APOD: Saturn's Iapetus: Painted Moon in 3D (2021 Jul 13)

[APOD Robot]

Saturn's Iapetus: Painted Moon in 3D

Explanation: What has happened to Saturn's moon Iapetus? Vast sections of this strange world are dark brown, while others are as bright white. The composition of the dark material is unknown, but infrared spectra indicate that it possibly contains some dark form of carbon. Iapetus also has an unusual equatorial ridge that makes it appear like a walnut. To help better understand this seemingly painted moon, NASA directed the robotic Cassini spacecraft orbiting Saturn to swoop within 2,000 kilometers in 2007. Iapetus is pictured here in 3D. A huge impact crater seen in the south spans a tremendous 450 kilometers and appears superposed on an older crater of similar size. The dark material is seen increasingly coating the easternmost part of Iapetus, darkening craters and highlands alike. Close inspection indicates that the dark coating typically faces the moon's equator and is less than a meter thick. A leading hypothesis is that the dark material is mostly dirt leftover when relatively warm but dirty ice sublimates. An initial coating of dark material may have been effectively painted on by the accretion of meteor-liberated debris from other moons.

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[jks]

Click (left or right) and manipulate. Very nice!

[nam888id]

What a great way of presenting a whole lot of images. Nice job on the computer programming. I suspect the same software can be used or has been used to display other moons or planets or asteroids, etc.

[Ann]

What a great way of presenting a whole lot of images. Nice job on the computer programming. I suspect the same software can be used or has been used to display other moons or planets or asteroids, etc.

Indeed, this is truly a great way of presenting the appearance of Iapetus (and other objects too, hopefully)!

Ann

[lmmt61]

I am missing the eye

[JohnD]

Indeed, a fascination toy, to roll and bauble with!
But I'm missing the "equatorial ridge". If I can see it then the axis that the image spins on passes through it, so it should be called the transpolar ridge, but this may be a product of the passage of Cassini past the moon, rather than its actual rotation.

John

[Stefan Hagel]

When moving around the moon, I find a bright half sphere and two dark half spheres, making about 1.5 spheres. Normally moons comprise only 1 sphere. What's going on here?

[orin stepanek]

I was trying to get the moon to rotate all the way around; I could not! Twas only what Cassini saw! But all in all; it was an interesting read with the videos and all!

[neufer]

The redder color of Iapetus's leading hemisphere can be seen in bright areas in a lower contrast image (left), and in dark areas in higher contrast images (right).

---

When moving around the moon, I find a bright half sphere and two dark half spheres, making about 1.5 spheres. What's going on here?

You are not positioned from infinity and
never see a full hemisphere at any one time.

<<The pattern of coloration is analogous to a spherical yin-yang symbol or the two sections of a tennis ball. The dark region is named Cassini Regio, and the bright region is divided into Roncevaux Terra north of the equator, and Saragossa Terra south of it.>>

[neufer]

But I'm missing the "equatorial ridge". If I can see it then the axis that the image spins on passes through it, so it should be called the transpolar ridge, but this may be a product of the passage of Cassini past the moon, rather than its actual rotation.

Iapetus equatorial ridge

---

<<Iapetus's equatorial ridge runs along most of Iapetus' equator. It was discovered when the Cassini spacecraft imaged Iapetus on 31 December 2004. It is not clear how the ridge formed. One difficulty is to explain why it follows the equator almost perfectly. Within the bright regions there is no ridge, but there are a series of isolated 10 km peaks along the equator. There are at least four current hypotheses, but none of them explains why the ridge is confined to [the dark] Cassini Regio.

Peaks in the ridge rise more than 20 km above the surrounding plains, making them some of the tallest mountains in the Solar System. The ridge system is heavily cratered, indicating that it is ancient.
The ridge forms a complex system including isolated peaks, segments of more than 200 km and sections with three near parallel ridges. It runs along most of Iapetus' equator. The ridge's origin is unknown. There are bright areas on the sides of the equatorial ridge near Iapetus' bright trailing hemisphere, which were already visible in Voyager 2 images appearing like mountains and were nicknamed the "Voyager Mountains".

