Approaching Iapetus - what makes it two-faced?

vogon13 · Mar 19, 2005

I have a question about the possible ring system of Iapetus which I will post here. The 'bump' process as described by Joseph Burns in the New Solar System book seems to be the process for bringing down the majority of the emplaced material. The process requires a minimum density in the ring system as particles in adjacent orbits need to experience physical contact with each other for the process to occur. Burns also describes the process by which material in a variety of inclined orbits around the parent body can interact and 'collapse' into a disk of material above the equator. In my simplistic view of the universe, I assumed the collapse phase would proceed to completion and then the bump phase would commence. I don't feel this is correct. As the bulk of the collapsing material reaches the Laplacian plane (sorry for earlier misspellings of this), the bump process may start before the final stages of the collapse to the Laplacian plane occurs. This implies a density of material in the ring system rather higher than what would be expected in my original view. And now the question: Does this affect what we expect to see? Probably, suspect this is the key to understanding the triple braid structure we see in the Cassini pictures. But I am clearly not smart enough to explain the symmetry of that feature. And why is the breadth of the parallel ridge tops similar? I think I understand the similarity of the elevations of the 2 converging ridge features. But the converging path of the flanking ridges at first glance, to me at least, seems mysterious. I would imagine the material of the rings 'destined' to emplace there to have been distributed in the form of symetrical 'shoulders' along side the ridge, not to be 'ridgy' itself. Would there be a 'wobble' phase in the collapse scenario to the Laplacian plane? Would there perhaps be a 'wobble' to the motion of Iapetus relative to the ring as it emplaced? I see a <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 19, 2005

Think I have a glimmer of why the ridge tops are similar in width and will post it and see if it 'jars' anything loose. In the above scenario where the ring material is coming down and accreteing against its previously emplaced material, and is therefore 'working' upstream, the incoming material is going to be impacting the leading edge of the formation at a constant velocity, ~1500 kph. It will 'splatter'. We have a constant speed of incoming material, 'impingeing' on a presumably homogenous material, therefore we get a consistent 'splatter' effect as the material builds the ridge. Picture a fire hose blasting at a brick wall, the spray deflects at the contact point. Size of the spray pattern relates to the pressure of the water exiting the nozzle. I'm thinking a constant width of the top of the ridge is 'plausible' in this scenario and consistent with what we see. And there will still be variations of smaller order from subsequent slides and craters, also, the incoming material may be 'clumpy' or otherwise not smoothly homogenous, which would vary the 'pressure', so to speak. And one other point, following the fire hose analogy, width of the splatter on the wall governs width of the puddle forming at the bottom of the wall from the dripping, as analagous to the 'spray' of the incoming ring material accumulating off to the sides a little as the ring emplaces. Need a diagram here, too bad I'm so uncomputer savy. Also, there is a detail here that suddenly occurs to me: The material in the 'splatter' moves side to side of course, some of it is directed downwards and some upwards. The material moving upwards, has just been violently decelerated from 1500 kph by its contact with the accreting edge of the ridge. As it moves upwards, before the 2 1/2% earth gravity of Iapetus pulls it back down, it will 'interact' with still orbiting, but very, very low altitude ring material. Think a <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>

lifebeyond · Mar 20, 2005

Iapetus is an artifical construct and the following link will provide you with the evidence to back up the theory. http://www.enterprisemission.com/moon1.htm The above link gives answers to the many questions which have been discussed on this board. Check it out! Iapetus is no sphere, no mere moon, and more than we could have ever expected!

claywoman · Mar 20, 2005

Hey guys, did you see this? Just wondering if I should believe this or is this another matrian snake view?

