Approaching Iapetus - what makes it two-faced?

3488 · Oct 18, 2008

[QUOTE]Thanks.  Yes, very fascinating and a good link with maps. Posted by silylene[/QUOTE]It is indeed silylene.Iapetus is certainly one of the more interesting of the Saturn moons, with Titan, Enceladus & Dione & is in fact one of the more interesting bodies of the entire solar system. These maps really show how the mountain belt, really does follow the equator & the bright / dark terrain is 50 / 50. This suggests to me that Iapetus has kept the same side facing Saturn for most, if not all of it's existence. Cratering generally appears uniform, in spite of the albedo differnence & apart from the enormous mountain belt, there appears that little has taken place, thus a treasure trove of data regarding the outer part of the Saturn system.No evidence of cryovolcanism, but that mountain belt, still seems to me as if something unique has happened here. Perhaps deoribited rings? Seems more likely now, after the suspicion that Rhea may have it's own tenuous ring.Andrew Brown.  <div class="Discussion_UserSignature"> "I suddenly noticed an anomaly to the left of Io, just off the rim of that world. It was extremely large with respect to the overall size of Io and crescent shaped. It seemed unbelievable that something that big had not been visible before". Linda Morabito on discovering that the Jupiter moon Io was volcanically active. Friday 9th March 1979.http://www.launchphotography.com/ http://anthmartian.googlepages.com/thisislandearthhttp://web.me.com/meridianijournal </div>

h2ouniverse · Oct 19, 2008

Replying to: <DIV CLASS='Discussion_PostQuote'>No evidence of cryovolcanism.  Posted by 3488</DIV> Hi Andrew, You might be interest by this:http://www.cosis.net/abstracts/EPSC2008/00274/EPSC2008-A-00274-1.pdfThe authors have not studied the equatoral ridge but have tried to explain the overall flattening (that is inconsistent with today's rotation period) as a frozen left-over of a fast-spinned past.They had to assume some amount of short-lived radio-isotopes but in no cases did their model evidence a liquid phase. That is due to the low density of Iapetus (hence its low ratio of radiogenic elements over total mass).Note that three considerations are in contradiction with Detsch's models (on the thread I recently opened on waterish TNOs discussing Detsch's paper):* some Al26 is required (and that would mean more heat source)* but thermal solid convection occurs (whereas Detsch had found that there was not enough energy in a 1000-2000km class body to initiate it without short-lived isotopes), and that would mean a shorter cooling time. A big reservation: Robuchon et al. compute Iapetus hence a surface temperature of about 100K wheras Detsch et al. computed Charon, ie a surface temerature of about 50K. A 50K difference can be very significant for rheology of ice/rock hence for solid convection. * Robuchon et al consider pure water (solid convection at play up to 250K) whereas Detsch et al consider water ammonia (hence liquid convection for temperatures beyond 180K)Detsch and Robuchon/Tobie would incidentally agree though that on a body the size AND density of Iapetus (too low density), differentiation through large-scale melting of ice and liquid convection should not have occured. Also they agree that the lithosphere limits the heat losses. Finally one team studied Iapetus and the other Charon, so they might be both right (or both wrong!).All in all I don't know how this raises prospects for extant liquid layers on other bodies (more heat but faster cooling).Best regards.

vogon13 · Oct 25, 2008

Replying to: <DIV CLASS='Discussion_PostQuote'>Hi Brellis and all I am wondering whether such thermal regulation is not the explanation of Hyperion aspect too. If accumulation of hot materials increase temperature locally under sunlight, then ice below may vaporize. On a very fluffy ground like on Hyperion, can't this lead the floor of the dark spots to collapse progressively? with the spots descending, surrounded by steep slopes? http://photojournal.jpl.nasa.gov/catalog/PIA07741 http://photojournal.jpl.nasa.gov/catalog/PIA06245 Best regards. Posted by h2ouniverse</DIV>  I have noted the sublimation idea being proposed for the brilliant white and black black dichotomy of Iapetus.   My understanding of that idea is once you get a bit of a dark smudge on an otherwise light colored proto-Iapetus, the smudge subsequently warms more, and evaporates out more ice and you are left with 'sludgy residue' (my term) that spreads, and the evap'd ice recondenses in more frigid regions of Iapetus. I like the idea of the brilliant white areas having an extent process for sustaining their clean appearance. However, it has been noted the black 'crud' is not entirely black, but more dark, dark brown.  Also, global tingeing of the dark areas has been noted, one end of Casini Regio is reddish dark, dark brown, and the other is greenish dark, dark brown. Erf!  How we explain that ?? The global tinting implies a global variance in distribution of 2 dark dark brown materials.  Somehow, we have more of one east and more of the other west.   Suddenly, the sublimation idea gets complicated.  The tinting, to my  understanding, is oriented  easty westy, and not noticeably northy southy.  (forgive verbiage, but it gets the point across)  This tinted primordial compositional variation revealed by sublimation needs to be explained. Not to be a wet blanket, but this is a major challenge for the sublimation idea.  Global east west color tinting variation is easily understood with an externally applied gaseous discolorant applied in sync with the Iapetan motion about Saturn, but as an inherent compositional anomalyof the very substance of Iapetus, it just seems startlingly unlikely.  My enthusiasm for the brighty whities is now suspect . . .        <div class="Discussion_UserSignature"> TPTB went to Dallas and all I got was Plucked !!So many people, so few recipes !!Let's clean up this stinkhole !! </div>

