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Saturn's outermost major moon, Iapetus (pronounced eye-ap-et-us), would be considered bizarre anywhere else in the solar system.  Ironically, it's one of the more normal worlds compared to its exotic siblings, but still has remarkable and baffling features.  Apart from being divided in color with one hemisphere dominated by dark terrain and the other bright white, it also sports a mysterious equatorial ridge large enough to visibly warp its shape and cause it to be known as the "walnut moon."

The progress of our adventure so far (current in bold):

1.  The Sun
2.  Mercury
3.  Venus
4.  Earth (Vol. 1)
5.  Earth (Vol. 2)
6.  Earth (Vol. 3)
7.  Earth (Vol. 4)
8.  Earth (Vol. 5)
9.  Earth (Vol. 6)
10.  Luna
11.  Mars (Vol. 1)
12.  Mars (Vol. 2)
13.  Mars (Vol. 3)
14.  Phobos & Deimos
15.  Asteroids (Vol. 1)
16.  Asteroids (Vol. 2)
17.  Asteroids (Vol. 3)
18.  Ceres
19.  Jupiter (Vol. 1)
20.  Jupiter (Vol. 2)
21.  Io
22.  Europa (Vol. 1)
23.  Europa (Vol. 2)
24.  Ganymede
25.  Callisto
26.  Saturn (Vol. 1)
27.  Saturn (Vol. 2)
28.  Saturn (Vol. 3)
29.  Rings of Saturn
30.  Mimas
31.  Enceladus
32.  Tethys
33.  Dione
34.  Rhea
35.  Titan (Vol. 1)
36.  Titan (Vol. 2)
37.  Titan (Vol. 3)
38.  Titan (Vol. 4)
39.  Titan (Vol. 5)
40.  Iapetus
41.  Minor Moons of Saturn
42.  Uranus
43.  Miranda
44.  Ariel
45.  Umbriel
46.  Titania
47.  Oberon
48.  Neptune
49.  Triton
50.  The Kuiper Belt & Scattered Disk
51.  Comets
52.  The Interstellar Neighborhood
53.  Updates
54.  Overview: Human Destiny Among the Worlds of Sol
55.  Test Your Knowledge
Iapetus' bright side in enhanced color, exaggerating the brownish tone of the dark terrain on the limb:

PIA08384 (Enhanced Color)

You can see the equatorial ridge on the right-hand limb, and the massive crater is Engelier, the second largest on Iapetus.

I.  Context

Iapetus is the 23rd moon of Saturn, and the seventh and outermost major moon.  Its orbital radius around Saturn is about nine times the distance between the Earth and Moon, but is still deeper in the planetary gravity well than the Moon is in Earth's.  It also has a pronounced orbital inclination of about 15° relative to Saturn's equatorial plane in which all of the other major moons orbit.  Orbital and gravity well diagrams:

IapetusOrbitalDiagram1

Iapetus_orbit_(side)

IapetusGravityWellComparison

Due to Iapetus' large distance from Saturn and major inclination, there haven't been opportunities to get Iapetus and Saturn in the same image.  However, one aesthetic benefit of the inclination is that the ring plane would be visible from Iapetus as more than a bright line seen edge-on.  From the surface, the planet (not including the rings) would cover a 2° arc, or about four times the size of the full Moon from Earth and only about 11% larger than the full Earth from the Moon.  Using our Apollo image template as a standard backdrop, here's what it might look like in the Iapetian (what an awful adjective) sky:

Imagining Saturn in the Iapetian Sky

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II.  History

As described in this section of Titan (Vol. 1), there is a hypothesis that attempts to explain the unusual profusion and arrangement of Saturn's moons with a sequence of major impacts shattering a smaller number of larger moons.  The cause of this hypothetical catastrophe would be a major gravitational disturbance due to the planetary migration of Uranus or Neptune early in solar system history, which would go a ways toward explaining why Iapetus is both so far away from Saturn and its orbit so inclined.  It definitely did not form where it is now, so it makes sense that it was flung or smashed out of Saturn's equatorial plane and into a distant, inclined orbit.

