Saturn’s rings cast shadows, affect the planet’s ionosphere

Data collected by the Cassini spacecraft, before it was deliberately crashed into Saturn's atmosphere in September 2017, show that the planet's illustrious rings are casting a shadow in ionized particles over the planet. Cassini has transmitted a hoard of valuable data from Saturn since it arrived at the planet in 2004. In its final months, the probe was sent on a series of orbital dips within the rings that reached very close to the planet itself. At altitudes between 2,600 and 4,000 kilometers, the spacecraft passed directly through the planet's ionosphere, a region of the upper atmosphere dominated by ionized particles. Data collected by the Radio and Plasma Wave Science instrument on board the spacecraft reveal a relatively cold, dense and dynamic ionosphere, Jan-Erik Wahlund and colleagues report. The ionosphere is surprisingly variable and structured on small scales. One reason for that is shadows cast by the rings, which block ultraviolet radiation from the Sun, thereby reducing the ionization in those regions. However, that cannot explain all of the variation, so Wahlund et al. propose that a "ring rain" mechanism may operate, whereby water ions originating from the planet's rings interact with free electrons in the ionosphere. Variation in extreme ultraviolet radiation from the Sun, strong longitudinal wind variations, or magnetic field interactions with the rings, are other possible explanations for the variability observed in Saturn's ionosphere, the authors say.

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http://dx.doi.org/10.1126/science.aao4134

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