Dust source of Jupiter's outermost ring discovered
Max Planck researchers explain how Jupiter's rings receive supplies of dust from its moons.
Jupiter is the largest and most massive of the eight planets in the solar system, with 80 known moons orbiting it. In addition, it is adorned by a system of several rings, although they do not reach the magnificence of Saturn's rings. Jupiter's rings are much fainter and can be observed only under optimal conditions. Predominantly they consist of tiny dust particles, finer than cigarette smoke. A team of researchers from the Moscow IZMIRAN Center of the Russian Academy of Sciences and the Max Planck Institute for Solar System Research (MPS) in Germany has now revealed a secret from the ring system using computer models. Even the most remote regions of the rings receive their "dust supply" from the impacts of micrometeorites on neighboring moons. Unlike previously thought, however, the velocities of the fastest released dust particles are so high that the forces in Jupiter's massive magnetic field accelerate them to the outer edge of the ring system.
Within the orbit of Jupiter's large moon Io, known for its intense volcanism, four small satellites circle the giant planet in close orbits. In particular, the two potato-shaped moons Amalthea and Thebe are important to the outer parts of Jupiter's ring system. They orbit the planet along the outer boundaries of each of the two outer rings. The rings are therefore named after these moons: Amalthea ring and Thebe ring.
Since bodies lacking an atmosphere like these moons are exposed to a constant bombardment of micrometeorites, the moons and the rings interact in a continuous interplay. Micrometeorites are tiny meteorites that typically measure barely a micrometer, smaller than a thousandth of a millimeter. Coming from space, they constantly pelt the moons’ surfaces thus knocking out dust particles. Another process helps lift dust from the surfaces of the satellites. "Especially at the poles of Amalthea and Thebe, dust particles at the surface of these moons become electrically charged by energetic electrons from the planet’s radiation belts and mobilized by Jupiter's magnetic field," says Nikolay Borisov from IZMIRAN, explaining the results of a paper already published last year with MPS scientist Harald Krüger.
It is undisputed that the rings are composed of dust that originally came from Jupiter's inner satellites. The current study by Borisov and Krüger now deals with the outermost part of Jupiter's ring system, the so-called Thebe extension, which borders on the Thebe ring outside the orbit of Thebe. It was photographed by NASA's Galileo spacecraft as it explored the Jovian system between 1995 and 2003. The dust detector on board, which had been built by the Max Planck Institute for Nuclear Physics in Heidelberg, also measured dust particles in the Thebe extension at that time. Although the instrument found fewer particles there than in the Thebe ring itself, their presence was nonetheless puzzling. What forces carry the dust away from Jupiter into the far-out extension?
A 2008 study involving Harald Krüger first attempted an explanation by relying on a "shadow resonance." The idea was that there was a differential charging of dust particles in sunlight and in Jupiter's shadow, which should ultimately populate the Thebe extension with dust particles. This idea was based on the assumption of a plasma of low density in the inner Jovian system. It is known that the electrically charged particles constituting this plasma originate mainly from the volcanoes of Jupiter's moon Io. In the absence of concrete measurements, a plausible value for the plasma density was assumed at that time. But due to NASA’s probe Juno, which has been orbiting Jupiter since summer 2016, the tide has turned: "In light of the new data, this explanation is outdated," notes Harald Krüger.
Although the Juno spacecraft has not yet advanced to Jupiter’s innermost system, the already existing measurements indicate that the plasma there is warmer and denser than previously suspected. The shadow resonance thus does not work as an explanation, and the researchers took another look at the old data from the Galileo probe. From the dust detector's measurements in the vicinity of the large moons, they were able to estimate the speed at which the dust particles are also knocked out of Thebe's surface by the micrometeorites. "The dust particles reach speeds of about 1.5 kilometers per second. That's enough to leave the small moon's weak gravitational field," the Max Planck researcher says.
Once freed from Thebe's gravity, the electrically highly charged particles are exposed to Jupiter's massive magnetic field. The team's calculations show that both effects work together: both the shock from the micrometeorites' impact and Jupiter's electromagnetic forces are jointly sufficient to hurl dust into Thebe's extension. Krüger says, "The behavior of the particles depends on their size: dust particles that measure several micrometers periodically move into the extension again and again. If they break into smaller particles, they can still stay there for years." The situation is different for dust particles that are already much smaller than one micrometer when they leave Thebe. They only make it into the extension for a very short time and then quickly drift back into the Thebe ring.
"It is remarkable that almost two decades after the end of the Galileo mission, new insights into the Jovian ring system are still possible with the data from that time," Krüger says. Borisov and Krüger are now eagerly awaiting new plasma measurements from the Juno spacecraft. Closer to the ring system, they will help test the new explanation.