3488 · Nov 7, 2008

Has anyone seen these remarkable, very close up Iapetan images. All narrow angle shots, from the September 2007 encounter.Iapetus from 4,298 KM. Looks like individual boulders are visible. <img src="http://sitelife.space.com/ver1.0/Content/images/store/1/4/b14855f3-4208-4769-b402-e54274cd3abd.Medium.jpg" alt="" />  Iapetus from 4,423 KM. <img src="http://sitelife.space.com/ver1.0/Content/images/store/12/6/3c40ac21-365e-4d8d-964a-8e16f1a3d50f.Medium.jpg" alt="" /> Iapetus from 3,110 KM. <img src="http://sitelife.space.com/ver1.0/Content/images/store/12/10/3cd7439f-548f-48c3-aa74-174ed9825b93.Medium.jpg" alt="" />Yet closer in over the Voyager Mountains, looks much smoother.  Iapetus from 1,808 KM. <img src="http://sitelife.space.com/ver1.0/Content/images/store/2/13/b2a7f228-b6c5-455f-9dc9-934d30a8a7db.Medium.jpg" alt="" /> Iapetus from 1,651 KM. <img src="http://sitelife.space.com/ver1.0/Content/images/store/3/8/7338eba2-d82c-44ab-88ab-338d7ace4d93.Medium.jpg" alt="" /> Andrew Brown.  <div class="Discussion_UserSignature"> "I suddenly noticed an anomaly to the left of Io, just off the rim of that world. It was extremely large with respect to the overall size of Io and crescent shaped. It seemed unbelievable that something that big had not been visible before". Linda Morabito on discovering that the Jupiter moon Io was volcanically active. Friday 9th March 1979.http://www.launchphotography.com/ http://anthmartian.googlepages.com/thisislandearthhttp://web.me.com/meridianijournal </div>

silylene · Nov 7, 2008

Replying to: <DIV CLASS='Discussion_PostQuote'>Has anyone seen these remarkable, very close up Iapetan images. All narrow angle shots, from the September 2007 encounter.Iapetus from 4,298 KM. Looks like individual boulders are visible.  Iapetus from 4,423 KM. Iapetus from 3,110 KM. Yet closer in over the Voyager Mountains, looks much smoother. Iapetus from 1,808 KM. Iapetus from 1,651 KM. Andrew Brown.  Posted by 3488</DIV> Thanks Andrew, I hadn't seen these yet. <div class="Discussion_UserSignature"> petet = silylenePlease, please give me my handle back ! </div>

3488 · Nov 7, 2008

Replying to: <DIV CLASS='Discussion_PostQuote'>Thanks Andrew, I hadn't seen these yet. Posted by petet</DIV>Thanks Silylene. I see your regular account has hosed up again.I had a very slow log in earlier after being timed out, the first time (do not usually have to log in, jusyt come straight in, but today had to log in) but since in, I've been fine on here. Strange. Almost as strange as to why Iapetus looks so smooth on the very close up images & rougher with ice boulders further out. Clearly Cassini passed over two distinct units, a cratered unit then the Voyager Mountains.It's going to take years to make sense of it. Andrew Brown.  <div class="Discussion_UserSignature"> "I suddenly noticed an anomaly to the left of Io, just off the rim of that world. It was extremely large with respect to the overall size of Io and crescent shaped. It seemed unbelievable that something that big had not been visible before". Linda Morabito on discovering that the Jupiter moon Io was volcanically active. Friday 9th March 1979.http://www.launchphotography.com/ http://anthmartian.googlepages.com/thisislandearthhttp://web.me.com/meridianijournal </div>