Scientists have more to work with in theorizing about how Iapetus came to be divided into light and dark hemispheres, although the question is still unresolved.  The current thinking is that the surface was originally all white, exposed water ice, but that slightly darker material ended up being deposited on the leading hemisphere (the side facing its direction of orbit) from impact debris among some outer moons with irregular orbits.  This slight reduction in brightness would have caused the leading hemisphere to absorb a little bit more sunlight during the long Iapetian day, which increased the rate at which ice in the region sublimated into space.

Some of the ice molecules that left the darker hemisphere would fall back down to the surface and re-accumulate in the colder, brighter one, leaving the darker hemisphere even darker (because the dark material would be left behind) and with less exposed ice while the bright side got brighter and colder.  This would have led to a feedback loop that would accentuate the color difference until a slightly less bright surface became outright dark while the slightly brighter surface became bright white.  It's not proven, but it sounds plausible to my semi-layman's ear.

The other mystery feature, the equatorial ridge, is much less understood because it's unique in the solar system.  One possibility is that Iapetus originally had a rapid rotation and the poles would have contracted more quickly than the equator as its interior cooled, leaving behind a bulge of material at the equator.  This seems plausible.  Another possibility is that it was generated volcanically and was so massive that it gradually altered the moon's rotation until the equator was on the ridge.  

I'm not sure I understand that theory, specifically why volcanic upwelling would occur along a single plane all over Iapetus.  However, it is true that the overall shape of Iapetus is "squashed" at the poles, so the ridge doesn't appear to be an isolated feature of the equator, but a consequence of a global phenomenon.  A third theory was that it was deposited by the infall of a ring system, but this appears to have been discredited by the cohesive structure of the ridge.  You might wonder if the sublimation process described earlier to explain the two-toned surface might have played some role, but it only changes a column of surface between 20 meters and 10 cm deep per eon (depending on longitude), so it couldn't have produced the 13 km-tall ridge by differential sublimation.

Unfortunately, these mysteries will very likely remain mysteries for the foreseeable future, as there are no more planned flybys of Iapetus for the remainder of the Cassini mission and no planned follow-up probes to Saturn by any space agency.  Even when additional probes are sent to the system, they'll most likely be targeted at Titan and/or Enceladus rather than being general system orbiters, and there's not much scientific motivation to dedicate a probe to Iapetus.  Since Iapetus is so far out in Saturn's boondocks and in an inclined orbit, there won't be much opportunity for synergy with probes aimed at closer moons.

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III.  Properties

1.  Orbital and Rotational Features

Like all major moons of Saturn, Iapetus is tidally locked to the planet, so it presents the same face to Saturn at all times and only completes a rotation when it completes an orbit.  The Iapetian day/month is about 79 Earth days and 8 hours, or the better part of three Lunar months.  The long days and nights are thermally significant, especially due to the two-toned surface, which we'll get into later.

The Saturn system as a whole is tilted about 27° off the ecliptic plane, meaning that either a lot of fuel or a lot of time have to be invested in changing orbital planes when entering the system, so Iapetus' 15° inclination from Saturn's equator is either a help or a hindrance in reaching it from Earth - depending on the orientation of the plane.  I'm not sure which it is, since the information I'm looking at either conflicts or I'm not reading it right.  Since it's a NASA page, probably the latter.  Whatever the case, the orientation of Iapetus' orbital plane relative to the ecliptic will play a major role in shaping its far-future prospects as a target for human activity.

2.  Size and Mass Characteristics

Iapetus is Saturn's third largest moon and its third most massive, as well as being the 23rd largest object overall in the solar system and the 24th most massive.  Its size is about 11.5% that of Earth and about 42% the size of the Moon.  So it is a large and massive moon, but because 80% of its mass is water ice, it has the second lowest density among Saturn's moons - only slightly denser than Tethys.  As a result, its surface gravity is only 2.3% that of Earth, so a 150 lb person would only weigh 3.5 lbs on Iapetus.  And that's actually the third highest gravity in the system - fourth if we count Saturn itself.  It's enough to work with, but some special methods would be needed to live there permanently.

