Approaching Iapetus - what makes it two-faced?

[bobvanx]

Suppose Iapetus is in fact keeping a record of how large Saturn's rings once were? As seen from Earth, the rings would have spread across a span of sky roughly 2/3 the diameter of the moon.<br /><br />There would be times in Saturn's orbit when the rings would quite clearly an ellipse, and a few months when it would disappear altogether.

 

[CalliArcale]

That's a very interesting thought. It might help explain why so many of Saturn's moons show such extensive battering (of which Mimas' Herschel crater is only the most famous example).<br /><br />The relatively high inclination of Iapetus' orbit (15 degrees) may be connected. Perhaps it suffered a tremendously cataclysmic event or series of events, and has not yet had time to collapse back down to a sphere.<br /><br />It does rotate synchronously, however, so one would assume that it has been in its current orbit and in something much like its current state for a very long time indeed. So I guess that means that either this cataclysmic event happened a long time ago, or it did not affect the moon's rotation significantly. <div class="Discussion_UserSignature"> <p> </p><p><font color="#666699"><em>"People assume that time is a strict progression of cause to effect, but actually from a non-linear, non-subjective viewpoint it's more like a big ball of wibbly wobbly . . . timey wimey . . . stuff."</em>  -- The Tenth Doctor, "Blink"</font></p> </div>

 

[bc_skunk]

Iapetus's surface is obviously frozen now, at issue is if it had ever coalesced (froze) in a manner that would allow for heavier liquids to migrate tidally to one side. I think the eveidence supports "yes" because the escarpment that runs the circumfarence of Iapetus, likens considerably to that of mercurys' and is the result of shrinkage through cooling where separated or layered materials, contracting at different rates, produce visable asymetry in objects.<br /><br />BTW that quote: "Remember: wherever you go, there you are." Thats something Ive been saying since kindergarten, and I put on my cs forum posts. Not an accusation but noteworthy of coincidence. =D

 

[alexblackwell]

Encountering Iapetus<br />CICLOPS Release<br />January 7, 2005

 

[alexblackwell]

Saturn's Moon Iapetus Shows a Bulging Waistline<br />Jet Propulsion Laboratory<br />January 7, 2005

 

[CalliArcale]

Thanks for the article links!<br /><br />This is absolutely incredible. I knew there was a chance that the flyby would raise even bigger questions about Iapetus, but I had no idea it would be something like this. It's just nothing I'd even have imagined.<br /><br />Very cool! <div class="Discussion_UserSignature"> <p> </p><p><font color="#666699"><em>"People assume that time is a strict progression of cause to effect, but actually from a non-linear, non-subjective viewpoint it's more like a big ball of wibbly wobbly . . . timey wimey . . . stuff."</em>  -- The Tenth Doctor, "Blink"</font></p> </div>

 

[bobvanx]

It's nothing anybody imagined!<br /><br />Just wait until those bright boys and girls map all these images onto a sphere, and discover that the anti-saturn side has this equatorial wierdness, too.<br /><br />I'm dissappointed they're all still arguing about emplacemetn possibilities from Iapetus alone. mark my words, someone will turn everybody on their ear and talk about ring emplacemrnt. I wonder how long I'll have to wait?

 

[yurkin]

I don’t accept the standard explanation for how Iapetus got its light side. <br /><br />That is the theory that it picked up the material long ago from Saturn’s rings. If that was the case then you would expect other tidally locked moons of Saturn moons to be two-faced. Dione, Rhea and Tethys are a lot closer to Saturn and nearly the same size as Iapetus. Yet they show no signs of two-faced-ness. Nor do they show any signs of recent geologic activity that would have changed their surface appearance. It is believed that the rings were caused by a moon or some sort of body breaking up within Saturn’s Gravity. If this is the case then material would have had to pass by every other moon before leaving Saturn’s gravity. Even on moons closer to Saturn where the material’s distribution would be denser you see no sign of it!<br /><br />The pattern of the debris on Iapetus would indicate that it came from a ring. But the obviously answer that it must be Saturn’s rings that caused it doesn’t work out. The only other possibility is that the ring came from Iapetus itself. <br /><br />My theory is that a large comet collided with Iapteus, and causes a number of changes to occur. The orbit of the moons is slightly changed. A massive ring of debris is created at the previous orbit or the moon. It also creates the unique surface geology of the moon. Twice an orbit Iapetus passes through its old orbit and encounters the ring of debris with its leading surface. This is what creates the light side. The collision must have occurred on the light side and is now hidden by the light debris. This would have also added to the lightness as well. Any trace of debris ring is long gone but I suspect when detailed measurements are taken of Iapetus and Saturn’s rings they will not match.<br />

