Sen— Astronomers studying the "rain" of charged water particles flowing into Saturn's atmosphere from the planet's rings have discovered there is more of it, and it falls across larger areas, than previously thought.
The study, using data captured by the W.M Keck Observatory on Mauna Kea in Hawaii, was funded by NASA and the analysis was led by the University of Leicester.
"Saturn is the first planet to show significant interaction between its atmosphere and ring system," said James O'Donoghue, the paper's lead author and a postgraduate researcher at Leicester. "The main effect of ring rain is that it acts to 'quench' the ionosphere of Saturn. In other words, this rain severely reduces the electron densities in regions in which it falls."
Artist's illustration of how charged water particles flow into the Saturnian atmosphere from the planet's rings. Image credit: NASA/JPL-Caltech/Space Science Institute/University of Leicester.
The rings' effect on electron densities explains why those densities seem to be unusually low at certain latitudes on Saturn and helps scientists understand the origin and evolution of Saturn's ring system and changes in the planet's atmosphere.
"It turns out that a major driver of Saturn's ionospheric environment and climate across vast reaches of the planet are ring particles located some 36,000 miles (60,000 kilometres) overhead," said Kevin Baines, a co-author on the paper, based at NASA's Jet Propulsion Laboratory. "The ring particles affect both what species of particles are in this part of the atmosphere and where it is warm or cool."
In the early 1980s, images from NASA's Voyager spacecraft showed two to three dark bands on Saturn, that scientists theorized could be water showering down into those bands from the rings. The bands were not seen again until this team observed the planet in near-infrared wavelengths with the Keck Observatory in April 2011.
The ring rain's effect occurs in Saturn's ionosphere, where charged particles are produced when the otherwise neutral atmosphere is exposed to a flow of energetic particles or solar radiation. The scientists tracked the pattern of emissions of a particular hydrogen ion with three protons (triatomic hydrogen), and observed a series of light and dark bands, with areas of reduced emission corresponding to water-dense portions of Saturn's rings and areas of high emission corresponding to gaps in the rings.
They surmised that charged water particles from the planet's rings were being drawn along Saturn's magnetic field lines and were neutralizing the glowing triatomic hydrogen ions. This leaves large "shadows" in what would otherwise be a planet-wide infrared glow. The shadows cover 30 to 43 percent of the planet's upper atmosphere surface from 25 to 55 degrees latitude, a significantly larger area than suggested by the images from Voyager.
Earth and Jupiter both have equatorial regions that glow uniformly, but Saturn shows dramatic differences at different latitudes.
"Where Jupiter is glowing evenly across its equatorial regions, Saturn has dark bands where the water is falling in, darkening the ionosphere," said Tom Stallard, a paper co-author at Leicester. "We're now also trying to investigate these features with an instrument on NASA's Cassini spacecraft. If we're successful, Cassini may allow us to view in more detail the way that water is removing ionized particles, such as any changes in the altitude or effects that come with the time of day."