Approaching Iapetus - what makes it two-faced?

[CalliArcale]

That's an interesting idea -- soft but not molten, and merging very slowly. I'd like to hear an expert's opinion on whether the ancient surfaces in the higher latitudes could endure such an event, but it's really interesting. Certainly it's original, and it's very compelling. I'd love to see an animation of it. <img src="/images/icons/wink.gif" /> <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>

 

[vogon13]

Current orbital velocity for a satellite in orbit around Iapetus at the minimum possible altitude is 1543 km/hr (per spacester), which is 900+ MPH in the US. Dividing Iapetus into two objects and letting them 'splat' is going to be rather more energetic than you suppose. The two proto-Iapetus objects will be both near spherical if roughly equal in mass. Distortion (which is quite an understatement) of the spherical surfaces at coalescence will be sudden and extreme.<br /><br />Having the 'seam' wind up at or near the equator seams unlikely (but not impossible), divide area of ridge structure into total are of coalesced Iapetus to get an estimate of the probability.<br /><br />Additionally, the two diverging and very symmetrical attendent ridges are another complication that I don't see this concept explaining. As I like to say, 'splain this and win the brass ring.<br /><br />A point in this ideas favor may possibly be in the apparent 'big end' of the ridge being antipodal to the sub-saturn point of Iapetus (not brought up but possibly relevent). Seems like the newly coalesced Iapetus would be rotating much faster than the current ~80 day period. Tidal lock (from Saturn gravity gradient) would 'prefer' the 'big end' aligned that way and Iapetus would have eventually 'spun down' into this alignment. <br /><br />On the other hand (does that make 3?) what I have referred to as 'landslide basin' is quite large, quite deep, and extends across the equator. At the resolution of the current Cassini pictures, there is no discernible trace of the 'seam' in this area.<br /><br /><br /><br />Thought experiment:<br /><br />Imagine yourself on Iapetus, with a variable muzzle velocity pistol. Spend an afternoon firing rounds up into space at all angles and all kinds of different velocities. Which ones move slowly away from Iapetus into independent orbit around Saturn?<br /><br />None of them.<br /><br />All the rounds fired at less than escape velocity from Iapetus don't make it. In the <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]

Vogon, you misunderstand my hypothesis....let me try to explain.<br /><br /><font color="yellow">Current orbital velocity for a satellite in orbit around Iapetus at the minimum possible altitude is 1543 km/hr (per spacester), which is 900+ MPH in the US. Dividing Iapetus into two objects and letting them 'splat' is going to be rather more energetic than you suppose. </font><br /><br />No I am not talking about a satellite orbiting Iapetus!<br /><br />Consider <i>two roughly equal sized</i> proto-Iapetus satellites co-orbiting <i>Saturn</i> (<i>not each other!</i>)....in nearly the same orbital radius from Saturn. In my hypothesis, the two proto-Iapetus satellites have an orbital geometries much like Empimetheus and Janus currently have (but at a radius correspnding to about where Iapetus is nowdays.)<br /><br />Tidal interactions between the two proto-Iapetus satellites gradually bring them closer and closer each time they both revolve around Saturn. They have nearly the identical orbital velocities. <i>So their delta-V is nearly zero!</i> The tidal interactions just preceeding the merge warm and soften the ice. They get closer and closer until they make a slow-motion inelastic merge. The merge, and heat from the slow-motion collision is enough that the soft iceball slowly collapses to a mostly-spherical satellite, with the ridge remaining as the remnant of this slow-motion collision. (Actually Iapetus is an irregular ellipsoid shape)<br /><br /> <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>

 

[alexblackwell]

This is a follow up to my response to jmilsom in the "AlexBlackwell are you out there??" thread.<br /><br />I've perused the presentations from the <i>Cassini Icy Satellites Workshop</i>, which was held April 12-13, 2005. So far as I know, these presentations are only available internally to the Cassini Program, specifically via Cassini SOST (Satellites Orbiter Science Team). The presentations were given by the following instrument teams: CIRS (Pearl/Spencer), ISS (Tilmann Denk), MAPS, RADAR (Steve Ostro), UVIS (Hendrix/Hansen), and VIMS. Needless to say, all were interesting. <br /><br />At any rate, I'm not sure of the differences, if any, from some of the Iapetus-related presentations at the 36th Lunar and Planetary Science Conference, which was held just one month before.<br /><br />Hope this helps ;-)<br />

 

[vogon13]