 

[yurkin]

<font color="yellow"> It's nothing anybody imagined!</font><br /><br />You would think we’d be getting used to that by now.<br />

 

[thalion]

I favor the old theory that the dark coating comes from outside, possibly from Phoebe. It's clear that Cassini Regio is just as cratered as the lighter portions of the satellite, and the imagery suggests--especially in the transition zone between the light and dark sides--that the coating, whatever it is, is relatively thin.<br /><br />I have no idea what that equatorial ridge means...I would have never imagined such a feature on any satellite. If we can prove some kind of tectonic activity, then maybe (as some have suggested) the ridge was some kind volcanic outlet onto the surface for the "dark stuff." But I still lean toward the old explanation.

 

[CalliArcale]

<blockquote><font class="small">In reply to:</font><hr /><p> I don’t accept the standard explanation for how Iapetus got its light side. <br /> <br />That is the theory that it picked up the material long ago from Saturn’s rings<p><hr /></p></p></blockquote><br /><br />If it helps, I don't believe that's the "standard" explanation -- in fact, I don't think there IS a standard explanation. There's nothing like a consensus yet, nor will be for a while. I actually haven't heard that theory very often (that the material came from the rings). I've heard the Phoebe theory more often, though the main problem with that one (besides "how?") is that Phoebe and Iapetus do not seem to have consistent spectra. (Or so I've read; I have not seen their actual spectra, and I'm not sure I'd be able read them properly anyway.)<br /><br />BTW, I think it is impossible that Iapetus' *light* side was produced by Saturn's rings. This is because the recent Cassini images clearly show that the dark material overlies the light material, not the other way around. Plus, the light side is the *trailing* hemisphere. I would expect the leading hemisphere to get "painted" in such a scenario. <div class="Discussion_UserSignature"> <p> </p><p><font color="#666699"><em>"People assume that time is a strict progression of cause to effect, but actually from a non-linear, non-subjective viewpoint it's more like a big ball of wibbly wobbly . . . timey wimey . . . stuff."</em>  -- The Tenth Doctor, "Blink"</font></p> </div>

 

[bobvanx]

The equatorial ridge has been reported to be 20km high, on a worldlet only 1400km in diameter. If you stood on top of this ridge, you'd see the curvature of Iapetus. You'd be able to look out over 75,000 square km, about 150km in all directions. Something the same proportions on the Earth would be 180km tall (roughly 115 miles, or more than 40 miles higher than SS1's hop). <br /><br />Here it is to scale:

 

[tom_hobbes]

That's quite a hill! <div class="Discussion_UserSignature"> <p><font size="2" color="#339966"> I wish I could remember<br /> But my selective memory<br /> Won't let me</font><font size="2" color="#99cc00"> </font><font size="3" color="#339966"><font size="2">- </font></font><font size="1" color="#339966">Mark Oliver Everett</font></p><p> </p> </div>

 

[yurkin]

I always thought it was the other way around. I guess because the light side looks so much like frosting. But its got chocolate frosting, that’s even better.

 

[CalliArcale]

*big grin*<br /><br />Mmmm....chocoloate frosting.....<br /><br />Here's a good pic of the leading hemisphere of Iapetus, showing that bizarre equatorial ridge and Cassini Regio. If you look closely towards the polar region at the top of the image, you'll notice what looks like wisps of dark material extending into the polar region. It's also interesting to see how dramatically less material is at the poles. <div class="Discussion_UserSignature"> <p> </p><p><font color="#666699"><em>"People assume that time is a strict progression of cause to effect, but actually from a non-linear, non-subjective viewpoint it's more like a big ball of wibbly wobbly . . . timey wimey . . . stuff."</em>  -- The Tenth Doctor, "Blink"</font></p> </div>

 

[bobvanx]

It sure looks like one big space walnut.

