Enigma of Iapetus

[Eroica]

... the overall shape of Iapetus is neither spherical nor ellipsoid - unusual for a large moon; parts of its globe appear to be squashed flat, and its unique equatorial ridge (see below) is so high that it visibly distorts the moon's shape even when viewed from a distance ... In the seventeenth century, Giovanni Cassini observed that he could see Iapetus only on one side of Saturn and not on the other. He drew the conclusion that one side of Iapetus was darker than the other, a conclusion confirmed by images from the ''Voyager'' and ''Cassini'' spacecraft.

The difference in colouring between the two Iapetian hemispheres is striking. The leading hemisphere is dark ... with a slight reddish-brown coloring, while most of the trailing hemisphere and poles is bright ... The pattern of coloration is analogous to a spherical yin-yang symbol...

The orbit of Iapetus is somewhat unusual. Although it is the Saturn's third-largest moon, it orbits much farther from Saturn than the next closest major moon, Titan. It has also the most inclined orbital plane of the regular satellites; only the irregular outer satellites like Phoebe have more inclined orbits. The cause of this is unknown.

Could Iapetus have been formed from the "gentle" collision of two asteroids, one bright and one dark? Its inclined orbit suggests that it's not one of Saturn's original satellites but was only recently captured - too recent for its orbit to be pulled into the equatorial plane by Saturn's bulge. And the equatorial ridge might have been puished up by the collision.

Iapetus

[antoniseb]

Could Iapetus have been formed from the "gentle" collision of two asteroids, one bright and one dark?

Iapetus is very interesting to me. I'm looking forward to the Cassini close encounter shots, though I doubt they will reveal the secrets.

I find it hard to imagine it forming in a soft enough collision of dark and light asteroids/KBOs. I suspect from the images I've seen that the surface coloration differences do not go very deep. As to the formation of the ridge... THAT is a real mystery.

[baric]

Iapetus is very interesting to me. I'm looking forward to the Cassini close encounter shots, though I doubt they will reveal the secrets.

I find it hard to imagine it forming in a soft enough collision of dark and light asteroids/KBOs. I suspect from the images I've seen that the surface coloration differences do not go very deep. As to the formation of the ridge... THAT is a real mystery.

I agree. The light/dark differences could be the result of natural processes initiating away from Iapetus.

The large geological anomalies suggests, to me, that Iapetus is still in the process of gravitationally "rounding" itself after a recent impact.

[Macro Mouse]

Has Iapetus's dark material been thoroughly compared to Titan and Hyperion's spectrums? Titan's composition seems to match that of Iapetus's dark side (including tholins and amorphous carbon) and Hyperion seems very much like it formed after a large impact. Perhaps the three satellites are tied together somehow.

[Frying Tiger]

One of the theories they were discussing on the Cassini site was that the moon might have swept up a ring, leading to the ring material forming the equatorial ridge. Sounded kinda cool.

[Kullat Nunu]

It sounds implausible considering how inclined the moon's orbital plane is compared to the rings.

A newer study suggest that the equatorial ridge is indeed a remant of a ring--around Iapetus.

It looks like the dark stuff on Iapetus is material fallen onto it either from inside Iapetus or from elsewhere (another possibility is that it is leftover material after the brighter ice sublimated). In any case, Iapetus' "natural color" is bright white.

[antoniseb]

It looks like the dark stuff on Iapetus is material fallen onto it either from inside Iapetus or from elsewhere (another possibility is that it is leftover material after the brighter ice sublimated). In any case, Iapetus' "natural color" is bright white.

Hmmm. To me it looks like the dark stuff is the original surface color, but that some event caused the surface to boil in the light colored areas, and brought a mix of lighter colored material to the surface. I note that the surface near the edge of the light area is covered with uniform sized little craters that look like bubbles on the surface of the uncooked side of a pancake. Also, there are stray bits of the light color on high points in the dark area.

In any case, it is just a guess based on pictures and what they look like. My guess is no more reliable than yours.

[Kullat Nunu]

Look how the dark material forms streaks at the north polar region. Also, can you see that many of the craters have bright north-facing slopes, especially near the upper left limb? To me it really looks as if the dark material originates from the equatorial ridge.

[antoniseb]

A year from now, Cassini will get ome close up images. We'll hopefully get better insights then.