Why is Saturn’s moon Iapetus so freaking weird?
Well, everything about Saturn and its moons is weird. But the 1500 km (900 mile) wide moon Iapetus may win the prize for the most bizarro. It has a weird equator-ringing ridge that may have formed when the moon solidified and shrank. It’s walnut shaped! It’s got several whopping huge impact basins on it.
But the really odd thing is that the two hemispheres of the moon are so wildly different in color:
That Cassini image shows the real mystery. One half of the moon is substantially darker than the other! Why?
Iapetus takes 79 days to orbit Saturn, and it spins at the same rate, so the same hemisphere always faces into its direction of motion (into the wind, as it were) and the opposite half faces away. Astronomers have supposed that there must be reddish dust or some other junk floating around Saturn, and the leading half of Iapetus slams into and accumulates it like bugs on a windshield. The thing is, the entire hemisphere is not coated! Plus, the boundary is complicated and sharply-defined, so clearly there must be something else going on.
Astronomer now think they have the answer. Using visible and infrared data from Cassini, a new model has been proposed where melting and migrating water ice is the key. As the dust piles up on the leading hemisphere, that area gets warmer when sunlight hits it (dark material absorbs heat better). Underneath this material, the ice warms as well. Even though the temperature is -140° C (-220° F), the sunlight can evaporate the water ice, which then migrates around. When it reaches the trailing hemisphere, where there is no dust and is therefore cooler, the water recondenses and freezes out.
Since the bright, shiny ice is leaving the moon’s leading half, that half gets darker. That ice, sans dust, reaccumulates on the trailing half, making that part brighter. This is a positive feedback loop, and over billions of years we now have this goofy two-faced moon. The model does a great job predicting what Iapetus should look like; the image here shows the result (click it to see the full graphic of how Iapetus changes with time).
According to the model, Iapetus got to its current condition a couple of billion years ago, and reached more or less a steady state. Note that the poles of the moon are colder than the equator, so they don’t have as strong an effect and therefore have relatively less dust.
This model of ice migration is still a hypothesis and will need to be tweaked as more details of the moon come in. But it does explain a lot of the features on Iapetus quite nicely, and may very well turn out to be correct. I’ll note that the wierdness of Iapetus has been known for a long time. Heck, Giovanni Domenico Cassini (our spacecraft’s namesake) noticed its bipolar* nature shortly after he discovered Iapetus in 1671!
This model of a moon of Saturn was made possible because we did something extraordinary: we went there. Mysteries are easy, solutions difficult. But because we are not afraid of difficult exploration, and we dedicate ourselves to the mystery, the solutions become possible. As I have said so many times before, and will continue to in the foreseeable future, that’s what science does. And as much as we scientists like answers, you better believe that it’s the questions that keep us going.
* Haha! It’s a pun! It’s not really polar, since we’re talking longitudinal difference here, not latitudinal, but still: haha!
December 10th, 2009 at 12:48 pm
Oh no! Not red matter! I heard that red matter creates black holes out of planets.
December 10th, 2009 at 12:49 pm
That’s quite a pronounced faultline. Iapetus must be a hit with the other moons.
December 10th, 2009 at 12:54 pm
LIES, LIES, LIES! The model should only be allowed to run for 6000 years, and as such cannot be that way as a result of natural processes. It was made that way just to fool you with all that pesky evidence and science.
Okay, the kidding aside, it is pretty cool that us clever hairless (some more than others) apes can send instruemnts that far, look at stuff, and figure ths out to a reasonable explanation. It’s like science keeps working over and over again!
December 10th, 2009 at 12:59 pm
That’s clearly a PhotoShopped Death Star, c’mon now…
December 10th, 2009 at 1:08 pm
Very cool. But what I want to know is: has Cassini spotted a monolith on the surface yet? =)
December 10th, 2009 at 1:09 pm
Ack! Saturn is going to implode! Damn you Nero!
December 10th, 2009 at 1:09 pm
“That’s no moon”
December 10th, 2009 at 1:11 pm
The real answer: the teenage punks ran out of spray paint before they were done.
