The Cassini spacecraft reached Saturn in June 2004, and the Magnetosphere and Plasma Science (MAPS) team has been intensely studying the planet’s space environment ever since. Do you remember Enceladus, Saturn’s sixth-largest moon, and the geysers spewing gas plumes from the “Tiger Stripes” chasms in the moon’s South Polar Terrain? The analysis of MAPS measurements played a key role in discovering these geysers, completely changing our understanding of the mass distribution within Saturn’s magnetosphere—the magnetic bubble around the planet. Cassini has made a robust tour of Saturn’s magnetosphere, including regular coverage both near the planet and in the distant magnetotail (the part of the magnetosphere that extends away from the Sun, or the “nightside”). Cassini has traversed over the polar regions and out to the dayside magnetopause where Saturn’s magnetic influence gives way to the supersonic electrified gas from the Sun, the solar wind.
In May 2016, the MAPS team held its annual meeting at the University of Michigan, and I was fortunate enough to be able to attend most of the proceedings. You might think that after a dozen years of data collection and community-wide analysis that everything has been seen and done. Not so! Discoveries are still being made with each new stage of Cassini’s orbit and the continued investigations of a dedicated team of scientists.
For instance, let’s go back to the Enceladus geysers. Once ionized, the now electrically charged gas, or plasma, of Enceladus’ plume becomes subject to the electric and magnetic forces in Saturn’s magnetosphere. The planet’s strong magnetic field makes this plasma circulate with the planet’s relatively fast rotation rate, and an unbalanced centrifugal force transports these particles outward from Enceladus’ orbit. This cold, dense plasma mixes with the hot, tenuous populations coming in from the outer magnetosphere [e.g., Thomsen et al., 2015; Lai et al., 2016], shedding mass from Saturn’s magnetosphere in a complicated and multi-step scenario that still offers many intriguing problems to be solved.
The radio wave instrument on Cassini was key to the discovery of strange periodicities in Saturn’s magnetosphere. The high-latitude auroral regions (northern and southern lights, just like Earth) produce radio-wave emissions that undergo a systematic pulsing, very close to the rotation period of the planet (10.7 hours) but not quite exactly with it. Even more strangely, the northern and southern auroral regions pulse with slightly different periodicities. Similar periodicities have been found in numerous other magnetospheric data sets [e.g., Hunt et al., 2015], and several models have been put forward to explain this mysterious pulsing, like the “new approach” by Carbary . There is still no resolution on this issue.
Additional quasiperiodic phenomena exist in Saturn’s magnetosphere. For instance, the outward flow of particles from Enceladus’ orbit near the planet, along with a charged particle source from the planet’s upper atmosphere [e.g., Felici et al., 2016], steadily builds up particles and magnetic flux in the magnetotail. This is explosively released when magnetic reconnection (the clash and reformation of magnetic field lines) snaps off a big coil of magnetic field and the plasma contained within it. They are expelled downtail and out into deep space [e.g., Smith et al., 2016]. This reconnection also causes a snapping of magnetic flux back towards the planet, creating an injection of hot plasma into the inner magnetosphere.
While Enceladus has been a surprise mass source, Titan has provided its own excitement. This moon, the biggest in the solar system, is 20 Saturn radii (759,000 miles) from the planet, orbiting near the edge of the magnetosphere. New observations have discovered H3+ in the moon’s ionosphere [Woodson et al., 2015], a sign of active chemistry in the upper atmosphere. Other observations show that Titan can enter the solar wind [Bertucci et al., 2015], which exposes its thick atmosphere of nitrogen and methane to the scavenging forces of this fast-streaming plasma.
I am sure that Cassini will reveal more surprises for us. Before Cassini plunges into Saturn’s atmosphere in September 2017, the F-ring (Saturn’s outermost discrete ring) and proximal orbits will bring the spacecraft very close to the planet: this will be Cassini’s “Grand Finale” before it descends into the planet’s atmosphere and burns up. These new measurements in a thus-far unsampled region could prove to be tremendously valuable in understanding its celestial neighbor, the beautifully ringed gas giant Saturn.
—Mike Liemohn, Editor in Chief, Journal of Geophysical Research: Space Physics; email: [email protected]