Moonday: The Tao of Iapetus

The two sides of Iapetus: the leading edge (left), which is darker in color and exhibits an odd equatorial ridge, while the trailing edge is an icy white color. The moon is also distinctly non-spherical. Both images are from the Cassini space probe.

(The Moonday series returns at long last, after an extended hiatus!)

Saturn’s moons are wonderful in their strangeness and variety. I’ve already written about my favorite moon, Hyperion (with its oddly spongy consistency and chaotic rotation), as well as Titan (with its thick atmosphere and methane seas) and of course Enceladus. Today’s moon is also weird and marvelous, with a number of mysterious aspects: Iapetus, Saturn’s third-largest satellite.

From the start, Iapetus seems weird: one side is a lot more reflective than the other, a fact first noted by its discoverer, the astronomer Giovanni Cassini, who first observed it in 1671. As we’ve seen with most of the other moons in the series, Iapetus is tidally-locked to Saturn, presenting the same face to its host planet. The side facing the direction the moon orbits—called the leading edge—is a dark reddish color, as are the polar regions, while the trailing edge (the side facing opposite to the direction of orbit) is nearly white, and very reflective. The entire moon has a yin-yang pattern when viewed from above the north pole, and in general looks a lot like a baseball, if the two pieces of leather stitched together were of different colors.

The orbit of Iapetus, showing the distinction between the leading edge and trailing edge. Sizes are not to scale!

Iapetus, like Enceladus and Hyperion, is largely icy in composition. The color difference seems to be from the collection of debris as Iapetus orbits: little chunks of rock lodge in the surface of the moon, which when exposed to sunlight heats up faster than ice. The heated rock then sublimates the ice (changing it directly from solid to vapor), leaving just the gray-brown residue we see on the leading edge. The trailing edge, which is spared from bombardment of this kind, remains mostly icy.

An even odder and more puzzling feature of Iapetus is its shape. It’s distinctly non-spherical: like many other moons and planets, it bulges at the equator and is flattened at the poles, but the effect is far more pronounced on Iapetus than is usual for objects of its size. In addition, a tall sharp ridge, nearly 20 kilometers (12 miles) high, stands exactly at the equator and extends about 75% of the way around the moon. No analogous formation is known anywhere else in our Solar System: it would be like having a mountain range more than twice the height of Mount Everest spanning the equivalent of several continents on Earth. Combined with the flattened spheroid shape of the moon, Iapetus is rather walnut-like in appearance. (Pardon me, it’s nearly lunchtime.)

Animation: the Cassini flyover of the equatorial ridge on Iapetus. (Image is from the Planetary Society blog.)

The fact that the ridge lies precisely along the equator is obviously no coincidence, but why it should be so is a mystery. One possible solution involves the tidal destruction of a smaller body by Iapetus. As described by Emily Lakdawalla (based on a paper to be published in the Journal of Geophysical Research), perhaps a small moonlet fell into orbit around Iapetus after it formed, or was made from an impact that broke pieces of the moon off. If the moonlet was close enough to Iapetus, the gravitational force on its near side would be substantially stronger than the force on its far side, exceeding the internal forces holding it together. The moonlet then would break apart, making a ring around Iapetus, which over time would collect precisely along the equator.

Iapetus compared to Earth and the Moon. It's not spherical, but the diameter at its equator is slightly less than half that of the Moon. However, since it is composed largely of ice, its mass is a lot less than a rocky body of the same size would be.

I’m not enough of a planetary scientist to judge this model against other possible formation scenarios, so please go read the analysis on the Planetary Society Blog. It seems somewhat plausible to me, since ring systems (such as Saturn’s) are indeed very flat and aligned perfectly with the equator. Most moons won’t form ring systems, due to the crazy interaction of gravity from the moon and the host planet, and such rings aren’t likely to be stable over the long term, due to bombardment and other influences, so the uniqueness of Iapetus isn’t too surprising. (The authors of the study think Oberon, which orbits Uranus, may also have such a ridge, but there aren’t any sufficiently good images of that moon right now. Voyager 2 is the only probe to have visited Uranus, and its flight path wasn’t aligned to get shots of Oberon’s equator.)

Whether this particular scheme is correct or not, Iapetus is an interesting moon. Comparing moons, planets, and other objects across our Solar System helps us understand the entire history of our local environment. Why objects appear the way they do reflects their evolution, the forces that forged them over time. Iapetus’ weirdness is no different: its odd features are the product of its long history, just like Earth’s continents, Mars’ channels, and Jupiter’s storms.

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