The Cassini-Huygens probe is approaching nearly 12 years of fruitful service in imaging and analysing the Saturnian system. However, it has only observed and photographed a few instances of moon-moon occultation, the most recent being Rhea and Titan in 2013. Occultations can help scientists to make more accurate predictions on the orbits of objects, by knowing the size of the object in front and the total duration of the occultation event. The upcoming Rhea-Tethys occultation has Rhea almost directly crossing in front of Tethys, providing a golden opportunity for recalibration of both moons’ orbit durations and shapes, which are prone to slow and gradual alteration.
Apart from the rare occultation event, both Rhea and Tethys each have their unique traits worth inquiring into. Cassini recorded a few flashes of ultraviolet light occurring around Rhea’s equator in 2009, ending many to believe they were caused by ring system particles de-orbiting and crashing into the equator, where the ring’s orbit must have been aligned along. However, recent pictures of Rhea showed no visual evidence of a ring system there, so further observation would be required to determine the exact cause of this unusual phenomenon.
Tethys has its own slew of anomalies as well. In 2011, false-colour images of Tethys revealed a faint blue band of material lying roughly along the leading hemisphere’s equator. Similar blue-coloured markings have been found on another Saturnian moon, Mimas and even on Rhea, but to a much less extent. Several studies predict that the bands might be caused by charged electrons moving due to Saturn’s magnetic field colliding with the leading hemisphere of these satellites. Just recently in 2015, Cassini had imaged a few reddish streaks running across the Northern hemisphere of Tethys. These colourations seem to have been overlaid on existing geological formations like craters, suggesting their formation to be relatively recent. Both sets of markings require in-depth analysis in order for more information to be obtained.
Finally, there is a trend in hemispherical colouration and brightness shared by Tethys, Rhea and Dione, the moon in between them. In all three cases, the trailing hemisphered have been darkened to a reddish-brown colour, while the leading hemispheres are brighter. Given the orbital distance of all three moons to be within or near the E-ring and within Saturn’s extended plasma sheet, a few researchers believe that plasma particles are bombarding the moons’ trailing hemispheres, caused by Saturn’s magnetosphere speeding up the orbits of the plasma sheet and causing particles to collide with the slow-moving moons. Reversely, brighter E-ring particles of ice and carbon dioxide may have deposited on the leading hemispheres of the moons, since the E-ring is unaffected by Saturn’s magnetic field, resulting in the moons’ orbits being faster than the E-ring. The Tethys-Rhea occlusion will provide a new opportunity to observe their shared hemispherical colouration side by side.