Some size comparisons with comparable bodies - mouse over to see the title if you don't recognize something:

IapetusMercuryComp

IapetusVenusComp

IapetusEarthComp

IapetusMoonComp

IapetusMarsComp

IapetusCeresComp

IapetusIoComp

IapetusEuropaComp

IapetusGanymedeComp

IapetusCallistoComp

IapetusMimasComp

IapetusEnceladusComp

IapetusTethysComp

IapetusDioneComp

IapetusRheaComp

IapetusTitanComp

IapetusMirandaComp

IapetusArielComp

IapetusUmbrielComp

IapetusTitaniaComp

IapetusOberonComp

IapetusTritonComp

3.  Temperatures

The three-month day allows Iapetus to have some of the warmest daytime temperatures in the Saturn system, particularly when the dark side is in daylight and captures more solar energy.  It can get as high as 130 K (-143 °C / -226 °F), which is second only to the temperature of active geysers on Enceladus among Saturn's moons, and is comparable to average temperatures on Callisto, over 600 million kilometers closer to the Sun than Iapetus.  The lows, at 90K, aren't that bad compared to Mimas, Enceladus, and Rhea, but are comparable to Tethys, Dione, and Titan.  Here's a partial temperature map of the dark side in daylight (the star is the subsolar point directly beneath the Sun):

PIA07005

You can see the unusually large thermal range, made possible by the very long time spent soaking in the Sun.  The lows are probably not as extreme because of all the energy absorbed during the day.  In speculative terms, this means one would probably want to locate machinery and human habitations in the dark regions to cut down on the fuel needed for heating and insulating material.

4.  Internal Structure

Iapetus is almost certainly geologically dead, and its odd shape requires it.  An internally dynamic world is basically a slow-moving fluid body that will always settle down into a spheroidal shape, so you can't maintain a squashed ovoid shape with a protruding equatorial ridge like that if it were still fluid inside - not unless it were spinning so fast that centrifugal effects could explain it.  But it barely spins at all, so if it still had a fluid mantle, it should have settled into a sphere by now.  So the most likely explanation is that it's frozen solid, and perhaps froze at different times in a fast-moving early history that let the poles contract first and left the equatorial regions bulging out.  

If Iapetus was born in the theoretical giant collision mentioned earlier, a very rapid and unstable rotation in its early life would be plausible.  Still, I haven't found any speculative illustrations of its internal structure like there often are for other moons, so this is another bit of unfinished business that will have to wait for a future probe, likely several decades from now.

5.  Surface Features

As a general rule, the longitudinal hemispheres of a solid, airless moon are the biggest determinant of its features, particularly the dichotomy between leading and trailing hemispheres.  Our Moon appears to be a rare exception where near and far hemispheres are more important than leading and trailing.  As for latitudinal hemispheres (North and South), usually you only see a lot of difference when there are strong tidal forces and geologic activity involved (as in the case of Enceladus), although strong tides and volcanism don't necessarily imply North/South dichotomies.  

Iapetus is relatively normal in this respect: Its Northern and Southern hemispheres are very similar, as are its near and far sides, but its leading and trailing hemispheres are radically different, with leading being the dark one and trailing the bright one.  However, the border between them does somewhat curve into a "yin-yang" pattern, and the poles are exclusively bright.  Some global shots showing these patterns:

PIA12706

PIA12697

PIA12604

PIA12521

PIA11632

PIA11608

PIA11474

PIA09756

PIA08848

PIA08234

PIA08177

PIA08125

PIA07766

PIA06646

PIA00348

Iapetus_Roncevaux

PIA11690 (IR, UV, and Green Filter)

I've always thought Iapetus in the image pair above looked like some kind of chocolate confection covered in powdered sugar, and it makes me hungry.  Is it wrong to want to eat a celestial object?  :D  Anyway, I mentioned Engelier crater on the bright side (the right-hand image) being the second largest on Iapetus, and the largest overall is called Turgis and is visible in the left-hand image of the dark side toward the rightward limb North of the equatorial ridge.  You can see it's a little bit bigger than Engelier.