December 10th, 2009 at 1:12 pm
Are there other moons where the rotation period matches the orbit period? What causes such a lock? I’ve never heard of such a thing.
December 10th, 2009 at 1:13 pm
Not only a poor spraypainting job, but they also forgot to file the mold lines off. Shoddy workmanship on Iapetus all around.
December 10th, 2009 at 1:17 pm
Interesting… very interesting. The equatorial bulge, I mean. I believe the astute Richard Hoagland has some thoughts about it. He also thinks there is advanced architecture on Iapetus, and a “shard”-like tower there, too.
December 10th, 2009 at 1:17 pm
Michael (#9):
Umm… Look up at the sky early tomorrow morning.
December 10th, 2009 at 1:19 pm
@ #9 Michael: it happens all the time. Our own moon’s orbit is tidally locked to Earth (that’s why we only ever see one side of it). A link will put my post in moderation purgatory, so just do a Google search on “tidal locking”. You might even end up back on the Bad Astronomy site (that’s where I first learned about it).
December 10th, 2009 at 1:26 pm
@Michael: or read Bad Astronomy even….it’s a hoot! (no charge for the book plug, Dr. Plait
)
December 10th, 2009 at 1:38 pm
Actually, it’s probably caused by aliens!
http://www.enterprisemission.com/moon1.htm
Ok, ok…sorry about that. I had to post that because that article actually got me into skeptical thinking, since I was fooled by it at a younger age.
December 10th, 2009 at 1:42 pm
Is it just a coincidence, or are the large bright-side southern crater and the nearly-antipodal large dark-side northern crater related?
December 10th, 2009 at 1:54 pm
In fact Poul Anderson wrote about that possibility in a science fiction story some years ago.
Nice, isn’t it?
The Saturn Game (1989)
December 10th, 2009 at 2:18 pm
Hah! If you think Iapetus is weird, check out Haumea. It’s a Kuiper belt object, about 1/3rd the mass of Pluto. It has two moons, is covered in water ice and has a big red spot on its southern pole.
Now, THAT’S weird.
Gary 7
PS to 17. El Corsario Negro: How’s THIS for an extension of the Poul Anderson story:
“Our colony is massive, coated in ice. It even has it’s own moons. The power system is damped down to normal maintenance levels, barely 10 terra watts. Waste heat is being excreted from the south polar region. The residents of this solar system think it is liquid upwelling from the interior but they have no explanation for the heat signature. The waste system heats subsurface ice/methane composites. As these migrate to the surface of the colony, methane evaporates and carries away the excess heat while the water condenses on the rocky exterior. Rotating in only 3.9 hours, the colony is able to experience nearly a full gravity on the levels closest to the surface.
Soon, it will be time to move along. No sense confronting the locals over a little ice mining. We are pretty certain they’ll never miss a few hundred billion tonnes.
By the time they get their spacecraft far enough out to notice our thievery, we’ll be long gone.
Hopefully, they’ll never know about the specimens we’ve collected, over the years.”
(I wrote this last month)
December 10th, 2009 at 2:23 pm
Because it´s not a moon, it´s a space station.
December 10th, 2009 at 2:25 pm
In the original novel of 2001, Arthur C. Clarke placed the monolith on Iapetus, bringing up it’s albedo differential as a plot point. When I first read it, I thought he was making the thing up out of whole cloth. Imagine my reaction when I found out that it was based on reality. It was one of those moments that showed the young adult version of me just how freaking cool astronomy actually was… filled with mysteries!
The equatorial ridge is really bizzare… (I’ve been trying to paint it for a couple of years now, but I still can’t quite find a composition that will do the feature justice.)
December 10th, 2009 at 2:25 pm
So it wasn’t the work of a flock of galactic seagulls?
December 10th, 2009 at 2:45 pm
@ 9. Michael
I believe Mr Plait describes tidal locking of our moon in his bad Astronomy book… (IVAN3MAN AT LARGE…zip it!)
December 10th, 2009 at 3:22 pm
Since there is no permanent “dark side” of Iapetus, what is the cause of the initial dusty ice that planted the seed, so to speak, for the whole runaway temperature variation? Was it just random chance that the one hemisphere happened to be a wee bit darker to begin with? Or did one of those giant impact craters perhaps stir up a bit more dust that settled more on one side?