vogon13 · Mar 20, 2005

OK, here goes; Let's imagine Iapetus in space, around Saturn with the ring system partially emplaced ( and by implication, the ridge partially formed from the low end, stopping at a height intermediate between the currently existing 20km high spot and the low end. Additionally, (mostly for my comfort) Iapetus is rotating on its' axis quite a bit quicker than the current once every ~80 days. What if; A pretty big rock or asteroid clobbers Iapetus at this point. Does the spin axis of Iapetus change orientation? Yeah, if rock is big enough and hits 'off center'. Does the spin axis stay that way? I dunno. Does the spin axis start tracing out a cone? (like earth's axis does in those diagrams that explain the precession of the equinox) Does this motion damp out? Eventually? Never? Right away? I dunno. What happens to partially completed ring emplacement scenario? Well............I don't think the ring material automatically changes inclination to the new 'couple of degree' offset equator. OK. 'Bump' process doesn't know anything has happened so material still wants to emplace. As the equator now passes under the (form its point of view) inclined ring system, twice each 'day' that partially 'installed' ridge passes under the ring, once moving from south to north, then 180 degrees later, north to south. What happens when the high spot passes thru the orbitng material? It knocks it down into 2 diverging paths centered on the ridge? Right? What else happens? Does this 'dampen' the precession of the Iapetan axis back down to 'normal'? And then the ridge resumes growing along its original track? Does this make sense? I'm having a wicked time trying to 'watch' this occur in my minds 'eye'. But typing this all out seems to help. Will be running errands all afternoon so I get to contemplate some more. I realize figuring out the 'triple track' is the 'bi <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 22, 2005

A few more thoughts about explaining the triple braid aspect of the ridge: I thought about intersecting inclined rings in the Iapetan ring system. The collapse to the Laplacian plane is something you can't get around. Additionally, it appears to me the triple ridge aspect formed towards the end of the emplacement, something would have to have happened at the right time to create inclined elements at that stage, too improbable. I also considered adjacent (non intersecting) inclined ring elements. This idea trips up with the 'bump' emplacement scenario. Also just keeping all this organized as it comes down just gets too weird. Why would adjacent inclined rings be exactly equally inclined and exactly equal mass wise to make the highly matched outer strands? Additionally, if process lays down, let's say, the north one first, and then the south one, you now have two weird things to explain. Explanation that deposits both adjoining ridges at same time from same material is simpler and presumably more likely. An inclined ring, orbiting around Iapetus will, (I'm pretty sure about this) precess around Iapetus. Having Iapetus move beneath the ring in a synchronized fashion so as to emplace the material in the tightly collimated ridge structure is really tough. And with three strands in the ridge structure, the improbabilities multiply. Having a point along the equator common to all 3 ridges is what 'syncs' up the whole scenario. Especially, if that point is quite high. It's that high point passing through the ring plane (after the Iapetan axis tilt event) that triggers the formation of the north and south 'attendant' ridges. This also tells us the central portion up to the high spot formed first and that it formed from the 'low' end towards the 'high' end. I'm starting to feel (except for some scattered 'piles' and close-ups of the attendants) we do have photos of the majority of the 'interesting' portion of the <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 22, 2005

And how about the 'quick' version: Long ago, when Iapetus was rotating on its axis faster than it is now, there was a major impact on Iapetus. A great deal of material wound up in all kinds of orbits around Iapetus. Over time, all the debris in orbit 'collapsed' into the Laplacian plane and formed a ring system around Iapetus. Differential shear motion (the 'bump' process) causes the low side of the ring system to lower further (at the expense of 'raising' the high side higher). Eventually, particles in the ring system are low enough to strike the highest spot on the equator of Iapetus (at that time). Material starts to accrete at this spot and it accumulates 'upstream' relative to the incoming material. Process continues for quite some time. Material forms a 'ramp like' structure on the equator as the accreting surface forms into the oncoming material. We see this in the Cassini pictures, it is now discontinuous due to subsequent impactor damage, and perhaps some faulting or ductile flow of the crust, or avalanches. Eventually, sufficient mass accumulates on the equator to 'wobble' Iapetus slightly as it rotates, ie. precession of the axis. In this phase of the process, the ring system stays 'fixed' in space relative to the distant stars, but Iapetus is now 'nodding' beneath what is left of the ring system. At this point, the 'high' end of the 'ramp' passes through the ring plane twice each Iapetan precession cycle During these encounters, emplacement continues as the 'bump' process still is lowering material into its path. As the ridges' highest spot passes thru the ring plane, once from north to south and once from south to north every precession cycle the emplacement starts again only now it is depositing in line with the currently 'inclined' (because of the axis precessing) surface. This forms the 'attendant' ridge <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 24, 2005