• A team of scientists associated with the Cassini mission have argued that the ridge could be a remnant of the oblate shape of the young Iapetus, when it was rotating more rapidly than it does today. The height of the ridge suggests a maximum rotational period of 17 hours. If Iapetus cooled fast enough to preserve the ridge but remained plastic long enough for the tides raised by Saturn to have slowed the rotation to its current tidally locked 79 days, Iapetus must have been heated by the radioactive decay of aluminium-26. This isotope appears to have been abundant in the solar nebula from which Saturn formed, but has since all decayed. The quantities of aluminium-26 needed to heat Iapetus to the required temperature give a tentative date to its formation relative to the rest of the Solar System: Iapetus must have come together earlier than expected, only two million years after the asteroids started to form.
  
  The ridge could be icy material that welled up from beneath the surface and then solidified. If it had formed away from the position of the equator at the time, this hypothesis requires that the rotational axis would have been driven to its current position by the ridge.
  
  Iapetus may have had a ring system during its formation due to its large Hill sphere, and the equatorial ridge could have then been produced by collisional accretion of this ring.
  
  The ridge and the bulge could be the result of ancient convective overturn. This hypothesis states that the bulge is in isostatic equilibrium typical for terrestrial mountains. It means that under the bulge there is material of low density (roots). The weight of the bulge is compensated by buoyancy forces acting on the roots. The ridge is also built of less dense matter. Its position along the equator is probably a result of the Coriolis force acting on a liquid interior of Iapetus.>>

[JohnD]

Thnak you Art! "Within the bright regions there is no ridge, but there are a series of isolated 10 km peaks along the equator. "
Now I see it! Within the dark area.
John

[neufer]

<<2001: A Space Odyssey is a 1968 science fiction novel by British writer Arthur C. Clarke. It was developed concurrently with Stanley Kubrick's film version and published after the release of the film. Clarke and Kubrick worked on the book together, but eventually only Clarke ended up as the official author.

The name of the Saturnian moon Iapetus is spelled Japetus in the book. This is an alternative rendering of the name, which derives from the fact that "consonantal I" often stands for "J" in the Latin language. In his detailed 1970 book on the film, The Making of Kubrick's 2001, author Jerome Agel discusses the point that Iapetus is the most common rendering of the name [but] goes on to say that "Clarke, the perfectionist", spells it Japetus. Agel then cites the dictionary that defines jape as "to jest; to joke; to mock or make fun of". He then asks the reader, "Is Clarke trying to tell us something?" Clarke himself directly addressed the spelling issue in chapter 19 of The Lost Worlds of 2001,[19] explaining that he simply (and unconsciously) used the spelling he was familiar with from The Conquest of Space (1949) by Willy Ley and Chesley Bonestell, presuming that the "J" form is the German rendering of the Greek.

https://www.etymonline.com/search?q=jape

jape (v.) late 14c., "to trick, beguile, jilt; to mock," also "to act foolishly; to speak jokingly, jest pleasantly," perhaps from Old French japer "to howl, bawl, scream" (Modern French japper), of echoic origin, or from Old French gaber "to mock, deride." Phonetics suits the former, but sense the latter explanation. Chaucer has it in the full range of senses. Around mid-15c. the Middle English word took on a slang sense of "have sex with" and subsequently vanished from polite usage. It was revived in the benign sense of "say or do something in jest" by Scott, etc., and has limped along since in stilted prose.

.....................................................
A mission, Discovery One, is sent to Saturn. En route, Dr. David Bowman and Dr. Frank Poole are the only conscious humans aboard; their three colleagues are in suspended animation, to be awakened near Saturn. The HAL 9000, an artificially intelligent computer, addressed as "Hal", maintains the ship. While Poole is receiving a birthday message from his family on Earth, Hal tells Bowman that the AE-35 communication unit of the ship is going to malfunction. Poole takes one of the extra-vehicular pods and swaps the AE-35 unit; but when Bowman conducts tests on the removed AE-35 unit, he determines that there was never anything wrong with it. Poole and Bowman become suspicious at Hal's refusal to admit that his diagnosis was mistaken; Hal then claims that the replacement AE-35 unit will fail. In communicating with Earth, Poole and Bowman are directed to disconnect Hal for analysis. These instructions are interrupted as the signal is broken, and Hal informs them that the AE-35 unit has malfunctioned. As Poole is removing the unit he is killed when his pod accelerates into him, crushing him. Bowman, uncertain of Hal's role therein, decides to wake the other three astronauts, and therefore quarrels with Hal, with Hal refusing to obey his orders. Bowman threatens to disconnect him if his orders are not obeyed, and Hal relents. As Bowman begins to awaken his colleagues, he hears Hal open both airlocks into space, releasing the ship's internal atmosphere. From a sealed emergency shelter, Bowman gains a spacesuit and re-enters the ship, where he shuts down Hal's consciousness, leaving intact only his autonomic functions, and manually re-establishes contact with Earth. He then learns that his mission is to explore Iapetus, in the hope of contacting the society that buried the monolith on the Moon. Bowman learns that Hal had begun to feel guilty at keeping the purpose of the mission from him and Poole, against his stated mission of gathering information and reporting it fully; and when threatened with disconnection, he panicked and defended himself out of a belief that his very existence was at stake, having no concept of sleep.

