Approaching Iapetus - what makes it two-faced?

vogon13 · Feb 27, 2005

Considering some more angles concerning our little moon friend... I am still trying to demonstrate ridge and spot don't have to have genesis in same process. All the major satellites of Saturn are 'tide locked' to Saturn. That is, they present one hemisphere continously towards Saturn, and the other away (Phoebe doesn't does this in case you were wondering). This also implies a leading and trailing hemisphere as the satellite revolves around Saturn. The 'forward' side on a few Saturn satellites show large craters (like Mimas and Tethys) where presumably, the satellite 'encountered' something in 'the way'. Iapetus has a 'stain' or 'spot' eerily centered on the leading hemisphere, with a pronounced symmetry north and south (apparently though modified by surface topography along boundary). Pretty clear 'spot' occured after 'tide lock' happened. Iapetus also has the 'ridge' structure crossing leading hemisphere, but, I hasten to add, there does seem to be some 'peaks' aligned with equator on side not yet photographed very well by Cassini. Tide locking phenomena is related to gravity strength of Saturn 'falling off' across diameter of object being 'tide locked'. This effect has been used to stabilize orientation of satellites in earth orbit so effect not peculiar to Saturn (magnetic trick also used in earth orbiting craft but not relevant to this discussion). Effect is related to distance from Saturn (becomes weaker with distance) and is believed to take an amount of time proportional to angular momentum of object being 'braked' and it's distance from Saturn (there is also an effect from the rigidity of the object which I am neglecting here to simplify discussion). Where this leads is, of all Saturn's tide locked satellites, it looks like Iapetus achieved tide lock last. Possibility exists, Iapetus was not in tide lock with Saturn during formation of ridge structure. Would this effect what we see? <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 · Feb 27, 2005

The spot: What we know about it 1) Spot seems related to topography in some places, around edge, low areas seem to favor darkening, and in my opinion, depth of darkening seems to corolate with depth of topography 2) Excellent work by j8hart and thalion (and their references) indicates Iapetus can retain some gases for some length of time. 3) Spot likes areas around equator on leading hemisphere, but around equatorial region, spot trails into other hemisphere, from both edges What is this telling us? How about a gas that lingers in Iapetus realm long enough for something interesting to occur? A gas that upon exposure to the UV, light, or heat of the sun changes into a staining material? Whatever the reaction is (probably temperature sensitive) it likes the equatorial clime. I suspect heat (temperature) is the catalyst for the change. It never gets warm enough in polar region for reaction to take place. Bright areas of Iapetus are resistant (but not immune) to the reaction taking place because they stay colder. Dark areas warm up and can 'grow' into lighter areas if gas 'depth' is sufficient and if temperature is high enough. This is a naturally occuring thermographic process. I am not implying a life form of any kind, low temperature chemistry should be sufficient to explain process. This is why spot is symetrical north and south. Solar insolation on Iapetus would be symetrical about the equator north and south. Spot 'grows' east and west due to favorable conditions. Initial 'seed' (for lack of better term) for spot apparently occured on leading hemisphere of Iapetus. Column of gas over peak of ridge is sufficient to produce stain on top of ridge because it's warm enough there. It's also warm enough 45 degrees north and south of the equator because dark spot warms region enough for it to spread that far. Need better pictures of opposite side of Iapetus to evaluate suitability of condition <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>

j8hart · Feb 28, 2005

Not much time to post tonight. Lots of things I would like to say if I did have time but having difficulty keeping up! Top three of the many points I want to make are. 1) I remain very sceptical of the collapsing ring. I still find it difficult to believe it could be there in the first place. Working on the atmosphere is a bit of a diversion, basically I’m putting off the time when I have to consider the much more difficult problem of the ring forming. 2) Sorry Vogon13, but at present I believe my work on Atmosphere retention for Iapetus is not adequate to prove either that the Atmosphere could exist, or that it could not. I have just not got to the bottom of it yet. I need to do more research. 3) On a positive note for your ring theory I think you do not have to worry about the ring ending up in a pile, nor do you have to do complex things with the centre of gravity to get you out of this. This is because we can assume that the angle of decent of a ring particle would be steep enough for it to miss the highest point on the equator and land somewhere else. Lets say the highest point before the ridge existed was 5Km (I doubt it would be as high as that), then in order for a particle to have even a 50% chance of hitting it the decent would have to be about .22%. In other words the particle would have to be dropping no more than 2.2 meters for every Km of its orbit.