Some information on the radar instrument on board Cassini: *At close range, the Cassini radar can produce 'maps' of surfaces the craft passes. Due to the lengthy computer processing required, 'real time' imaging is not possible. Similar instuments have been used on earth and Venus from orbitng spacecraft. The Ariecibo Dish in Puerto Rico has been used similarly to examine objects like Mercury, Titan, and earth's moon. *the Cassini radar instument can also be used as an altimeter. It will provide very accurate distance information between the target and the spacecraft. When the spacecraft position is correlated with the altimetry data, accurate elevation differences on the target can be derived. *Radiometery. The radar instrument can also passively 'listen' to the natural radio emissions of a target. The 2 above mode are 'active' in that the spacecraft transmits a signal which is reflected by the target, hence the limited range. In this radiometry mode, the craft does not transmit a signal. Objects can emit detectable signals from a variety of processes. Temperature, for instance. Auroral type emissions, ionization, etc. When observations are made with the radar instrument, it is important to understand which mode is employed and what the project scientists are trying to discern from the observation. <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 27, 2005

There seems to be great interest in the degree of 'unsphericalness' exhibited by Iapetus. Some of the conclusions I feel we can derive from the Cassini photos are that the mechanical strength (rigidity) of the surface is sufficient to maintain the unevenness of the features. This seems plausible, as when we look at the ridge structure, we do not see parallel faulting presumably caused by the mass of the feature on the Iapetan surface. (keeping in mind the resolution of the current photos may not have been sufficient to reveal such features, but obviously, the ridge has not 'plunged' through the surface or 'settled' to any great degree) An additional consideration in the rigidity of materials in this part of the solar system is the example set by Hyperion. In a side by side comparison (to scale) with Mimas, Hyperion shows amazing divergence from sphericity. This is simply a result of the low gravity and the rigidity (mechanical strength) of the materials making up Hyperion. We can also compare to Enceladus. It is very spherical and quite small. What's different? Heating (which is producing the resurfacing flows) drastically lowers the rigidity of the materials making up Enceladus. When we look at Iapetus, a reasonable conclusion would be that there apparently has been no global heating to a degree necessary to 'flatten out' the irregularities since they were formed. If these irrregularities are resultant from Iapetus final accretion phase, then we are looking at an extremely ancient surface. I have used that phrase previously in this thread in describing the formation of another feature on Iapetus. Having a set of surface characteristics on Iapetus that are consistent with each other seems logical, and can be viewed as mutually confirming each other. Discovery of large Olympus Mons type volcanoes or a large area of Iapetus that is volcanoed like Io would be inconsistent with this line of reasoning. To date, there are no obvious classical v <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>

telfrow · Mar 27, 2005

Bravo. Well said. <div class="Discussion_UserSignature"> Made weak by time and fate, but strong in will to strive, to seek, to find and not to yeild. - Tennyson </div>

igorsboss · Mar 28, 2005

vogon, I've spent the last couple of days reading this whole thread. I have some new ideas you might want to ponder. Much of your theory is predicated on the existence at one time of an Iapetian ring system, a miniature of the saturnian ring system, which collapsed onto or near the equator of Iapetus. The Iapetian rings formed as a cloud of debris collapsed into the Laplacian plane due to internal collisions. In doing so, you've had to consider impacts by fast-moving particles, which tend to impact the Iapetian highlands. I think there may be even more to the story. I'd like to consider slow-moving particles which are confined to the surface of Iapetus. I'm not suggesting an atmosphere. I'm considering individual ejecta of rocks and dust grains which may happen to be jarred loose by 1543KPH (spaceter) impacts. Particles in motion while confined to the surface of a spinning (and oblate) spheroid experience a (virtual) coriolis force, and a (virtual) centripedal force. The upshot of this is that the moving particles experience a turn to the right in the Northern hemisphere, and a turn to the left in the Southern hemisphere. Consider a small Saturnian ring particle impacting Iapetus at a low angle, somwhere between 30 degrees North and South lattitude, on the side of Iapetus which faces Saturn, at 1543KPH. This low-angle impact would form a small ejecta cloud, moving much slower than 1543KPH. Since the ring particle was moving from West to East on Iapetus, the ejecta cloud would also tend to move more-or-less West to East (by conservation of momentum). That ejecta won't continue to travel due East along Iapetus. Instead, the debris flow will turn towards Iapetus' Equator! Come to think of it, if there was a tremendous dust load, hitting both North and South lattitudes simultaneously, these slow ejecta particles would tend to collide over the Equator (aka. the Laplacian plane) and would precipitate out al

