Astronomy Blog

Occultation animation

Rob over at Dirty Skies has a fantastic animation that he made from raw images from the Cassini probe. It shows Saturn's moon Enceladus occulting, by his deduction, Tethys. Not only do we see one of Saturn's moons occult another, but the image seems to show the day side of one and the night side of another, seemingly defying logic.

After a bit of thought, Rob managed to work out that the images actually show Tethys illuminated by Saturn-shine; sunlight reflected from Saturn. The positioning of the three bodies is tricky to understand and I needed to draw a diagram on paper to help me to get my head around it. The reason Enceladus isn't also illuminated (especially considering that it is more reflective than Tethys) is because it was in a part of its orbit that meant it couldn't see as much of the daylight side of Saturn. However, the plumes of material from the cryo-volcanism on Enceladus are nicely back-illuminated by the direct sunlight.

Cassini keeps providing wonderful and mind-bending images.