New observations provide evidence that the origin of Saturn's E ring is in material ejected from the moon Enceladus
20.5.2006
By: Michal Saaf, Galileo
The planet Saturn is surrounded by several rings, whose width and the spaces between them are controlled by the gravitational forces of the moons surrounding the planet. The E ring is the outermost ring of Saturn. Researchers from Great Britain and the United States, led by Michelle Dougherty, discovered that the E ring receives a supply of material from the moon Enceladus.
During flights of the Cassini spacecraft near the moon Enceladus, an active atmosphere surrounding this moon was discovered (and see: Yoram Ored, "The oxygen surrounding the rings of Saturn", "Galileo" 88). The spacecraft made three flybys near the moon, the last one at a distance of only 175 km from it. Findings from various instruments on the spacecraft confirmed that the moon is surrounded by an atmosphere whose origin is, apparently, a kind of "geyser" that emits particles and is located near the south pole of the moon. These particles do not accumulate on the surface of the moon, but are ejected into space and provide the material from which Saturn's E ring is composed.
The initial evidence for the existence of an atmosphere was magnetic. Saturn is surrounded by a magnetic field that traps charged particles (plasma). In Cassini's first flyby near Enceladus, it was possible to notice disturbances in the movement of the particles trapped in the magnetic field. This means that there is a distortion in Saturn's magnetic field, caused by the moon. Due to the size of Enceladus, its estimated structure and composition, it is unlikely that an internal magnetic field (such as that of the Earth) caused the disturbances. The more likely hypothesis, which was verified later, was that an electrically conductive atmosphere is the cause of these disturbances. If there is an atmosphere containing charged particles - ions - that are free to move, this would explain the distortion in the magnetic field.
Enceladus' gravity is not strong enough to keep the atmosphere on its surface, so there must be a source on the surface of the moon to supply particles and replenish the atmosphere. Measurements carried out on the second and third flights revealed that the deformation occurs mainly near the south pole of the moon, and hinted that the source of the particles apparently resides near the south pole.
The jet apparently emits neutral particles (mainly water) which turn into ions following interaction with the plasma in Saturn's magnetosphere. In subsequent flights, it became clear that the disturbances in the magnetic field are changing, which suggests changes in the atmosphere, and therefore changes in the activity of the jet that emits the particles.
From the simulations conducted, it becomes clear that the existence of a jet near the South Pole explains well the changes in the magnetic field. It also serves to explain the source of the material in the E ring. However, it must be remembered that this interpretation of the findings is preliminary, and further processing and perhaps additional observations are needed to reach a deep understanding of the dynamics in the system.