I’m just curious to know how the whole process got started. Apart from a malfunction in the space station’s exhaust port, or course, which we all know is not much bigger than a womp rat.
December 10th, 2009 at 3:23 pm
I mean, “Bad Astronomy book”. It is not ‘bad’. It’s not even ‘not bad’. It was awesome.
December 10th, 2009 at 3:25 pm
Phil,
Does this mean that the ice migration process is more important than the fact that it is plowing into the most recently discovered ring of Saturn (http://www.universetoday.com/2009/10/07/spitzer-sees-giant-ring-around-saturn/), or is it in conjunction with the continued application of the dust, or does it simply help explain why the dust is so clearly concentrated in certain areas?
December 10th, 2009 at 3:59 pm
@Bandsaw, in answer to your question:
At least that is what Dr. Plait said about it (emphasis mine).
December 10th, 2009 at 4:30 pm
Gotta say I’m no fan of the “Red matter” nonsense that Star Trek the 2009 prequel /alternative universe came up with.
Don’t get me wrong, I thought the most recent Star trek was a good movie in many ways – great dialogue, good characterisation, fast paced and surprising twists, we finally got Uhura’s first name and so forth but that “red matter” gimmick had me thinking : “Oh Puh-leese that’s just dumb, coudn’t you have come up with something better!”
Also I thought that theory about dust and ice migration explaining Iapetus’es two-faced aspect had been around for a while? Wasn’t it one of a couple of theories proposed ages ago along with the dust shed by Phoebe and other moons idea? Not that I mind hearing about it again – any excuse to visit the Saturnian system is cool with me!
While I’m on being nit-picky and contrary :
Well, everything about Saturn and its moons is weird.
Not its chemical composition – being mostly hydrogen and a bit of helium is pretty much standard for gas giant planets – and stars, brown dwarfs and interstellar nebulous clouds too! Hey, there’s always an exception that proves the rule!
@ 7. ND Says: “That’s no moon” & 19. cardoso Says: “Because it´s not a moon, it´s a space station.”
I think you might be thinking of Mimas with its huge Herschel crater doing the “Death Star” impression instead!
See : http://en.wikipedia.org/wiki/Mimas_(moon)
December 10th, 2009 at 4:47 pm
@10. ellindsey Says:
Not only a poor spraypainting job, but they also forgot to file the mold lines off. Shoddy workmanship on Iapetus all around.
And not a single fijord either!
Very poor job – all that ice and no fijords, poor workmanship indeed!
December 10th, 2009 at 4:55 pm
Phil, is this new? I am certain I read this explanation several months ago. I remember at the time thinking that this was a chicken-and-egg explanation – the ice is where it is because the dark material is where it is. I think I need to re-read this more closely, and maybe try to dig up where I have seen this explanation before.
EDIT: I don’t know if this is the same explanation from over two years ago, or a slightly different one, or maybe this was the explanation at a much earlier stage:
http://blogs.discovermagazine.com/discoblog/2007/10/11/two-faced-moon-shows-its-true-colors/
December 10th, 2009 at 5:26 pm
Well, as is often the case here, not exactly: “This model of a moon of Saturn was made possible because we did something extraordinary: we went there.” This model was certainly at least partially MOTIVATED by new and more detailed imaging of Iapetus but the model itself could well have come BEFORE the new data and in fact could have PREDICTED much of the new data.
Just ask the authors of either of the two new papers whether they’d rather have this nice new result NOW, or have had it 5 years ago….(I know one of the authors slightly, so I might get a chance to actually do that).
December 10th, 2009 at 5:40 pm
Considering the shape of these impact craters, I believe it’s obvious that Iapetus is simply an advanced Death Star model in disguise. The raised equator is shielding over the trench, an attempt to avoid a repeat of the Battle of Yavin.
December 10th, 2009 at 10:28 pm
Steveo R @ # 27 sez:
What does that mean?