Bowman spends months on the ship alone, slowly approaching Iapetus. During his approach, he gradually notices a small black spot on the surface of Iapetus, and later finds it identical in shape to TMA-1, only much larger. The scientists on Earth name this monolith "TMA-2", which Bowman identifies as a double misnomer because it is not in the Tycho crater and gives off no magnetic anomaly. When Bowman approaches the monolith, it opens and pulls in Bowman's pod. Before he vanishes, Mission Control hears him proclaim: "The thing's hollow – it goes on forever – and – oh my God! – it's full of stars!">>

[MarkBour]

Ah, back to Iapetus! This construction is a great way to have a look around all over it.

I'm wondering about a part of today's caption:

Close inspection indicates that the dark coating typically faces the moon's equator and is less than a meter thick.

I don't know what "faces the moon's equator" would mean. Did the author mean to say "faces in the moon's leading direction"?

I find the following note from Wikipedia amazing:

>>>> https://en.wikipedia.org/wiki/Iapetus_(moon) <<<<
Iapetus was discovered by Giovanni Domenico Cassini, an Italian-born French astronomer, in October 1671. He had discovered it on the western side of Saturn and tried viewing it on the eastern side some months later, but was unsuccessful. This was also the case the following year, when he was again able to observe it on the western side, but not the eastern side. Cassini finally observed Iapetus on the eastern side in 1705 with the help of an improved telescope, finding it two magnitudes dimmer on that side.

Cassini correctly surmised that Iapetus has a bright hemisphere and a dark hemisphere, and that it is tidally locked, always keeping the same face towards Saturn. This means that the bright hemisphere is visible from Earth when Iapetus is on the western side of Saturn, and that the dark hemisphere is visible when Iapetus is on the eastern side. The dark hemisphere was later named Cassini Regio in his honor.

At least, in my opinion, those are impressive deductions. And also, serious perseverance. He only finally observed the dark side 34 years later.
(Also, in this way, Cassini anticipated the progression of the typical Jedi knight. )

Finally, I noticed an even larger crater than the APOD caption mentions in the darker region (Cassini Regio); Wikipedia mentions that this crater, at 580km diameter, is called Turgis.

So, Iapetus is a wonder. With that large ridge, and some of its orbital features, I can understand the belief that it is a primordially-formed moon. But with its odd orbit and huge impact craters, I wonder if an asteroid or even another moon knocked its orbit somewhat out of kilter. The theory that Iapetus was formed after an impact involving Titan is also intriguing.

[neufer]

Click to play embedded YouTube video.

Close inspection indicates that the dark coating typically faces the moon's equator and is less than a meter thick.

I don't know what "faces the moon's equator" would mean. Did the author mean to say "faces in the moon's leading direction"?

face (v.) From c. 1400 as "deface, disfigure."
Meaning "to cover with something in front" is from 1560s.

[johnnydeep]

Yeah, I don't know what "faces the equator means" either or what that phrase is intending to imply about the nature of the "dark material". Neufer's cinematic and etymological seeming non-sequiturs didn't really help me.

Unless is means that the dark material is predominantly on the inclined surfaces of mountains and ridges that face the equator. And that it's the equator-facing surfaces that experience the most sublimation. Although that doesn't make sense either since the dark material seems to be entirely coating the inner surface of craters as well.