In closer, regional images, the dichotomy really starts to look like the white stuff is snow or ice deposited on to a dark surface, but that's actually the opposite of what's going on here.  The white is the "rock" - it's the underlying ice surface of the moon that goes straight through to the boundary of the rocky core.  The dark stuff, however, is the deposited material that's resting on top of the ice and obscuring it:

PIA07660

PIA06166

The illusion gets even more pronounced with closeups of the surface - our brains automatically want to see this as snow covering rock when it's the other way around, with the ice being the exposed "real" surface:

PIA06170

PIA08383

PIA08381

So when you look at this, you're not looking at snowpack with rocks sticking out - it's solid ice with dark dust pooled on the surface in some craters and gulleys:

PIA08382

PIA08374

PIA08375

And now we can actually see a up-close dimensional view that gives us a sense of the terrain, and can really let us imagine being there:

PIA08373

The two sides of Iapetus are given different names: The dark region is Cassini Regio, and the light region is Roncevaux Terra.  It's actually possible to see these regions from Earth with an expensive amateur telescope.

Hemisphere Names

Here are some zooms from our HD signature image at the top:

First, limb shots of Ronceveaux show the terrain in relief through shadows - you can also catch part of the rim of Engelier in the first one:

PIA08384 (Enhanced Color) (1)

PIA08384 (Enhanced Color) (2)

PIA08384 (Enhanced Color) (3)

PIA08384 (Enhanced Color) (4)

PIA08384 (Enhanced Color) (5)

A closer image of Engelier:

PIA08384 (Enhanced Color) (7)

Turgis, showing where a secondary impact caused part of the rimwall to collapse into the new crater:

PIA06171

Image strips of various terrains:

PIA08381

PIA08380

PIA08378

PIA08377

A global crescent view of the ridge:

PIA08376 (False Color)

Cassini Regio terrain is less distinct, perhaps because the surface is more dusty than the solid ice of Roncevaux.  Here is a mid-range closeup of the equatorial ridge on the horizon:

PIA08384 (Enhanced Color) (6)

The equatorial ridge is not continuous around the entire equator, but is complete mostly across Cassini Regio and then breaks up into isolated peaks and ridges in Roncevaux that nonetheless remain on the same line.  Closer images of the ridge, in monochrome:

PIA08404

PIA08372

The continuous portion of the ridge is about 1,300 km long, 20 km wide, and 13 km (or about 8 miles / 43,000 feet) high.  At its tallest, it's about the same height as the Matterhorn stacked on top of Mt. Everest.  And unlike other tall mountains in the solar system like Olympus Mons on Mars, Maxwell Montes on Venus, or Boösaule Montes on Io, the Iapetian ridge is not very spread out at its base, so it would be extremely visually prominent.  However, since it's in vacuum, you might not be able to tell how far away you are from it at any given time, so your perspective could be distorted.  A stop-motion video of Cassini's flyby of Iapetus:

The IAU maps of astronomical bodies on the USGS site are unavailable due to the federal government shutdown, but here are some other maps if you're interested:

Iapetus Polar Regions Map
Iapetus Cassini Regio Map
Iapetus Roncevaux Terra Map

Oddly enough, the International Astronomical Union hasn't yet named the ridge, and I don't know if they ever intend to, but it seems like something that profound deserves a more characterful name than "equatorial ridge."   Hopefully if they do name it, it will be something important enough to reflect its stature: Like Clarke, Sagan, etc.

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IV.  Modern Relevance to Humanity

The novel version of 2001: A Space Odyssey sets Iapetus as the location of the stargate monolith through which Dave Bowman travels, but Stanley Kubrick changed it to Europa in the film because his effects people couldn't make a convincing enough Saturn.  In the novel, the white area is perfectly smooth, and the monolith is an optical illusion that's actually a hole in the surface leading to the stars.  In the Odyssey universe, Iapetus was an artificial construct whose creation destroyed another moon and created Saturn's rings.  In reality, its origins seem to be involved with cataclysmic events, but otherwise it's just a moon with some weird albedos and morphology.  FYI, Clarke writes it as "Japetus" due to the Latin interrelationship of I and J, but "Iapetus" is now standard.

