back to article Sun to blame for yin-yang Moon's dark side

The fearsome heat of the Sun is being blamed for the strange yin-yang appearance of Saturn's moon Iapetus. Even at an average distance of roughly 1.5 billion kilometres from the sun, Iapetus is being gradually toasted on one side. False colour image of Iapetus' bright leading edge. Credit: NASA False colour image of Iapetus …


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  1. Anonymous Coward
    Anonymous Coward


    Is Iapetus tiedlocked to the Sun, then? Seems like a very long way off for that, and I'd've thought Saturn would be the greater gravitational influence... What happens when the moon is on the other side of the planet to the Sun? Does the process start to reverse itself?

    Or is there some confusion here?

  2. Chris

    re: Tidelocked

    My thoughts exactly - surely Iapetus is locked to Saturn and should get sunlight all over at different points in its orbit of Saturn, like our Moon does with us, no?

  3. Martin Gregorie

    Re: Tidelocked

    Iapetus is tide-locked to Saturn, not the sun, and the blackening is on its leading edge. Its rear edge is the white one: the photo caption is wrong.

    As Iapetus orbits Saturn it gets a similar amount of solar heating all round its equator. In consequence the only differential heading is due to the blackened leading edge absorbing more radiation and getting (relatively) warm while the white trailing edge reflects much of the radiation and so gets (relatively) cold.

    Lastly, Iapetus' poles pick up the least solar energy of any part of the moon and so are the coldest parts. Hence water could well sublime from the leading edge and condense at the trailing poles. If this is what actually happens you'd expect the leading edge to be darkest, the trailing edge to be much paler and the poles to be the whitest parts. As it happens, this is exactly what the photos show.

    For the latest set of photos, see

    The one shown in this article is called "The Other Side of Iapetus".

  4. John

    Re: Tidelocked

    Tidal locking isn't necessary. As the article explains, if part of the moon is darker, it will get warmer than the rest given _even_ heating. This produces the runaway effect described in which part of the moon gets warmer and darker, even though all of the moon gets the same amount of sunlight.

  5. Anonymous Coward
    Anonymous Coward

    A little Googling...

    ...suggests that Iaptus is, indeed, tidelocked to Saturn and the dark and light sides are the leading and trailing faces of the moon in its orbit.

    But re-reading the article, it's saying that the darker side gets warmer because it's darker, not because it's facing the Sun the whole time or anything. Given that the moon's density is nearly that of ice, I'm a bit puzzled as to why they think that ice melting is going to darken the surface like it would if there was ice on top of rock...

  6. Keller Drozdick

    re re tidelocked

    Perhaps being tidelocked with Saturn caused deposition of dust on the leading edge, and whenever that leading edge faces the sun (or the part that faces the sun) experiences the evaporation/sublimation explained. So you have a continual deposition on one face due to tide lock, and a second cyclical process that reinforces the apparent contrast through shifting the moisture.

    Or they are full of crap. :)

  7. Olly Houlton

    Dark Bodies

    I think what they are saying is that because of the moon is tidally locked to Saturn the side facing away from Saturn is covered in dark dust from the outer moons which was falling in towards Saturn.

    This dust changes the natural absorption of heat of the moon causing the darker side to lose more water through evaporation to the lighter more reflective and cooler side. This in turn further increases the absorption of the dark side leading to more heat and evaporation etc etc and so on.

    So in fact; dust, heat and evaporation, not; fear, anger and hate leads to the dark side...what a let down.

  8. bob, mon!

    leading edge is the key

    Iapetus is indeed tidelocked to Saturn, but that means that there is a constant leading edge to get covered with impact debris. So equal solar input on all sides will have more effect on the darkened leading edge.

    Add in the possibility that reflected light from Saturn contributes to the warming --- but only on (part of) the leading edge.

  9. Anonymous Coward


    Yes. It's a synchronous orbit. But that's not the point. The point is that the dark side here is the front side, which is described as dark because it doesn't reflect light much, rather than being dark because light does not fall on it. Obviously, there is another dark side - the one that's always facing away from Saturn - but that's clearly not the dark side in question.

    I hope that clears things up.

  10. Jon

    @ Tidelocked

    "Dusty material spiralling in from outer Moons hits Iapetus head-on, and causes the forward-facing side of Iapetus to look different than the rest of the Moon."

    Hmm.. This comment would seem to suggest that the moon is indeed tidelocked to Saturn, with the dark side leading the orbit.