vogon13 · Mar 1, 2005

I was busy thinking and posting all during the weekend. Was tied up working (real job type work, egad!) all afternoon and Monday night. Have had a few more thoughts about the 'naturally occuring thermographic process' on this bizarro little moon. And an idea or two on the ridge/ramp 'forked tail'......... Think there is a another reason we don't see a ridge on Rhea, Oberon or Callisto too...... <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, 2005

Some more speculation on the 'gas' that I feel is behind the staining mystery. 1) Probably methane (at least) since science seems pretty goods regarding its behavior on Titan, seems reasonable to suppose a parcel of methane would react wherever it was if the conditions were suitable 2) If the staining material is derived from methane, where does methane come from? Perhaps Iapetus incorporated methane 'ices' during its formation and they have been slowly 'breaking down" ever since, keeping a small steady trickle of methane around Iapetus since its accretion. Possibly methane containing impactors strike Iapetus from time to time. Another possibility is these 'ices' exist naturally on Phoebe (and/or the other asteroidal moons) exterior of Iapetus orbit, and are 'released' during impact events and 'fall' inward towards Saturn and intercepted by Iapetus as it goes by. This mechanism would also apply to Hyperion, except in its case, gas would be moving away from Saturn. And finally, a scenario I have mentioned before, largish impacts on Titan exceed 'Teller treshold' for blasting gas away from Titan were it is eventually entrained in Saturn's magnetotail and 'paints' Iapetus. This last scenario would also seem doable with a steady stream of 'leakage' methane from Titan, appreciating thalion and j8hart working out atmosphere retention numbers, a very slow steady gas loss from Titan seems reasonable. The other satellites of Saturn may have incorporated methane 'ices' but if they all did, so most likely did Iapetus so it becomes moot. 3) If gas from Titan is 'staining' Iapetus, why isn't Hyperion discolored? Hyperion is not massive enough to retain gas long enough. Also, if speculation regarding temperature being important is correct, recall Hyperion rotates drastically faster than every 80 days like Iapetus. Dark areas on Iapetus experience ~40 days of sunlight every orbit of Saturn, and ~40 days of night! Seems like dark spot of Ia <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 3, 2005

Where this leads, is that of all Saturn's tide locked satellites, Iapetus achieved tide lock last. This is probably still true even if you include satellites of Jupiter and Uranus, too. As satellites achieved tide lock (assuming of course, they did not form that way, and I don't recall any scenario that postulates that) the energy of rotation is dissipated as heat. (angular momentum transfers to, but not interested in that right now). A molten (weird to say 'molten' for something primarily water and volatiles) object, would stay molten longer with this added energy, an object barely crusted over might (re)liquify if heat is dumped quickly enough. Where I'm going with this is, tidal braking heating for Iapetus is uniquely low in the outer solar system. Iapetus, by this line of reasoning, is 'most' likely to have a chance at preserving the most ancient surface of all the satellites in the outer solar system (forget Triton for now, it's a freak). Perhaps the equatorial ridge is the remnant of the final stages of the original accrection process that formed Iapetus! Final leftovers from formation process lined up in equatorial plane and came down over a period of time. Ring around Iapetus may not have formed in an event similar to Odyseus impact believed to have formed earth's moon. Just formation residue is all we see in the ridge. Iapetus needs to have 'firmed up' in time for this ancient record to be preserved, and looking at tidal dissipation rates might be a missing piece of the puzzle. Still not seeing a need to tie 'spot' to ridge formation scenario, spot still looks like an ongoing process to me. JPL lecture (2/25) revealed dust origin of 'spot' from Phoebe not looking too good, but they did not rule out other outer satellites (didn't mention them either). <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 4, 2005

