.
Until now, H₃⁺ had mainly been studied remotely through the infrared light it emits. This method provides information about the temperatures and movements of the upper atmosphere, but it combines emissions from different altitudes. This time, Juno made it possible to measure these particles directly on site, above the poles.
The probe’s instruments identified intermittent flows of H₃⁺ high above the ionosphere. Some particles were moving faster than the speed needed to permanently escape Jupiter’s gravitational pull. This measurement therefore confirms that some of these ions are genuinely escaping the planet.
Parameters measured in several H₃⁺ flows at different distances from Jupiter.
Source: Nature Astronomy, figure from the scientific publication.
How are they accelerated? The authors propose a two-step mechanism. Interactions between particles and waves in the plasma could first initiate the upward motion. Electric fields located above the ionosphere would then accelerate the ions to the observed speeds.
The phenomenon represents a transfer of matter and energy between the ionosphere, the electrically charged upper layer, and the magnetosphere controlled by Jupiter’s magnetic field. The researchers estimate the loss at an order of magnitude of 1026 H₃⁺ ions per second. This number is enormous, but each ion is extremely light.
This escape therefore does not mean that Jupiter will quickly lose its immense atmosphere. Its significance lies elsewhere: it directly reveals one of the paths taken by matter in the polar regions. It also helps explain how auroras, electric currents, and the upper atmosphere are connected.
A comparable mechanism could operate on other planets with hydrogen-rich atmospheres, strong magnetic fields, and infrared auroras. Juno’s data, publicly available in NASA’s archives, will make it possible to search for other episodes and determine their frequency more precisely.