Its current scientific importance has been as an outlier in the Saturn System that informed the giant impact formation hypothesis discussed earlier.  Any formation theory will have to account for such a large moon existing so distantly from the planet and in such an inclined orbit, so it proves useful in that respect.  Beyond that, we don't really know enough about it to be more significant, but it remains an exotic and visually distinctive world.  However, there are no current plans to explore it further, so it may be decades before any major discovery is made about it or more detailed photography taken of it.

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V.  Future Relevance to Humanity

My sense of Iapetus' future relevance to humanity would depend on whether its orbital plane is closer to the ecliptic than Saturn's equatorial plane or farther away from it, which determines the relative economics of going there.  If it is closer, then Iapetus could be a gateway between the rest of the Saturn system and other planetary systems, but if it's farther away, it might be cheaper to go straight to the other moons without stopping on Iapetus.  It's the difference between potentially being a trading post and waystation vs. being way out in the middle of nowhere that everyone would just skip.  

It has all the same material advantages as Rhea and Dione - i.e., inexhaustible ice - in addition to whatever might be offered by the dark dust, whose composition is still unknown, and the view of Saturn would be pretty due to the angled view (albeit much smaller than from the other moons).  However, in terms of aesthetics, the other moons would just be dots in the sky, whereas from some of them you could see the other close ones as actual crescents or globes during close approach.  

You would gain some fuel advantage by Iapetus being high up in Saturn's gravity well, letting you leave the system more cheaply, but that doesn't necessarily help much if you have to then spend a lot of fuel changing your plane to align with another planet's orbit.  Still, there might be times of the Saturnian year when Iapetus' orbital plane is advantageous for reaching (or arriving from) some destination or other outside the system.  Its prospects would all come down to fuel economics: Even if you could afford to ship something to/from Iapetus from any given place, it might not be the best deal.  

I'm sure people will go there and set up shop anyway, because that's just what people do.  There was no reason to colonize Iceland, but it wasn't so bad as to be prohibitive, so they did.  And it's a nice place, but it was never teeming with people or business.  So I would figure Iapetus for a far-flung island kind of society and economy once there is a large human population in the Saturn system, say about 800 to 1000 years from now.  The ridge could serve as a tourist attraction, and since Iapetus has the same resources as the other airless moons in the system, you could expect they'd have similar lifestyles  - though perhaps with fewer luxuries due to relative economic isolation.

Maybe over time they might want to do something with the ridge, like artificially complete it so it completely circles Iapetus.  And/or sculpt into it with colossal statues, relief murals, etc, making it even more of an attraction.  Pure speculation, of course, but we can guarantee ourselves that the new civilizations flung across the solar system will create artistic wonders on a whole new scale in their new environments.  It's anyone's guess who will shine the most in art and architecture, but out of the dozens, hundreds, thousands of new civilizations that will be born over coming centuries, there won't be any lack of cultural options.

As to Iapetus itself, in summary I don't have the slightest clue what humans will make of it or how it would fit into a Saturn system economy.  The best I can do is guess that people who do set down there would probably settle in Cassini Regio because the higher temperatures due to the dark terrain would mean somewhat less energy consumption for heating.

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VI.  Future of Iapetus

Iapetus is far enough away from Saturn that it would probably leave the system when Saturn loses mass to the increased solar wind in the late stages of the Sun.  It would probably stay in the solar system though, out at great distances with the likes of Pluto and Eris, accumulating organic ices on top of its water ice and rocky dust and becoming tinted pink like Triton.  Although there would be no appreciable light to see it by, so it would just be one of the many thousands of such objects wandering in the dark, but still orbiting the dead Sun in its white dwarf phase.  Still, that's just my amateur sense of it - I yield to any actual scientists who know otherwise.

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VII.  Catalog of Exploration

1.  Past and Current Probes

Voyager 1 (USA, 1980 flyby)
Voyager 2 (USA, 1981 flyby)
Cassini-Huygens (USA & Europe, 2004-ongoing Saturn system orbiter)

2.  Future Probes

(none planned)

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This concludes our tour of the major moons of Saturn.  Next time we'll be meeting its mesmerizing menagerie of oddly-shaped and irregular minor moons.

Poll

What is your favorite major moon of Saturn OTHER than Titan?

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