Is Hyperion what's left of Iapetus's grazing impactor that generated orbiting ring material? Is Hyperion 'parent' body for some/most/all of the debris in Saturn system? <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 7, 2005

See where I'm at now...... If Iapetus had a ring system, not unlike Saturn's, it could have been 'leftover' material from accretion of Iapetus, or perhaps it was material 'blasted into orbit' in a scenario similar to that believed to have birthed earths' moon. Additionally, several posters like the idea of Iapetus 'gathering up material' in orbit around Saturn. Not sure any of these scenarios would generate an equatorial ridge structure dramatically different from what we have seen so far from Cassini. I like first idea as it gives us a glimpse of an extremely ancient feature, apparently unique in the solar system. I like the second idea, also, since if process is related to formation of our own moon (and maybe Pluto/Charon, too) we have a fascinating look at a process that was at work right here on our own planet. Third idea is also quite interesting to me, as I just have no 'feel' for how it happened, therefore it is going to be a major learning experience in accretion and dispersion processes. Then we get to 'getting the ring down' onto the surface of Iapetus and having it wind up in a form that we recognize from the the Cassini pictures. We have looked at tenuous and extremely tenuouser atmospheres to provide 'drag force'. I have invoked solar perturbations and Poynting/Robertson forces. Suprised someone didn't post something about a possible magnetic field of Iapetus interacting with salty conductive ice particles in possible ring and dissipating ring energy electrically into ohmic heating (darn, that's a good one!). If ring was 'leftover 'accretion material we have age of solar system almost to get it down. Potentially, the other 2 scenarios are apparently constrained to occur more quickly. If the ridge is orbitally decayed ring material, it did get down, whatever the mechanism. Like atmospheric drag, it just seems so cool for it to be that. Poynting/Robertson effect is well understood by real ast <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 · Mar 7, 2005

Re: Ridge formed by Iapetus overtaking an object. Earlier I posted this idea Could a large cloud from an impact shoot out in front of Iapetus, which it would slowly sweep up? I think that you have debunked this idea quite effectively. It is not possible for such a trajectory to exist. Ok. So such a scenario could not occur through an impact with Iapetus and the ejecta from that impact. More questions then: Just say a moon further out from Saturn was destroyed by an impact. What happens? Would it form a ring? Could it form an agglomeration of rubble that slowly spirals into Saturn (decaying orbit)? Just say this eventually crossed the path of Iapetus. It would slowly turn and sweep through this agglomeration forming the ridges. Sort of half way between the ‘ring sweep’ and ‘collision of an outer Saturnian satellite with a heliocentric object with the result of a retrograde debris cloud crossing Iapetus’ orbit’ theories. A lander would settle this question! Alternatively, does the instrumentation on the Cassini allow a cursory analysis that could connect the ridge material with other bodies in the Saturnian system? I see from my Iapetus schedule that Cassini will not visit again until August 2007? AlexBlackwell lists all those papers above, but not one of those has dared release a theory yet – they must all be stumped! You are bursting with ideas Vogon13 - keep them coming. Onwards to Mimas, 2 August 2005 <div class="Discussion_UserSignature"> </div>