vogon13 · Jan 4, 2009

I found this at Wiki:   <ol><li>It has also been suggested that Iapetus could have had a ring system during its formation due to its large Hill sphere, and that the equatorial ridge was then produced by collisional accretion of this ring.[20] However, the ridge appears too solid to be the result of a collapsed ring. Also, recent images show tectonic faults running through the ridge, apparently inconsistent with the collapsed ring hypothesis[14].</li></ol>  Too solid?  It would depend on the size of the ring particles and how much they fracture during deposition.  We have seen NASA footage of small ice chunks contacting solid surfaces in the 500 kph range, and the pieces pulverize.  I suspect any plausible ring particle size orbiting at Iapetus would pulverize at contact with the surface too. So we would be looking at a 'snow drift'.  And recall, one emplaced in essentially vacuum conditions, a circumstance we as air breathers  would not be familiar with.  Be thinking much less 'fluff' and much more compaction as the pile grows.  So I am not too sure what the Wiki article means by 'too solid'.  Also, as I have noted before, the sides of the ridge structure are plausibly at the angle of repose expected for cryogenic icy granular materials.  -and- considering the extreme age of the ridge evident in its' extremely cratered appearance, would some subsequent minor faulting running under  the ridge be unexpected ?? Iapetus is noticeably 'lumpy' and has had 4+ billion years to settle and crack.  The only thing some fault damage to the ridge portion of the equatorial ridge structure tells us is the ridge structure is really old.  Additionally, just about any reasonable time scale for depositing a ring here is an insignificant fraction of the total age of Iapetus.  IIRC, deposition rates of 1 cubic meter per second  (an extremely fast deposition rate in my view) can emplace the entire ridge and Voyager mountain system in less than a 1000 years.  Adjust the rate to .001 cubic meters per second and the emplacement stretches out to 1 million years, and that is still negligible compared to the 4 billion years hence.     <div class="Discussion_UserSignature"> TPTB went to Dallas and all I got was Plucked !!So many people, so few recipes !!Let's clean up this stinkhole !! </div>

vogon13 · Mar 1, 2009

Search function test.

Search did not pick up "Iapetus" prior to this post. Will see if it works now.

h2ouniverse · Mar 1, 2009

Apparently the recall works.
Since you've woken Iapetus up, this paper to come at the EGU:

How did the Dark Crater Surfaces in the Bright Areas of the Saturn Moon Iapetus Evolve?
http://meetingorganizer.copernicus.org/ ... -10301.pdf

silylene · Mar 27, 2009

Slowing spin created the equatorial bulge??

VERY INTERESTING!!!
http://news.bbc.co.uk/2/hi/science/nature/7965332.stm

Slower spin 'made moon's bulge'
By Paul Rincon
Science reporter, BBC News, The Woodlands, Texas

The Cassini flyby returned exquisite pictures of the bulge

Scientists have come up with a novel theory to explain the unexplained terrain on one of Saturn's icy moons.

The most striking feature of Iapetus is a bulging ridge, which encircles the moon's equator and reaches an altitude of 20km in places.

A new theory suggests the ridge formed when the moon went from a relatively fast-spinning body to one spinning more slowly.

Details were presented at the Lunar and Planetary Science Conference in Texas.

The results are based on computer models of the interior of Iapetus created by James Roberts and his colleagues at the Johns Hopkins University Applied Physics Laboratory (JHUAPL).

Referring to the ridge on Iapetus, Mr Roberts explained: "It looks like somebody screwed two halves of the moon together and did a very bad job soldering the joint.

The ridge is unparalleled in the geology of Solar System bodies. At up to 20km high in places, it dwarfs Mount Everest, which stands only 8km high. The Olympus Mons volcano on Mars stands some 25km tall.

But Mr Roberts points out that Mars is over four times the diameter of Iapetus: "Proportionally, this is the tallest feature in the Solar System with respect to its host body," he said.

Spin cycle

Today, the 1,500km-wide Saturnian satellite spins at a rate of about once every 79 days. But Mr Roberts' data suggest Iapetus could have been rotating as fast as once every 16 hours early in its history.

Over time, tidal interactions with Saturn caused the moon's rotation to slow down until it matched the time taken to complete one orbit of its planet.

This process is known as "de-spinning", reaching a condition where the moon always keeps the same face towards its host planet.

Spinning celestial bodies tends to flatten out at the poles and bulge at the equator. The amount of flattening is controlled by the object's rotation rate: if the rotation slows, the flattening decreases.

De-spinning can cause compression along the equator, but it cannot have formed the ridge on Iapetus because this compression is acting in the wrong direction.

