NASA-funded observations on the W. M. Keck Observatory with analysis led by the University of Leicester, England, tracked the “rain” of charged water particles into the atmosphere of Saturn and found the extent of the ring-rain is far greater, and falls across larger areas of the planet, than previously thought. The work reveals that the rain influences the composition and temperature structure of parts of Saturn’s upper atmosphere.
“Saturn is the first planet to show significant interaction between its atmosphere and ring system,” said James O’Donoghue from Leicester. “The main effect of ring-rain is that it acts to ‘quench’ the ionosphere of Saturn, severely reducing the electron densities in regions in which it falls.”
O’Donoghue said the rings’ effect on electron densities is important because it explains why, for many decades, observations have shown electron densities to be unusually low at some latitudes at Saturn.
“It turns out a major driver of Saturn’s ionospheric environment and climate across vast reaches of the planet are ring particles located 120,000 miles [200,000 kilometers] overhead,” said Kevin Baines from NASA’s Jet Propulsion Laboratory in Pasadena, California. “The ring particles affect which species of particles are in this part of the atmospheric temperature.”
In the early 1980s, images from NASA’s Voyager spacecraft showed two to three dark bands on Saturn, and scientists theorized that water could have been showering down into those bands from the rings. Those bands were not seen again until 2011 when the team observed the planet with Keck Observatory’s NIRSPEC, a near-infrared spectrograph that combines broad wavelength coverage with high spectral resolution, allowing the observers to clearly see subtle emissions from the bright parts of Saturn.
The ring-rain’s effect occurs in Saturn’s ionosphere — Earth has a similar atmospheric layer — where charged particles are produced when the otherwise neutral atmosphere is exposed to a flow of energetic particles or solar radiation. When the scientists tracked the pattern of emissions of a particular hydrogen molecule consisting of three hydrogen atoms, rather than the usual two, they expected to see a uniform planet-wide infrared glow. What they observed instead was a series of light and dark bands with a pattern mimicking the planet’s rings. Saturn’s magnetic field “maps” the water-rich rings and the water-free gaps between rings onto the planet’s atmosphere.
The astronomers surmised that charged water particles from the planet’s rings were being drawn toward the planet by Saturn’s magnetic field and neutralized the glowing triatomic hydrogen ions. This leaves large “shadows” in what would otherwise be a planet-wide infrared glow. These shadows cover 30 percent to 43 percent of the planet’s upper atmosphere surface from around 25° to 55° latitude. This is a significantly larger area than suggested by the Voyager images.
Both Earth and Jupiter have a uniformly glowing equatorial region. Scientists expected this pattern at Saturn, too, but instead they saw 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 from 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.”