BS
December 11th, 2009 at 12:23 am
29. Harold
in re:
http://blogs.discovermagazine.com/discoblog/2007/10/11/two-faced-moon-shows-its-true-colors/
Lol, they both used “Like bugs on a windshield” in their descriptives. Although I can see small differences between Bad Astronomy and Discoblog’s reports, they do appear to be essentially the same.
December 11th, 2009 at 6:48 am
The real reason behind it is that God painted half of the moon, meant to come back to it, but got caught up in other things. If you were running the Universe, imagine how big your To Do list would be! Especially if you spend your time drawing bad crosses on cows.
December 11th, 2009 at 6:58 am
o.O Weird moon.
December 11th, 2009 at 9:10 am
23. kuhnigget:
Here’s an entry from Phil that’ll clear things up for ya:
http://blogs.discovermagazine.com/80beats/2009/10/07/enormous-ghost-ring-is-found-around-saturn/
Where my UVa pals Dr. Anne Verbiscer & her hubby Dr. Mike Skrutskie (and a guy from UMd) found a giant ring of dust around Saturn that emanates from Phoebe. Iapetus cruises through the inner portions of this ring a couple times with each of its orbits and accumulates it’s dust that way.
December 11th, 2009 at 9:59 am
Phil:
We really don’t know the color of the infalling material from the Phoebe Ring recently discovered by Spitzer, so “red matter” may be jumping the gun a bit. The thickness of the ring points to Phoebe, whose surface is most definitely *not* red but grey, as the source, and dynamical models tell us that the leading hemisphere of Iapetus is smacked “like bugs on a windshield” by small ring particles (while large ones stay resident in the ring).
Iapetus is not “sprinkled” by a gentle “rain” of ring particles either; impact velocities are on the order of 10 km/sec. So there must be some process by which the leading hemisphere of Iapetus reddens. Either Phoebe ring particles are grey and the surface of Iapetus reddens when ring particles mix with native Iapetus material or Phoebe ring particles redden as they make their way to Iapetus. Iapetus hasn’t given up all of its secrets yet!
@Bandsaw: Interesting question. The thermal migration model may indeed explain the distribution of dark material on Iapetus, but the ring dynamics tell us that dust impacts on Iapetus from the Phoebe Ring are still occurring.
December 11th, 2009 at 10:04 am
What she said!
:^)
December 11th, 2009 at 10:48 am
32. Blind Squirrel Says:
“Steveo R @ # 27 sez: ‘Hey, there’s always an exception that proves the rule!’
What does that mean?”
It’s a saying that is misused and misunderstood almost as much as “Money is the root of all evil” (instead of “love of money is the root of all evil”).
In this case “proves” was badly translated (I’m not sure the original language, I think it was French). In the original language it was closer to “tests.” This makes a lot more sense. Having a rule “proved” by its exception is like throwing out the outliers in a data set, it’s exactly the opposite of what it’s saying. Having the exception “test” the rule means that you have to think things through and make sure your theory is still valid.
- Jack
December 11th, 2009 at 12:05 pm
“And not a single fijord either!”
He’s not dead, he’s pining for the fjords!
December 11th, 2009 at 1:00 pm
So what you’re saying, Phil, is that they named the wrong moon Janus? What a missed opportunity.
December 11th, 2009 at 5:25 pm
Iapetus has a ridge at the equator because it’s not a moon, it’s a chocolate with a creamy center. The ridge is where the 2 halves were stuck together.
Mmmmmmmm, chocolate!
December 11th, 2009 at 10:19 pm
@ 32. Blind Squirrel Says:
Stevo R @ # 27 sez: “Hey, there’s always an exception that proves the rule!”
What does that mean?
What Jack Hagerty (39.) said. It was intended humerously.
BTW. You may want to change the ‘BS’ to “Blind Squirrel’ there as the acronym BS can be mistaken as being rude. I’m okay with it & am just sayin’ in case you don’t know.
December 12th, 2009 at 11:49 am
Thank you Jack Hagerty.
You are quite correct. A better interpretation of the phrase would be: “The apparent exception tests the rule.”
This is part of my one squirrel campaign against fuzzy thinking. I run into so many people who throw out the phrase as if it actually meant something.
Gosh, I hadn’t noticed.
BS