This image shows the limb of Saturn's moon Iapetus, captured by #Cassini on Sept. 10, 2007. A ridge of 10km-high (6.2 mile) mountains encircles the equator, possibly the remains of a former satellite that broke apart as its orbit degraded. Iapetus is 1,470 km (913 miles) wide. 
Here's a wide-angle view of Iapetus from the same observation set on 9/10/07. Color comp made from images acquired in IR, G, and UV.
Iapetus is an icy moon stained dark. This "telephoto" view of the eastern limb region from the previous image shows where dark material has coated its leading forward-facing side. This dark material originates from even more distant moon Phoebe, which is likely a captured comet.
This dark reddish material from Phoebe, which #Iapetus picks up along its orbit around Saturn, helps warm the surface enough during its 900-hour-long daytime to sublimate any ices present. These then redeposit on the cooler trailing side, making it even more reflective.
It's a feedback loop that prevents any bright areas from remaining very long on the dark side, and makes the bright side even brighter than it would have been otherwise. All because of Phoebe (and simple thermodynamics.) ☄️
This is Phoebe. It's a heavily-cratered irregularly-shaped satellite 212 km (132 miles) wide orbiting Saturn at a distance of 12.9 million km—four times as far as Iapetus. This is a composite of two monochrome images acquired by #Cassini in June 2004 from 33,600 km away.
This of course isn't a true color image but it's my attempt to synthesize one based on the coloring of the material covering Iapetus, which originates from Phoebe. (It's not easy to portray a world literally as dark as charcoal!)
I'd said earlier that Phoebe is a captured comet. It really may be even more complex than that; Phoebe could be a captured planetesimal from the Kuiper Belt. Still with its retrograde and highly-inclined orbit it most certainly didn't originate at Saturn. universetoday.com/94858/cassini-…
But Phoebe shows how moons can have physical effects across vast distances. Stuff knocked off Phoebe by impacts has created a huge ring of fine dark dust surrounding Saturn. This was discovered in 2009 with Spitzer—it's literally Saturn's largest ring! science.nasa.gov/science-news/s…
According to @verbiscer, who worked on the discovery, "If you could see the ring in the night sky, it would span the width of two full Moons." But it's so diffuse and the particles are so dark, it can only be detected in infrared.
So the takeaway here is that there's a giant donut of dust surrounding Saturn that was created by a stolen moon and that paints another moon brown. Happy #DonutDay! lightsinthedark.com/2009/10/07/rin…