vogon13 · Mar 7, 2005

I agree, from time to time, outer asteroidal moons of Saturn (and other gas giants too) are subject to large impacts. A collision that makes a crater on Tethys larger than Mimas diameter if it occured on an object in the ~50 km range would pulverize it. And this is where I see your idea hitting a problem. Debris may stay in orbit around Saturn, but dispersal will be sudden and extreme. See all satellites encountering debris randomly all over their surfaces. Picture earth crossing meteor stream like Geminids. Without atmosphere, all those meteors are going to impact all over a hemisphere of earth. Granted, watching display from earths' surface, you would note the 'radiant' of the meteor shower, but results not nearly as organized you would need to make a ridge. Also if you did get that far, what are the odds of having it line up close to equator? Recall tilt of Iapetus orbital plane to Saturn's ring plane, tilt of Saturns' ring plane to the ecliptic, and tilts of all the orbits of all the asteroidal moons. I concede disruption of a 'regular' satellite of Saturn exterior to Iapetus may have occured, but would its' orbital inclination to ring plane of Saturn just happen to match Iapetus in size and 'phasing' (sorry I don't know what the word is for 2 orbits with same inclination around parent body but are not aligned parallel to each other ). Getting a little too far out for me to 'see' in minds eye. I saw next scheduled close flyby of Cassini to Iapetus in 8/2007 also, but wonder if some useful <250,000 km flybys might be in the works before then. Frankly, any pictures from closer than Voyager II was would be very desirable. Some kind of rover working its' way across surface of Iapetus, like MER, and moving towards ridge would be so grand! I know ridge looks steep, but actual ground truth might allow for a gentler grade, if its pitched at 'angle of repose' though, climbing it will be most difficult. Maybe an orbiter in a low equatorial pa <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 8, 2005

Hey, a big tip-o-the-hat to Mssrs. Beatty, O'Leary and Chaikin (The New Solar System authors) for pointing out a mechanism for getting a ring down I had forgotten about. A ring around a body in space can come down all by itself. Under certain conditions. My understanding is that as long as ring is densely enough packed, objects in lower faster orbits will 'bump' objects in adjacent higher slower orbits. This results in a net lowering of the low side of ring system, and a net raising of the high side. By shedding material off the top side, ring can lower some of itself on to surface. Process stops when ring system thins enough to stop bumping. Once ring thins to this point, other forces (discussed already) will continue to act on ring system residue. Seems like a ring system big enough to leave 20 km high ridge on surface would have been more than thick enough for this process to occur. Also realize, a similar amount of material would have been lost to the void during the emplacement. And a certain amount of non-bumping material would have been left in orbit to be dissipated thru other mechanisms. <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 8, 2005

and part 2 (I needed to tend to annoying, but adorable, house cat) Authors also describe process that puts orbiting debris into plane of equator regardless of original orientation{I should have used word 'inclination' here, instead of orientation}of material. Material in a variety of orbital inclinations 'spreads' around the object. Individual objects pass thru the 'average' plane of all the material twice per orbit. Through collisions, system always 'collapses' into what authors refer to as Lapacian plane, which coincides with equator. Material in variety of orbital inclinations 'spreads' around object due to effects of oblateness(!) and other perturbations. Yeah, oblateness. It is not specifically stated that rotation of the oblate object is necessary to generate forces to put material into Laplacian plane, but from point of view of orbiting material, we have a rotating frame of reference anyhow. But if Iapetus needs to be rotating faster than every ~80 days or so, that drastically constrains when ring system formed and when it emplaced on Iapetus. Really starting to think this ridge formation is extremelyold. Also, am feeling dark 'spot' is an on going process. Two of the most bizarre features ever seen, and they are both on Iapetus, and apparently are not related in their origins. Are we starting to 'grok' Iapetus? Edit added in yellowbove, sorry for that <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 8, 2005

Just say a moon further out from Saturn was destroyed by an impact. What happens? Am starting to fathom your point and will go a little further with it. Material scatters. Some lost to limitless void, some 'falls' into Saturn, some burns up in Titan atmosphere. And some winds up near Iapetus. Some of that material impacts immediately, and apparently randomly on surface. Some of the material will be deflected as it passes by Iapetus. Some scatters to higher or lower inclination orbits, some deflected away and some towards Saturn. And a tiny amount will probably go into some kind of orbit around Iapetus. Now tiny = a rock or tiny = 1000 cubic kilometers? I dunno. I do know however, if there is enough, it will 'do' the Lapacian collapse tango and go thru the above process and some will scatter into the limitless void, some will wind up in scattered orbits inside Iapetan Roche limit, and some will wind up laying in ridge along equator of Iapetus. Thanx jmilson! (and others who have brought up this scenario) Calculating 'tiny' involves 'math' and I will happily leave it to others to figure out, as, I have mentioned before, I don't 'do' math. I have enjoyed working out tonights postings about as much as anything I've done recently. Appreciate Spacedotcom and Imaginova for providing sandbox for me to play in. <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 · Mar 8, 2005