vogon13 · Mar 28, 2005

Ejecta from impacts as a resistive media to dissipate orbital energy to facilitate emplacement of the ring material on the Iapetan equator? Cool! I have pondered some mechanisms to lower the remainder of the ring material once its density is too low to sustain the 'bump' process as descibed in the New Solar System chapter on rings. In fact, as you noted in your reading of this thread, prior to the discussion of the 'bump' process, many scenarios were brought up to accomplish the emplacement of the ring material on the surface. With the Enceledosian atmosphere findings from Cassini, I have been a little more comfortable with an extremely thin atmosphere around Iapetus doing the 'clean up', if you will, of the residual ring material. Don't be afraid of speculating about a possible Iapetan atmosphere, I've done it a lot here and every one has been very nice about it. I have even considered seasonal winds (spring and summer blowing north across equator, and fall and winter blowing south across the equator) to explain the symmetrical attendent ridges, but I really like the precession idea. A lot. Its simpler, I don't have to figure out how to make the wind blow that way on Iapetus if I just stick to the precession idea. Your idea may certainly be a contributing factor. Points in its favor would include realizing the more ancient the ridge is, the higher the cratering rate on Iapetus would have been. Iapetus is extremely battered (and no, I'm not going to do any crater counts, that is for people with better eyesight and more patience than I) and you positing a process that would appear to get more plausible as the feature becomes more ancient in its formation is certainly appealing. I have commented before on having consistency in parameters of the various features on this objects, and you have posited a process that is consistent with that. Additionally, earlier posts from j8hart evidenced concern for 'scatter' effects <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>

igorsboss · Mar 29, 2005

Your last paragraph, however...... Sorry, but its a non-starter. Consider it a spurious thought. I haven't run the numbers enough on this scenario to see if it would work or not. The coriolis force on an oblate spheroid is a vector equation involving cross products. I can do math at this level, but it is a substantial effort. Haven't had time to tackle it yet. One thing that would make the coriolis force more effective is if there were some kind of 'confining force' that would hold the particle close to the surface. One candidate is static electric charge. If the incomming particles were charged differently than Iapetus' surface, perhaps they could experience a brief coulomb attraction when very close to the surface. I think this stronger attraction would intensify the coriolis turn towards the equator. It might also help stuff cling to the surface upon landing. It sounds like a significant computer simulation may be in order, to sort this out.

vogon13 · Mar 29, 2005

As I mentioned before, I have an aversion to math (arithmetic is ok) and leave it to others with 'the knack'. Think understanding of Iapetus is sufficient to guide software process to generate code that will fill in the details. I fully expect to see a Pixar animation of the ring emplacement in 3D IMAX in my life time and am extremely eager to see it. Have watched this thing with minds eye enough as it is, would love to tie into a website and even see a low res thumbnail at this point. Electrostatic forces on such a large scale are rare in my experience and I have no feel for their capabilities and effects and what to look for in the pictures. However, we certainly cannot rule out any contribution they have made to the emplacement scenario. Coriolis effects would be weak on Iapetus now, however, during the 'spin down' phase following its accretion, the coriolis effects would have been stronger. <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 30, 2005

in reply to: This low-angle impact would form a small ejecta cloud..... Just was considering this regarding atmosphere. A thin atmosphere like Mars will 'burn up' small inbound particles. How much thinner an atmosphere can be and still do this is something I'm not sure of. I imagine the atmosphere density of Mars where incoming material is zorched is very low. With a sufficiently thin atmosphere, however, the ejecta cloud is actually a large number of small objects on ballistic trajectories. And of course, in a vacuum as well. Ejecta won't really act like a cloud. This idea is going to be constrained by needing atmosphere for cloud to be effected by coriolis force, yet too much atmosphere fries the incoming material prior to surface contact. <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 30, 2005

