APOD: Saturn's Iapetus: Moon with a... (2023 Feb 26)

[APOD Robot]

Saturn's Iapetus: Moon with a Strange Surface

Explanation: What would make a moon look like a walnut? A strange ridge that circles Saturn's moon Iapetus's equator, visible near the bottom of the featured image, makes it appear similar to a popular edible nut. The origin of the ridge remains unknown, though, with hypotheses including ice that welled up from below, a ring that crashed down from above, and structure left over from its formation perhaps 100 million years ago. Also strange is that about half of Iapetus is so dark that it can nearly disappear when viewed from Earth, while the rest is, reflectively, quite bright. Observations show that the degree of darkness of the terrain is strangely uniform, as if a dark coating was somehow recently applied to an ancient and highly cratered surface. Last, several large impact basins occur around Iapetus, with a 400-kilometer wide crater visible near the image center, surrounded by deep cliffs that drop sharply to the crater floor. The featured image was taken by the Saturn-orbiting Cassini spacecraft during a flyby of Iapetus at the end of 2004.

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[Nydhogg]

Has anyone considered the possibility that two equally sized objects, one dark and one light, collided so perfectly that they smashed together in such a way as to create the ridge seen?

[VictorBorun]

Has anyone considered the possibility that two equally sized objects, one dark and one light, collided so perfectly that they smashed together in such a way as to create the ridge seen?

merging that soft were successful for many Kuiper-belters, at low velocity orbits

[Chris Peterson]

Has anyone considered the possibility that two equally sized objects, one dark and one light, collided so perfectly that they smashed together in such a way as to create the ridge seen?

The likelihood of two bodies this close to the Sun and this close to a massive planet having orbital velocities so close to each other that they could just gently merge is close enough to zero to discount. At this location, any collisions are going to be violent affairs.

[bystander]

The destination of Discovery One in Arthur C. Clarke's novel 2001: A Space Odyssey.

This got rewritten in the sequel 2010: Odyssey Two to agree with Stanley Kubrick's film version.

[ptahhotep]

Has anyone considered the possibility that two equally sized objects, one dark and one light, collided so perfectly that they smashed together in such a way as to create the ridge seen?

They would have to have been two half light, half dark objects that collided so perfectly that their light and dark parts lined up exactly. The ridge runs around the equator while the colour boundary runs north-south.
Of course it's also possible that the aliens who built Iapetus simply never finished it properly; the excess from the joint in the mould hasn't been filed off and the paint job is only half done.

[zendae]

The destination of Discovery One in Arthur C. Clarke's novel 2001: A Space Odyssey.

This got rewritten in the sequel 2010: Odyssey Two to agree with Stanley Kubrick's film version.

Clarke and Kubrick worked together on the book, which resolves a bit differently than the movie, which ends at a point which in the book is not the end, as Bowman's evolved self actually returns to Earth.

[De58te]

I think I remember reading some time ago that the dark half is on the leading side of the satellite in its orbit. So it's most likely material shed off from a darker moon.

[Chris Peterson]

I think I remember reading some time ago that the dark half is on the leading side of the satellite in its orbit. So it's most likely material shed off from a darker moon.

This is not the only moon that has a different surface composition on its leading and following hemispheres. For exactly the reason you state. The big ridge is the unusual feature here, not the coloration.

[johnnydeep]

Speaking of movies based on Arthur C. Clarke's books, I'm getting mighty psyched for "Rendezvous With Rama", to be directed by Dennis Villeneuve, of Arrival, "Blade Runner 2049", and Dune fame, all pretty darn great movies IMHO.