... dispersal will be sudden and extreme.... Your argument is good but I am not quite convinced. My idea is closest to the Denk and Neukum paper of 2000. They postulated a debris cloud from a major collision between an asteroid and an "early days" satellite that had its orbit betwen Iapetus and Phoebe. This is a reasonable hypothesis for the leading dark side, but could it also explain the ridge? This depends on how long the debris cloud took to spiral in to Iapetus' orbit - and the dynamics of what happens to the debris 'cloud' during this journey. Would it, could it flatten out as has occurred with the rings? Could Iapetus have swept up a decaying spiral debris ring? I know you argued earlier that the chances of it being in the same plane are minute - but stranger things have undoubtedly occurred in our solar system. And as you now mention above, even ploughing into a cloud, there are processes that may draw some of the debris into a collapsing ring to be deposited on the equator. (Think how many lucky shots one has when playing snooker - how many lucky shots have been played in our solar system?) <div class="Discussion_UserSignature"> </div>

vogon13 · Mar 8, 2005

I really like 'plausible' over 'lucky'. Picked up phrase 'sudden and extreme' from a legal brief (Coyote versus Acme) and have been dying to use it in a post for some time. Feel the moon disruption scenario could go towards explaining what we see as described in my post just before your post I'm responding to now. Kinda think it's 'plausible' till some one else does math, not sure I would want to invoke 'luck' to demonstrate a scenario. Really would like to post the ring dynamics info from the Planetary Rings chapter from the New Solar System book, but have no idea how to secure copyright, or 'scan document' or any of the other computer savy stuff my un-computer savy brain would need to do. It's very convincing. Simply put, regardless of how material is transported to Iapetus vicinity, once enough material is in orbit inside Roche limit, ring automatically forms in equatorial plane and automatically deposits large percentage onto surface of body it's orbiting. IMO, ridge on Iapetus was formed of emplaced material from a ring system around Iapetus. Getting material in Roche limit can occur (with luck, as you put it) from the outer moon disruption scenario, it could be material leftover from the accretion of Iapetus itself, or material could have been orbited from an impact of a large body on Iapetus in a manner similar to impact believed to have created earth's moon and possibly the Pluto/Charon system. Additionally, (just thought of this) two sizable objects could have collided with each other in the vicinity of Iapetus and this would presumably leave a great deal of material to start the ring building process. All the scenarios that put material in orbit around Iapetus automatically yield an equatorial ring around Iapetus and the ring will (if dense enough) automatically emplace some of itself onto body its orbiting. I know it's fantastical! It's also amazing we have front row seats to this thing! Kind of glad this thread took 'the l <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 8, 2005

Have thought about dark colored ejecta from an impact quite a bit. Felt some of the details were intractable so didn't post thoughts. We see ejecta blankets around craters on objects around the solar system. Imagining in the minds eye of a photographic negative of a bright ejecta field photo from the moon (to make it look dark instead of light) doesn't resemble what we see on Iapetus, especially around the edge of the 'spot'. Most (not all of course) ejecta blankets show reasonable symmetry around 'parent' crater. Iapetus 'spot' extends ~45 degrees north and south and 180+ degrees east and west. Debris, particles, ejecta, etc. on ballistic trajectories do not 'know' of topography of terrain they are heading towards until they strike it, edge of 'spot' shows strong correlation to elevation, depth of darkening and depth of depressions follow right along. Craters near equator show (if any at all) bright white on equatorward (is that a word??) side and as you get into the +/- 45 degree from equator, percentage of crater bowl that is white 'grows' perpendicular to direction to equator. Some of the craters furthest from equator show only darkening on crater wall closest to pole, almost 'negative' image of the near equator craters. It would seem unlikely for ejecta to correlate so strongly with existing terrain, the trajectories would after all, be random. This effect, additionally, suggests the temperature clue. A large crater bowl in the northern hemisphere of Iapetus would be expected to be warmest on its northern arc. And that is how the darkening tracks. Its really quite bizarre, but it's in the pictures. Symmetry of spot north and south constrains location of 'parent crater'. Ridge seems to be nearly contiguous where perhaps we would expect 'parent crater'. We just aren't seeing bright fresh craters around the equator in the spot. My best feeling for this is that the 'spot' can 'regenerate'. W <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 10, 2005