My take on the co-orbital satellites is that their mutual gravitaional attraction is per Newtons' equations, but the proximity to Saturn heavily modifies how their attraction is exhibited. (sorry for the clunky verbiage, I am having trouble captioning what I see thru the minds eye).<br /><br />When the co-orbitals swap orbits, to model the trajectories you would have an n-body problem and the math would have to be excruciating. When you cleve Iapetus (in simulation) and let the parts mutual gravititation fields interact, you are far enough from Saturn that the situation simplifies greatly. IIANM, you are down to only a few posibilities, one or the other (but not both) is ejected from the Saturn system, they both drastically increase their eccentricity and / or inclination, they wind up orbiting each other as they both go round Saturn, (not sharp enough to do the math, but this has gotta be a real low probabilty outcome) or they collide. And I'm real sure the collision is going to be way more splat than smoosh. <br /><br />Sorry, I am too tired to figure surface gravity for half an Iapetus. Pretty sure it is between 1 and 2 % earth. Two such bodies would interact tidally, if close enough, and such an interaction would be interesting, but it just wont go the way you want. Making the split 60 / 40 or 20 / 80 or even 1 / 99, doesn't seem to push scenario where you want it to go.<br /><br />It's a neat scenario, love to see some CG sims of it, but it just seems so difficult to 'tune' starting conditions to generate something like what Cassini saw.<br /><br />Significantly, when Cassini gets real good close ups of the ring rocks, co-orbitals and the inner moon companions like Dione B, we very likely will see coallesced objects and they most likely will have done something very close to your idea. And we may see and be able to interpret collision generated topography, just along the lines of what your working on. But Iapetus, just doesn't seem to be on that page of the <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]

Thought of something just now.<br /><br />Saw a TLC or Discovery show on mining technology. They had old footage of hydraulic mining being done in California to search for gold. Hydraulic mining is a technique where something very much like a very large fire hose is used to blast terrain apart. (This was outlawed many years ago because of the gross damage it did to the environment)<br /><br />What would happen if this technique were tried in extreme cold?<br /><br />Water will freeze pretty close to the impact area. Will freezing face of accumulated jetted material move upstream into the oncoming flow? Where else is it going to go?<br /><br />Now look at Iapetus while thinking about that.<br /><br />In the decaying Iapetan orbiting ring scenario, we have something quite similar. The material in the ring on its final orbit will be moving virtually horizontally to the existing terain. Its speed is 1543 km / hr. This does not seem grossly dissimilar to the hydraulic mining analogy. True, the ring material is most likely chunkified ice, and the miners are using water, but I think the outcome will be quite similar in both scenarios. Material comes in at speed, encounters prior accumulation, and agglomerates onto the existing material, and 'builds' up stream.<br /><br />This is how the orbitally decaying ring material built the ridge. Process apparently starts near the low end of the Iapetan ridge (but not at it) and works towards the high end. Also, the material accreting on to what I'm refering to here as the freezing face, also interacts with material passing over the site of the freezing face. This material has just been violently decelerated at the freezing face and some part of the incoming stream wil be deflected upwards (also to the sides, but not what I'm interested in here). Any material deflected upward, will eventually in the weak Iapetan gravity, come back down. But if it encounters any material still in orbit, that material will not make it around Iapetus <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]

Vogon, I am happy too see that you understand my hypothetical co-orbital collision scenario.<br /><br />I too would love to see some good simulations of this! And I also agree that some of the tiny ring shepard satellites might be coalesced co-orbital satellites.<br /><br />As far as the Iapetus case, I don't know how much tidal heating would occur. I agree, the surface gravity is minute, exactly as you said. But I also think that the orbit-swapping that co-orbitals undergo might give just enough tidal heating to warm up the proto-moons somewhat (something has - just look at the surface crater morphology).<br /><br />Having consider the plusses and minuses, I think the co-orbital collision scenario is more likely than the ring collapse scenario. However, I do admit either is a bit of a stretch. <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]

Pretty neat to have the continued interest in this object and a wide diversity of threads exploring this interesting moon. <br /><br />Just noted in the New Solar System book that orbital resonances from Iapetus and Titan are evident in the rings of Saturn. Still amazing how the math can be generated to demonstrate all these subtle interactions.<br /><br />And how much is left to do.....<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>

 

[jmilsom]

Many thanks for taking the time to reply and providing that link. The papers available at that site will give me much food for thought and many hours of reading!<br /><br />Thanks. <div class="Discussion_UserSignature"> </div>

 

[vogon13]