Storyline
"The story is set in the twenty-second century. A thirty-mile-long cylindrical starship is detected traveling on a course to pass through the Solar system. A group of human explorers are selected and dispatched to intercept the ship in an attempt to discover its purpose, ascertain if there is any threat to Earth, and answer the mysterious questions regarding its origins and purpose. Because all extant names for Roman and Greek gods have been used on other newly discovered celestial objects at this point, the Hindu god Rama is invoked in naming the object, which is originally mistaken to be a comet. Arthur C. Clarke's novel won both the Hugo and Nebula awards upon its release, and coming hard on the heels of "2001: A Space Odyssey", "Rendezvous With Rama" is widely regarded as one of Clarke's best works, and is often cited as a quintessential example of "hard" science fiction." — Paul Wishengrad

[VictorBorun]

I think I remember reading some time ago that the dark half is on the leading side of the satellite in its orbit. So it's most likely material shed off from a darker moon.

This is not the only moon that has a different surface composition on its leading and following hemispheres. For exactly the reason you state. The big ridge is the unusual feature here, not the coloration.

Iapetus's orbital spin is nearly opposite to Saturn's one, so:
1) Iapetus formed elsewhere, possibly at Kuiper belt, where it was a merger product with a rim
2) Iapetus is heavily bombed with headwind of dust from the ghostly Phoebe ring, and the bombing evaporates water ice from the head-half; when the water vapour settles as ice at the tail-half, it makes the ice cover there dense and bright


By orbiting in the opposite direction to how the particles in the Phoebe ring orbit, Iapetus accrues somewhat darker material, preferentially, on one side only. As the volatile ices on that side preferentially sublimate, it leaves the darker deposits behind, while the ice-rich side gets thicker and more reflective. The Phoebe ring exists completely independently of Saturn’s main rings.

[Chris Peterson]

I think I remember reading some time ago that the dark half is on the leading side of the satellite in its orbit. So it's most likely material shed off from a darker moon.

This is not the only moon that has a different surface composition on its leading and following hemispheres. For exactly the reason you state. The big ridge is the unusual feature here, not the coloration.

Iapetus's orbital spin is nearly opposite to Saturn's one, so:

What do you mean by that? Iapetus is tidally locked to Saturn. It rotates and orbits in the same direction as Saturn and most of its moons.

1) Iapetus formed elsewhere, possibly at Kuiper belt, where it was a merger product with a rim

The consensus view is that it formed around Saturn by aggregation, like all the large moons. You have to go out to the much smaller and more distant moons like Phoebe before you have captured bodies.

2) Iapetus is heavily bombed with headwind of dust from the ghostly Phoebe ring, and the bombing evaporates water ice from the head-half; when the water vapour settles as ice at the tail-half, it makes the ice cover there dense and bright

Something like this had to happen once, maybe a billion years or more ago. Now the long orbital period creates thermal feedbacks where the dark side gets darker and the light gets lighter, even without encountering new material.

[orin stepanek]

I had to watch the cute squirrel eat the whole walnut!

[VictorBorun]

This is not the only moon that has a different surface composition on its leading and following hemispheres. For exactly the reason you state. The big ridge is the unusual feature here, not the coloration.

Iapetus's orbital spin is nearly opposite to Saturn's one, so:

What do you mean by that? Iapetus is tidally locked to Saturn. It rotates and orbits in the same direction as Saturn and most of its moons.

1) Iapetus formed elsewhere, possibly at Kuiper belt, where it was a merger product with a rim

The consensus view is that it formed around Saturn by aggregation, like all the large moons. You have to go out to the much smaller and more distant moons like Phoebe before you have captured bodies.

2) Iapetus is heavily bombed with headwind of dust from the ghostly Phoebe ring, and the bombing evaporates water ice from the head-half; when the water vapour settles as ice at the tail-half, it makes the ice cover there dense and bright

Something like this had to happen once, maybe a billion years or more ago. Now the long orbital period creates thermal feedbacks where the dark side gets darker and the light gets lighter, even without encountering new material.

my bad, I catched that Phoebe and Iaperus are anti-parallel but was confused as about which is which.
Still the Phoebe ring does make a headwind for Iapetus, right?
However the ridge for a natively formed Saturn's moon should be explained by some other way…