It would certainly be a Hobbes choice, but would consider sacrificing some up close high resolution photos of Iapetan ridge structure in exchange for using the Cassini radar instrument to evaluate surface roughness (sadly, Cassini cannot do pictures and radar at same time as they point in different directions). If radar instrument would yield useful information on feasability of surface for a future rover mission then the observations should be made. Also, has anyone considered the possibility of aerobraking in Titan's atmosphere along the way to approaching a different Saturnian satellite? Could save a great deal of fuel and could also bend flight path to achieve the rondevous. Recall Cassini only retro burned engine to the extent of 1500 kph to achieve Saturn orbit. Seems plausible a short hot decel through upper region of Titanian atmosphere would be survivable. MER decel from 12000 kph (IIRC) pretty quickly when it got to Mars. Be quite interesting to approach Saturn from deep space, graze Titan atmosphere, arc around Saturn system and wind up at a relatively low speed in vicinity of moon of your choice, do a braking burn and be in orbit. If moon was Iapetus, and craft had a rover, just think what a 50 kilometer traverse (over a couple of years) on the surface of Iapetus could tell you. <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 12, 2005

I have been picturing standing on a mountain top on Iapetus, close, but not toooo close, to where the possible ring is emplacing itself. Presumably in the earliy stages of the emplacement, from a high enough vantage, one could look downward across the surface of Iapetus and watch the ring whizzing by and actually accreting onto the ridge! And then look up to almost the zenith and see the ring system arcing out into space. Hard to imagine anything else as spectacular. Sure like to see this in IMAX! <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 14, 2005

And did a tiny amount of math this morning on our little moon. Diameter (from above) = 1436 km Circumference = 4511 km Velocity at equator for 10 hr Iapetan 'day' = 451 km / hr Velocity at equator for 20 hr Iapetan 'day' = 225 km / hr Velocity at equator for 80 earth day Iapetan 'day' = 2.35 km / hr I picked 10 and 20 hours as a 'best guesses' to bracket probable Iapetan rotation period at formation. Over time , tidal effects of Saturn have slowed Iapetus to the near 80 day value. Rate of decel pretty iffy in my opinion. Possible if decel rate low enough, ring emplacement occured during a significant rotational velocity period. Spacester calculated a velocity of 1543 km / hr for an object in the lowest possible orbit, motion of equator aligned with ring direction could decrease surface impact velocity by up to 25%, so we have an additonal possible bit of information on ring during possible surface emplacement. <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 14, 2005