The nature of Iapetus' surface colouring has also proved enigmatic
However, the computer modelling work carried out at JHUAPL shows that de-spinning also dissipates more heat at the equator than elsewhere.

Mr Roberts suggested that warm, buoyant ice rose to the surface from Iapetus' interior and pushed the brittle surface ice outward, forming a ridge around the equator.

The slowing down of Iapetus' spin is estimated to have taken 100 million years or so, at which point the heating stopped. As the moon cooled down, the ridge was frozen in place.

Extreme case

A number of other theories have previously been advanced to describe the development of the ridge.

One group has proposed that Iapetus oence had its own ring system, just as Saturn does today, and that these rings collapsed, falling on to the equator.

It has also been theorised that as the moon de-spun, forces created so-called thrust faults to appear in Iapetus, causing a ridge to pop up around the equator.

The idea of heat driving upwelling currents in the ice has already been proposed as a mechanism for pushing the surface up at the equator.

But Mr Roberts' theory shows that heat generated at the equator by de-spinning can result in the pattern of convection required to cause these currents in the ice.

Most moons are thought to have undergone de-spinning. But why this process should have caused a ridge around Iapetus, and not around the equatorial regions of other satellites, remains an open question, Mr Roberts told BBC News.

However, he added, going from 16-hour rotation to 79 days could mean Iapetus presents the most extreme case of de-spinning in the Solar System.

h2ouniverse · Mar 28, 2009

Now the confirmation by NASA of "what makes it two-faced" (tholins)

http://www.nasa.gov/topics/solarsystem/ ... faces.html

Extrapolation to Phoebe, Hyperion and reddish TNOs looks a little bit quick, although likely.

3488 · Mar 28, 2009

Thanks to both Silylene & Joel.

Fascinating stuff. The despinning of Iapetus makes sense, as does also a collapsed ring, perhaps a bit of both.

What would be interesting to extrapolate would be the original orbit of Iapetus, did it orbit closer in to Saturn in it's youth & the orbit was pumped up by Titan or as I suspect Iapetus has always been this far out from Saturn?

It is interesting Joel to see that Iapetus's colour is linked to the reddish Centaurs, KBOs, etc. How about Centaur 5145 Pholus.

Also 5145 Pholus like Iapetus also appears to show a 'bluer' hemisphere (based on spectral analysis of it's rotation), despite being such a red object.

Also it shows the wisdom of NASA Cassini mission planners to have made the most of the incoming Phoebe encounter in June 2004 (which they certainly have). It seems to bolster the plausible theory of Phoebe being a captured Centaur or KBO, by Saturn.

Andrew Brown.

3488 · Dec 10, 2009

Dichotomy of Iapetus apparently explained.

CICLOPS news release.

Below the trailing hemisphere of Saturn's third largest moon the 1,471 kilometre wide Iapetus.

Leading hemisphere of Iapetus.

Andrew Brown.

3488 · Mar 14, 2010

These observations made on Saturday 6th March 2010 may help explain the dichotomy on Iapetus.

Overexposed distant views of Iapetus. These observations are to see if dust from the newly found ring is accumulating on Iapetus, or perhaps even a thin dust ring orbiting Iapetus. Iapetus was approx 3.5 million KM away.

Trailing stars can be seen in the background. Spots are instrument noise & cosmic rays. Iapetus was passing in front of the constellation of Coma Berenices as seen from Cassini during these observations.

Iapetus just off the frame at lower left. Deliberate to see if dust concentrations & / or a dust ring could be detected within the Hill Sphere of Iapetus.

Andrew Brown.

3488 · Sep 4, 2010

Copied Post by EarthlingX on the Cassini Mission thread.

Amazing animation of one of the tallest mountains in the solar system not on Mars or Io.

From EarthlingX.
The equatorial ridge - Valterne Mons, Iapetus
[youtube]http://www.youtube.com/watch?v=o6J69uqPVGY[/youtube]

galsat400 | September 03, 2010

Perspective rendering of one of the highest parts of the amazing equatorial ridge on Iapetus. The ridge, 20 kilometers wide, stands roughly 18 kilometers high in this region. Despite the length, there are several gaps in the ridge. The origin of the ridge remains unknown but is still one of the most spectacular edifices in the Solar System..

Andrew Brown & EarthlingX.

3488 · Sep 4, 2010

Cassini September 2007 Iapetus pass video.

[youtube]http://www.youtube.com/watch?v=baaGOqIJaFM[/youtube]

Andrew Brown.

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