Have been contemplating features we may want to search for in existing or planned Iapetus photos. The 'biggee' would be the source crater for the ring material (should it exist). What can we speculate about its' appearence?<br /><br />It will be large. The material of the ridge is hundreds of cubic kilometers. Assuming a substantial fraction of the excavated crater material did not wind up in the ridge (and the 2 symmetrical attendent ridges) all the craters below a certain size can be eliminated.<br /><br />I'm thinking that an impactor that could deliver material to orbit will more than likely be an oblique impactor. Many oblique craters are known on Mars. Much has been made of irregular craters on Iapetus, I'm not sure any have been pointed out with 2 to 1 ratio length to width ovalness. Due to the inevitable collapse to the LaPlacian plane of randomly inclined orbits of incipient ring forming material, a latitude of a potential source crater probably is not predictable. In other words, even a polar location for the source crater cannot be ruled out or even an equatorial one. I suppose students of probability would say start looking at 45 degrees N and S, but for a single crater, stats not of much help. Also, the orientation of the long axis of this putative crater is going to be random, too.*<br /><br />Another possibility, the original impactor is the cause of the axial precession of Iapetus that I feel explains the two symmetrical diverging attendant ridges, rather than the 'big end' of the ridge 'unbalancing' Iapetus towards the end of the emplacement. In this view, the attendant ridges formed first, then as the precession 'damped out' the main equatorial ridge formed between the two existing ones. I am not as comfortable with this scenario, but admit either would generate the structure we see. If the original impactor did initiate the precession, presumably, a reasonable mass estimate for the impactor would constrain the latitude (but not longitude) <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]

Will try and flesh out an overview of the 'process':<br /><br /><br />Imagine a random small generic iceball in the universe somewhere. In size it is pretty close to Iapetus or Rhea or Oberon or Titania. Lets assume it has a ring system very similar to Saturn (but scaled to its' smaller size) made of icy materials. Due to dynamical effects of the ring particles, the lower edge of the ring system descends (apparently slowly) towards the surface of the generic iceball. Lower and lower it goes. Then what happens?<br /><br />Splatt! The lower edge of the ring contacts the surface. Anything special about that spot? Yes, it will be the highest spot along the ground track of the descending ring material. What does this look like? By saying it as the descending edge of the ring contacting the surface, the minds eye pictures something coming down, but actually, the downward motion will be imperceptible. What you will notice is the large horizontal velocity of the material. It is in fact, orbiting the iceball right up until it contacts the surface. As mentioned before, in the case of Iapetus, the horizontal velocity is ~1540 kilometers per hour. <br /><br />This is actually an interesting speed range. Slightly supersonic in an earth type atmosphere, but certainly beyond our everyday experience. This velocity is insufficient to vaporize a large quantity of ice (or icy) ring material in a -300 degree C environment.<br /><br />So what happens as more and more ring material contacts the high spot? I had trouble accepting this at one time, but you do in fact get a 'pile'. What else is it going to do? Especially with an enormous quantity of material still coming in. Eventually, there is enough material in the 'pile' that the accretion face will start moving away from the pile. In what direction does it move? To the right (as seen from an incoming particle) (south to an observer on the iceballs surface)? No, at least no further to the right than the dissipation of the <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]

Getting a little sentimental in my old age. Wanted this post in this thread today...<br /><br />Look around a little bit and you'll see why.<br /><br /><img src="/images/icons/laugh.gif" /><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>

 

[najab]

Congrats!

 

[vogon13]

Almost missed it this morning.<br /><br />Couldn't have done it with out everyones good example.<br /><br />{sniff}<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]

Found a little tidbit on the Dark spot of Iapetus:<br /><br />D-type asteroids are believed to have a susbstance on them similar to kerogen. The kerogen like material is believed common in the solar system from the main asteroid belt out to at least Saturn. D-type asteroids seem common in Jupiters trojan complexes. Also, the kerogen material provides a very close infrared spectral match between the D types and Iapetus.<br /><br />With the recent Hyperion pictures, and all the dark crater bottoms evident upon it, I suspect we now have another body with this kerogen like substance on it.<br /><br />If Hyperion's rotation is indeed chaotic (no dispute about that I am aware of) and if the kerogen material is formed via a temperature sensitive reaction, I think we can expect the dark bottomed craters on Hyperion to occur all over its surface. <br /><br />It would seem the small deep craters can produce a local hotspot (relatively speaking) at their bottoms when the sun is directly overhead. This condition will occur everywhere on Hyperion (and apparently, uniquely to Hyperion) due to the chaotic rotation. <br /><br />Looking forward to the upcoming Hyperion flyby. Have something to look forward too, a surface stippled with dark bottom craters. Is it possible to know now what hemispheric photo coverage we will get from Cassini? <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]