 

[vogon13]

Could we consider a ring system around Iapetus itself, perhaps orbiting debris from a major impact (like the theory about our own moon's origin)? The equatorial ridge seems to show three strands in one area. This may have been the result of inclined structures in the possible ring system. Iapetus surface gravity of 2 1/2% that of earth would imply orbital velocity of inner ring edge just prior to contact with surface of approximately 1500 kph. This seems low enough for a fraction of the orbitally decayed ring material to accumulate on the surface without vaporizationation/crater formation. Material that did vaporize may be stain on leading hemisphere. With iapetus inclined in it's passage around saturn, it would transit a variety of paths thru saturn,s magnetotail. Material in orbit around iapetus would interact with the magnetotail, material over leading hemisphere of iapetus would experience this force as opposing its forward motion, material orbiting over trailing hemisphere would experience acceleration in its orbit. Net effect would be to keep ring particles orbital eccentricty higher than normal in a non perturbed ring system. Additonally, if the cratering rate on iapetus was high enough, a tenuous impact derived atmosphere may have existed long enough to have also interacted with the possible ring structure. <br />The equatorial ridge seems to lie on top of existing craters but is also cratered itself. This seems to imply formation during cratering event, therefore, cause of the ridge may be external to Iapetus. This equatorial ridge is a stunning feature, whatever it's cause, I hope additional Cassini flybys are achieved during the extended mission.<br /><br /><br /><br /><br /><br /><br /><br /><br /><br /><br /><br /><br /><br /><br /> <div class="Discussion_UserSignature"> <p><font color="#ff0000"><strong>TPTB went to Dallas and all I got was Plucked !!</strong></font></p><p><font color="#339966"><strong>So many people, so few recipes !!</strong></font></p><p><font color="#0000ff"><strong>Let's clean up this stinkhole !!</strong></font> </p> </div>

 

[bobvanx]

I'm sure this will turn out to be very close to what gets accepted as the cause of Iapetus' equatorial ridge and its dark/light dichotomy.<br /><br />Since it's in orbit around Saturn, a ring would certainly be affected by Saturn's gravity, the magnetosphere, and Iapetus' gravity. Thank you for working out that the orbital velocity is low enough that we can consider it to be a "landing" velocity. It is probably even lower than that speed, if the ring material had any density to it at all. Outer particles would act like "brakes" on inner particles.<br /><br />The system would have been so chaotic, that a ring around Iapetus would be highly distorted.<br /><br />Oh! The little moonlet in the Encke gap, I wonder if it has an equatorial ridge?

 

[spacester]

Hi vogon13! Welcome to sdc!<br /><br />Please forgive me, but if I can find the time, I try to check numbers . . .<br /><br />And I get the same value! <img src="/images/icons/cool.gif" /><br /><br />Using values from JPL Horizons for Iapetus:<br />GM = 132 km^3/s^2<br />r = 718 km<br />and the simplified verion of the vis-viva equation:<br />v = sqrt(GM/r)<br />v = 0.428 km/s (3600 sec/hr) = 1543 km/hour<br /><br />For us poor backward Americans, that's 959 miles/hr<br /><br />So anyway . . . <br /><br />This feature on Iapetus is one of the coolest things I've ever seen! Space walnut indeed!<br /><br />Does anyone know if the ridge is right smack dab, exactly on the equator? Or just close?<br /><br />(bobvanx: y'know, the world needs stutes just as badly as lerts)<br /> <div class="Discussion_UserSignature"> </div>

 

[bobvanx]

Dear stute,<br /><br />No-one has released images with lat/lon lines superimposed on these pics of Iapetus (yet) but they <i>have</i> done so for the anti-Saturn face. In which you can see a string of white "mountains" and the faint suggestion of the ridge wrapping around to the leading hemisphere.<br /><br />

 

[vogon13]