Am in rare form today, feel like a little more number play. If the circumference of Iapetus is 4511 km, and per Spacester the 'contact' velocity is 1543 km / hr, then the final orbits of our hypothetical ring particles will take a little over 2 hours and 55 minutes. Something that could be confirmed by a brave local observer with a can of spray paint and a clock. Just kidding, don't try that if you find yourself in this scenario. Let's say all the ring particles are chunks of ice 1 meter in or smaller in diameter (who knows after all?). And lets say the sink rate is 10 meters in altitude per hour. Any ring particle that strikes the high spot sticks after rolling to a stop. Any ring particle that clears the highest spot, faces a couple of possibilities. If it clears a pinnacle, it can complete one more orbit of Iapetus and then strike the pinnacle, on its leading side, roughly 30 meters from the top. Another possibility is the high spot is in the form of a plateau. Objects that just clear the leading edge of the plateau have the possibility of landing on the plateau if its far edge is high enough to catch the 10 meter/hour sink rate. If you don't have a plateau, as ring particles 'pile up' on and around the pinnacle, due to the forward motion of the incoming material, you start to form a plateau in the direction of flight of the incoming ring debris. The slope of the plateau is governed by the sink rate. Width of the plateau will be governed by the north / south spread of the incoming material, and possibly inclined structures in the ring system as it emplaces. Also, two or more high spots on the equator can participate in the emplacement of the ring material, if their heights are compatible with the sink rate (as you go around Iapetus, falling at 10 meters per hour, 3080 km down range for example, if there is a high spot less than 20 meters shorter than the other high spot, it will emplace simultaneously.) <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 16, 2005

And, from the above, have been thinking about that pinnacle being battered on its leading side on the top 30 meters. These ring 'particles' if, let's say 1 meter in size on average, are going to be like cannonballs if they are striking a steep surface. There is going to be a lot of 'incoming'. While the 1500+km/hr touch down speed isn't going to be 'meteoric', I still think we can expect it to be 'erosive'. Presumably, the pinnacle is going to consist of materials similar to the ring. And therefore, of similar strength. That pinnacle, in the example, is going to be machine gunned by a nearly unending stream of material. This will also be a factor in generating the 'ramp' structure along the equator. Am thinking the 'sink rate' of the emplacing material is going to strongly be evidenced in the slope of the ridge structure. I realize in the preceeding post, I used 10 meters/hour as the sink rate. But looking at the ridge, ~20 km high at one end, and we see it taper to virtually nothing across, what, roughly 90 degrees of Iapetan longitude? The sink rate is apparently quite bigger than my 10 meters/ hour! Now I realize, pre-existing slope of terrain, may account for quite a bit of the taper but just from the pictures on hand now, 10 meters/hour is too low. Another characteristic of a high sink rate though, is simultaneous emplacement at more locations around the equator. The 10 meter/hour sink rate just won't emplace in too many spots due to the matching of the elevations effect as described above. A high sink rate also implies a more rapid emplacement of the ridge feature. At least until the 'bumping' process slacks off. And consequently may be another reason why this stucture is not seen on other satellites in the solar system. A process that is over relatively quickly, can occur during a presumably 'molten' epoch of the body being emplaced upon, and be finished before a solid surface capable of preserving the feature exists. <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 17, 2005

The 'bumping' process for the self-emplacement of the ring material is a fascinating process. And an idea I have really jumped on. Also have been thinking about the apparently high 'sink rate' implied by the Cassini photographs. How fast can the 'bump' process propagate through the ring material? We really can't look at Saturn's rings for clarification of this process since it appears all the different resonances across that ring system act like 'curbs' to reign in how fast and where the material is allowed to go. Additionally, the rings closest to Saturn do not appear to be sufficiently contiguous for the bump process to work. The New Solar System chapter doesn't spell out how fast either. Additionally, at the time it was written, don't suspect the writers specifically envisioned a small scale ring system around an object the size of Iapetus. Additionally, since it seems unlikely that Iapetus had (sub)satellites of its own, the resonance 'curbs' would not have been active in the Iapetan ring system. We would be looking at something that could, perhaps, be potentially much easier to model than Saturn's ring system. Visually, the Iapetan ring system would most likely be more 'smoothly' arranged than Saturns with all its breaks and discontinuities. So if the apparent sink rate seems somewhat high for the bump process, some of the other orbital dissipation ideas explored here may have operated simultaneously with the bump process. In fact, if the sink rate is high enough, bump process may have only played a minor role in bringing down the ring. Am aware of the Enceladosian atmosphere findings, and understand its relevance to the Iapetan spot, and am starting to again appreciate the possibility of atmospheric drag as a player in bringing down the Iapetan ring system. Looking at the 'sink rate' and pondering how to get the ring down that fast! Who'd have thought it ! <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>

