High-energy oxygen and sulfur ions have been discovered in Jupiter’s inner radiation belts, as well as a previously unknown ion source. Nearly two decades after NASA’s Galileo mission to Jupiter ended, scientists led by the Max Planck Institute for Solar System Research (MPS) in Germany have unearthed a new secret from the mission’s massive data sets. The research team was able to prove beyond doubt for the first time that the high-energy ions surrounding the gas giant as part of its inner radiation belt are primarily oxygen and sulfur ions. They are thought to have formed as a result of volcanic eruptions on Jupiter’s moon Io.
The team discovered an unexpectedly high concentration of high-energy oxygen ions near the orbit of Jupiter’s moon Amalthea, which orbits Jupiter further inward, which cannot be explained by Io’s volcanic activity. An unknown ion source must be at work here. The study’s findings were published in the journal Science Advances.
Planets with their own global magnetic fields, such as Earth, Jupiter, and Saturn, are surrounded by radiation belts: Fast-moving charged particles such as electrons, protons, and heavier ions whiz around in the magnetic field, forming the invisible, torus-shaped radiation belts. With speeds approaching the speed of light, the particles can ionize other molecules when they collide, creating a hazardous environment that can also endanger space probes and their instruments.
In this respect, the gas giant Jupiter sports the most extreme radiation belts in the Solar System. In their new publication, researchers from the MPS, the California Institute of Technology (USA), the Johns Hopkins Applied Physics Laboratory (USA), the Laboratory of Instrumentation and Experimental Particle Physics (Portugal), and the Academy of Athens (Greece) now present the most comprehensive study to date of the heavy ions in Jupiter’s inner radiation belts.
The radiation belts of Jupiter, like Jupiter’s massive magnetic field, extend several million kilometers into space; however, the region within Europa’s orbit, an area with a radius of about 670,000 kilometers around the gas giant, has the highest energetic particle densities and velocities. When viewed from Jupiter, Europa is the second of four large Jovian satellites known as “Galilean moons” after their discoverer in the 17th century. Io is the smallest of the Galilean moons. Three space missions have so far ventured into this innermost part of these radiation belts and performed in-situ measurements: Pioneer 11 in the mid-1970s, Galileo from 1995 to 2003, and now Juno.
Because of the exposure to strong radiation, it was to be expected that the measurement data from HIC and EPD from the inner region of the radiation belt would be heavily corrupted. After all, neither of these two instruments was specifically designed to operate in such a harsh environment.
Dr. Elias Roussos
“Unfortunately, the data from Pioneer 11 and Juno do not allow us to conclude beyond doubt what kind of ions the spacecraft encountered there,” says MPS scientist Dr. Elias Roussos, lead author of the new study, describing the current state of research. “Therefore, their energies and origin were also unclear until now,” he adds. Only the now rediscovered data from the last months of the Galileo mission is detailed enough to improve this situation.
Venturing into the inner radiation belts
NASA’s Galileo spacecraft reached the Jupiter system in 1995. Equipped with the Heavy Ion Counter (HIC), contributed by the California Institute of Technology, and the Energetic Particle Detector (EPD), developed and built by Johns Hopkins Applied Physics Laboratory in collaboration with the MPS, the mission spent the following eight years providing fundamental insights into the distribution and dynamics of charged particles around the gas giant.
To protect the spacecraft, it initially flew only through the outer, less extreme regions of the radiation belts. Only in 2003, near the end of the mission, when a greater risk was justified, did Galileo venture into the innermost region between the orbits of the moons Amalthea and Thebe. Amalthea and Thebe are Jupiter’s third and fourth moons, as seen from the giant planet. Io’s and Europa’s orbits are further out.
“Because of the exposure to strong radiation, it was to be expected that the measurement data from HIC and EPD from the inner region of the radiation belt would be heavily corrupted. After all, neither of these two instruments was specifically designed to operate in such a harsh environment,” Roussos describes his expectations when he started working on the current study three years ago.
Nonetheless, the researcher wanted to see for himself. He had seen Cassini’s final, similarly daring orbits at Saturn two years earlier as a member of NASA’s Cassini mission and analyzed the unique data from that final mission phase. “The long-completed Galileo mission kept coming to mind,” Roussos recalls. To his surprise, among the many useless data sets were some that could be processed and analyzed with considerable effort.
Enigmatic oxygen ions
The authors of the current study were able to determine the ion composition within the inner radiation belts, as well as their velocities and spatial distribution, for the first time using this scientific treasure. In contrast to the radiation belts of Earth and Saturn, which are dominated by protons, the region within Io’s orbit contains significant amounts of the much heavier oxygen and sulfur ions, with oxygen ions predominating.
“The energy distribution of the heavy ions outside Amalthea’s orbit suggests that they are largely introduced from a more distant region of the radiation belts,” Roussos says. The moon Io with its more than 400 active volcanoes, which repeatedly hurl large amounts of sulfur and sulfur dioxide into space, and to a lesser extent, Europa, are likely the main sources.
Within Amalthea’s orbit, the ion composition shifts dramatically in favor of oxygen. “The concentration and energy of oxygen ions there are much higher than expected,” says Roussos. Actually, the concentration in this region should be decreasing as the moons Amalthea and Thebe absorb incoming ions; the orbits of the two small moons form a kind of natural ion barrier. This behavior has been observed in the radiation belts of the Saturnian system, which has many moons.
The only explanation for the increased concentration of oxygen ions is thus another, local source in the innermost region of the radiation belts. According to computer simulations, one possibility is the release of oxygen as a result of sulfur ion collisions with fine dust particles in Jupiter’s rings. The rings, which are much fainter than Saturn’s, extend approximately as far as Thebe’s orbit. However, it is also possible that low-frequency electromagnetic waves in the magnetospheric environment of the innermost radiation belts heat oxygen ions to the observed energies.