Would like to discuss misshapen craters a little. There seems to be a relationship between the percentage of the crater bowls darkened fro the 'spot' effect, the distance from the equator, and the geometric effects of the mostly northern walls exhibiting the darkening. As I have posted here before, the northern walls of craters in the northern hemisphere of Iapetus near the edge of the 'spot' feature are expected to be warmer than the southern walls of those same craters due to the northern arc of the bowl receiving more direct sunshine than the southern arc due to the steep angles of the crater bowl interiors. It occurs to me that if photos of this area are too heavily processed regarding contrast, light /dark edges parallel to the equator are going to be seen in all these crater bottoms and this contrasty effect is spurious. Please compare to the raw images at the JPL Cassini website, and please click on the full resolution view. Without the excessive processing, you will see a gradation in dark to light tone across the crater floors. And as I have posted before, this is literally the 'essense' in understanding the 'spot' mystery. The dark coating is stongly suggested to be the result of a temperature sensitive chemical reaction involving a (apparently) tenuous gas, most likely methane. Additionally, keep in mind when viewing all of thes Cassini photos, the effects of video data compression. The internet could not work if all the video information transmitted upon it wasn't digitally compressed to save bandwidth. Do not mistake compresion artifacts for actual features on the surfaces of these celestial objects. Compression artifacts in images can take the form of 'mosquito' noise around contrasting parts of the image, loss of resolution, and blocking. Particularly blocking. And as an aside, tonight, if I have counted correctly, this is my first post as a star Yee haw.<</safety_wrapper> <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>

igorsboss · Mar 31, 2005

With a sufficiently thin atmosphere, however, the ejecta cloud is actually a large number of small objects on ballistic trajectories. And of course, in a vacuum as well. Ejecta won't really act like a cloud. Agreed. This idea is going to be constrained by needing atmosphere for cloud to be effected by coriolis force, yet too much atmosphere fries the incoming material prior to surface contact. Even objects on orbital or ballistic trajectories are affected by the coriolis (fake) force. There is no requirement for an atmosphere. The coriolis (fake) force relates a spinning frame of reference to an inertial frame of reference. For ballistic and orbital ejecta, various ejecta particles would experience a variety of different coriolis turns before they land again. Since the ejecta paths are not confined to the surface, the turns will depend on the exact particle velocity. That's the whole point of coriolis, anyway. Things that you think "should" go straight don't look like they arre goings straight. Each prograde northern particle appears to turn right, each prograde southern particle appears to turn left, relative to the surface of Iapetus. Remember the collapse of orbital particles into the Laplacian plane that created your hypothetical ring? This is a big part of that same process. Southgoing and Northgoing orbital particles have an opportunity to hit each other each time they cross the equator (the Laplacian plane). This is the "nonstarter" idea I had earlier. Is it still a nonstarter? I hope I'm not confusing the issue here. This is not some kind of new physics. It's about the point of view.

vogon13 · Apr 6, 2005

Sorry it took so long to get back to this, family matters, etc. One thing about the coriolis effect on ejecta, it only lasts until the objects (re)contact the surface. Not sure if this effect is discernible in the low res (comparatively) pictures we now have. The electrostatic levitation of the small stuff would of course allow longer time interval, but then the strengtht of the electostatic interaction is going to be so much larger than the coriolis effect that I'm not sure what to look for in the hopefully super pics we are getting in 2004. Additionally, if there are static charges involved, presumably, we would see geographic or topological effects.... This is sounding familiar now. Will need to sleep on that overnight. <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 · May 11, 2005

{bump} <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>

alan714586 · May 12, 2005

vovon13 someone else has the same idea http://www.universetoday.com/am/publish/iapetus_consume_saturn_ring.html?1252005

vogon13 · May 12, 2005

That article didn't address the 2 symmetrical attendant ridges. My thinking is, anything that plausibly accounts for that detail of the feature wins the brass ring. Really, really strongly feel material orbited Iapetus as it emplaced on Iapetus. Plausibilty of accumulating ring material orbiting Saturn onto Iapetus and generating the main ridge and the 2 attendant ridges is remote at best. There is no mechanism for maintaining the 'collimation' of Saturian ring material at Iapetus's distance from Saturn. In the other scenario, there is no mechanism for having Iapetus within Saturn's rings current realm, and then lofting it ~3 million kilometers into a circular orbit tilted only 14 degrees to Saturn's equatorial plane, and simultaneously leaving all the intervening satellites unscathed. Putative Odysseous impact with earth orbited enormous quantity of debris. Why the resistance to a similar impact and result to Iapetus? Also suspect similar event involved in current configuration of Pluto and Charon. <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>