    All sides of the moon are exposed to equal amounts of Solar radiation, however dark matter (As opposed to "Dark Matter") absorbs heat faster than lighter coloured matter, suggesting that the dark side of the moon (roll over Pink Floyd) has a greater temperature fluctuation than the lighter spots, meaning that over the billions of years of the moons existance, water gets vapourised to a greater extent on the dark side, and slowly over time, that side of the moon gets darker and the trailing edge gets lighter (Ice?) in a sort of global warming catastrophe. More Ice accumulates on the trailing edge, more dark matter accumulates on the leading edge until it gets to the point where, despite the temperature fluctuations brought about by exposure and lack of exposure to sunlight ice cannot form on the leading edge because it's always too warm and does not melt on the trailing edge because it's always too cold.. Closest example I can come up with on earth is a Glacier in the Alps that does not melt in the summer months to the point that it dissappears entirely, compared to regions of a similar latitude where Ice almost never forms (Death Valley California?)

  11. Anonymous Coward
    Anonymous Coward

    re: Tidelocked

    Iapetus is indeed tidelocked to Saturn and so gets toasted evenly by the sun. This locking means, however, that it has a distinct leading and trailing hemisphere in relation to it's orbit, so only one side picks up most of the detritus floating around in the Saturn system. As the material it picks up is darker than the moon's mostly water-ice geology it tends to retain more heat and this sets up a cycle of evaporating the water from the 'wamer' leading hemisphere and depositing it on the 'colder' trailing one. 'Warmer' is relative in this case of course.

  12. Anonymous Coward
    Anonymous Coward

    re: Tidelocked

    Yes, Iapetus is tidally locked to Saturn, just like all the planet's other major moons. You are right, Iapetus rotates as it orbits (just like our moon), and for part of its "day" the bright trailing hemisphere is exposed to sunlight, just like the dark leading side.

    The difference the scientists point out is that on the leading side (the side that always faces forward as the moon orbits Saturn), some thin coating of dark material from elsewhere (likely an outer moon or moons) settled, just *slightly* darkening the surface there. (Think bugs hitting the windscreen of a speeding car.) That slight darkening was enough to allow sunlight to heat the surface and vaporize ice there, leaving behind the dark stuff in a runaway process. The vaporization/sublimation happens also on the trailing side, but because the dark stuff from elsewhere isn't falling onto that side, you only see the darkening process on the sunward-facing slopes and crater walls.

  13. Ru

    re: Tidelocked

    If Iapetus is tidelocked to Saturn, debris falling on to it would stick largely to one side (the leading edge, presumably). Assuming all different parts of the moon receive equal sunlight, the debris-coloured side will absorb more heat simply because it is darker. It need not spend any more time facing the sun.

  14. Anonymous Coward

    all washed out

    You guys are all washed out.

    Iapeus is obviously one large ball of ice cream, vanilla no less. The dark areas are candy sprinkles that have accumulated over the years. You see, on the light side where every one lives, they have eaten the tasty sprinkles and on the dark side they have been left to accumulate.

    The next time you debate the worst that can happen given our own global warming perhaps we should realize that just about every planet and moon in this solar system is a pretty good example of the worst that can happen. (Except perhaps for Iapeus with all those candy sprinkles.)

    I'll get me coat.

  15. Luther Blissett


    So they don't know what "it" is, but they know how "it" is doing it. Clever.

    Now, let's see if these boffins are clever enough to explain how there is that bloody great crater sitting there. It's far too big to be a volcano. And far too big to be an impact strike. Sublimation? Fungus? amanfromMars?

  16. Anonymous Coward
    Anonymous Coward

    Re: Hmmmm

    Bloody great crater = bloody great impact. There are something like nine such large impact basins on this moon. It has really been hammered through the ages. It's not "too big for an impact strike." Look at Mimas, Tethys, Rhea, etc. Most of Saturn's major moons have BIG craters.

    And, if you see dark powdery material on a snowbank, causing it to melt more rapidly, you need not know whether that powder is dirt or cocoa to explain what's happening.

  17. wayne


    They have finally found mysterious dark matter ;) .

  18. Anonymous Coward
    Anonymous Coward

    Just like yellow snow

    Yellow snow is darker than white snow and it melts faster too. At last I understand astrometeorology.

  19. Johan Struwwelpeter

    Ohhh ...THAT Sun !

    just briefely reading the headlines, I thought Sun Corporation of the Servers and Other Stuff did something to our moon ... yes ,would be incredible .

  20. Steve Roper

    Why Iapetus?

    The question the nobody has asked here and is bugging me is, why has just Iapetus been affected by this phenomenon? All of Saturn's moons are tidelocked to Saturn and most also contain traces of water ice. So why hasn't this happened to Mimas, Enceladus, Dione, Rhea and the other Saturnian moons? (Excluding Titan, which has a dens atmosphere and therefore subject to different effects.) What is so special about Iapetus' composition that this should happen only to it and not to any other moon? And why has it not happened to any of Jupiter's moons, like Europa (which has plenty of water ice) or Ganymede (which also has water ice AND dark rock)? Similar conditions obtain in the Jovian system as well as the Saturnian one.

    The answer to that is what I'd like to hear from the scientists.

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