silylene old · Mar 17, 2005

If you stare very closely at the circumferential ring, at the full photo sizes, you will see that the ring appears not to be continuous, but rather appears to be a series of long, parallel, shallow arcs arranged circumferentially and slightly overlapping. Maybe I should mark up a big photo, if this isn't clear. <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>

vogon13 · Mar 17, 2005

Quite a few of the breaks in the ridge seem to be (subsequent) crater damage. And some of the breaks seem to be 'inherent'. (!) With out the 3D pairs (or whatever else we get from Cassini that fufills this function) to figure out the elevations, I can't state definitively that the stop points and start points follow the 'down range elevation proportional to the sink rate' rule. They look close, real close. But between my almost 50 year old eyes, bifocals, and a monitor that a certain family member keeps playing with the contrast, brightness and gamma on, just don't want to push the analysis coclusions till we see the other side of Iapetus along the equator. Those 'peaks' south of Basan crater formation are intruiging. Would love to have a nice topographic relief map of ridge and point to a gap, measure its width and predict altitude of ridge at point where it resumes, but, wow, that would be so intense. Realizing ring can 'emplace' simultaneously around Iapetus just seems so counterintuitive, but sink rate, slope, distance around equator are all related, in my view. You have sharp eyes to have noticed 'differences' in the breaks. Spend some time, if you will, looking at the pattern the darkening makes in the Iapetan craters as you move north. And then look at features that seem to be depressions, and see how the darkening developes. All the answers we might ever get are in these pictures. I'll move up the thread with all the great pix in it so it will be easier to find. <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 18, 2005

Here's a twist to the scenario: In the photos of the ridge structure, what if the original 'high spot' on the surface, the place where the emplacement starts, is on the LOW end of the 'ramp' stucture? The 'pile problem' (several posts ago) is not a problem at all. It's how it happened, with one more detail. The 'pile' grows 'upstream' relative to the incoming materials' orbital direction. This puts the scenario firmly back at a low 'sink rate' (which just has to be). A sink rate of kilometer or kilometers per hour emplaces most of the material in weeks, that just feels way, way off. Above I pictured the upper 30 meters of a pinnacle being 'machine gunned' off. Don't think it works that way. Material starts to accumulate in the area of the 'obstruction' (whatever its form) and eventually, the emplaced material is the obstruction. A better analogy might be to picture a dump truck backing in to dump its load. Subsequent dump trucks back in from the same direction and dump and the pile 'grows' upstream, if you will. A strange characterisic of this empalcement scenario is, it would be 'safe' (I'm not sure I would say real safe) to stand on the ridge during emplacement, as long as you were well away from the 'building' end of the ridge. You would be 'in the shadow' so to speak, in a place where the incoming material that would have reached your position has, instead, been intercepted before reaching you. Now how far above would there be material whizzing by you? Probably close enough to be very, very scary. Anyhow, I like this as it gets the 'sink rate' back into feasible range (IMO). Unfortunately, slope of ridge and sink rate aren't tied together as much except to the extent as you go around Iapetus, the 'shadow effect' still relates heights of discontinuous sections of the emplaced material. Will be driving around all day today, gathering supplies for my business, so will be contemplating <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>

Approaching Iapetus - what makes it two-faced?

vogon13

Guest

vogon13

Guest

j8hart

Guest

vogon13

Guest

vogon13

Guest

vogon13

Guest

vogon13

Guest

vogon13

Guest

jmilsom

Guest

vogon13

Guest

vogon13

Guest

vogon13

Guest

vogon13

Guest

jmilsom

Guest

vogon13

Guest

vogon13

Guest

vogon13

Guest

vogon13

Guest

vogon13

Guest

vogon13

Guest

vogon13

Guest

vogon13

Guest

silylene old

Guest

vogon13

Guest

vogon13

Guest

Similar threads

TRENDING THREADS

Latest posts

Share this page