(bump) <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]

Have been thinking about something we <i>haven't</i> seen in the Cassini pictures:<br /><br />Moons interior of Titan's orbit don't seem to exhibit the dark bottomed crater floors seen on Hyperion. My thinking is, the process doing this is largely related to methane released from Titan's atmosphere and it being procesed by solar radiation. I feel a strong mechanism in directing gas escaped from Titan's atmosphere is the Saturnian magnetotail. If this is the mechanism, we would not expect these materials interior of Titan's orbit. Additionally, I expect the dark spot of Iapetus and the dark bottomed crater floors of Hyperion to be generated by the same process.<br /><br />Reviewing all these Cassini pictures of the Saturnian moons is quite interesting, and comparisons amongst the moons seems to help our understanding of all of them.<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]

And it happens again.<br /><br /><img src="/images/icons/wink.gif" /><br /><br />It doesn't seem like all that long ago.........<br /><br />Memories light the corners of my mind, misty, watercolor memories...<br /><br />I feel a haiku coming on.<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>

 

[vogon13]

Might have some details on a factor I glossed over. (gasp)<br /><br />In the dynamical ring spreading process, as ring material is lowering to the surface, material is also being 'lifted' towards the outer edge of the ring system.<br /><br />What becomes of this material on the high side?<br /><br />I had thought perhaps, like we see in the big rings of Saturn itself, the material might become clumpy. And then what? Perhaps we should see some small satellites in orbit about Iapetus, a couple of Iapetus radii above the surface. With the recent asteroidal satellite discoveries, it seems likely some of the high side left over ring material should be around Iapetus somewhere.<br /><br />Well......<br /><br />At Iapetus' distance from Saturn, the tidal effect of Saturn is rather small. Small enough that I consider it likely that Iapetus was rotating on its axis (much) faster than the current ~80 days when the ring system existed and piled up on the surface. But obviously, the tidal influence of Saturn is strong enough that during the age of the solar system, Iapetus has in fact become tide locked and now presents one hemisphere continuously towards Saturn.<br /><br />I had not considered the effect of the Saturnian tides on the Iapetan ring material as it would have been so much weaker than the strong (relatively speaking) Iapetan gravity. However, I think we can expect the ring material to be actually quite sensitve to the effect of the tidal influence of the distant Saturn.<br /><br />As the individual particles in the ring circle Iapetus, the tidal effect of Saturn will subtly change the shape of the particles orbits. If Iapetus were alone in the universe, the ring particles orbits would approach perfect circularity. With Saturn though, the difference, although slight, is significant. Any deviation from circularity will work with the existing ring spreading effect working on the ring material. Assuming particles of sizes ranging from inches to feet, an additional pertur <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]

Ever notice in the Iapetus picture that there seems to be a possible very old highly degraded <b>huge</b> crater on the right side......again with that weird hexagon shape?<br /><br />It's easiest to see if you look at the big, original Iapetus photos, not these jpg's I posted. Also, there is another picture available of Iapetus turned a bit more, and yuo can see the same possible crater in that picture too. I haven't seen anyone identify this as a crater....but I think it could be. We need better photo imagery to confirm.<br /><br />I think the southern walls of my proposed crater is what you are calling one of the off-centered ridges. <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]

The symetrical diverging attendant ridges are on the left side of your pix. At this level of resolution, they are not apparent. Those nice blowups in the Saturn image thread show them.<br /><br />The large feature you describe is what I have been calling 'landslide crater'.<br />{Cassini imaged an enormous landside in its' southwest quadrant.}<br /><br />I have not encountered any updated nomenclature for Iapetan surface features yet. May I propose consideration of naming this large impact feaure Silylene?<br /><br /><img src="/images/icons/wink.gif" /><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>

 

[vogon13]