(PS - I know everyone has been suitably impressed with this 3D rotatable map of Iapetus, but I can't help but be disappointed by the blurry sections. Yes, I realize that a limited dataset is the likely cause, and I could never manage to create this 3D map myself, so I'll chalk it up to being a spoiled American.)

[BillBixby]

I was trying to get the moon to rotate all the way around; I could not! Twas only what Cassini saw! But all in all; it was an interesting read with the videos and all!

Use the mouse left click button to drag the image, causing it to rotate.

[orin stepanek]

I was trying to get the moon to rotate all the way around; I could not! Twas only what Cassini saw! But all in all; it was an interesting read with the videos and all!

Use the mouse left click button to drag the image, causing it to rotate.

10-Q! Works good; Tried that before though!

[johnnydeep]

FYI, there’s a great annotated map of Iapetus with craters and other features named here:

https://www.planetary.org/space-images/ ... lace-names

[VictorBorun]

Thnak you Art! "Within the bright regions there is no ridge, but there are a series of isolated 10 km peaks along the equator. "
Now I see it! Within the dark area.
John

My eyes make the ridge into the light area:

Click to view full size image

[MarkBour]

When moving around the moon, I find a bright half sphere and two dark half spheres, making about 1.5 spheres. Normally moons comprise only 1 sphere. What's going on here?

You're right! I guess the image is stretched and gives this appearance when rotating with the equator staying at the midline. It creates a bit of a fisheye-lens effect.

I don't know what "faces the moon's equator" would mean.

face (v.) From c. 1400 as "deface, disfigure."
Meaning "to cover with something in front" is from 1560s.

Ah. That makes sense. And now my face is sur-faced with redness.

[johnnydeep]

FYI, there’s a great annotated map of Iapetus with craters and other features named here:

https://www.planetary.org/space-images/ ... lace-names

And here’s my attempt at mapping the map to the dark side of the 3D model:

Iapetus’ Dark Side

[bboyne3@gmail.com]

Dear APOD

very nice and informative, great job!

[JohnD]

John Milton knew a thing or two about dark material

Into this wild abyss,
The womb of Nature and perhaps her grave,
Of neither sea, nor shore, nor air, nor fire,
But all these in their pregnant causes mixed
Confusedly, and which thus must ever fight,
Unless the almighty Maker them ordain
His dark materials to create more worlds
Into this wild Abyss the wary Fiend
Stood on the brink of Hell and looked a while,
Pondering his voyage; for no narrow frith
He had to cross.

Paradise Lost, Book 2, from line 910

[johnnydeep]

John Milton knew a thing or two about dark material

Into this wild abyss,
The womb of Nature and perhaps her grave,
Of neither sea, nor shore, nor air, nor fire,
But all these in their pregnant causes mixed
Confusedly, and which thus must ever fight,
Unless the almighty Maker them ordain
His dark materials to create more worlds
Into this wild Abyss the wary Fiend
Stood on the brink of Hell and looked a while,
Pondering his voyage; for no narrow frith
He had to cross.

Paradise Lost, Book 2, from line 910

Cool. I guess I never explicitly realized that Philip Pullman's His Dark Materials trilogy took its title from, and was largely influenced by, Milton:

https://en.wikipedia.org/wiki/His_Dark_ ... Influences

Influences
Pullman has identified three major literary influences on His Dark Materials: the essay On the Marionette Theatre by Heinrich von Kleist,[13] the works of William Blake, and, most important, John Milton's Paradise Lost, from which the trilogy derives its title.[14] In his introduction, he adapts a famous description of Milton by Blake to quip that he (Pullman) "is of the Devil's party and does know it".

[Chief Dan of Turtle Island]

Great picture! I appreciate the work that it took to make this image for us earthlings.
But I don't buy your hypothosis that the dark material is overlaying the white material, rather the opposite seems to be true.
I see the dark material to be the body of the sphere and the white material to be painted over the dark older material.
When I read your text explaining what we are looking at, after looking at the image, I thought this was a joke to see how many people are paying attention. None of the comments sent in seemed to miss what I thought was obvious. Soon I realized the image has an "Esher-like" quality with the three half spheres. Plus I see the equitorial ring going under the near white area. I think the white material is snow or salt that was splashed onto the rock base dark material. The giant craters were made by a large wet or loosely held together ball of stuff moving slowly, compared to the high impact craters caused by fast moving solid rock.