jmilsom · May 14, 2005

"Really, really strongly feel...." It seems the discussion on Iapetus has gone full circle. Way back at the start AlexBlackwell posted a summary of the leading theories. What we need to do now is summarise the leading theories again (including yours Vogon13! - which still could win the gold ring - as you say how could Iapetus have shifted its orbit unscathed?) - and then systematically try to logically disprove each to see which holds up the best. This will be better than emphasising feelings!!! <img src="/images/icons/wink.gif" /> <div class="Discussion_UserSignature"> </div>

silylene old · May 15, 2005

Remember the (facetious) "tennis ball hypothesis" I posted of Iapetus back when the pictures first got transmitted? I noted that the equatorial ridge looked like a seam formed by the merger of two half-spheres. I made the analogy that it looks like the seam on a tennis ball, if you remove the felt covering. Well, I was thinking more about this highly speculative idea. And I will then propose a more serious speculative concept, based on the same line of thought. Let's suppose, long ago, there were two equal sized proto-ice-moons sharing a nearly common orbit around Saturn. These two proto-ice-moons perhaps had an orbital arrangement much like Janus and Epimetheus, and as the two moons approach each other they exchange momentum and trade orbits with each other. Because of gravitational drag from the other moons, and induced tides from Saturn, this is not a stable system. So slowly, these ice-moons move closer and closer to each other. As they get real close the two moons start warming each other up from mutual tidal interactions (I wonder what's their Roche Limit?). The moons finally get warm enough from mutually-induced tidal interactions that they become softer-slushier ice, instead of -200C rock-hard ice. I now note that the relative velocities of these two moons is minimal....... Then suppose the two soft-ice-balls slowly close the gap between themselves, and have a slow-motion collision and merge? Imagine two snowballs slowly pushing into each other. And then the moons re-freeze? Could the ridge be the trace of the merger? And maybe the dark deposits are bits of dark icey debris tossed out by this slow-motion collision, which are then gradually swept back up afterwards.... Is this plausible? <div class="Discussion_UserSignature"> <div class="Discussion_UserSignature" align="center">- - - - - - - - - - - - - - - - - - - - - - </div><div class="Discussion_UserSignature" align="center">I really, really, really miss the "first unread post" function. </div> </div>

CalliArcale · May 17, 2005

Wow, now that's an interesting concept. We'd be seeing the process of two moons becoming one, slowly settling down into a rough sphere. However, I do see a problem with it. Most of Iapetus is extremely battered, and therefore ancient. If two moons collided hard enough to get them to become a single body, wouldn't you expect them to have liquified at least most of the way? I would expect their surfaces to have been obliterated. Yet the ridge is so much younger than the polar regions. I'm not sure that works. But then, I don't know. Is it possible for a moon to liquify only partly during such a collision, if it's sufficiently low-speed? Better analyses of the interior of Iapetus would be helpful in evaluating that idea, and I suspect we won't have that data for a couple more years or so, depending on how long it takes to process the data from the next Cassini flyby of Iapetus and correlate it with past flybys. <div class="Discussion_UserSignature"> "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." -- The Tenth Doctor, "Blink" </div>

silylene old · May 17, 2005

Calli - I was assuming that the two proto-moons were already tidally warmed by their interactions resulting from increasingly close-passage dances around each other. Thus the two moons, at the time they slowly collided were rather soft ice-balls, perhaps as soft as ice is at -20C. If the proto-moons were soft, then their collision would be completely inelastic and absorptive. I would love to see my hypothesis modelled!! I wonder how slow a pair of co-orbital proto-moons would collide? How much tidal energy would they deposit into each other as they approach? What would a finite-element model for a slow-motion collision predict? Would the merged moon gain some angular momentum and rotate before becoming tidally locked with Saturn? I also notice that the older craters on Iapetus are unusually "soft"-edged, compared to craters on other (cold) ice-moons of the outer planets. I think the soft-edged craters are a bit unusual too, and their existance is consistent my hypothesis that the proto-moons had been tidally warmed at some time in their existance. Ice at -200C is as hard as steel and will not become soft. But ice at -20C is rather soft and slowly will deform (such as Terran glaciers exhibit). <div class="Discussion_UserSignature"> <div class="Discussion_UserSignature" align="center">- - - - - - - - - - - - - - - - - - - - - - </div><div class="Discussion_UserSignature" align="center">I really, really, really miss the "first unread post" function. </div> </div>

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