I think one of the captioned Iapetus photos at the Cassini Huygens site indicated the equatorial ridge was aligned within a few degrees of the current equator. What with comet/asteroid impacts over the eons, not being exactly on the equator is probably to be expected for a presumably ancient feature. I am trying to imagine standing on Iapetus and seeing its' own ring in the sky and distant Saturn and its' ring at the same time. Awesome. Too bad Cassini didn't get there a few billion years ago. <br /><br /> <div class="Discussion_UserSignature"> <p><font color="#ff0000"><strong>TPTB went to Dallas and all I got was Plucked !!</strong></font></p><p><font color="#339966"><strong>So many people, so few recipes !!</strong></font></p><p><font color="#0000ff"><strong>Let's clean up this stinkhole !!</strong></font> </p> </div>

 

[vogon13]

May have noticed something else here. The large depression (crater/basin) where the large landslide was noted, seems to extend over the equator a ways. The ridge doesn't seem to exist there. The small crater density in the basin seems similar to other areas where the ridge is seen so the surface age should be similar. If the ridge is the accumulated residue of a decayed ring, a ring particle, just skimming the surface before contacting the surface, would probably have enough momentum to clear a sizeable gap without coming down in it. Maybe with less material between the particle and the center of Iapetus, the particle would also'loft' slightly. This would be the inverse of the famous mascons detected in the Earth's moon. In other words, where the equatorial ridge isn't may be telling us some more about how it formed. This is a tough detail to make out in the photos we currently have, especially since we don't, as yet, have any of this side of Iapetus with lat. and long. markings. Do we know if there will be any distant (but usefull) Cassini flybys before the really close one late in the primary mission? <div class="Discussion_UserSignature"> <p><font color="#ff0000"><strong>TPTB went to Dallas and all I got was Plucked !!</strong></font></p><p><font color="#339966"><strong>So many people, so few recipes !!</strong></font></p><p><font color="#0000ff"><strong>Let's clean up this stinkhole !!</strong></font> </p> </div>

 

[silylene old]

An interesting hypothesis vogon13. I hadn't considered that aspect. <div class="Discussion_UserSignature"> <div class="Discussion_UserSignature" align="center"><em><font color="#0000ff">- - - - - - - - - - - - - - - - - - - - - -</font></em> </div><div class="Discussion_UserSignature" align="center"><font color="#0000ff"><em>I really, really, really miss the "first unread post" function.</em></font> </div> </div>

 

[vogon13]

Consider the Voyager II flyby of Uranus in 1986: Uranus and all its' regular satellites (Miranda, Ariel, Umbriel, Titania, and Oberon) were all delibrately and capriciously oriented with their polar regions pointed towards the sun, thus obscuring through darkness and limb foreshortening their EQUATORS!!!!! Arggggh! We have got to get some more probes ready right away...... <br /><br /><br /><br /><br />I'll go get the legos.<br /> Opus, Bloom County <div class="Discussion_UserSignature"> <p><font color="#ff0000"><strong>TPTB went to Dallas and all I got was Plucked !!</strong></font></p><p><font color="#339966"><strong>So many people, so few recipes !!</strong></font></p><p><font color="#0000ff"><strong>Let's clean up this stinkhole !!</strong></font> </p> </div>

 

[vogon13]

Found map of Iapetus from USGS in the Rand McNally Saturn book by Hunt/Moore. Shows row of white dots 30 degrees south of Basan crater formation. Same featurs seen better in above pix. Line passing thru dots is parallel to and approx. 3 derees south of equator (as laid out by USGS). These features are visible near limb of Iapetus in Voyager II pix but very hard to see. No indication on map of accuracy of placement of features, and I know USGS was motivated to do good job, but with Voyager over 900000 kilometers away, error margin probably much higher than what Cassini is going to do for us. I assume polar axis of Iapetus(and therefore equator alignment follows) perpendicular to orbit around Saturn, and libration, nutation, precession, eccentrity, polar drift, etc are all small/negligble effects. <br /><br /><br /><br />Don't make me come down there!<br />-God <div class="Discussion_UserSignature"> <p><font color="#ff0000"><strong>TPTB went to Dallas and all I got was Plucked !!</strong></font></p><p><font color="#339966"><strong>So many people, so few recipes !!</strong></font></p><p><font color="#0000ff"><strong>Let's clean up this stinkhole !!</strong></font> </p> </div>