Let's take a little ride on an Iapetan ring particle:<br /><br />First, how big is it?<br /><br />Well, bigger than a softball, and smaller than a house. For purposes of discussion, let's say a chunk roughly a meter across. (don't think it matters to much at the level of detail I think about about these things)<br /><br />Where are we in the rings?<br /><br />Low, really low. Last pass over the high point on the equator we just cleared by a few meters.<br /><br />What's aound us?<br /><br />For purposes of discussion, we are in a very thin part of the system. There is a similar sized chunk a couple of meters below us and it is moving ever so slightly faster than we are due to its lower orbit.<br /><br />What happens on the next pass over the high spot?<br /><br />Our unimpeded trajectory will clear the high spot on this path, barely. The piece below us hits as we watch.<br /><br />What do we see?<br /><br />Assuming a solid surface impact, the icy chunk will shatter into a cloud of snow like material. Some will coalesce or stick to the solid surface, the 'snow cloud' will disperse.<br /><br />Do we hit the snow cloud?<br /><br />Yep, direct hit through just about the densest zone of it.<br /><br />Is this a problem?<br /><br />Well life is going to be rather more interesting for the next ~3 hours. We encountered a small amount of material that was virtually stationary in regards to our motion.<br /><br />Computer is making weird noise. Will be back, I hope.<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>

 

[vogon13]

Ok it quit, due to nothing I did.<br /><br /> As a result of the impact with the small snow cloud, we are a little 'snowblasted' (like sand blasted). We have also decelerated slightly and are now on a different orbit around Iapetus.<br />Our orbital high point is virtually unchanged, but our circular orbit is now elliptical. We will slowly descend for 180 degrees around Iapetus, then re-ascend to virtually our height at the time of the snow encounter.<br /><br />Unless...<br /><br />We hit something. How low can we go? Well, the highest spot on Iapetus is rather high, ~12-20 kilometers. Assuming no dangerous crater rims or mountains (mostly around the area 180 degrees around the equator, as our direction would most likely not have changed, If inclination increased a tick or two, will still return very closely to the high spot, barring an impact along the way.<br /><br />If our velocity through the snow cloud is rather more dissipated, we impact down range a distance reflecting our short fall in velocity. Big decel, we come down on the higher parts of the 'ramp', lesser decel, we hit farther 'downhill'. Even less decel, we take our chances on making it around the equator of Iapetus one last time.<br /><br /><br />This is why the ridge slopes away from the high spot. There will be a specific relationship between the distance around Iapetus and the height of particular feature located there.<br /><br />I have worked on the slope thing a great deal (see the rest of the thread). This is the clincher for the slope of the ridge. Should Cassini photograph enough of the equator, the relationship of the heights of the features along it will intersect an ellipse around Iapetus. The ellipse will be defined by the absolute altitude of the material antipodal to the high spot and the high spot itself.<br /><br />There will be a small correction to this figure due to the roatation of Iapetus. The current rotatition rate of ~80 days, makes the correction tiny, a rotation rate for Iape <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]

I am also thinking the 'source' crater for the ring material is going to be rather unusually shaped.<br /><br />As the incoming body (destined to trigger the ring formation) approaches Iapetus, a projection of its flight path will show an impact zone near the limb of Iapetus (as viewed from the incoming object)<br /><br />I anticipate the processes that put the resultant ring material into orbit to be rather inefficient.<br /><br />The incoming object will not be an 'orbit sharer' with Iapetus, the incoming velocity needs to be higher. Object most likely in solar orbit and Iapetus nails it. Projected path of center of mass of incoming object may in fact not intersect with surface of Iapetus, but the objects surface will. Only material liberated at speeds less than roughly SQR 2 times 953 MPH has a chance of acheiving orbit. Material sent at less than 953 MPH, will impact surface prior to completing an orbit.<br /><br />This scenario should yeild a (uniquely) elongated crater somewhere on Iapetus, and hopefully, a big pile of debris scattered roughly 180 degrees away. Perhaps the jumbled terrain of Mercury will be reminiscient of this locale on Iapetus.<br /><br />I cannot rule out a fairly large impactor making a very shallow divot either. (maybe 200 km range). Although smaller objects are quite feasible too.<br /><br />As mentioned before, the latitude and longitude of the crater cannot be inferred from the scenario beforehand, although a hit directly on a pole is ruled out by the collapse to the LaPlacian plane thing.<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]

Back on the ring particle final orbit ideas:<br /><br />The point of all that was to illustrate a posible mechanism for depositing material at different points around the equator of Iapetus. Materials that are on the 'elliptical final orbit' path will have a wide variety of periapsis altitudes, from barely differant from their final circular orbit to a path that skims the surface 180 degrees around. The apoapsis of all these paths don't converge again till at or near the 'high spot'. This allows debris to accumulate at various places around the equator depending on the height of the obstacle relative to the 'high spot'.<br /><br />A smallish mountain 45 degrees around (either way) won't intercept material at that location that it would at 135 degrees around (either way).<br /><br />Plotting all these potential 'impediments' (and note we need to accurately ID emplacement spots) will confirm their heights are consistent with their location along the equator relative to the 'high spot'.<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>