Approaching Iapetus - what makes it two-faced?

vogon13 · Feb 19, 2005

I am aware that tilt of moons orbit around earth mostly negates axial tilt of earth, thus rendering plane of moons orbit closer to plane of earth's orbit around sun. This is good for us as we get more total solar eclipses. Have noted Iapetus orbit is tilted to plane of Saturns' equator thus, if aligned correctly, could put plane of Iapetus orbit closer to plane of Saturns orbit around sun. Or even further away. Do we know which way it is, and does it stay that way for longish periods of time? (like 4000000000 years). If Iapetus orbit around Saturn is situated closer to plane of Saturn's orbit around sun rather than more inclined, then it would appear that Iapetus would be closer to Saturn's magnetotail every orbit around Saturn rather than for some periods every 14 years when Iapetus, Saturn and sun are aligned. This would boost efficiency of hypothetical process that would sweep Titan derived atmospheric materials towards Iapetus to 'paint' the leading hemisphere. In this case we would be looking at periods every 14 years when coating process 'slacks off' rather than reverse. Where I'm heading with this is, I would like to be able to demonstrate possibility exists that leading hemisphere staining of Iapetus is an on going process, and the equatorial range is an ancient feature that coincidentally exists on Iapetus. It is not necessary to postulate a process that creates both features at the same time. <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 · Feb 21, 2005

Your argument is very good – i.e. leftover debris from earth-moon-type collision, but this would mean it is likely that we would see equatorial ridges on other moons. I note from your posts above that you are searching for such evidence. Good luck! Freak collision with heavy meteor / debris cloud? Orbital mechanics must give billions of possibilities. Could the ridge be the result of a freak / rare event? If no examples on other moons, there will be lots of work on modelling such events. Thanks for your detailed consideration of my ‘moonlet’ question. Maybe Iapetus slowly ploughed into the remains of something going in the same direction. What if a ‘glancing’ collision occurred – i.e. a large object going in a similar direction. Could a large cloud shoot out in front of Iapetus, which it would slowly sweep up? I still feel the two phenomena are likely to be linked. Don’t most icy bodies in the solar system eventually darken after long exposure to solar radiation and solar wind? I don’t see how you would get such a clear line from any long term staining process. Surely you would get more of a gradation from light to dark over a greater distance. Have any of the authors of the aforementioned papers dared to release a hypothesis yet? I am really keen to learn whether analysis of the imagery can give us a more clear definition of the boundary between light and dark – it may also give some indication whether the phenomena are linked or not. PS. Congratulations on attaining planethood! <div class="Discussion_UserSignature"> </div>

vogon13 · Feb 21, 2005

Appreciate your post, am not ignoring it at this juncture, had thought all night about a different aspect of Iapetus riddle which I will go into now. Consider the mass of Triton, 2.14 X 10^22 kilograms, and Pluto, estimates range from 6.6 X 10^21 kilograms to 1.6 X 10^22 kilograms and compare to Iapetus at 1.93 X 10^21 kilograms. Now my figure for Iapetus is derived from Voyager flybys, I'm sure Cassini gets us a better number, but for now, it's close enough (consider range of values for Pluto, and we have to wait for New Horizons for more accuracy). For pratical purposes, lets say Triton has 10X mass of Iapetus. Triton has a thin atmosphere, presumably for a very long period of time. I am suggesting a very thin, very temporary atmosphere around Iapetus isn't just possible, but likely. As Cassini showed regarding Phoebe, volatile rich objects exist in this part of solar system. So existence of volatile materials on Iapetus seems likely. Continued outgassing from final accretion phase would also supply material for possible atmosphere. This possible atmosphere only has to stick around long enough to provide drag force on ring to bring it down to accumulate on surface along equator. Composition of this possible atmosphere would be governed by low gravity and low temperatures, not much hydrogen is going to stick around, carbon dioxide will freeze out real fast, I'm thinking methane, nitrogen, and the heavier inert gases. Occultation data from Pluto and physical properties of atmospheres imply a possible Iapetus atmosphere will be quite deep (extending to high altitude). Roche limit of Iapetus not all that high either, so expect most/all of the possible ring to experience drag force. Please keep in mind, this atmosphere can be extremely thin and very temporary on the age scale of the solar system and still do it's job on the possible ring system. I not sure this speculation relies so much on a continuing flux of meteors to maintain a possible at <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 21, 2005

To clarify, don't expect equatorial ridge on : Mercury-high orbital speed around Mercury, suspect its elliptical orbit around sun cranks up perturbations of objects orbiting Mercury, planet proabably molten during interesting epoch Venus-radar probes imply surface melted and resurfaced <1000000000 years ago, bets off Earth- plate tectonics, weather, volcanoes, yada, yada, yada Moon- proximity to earth and/or sun, may have been molten during correct epoch Mars- too massive, ring debris burns up in presumed early thicker atmosphere, volcanic resurfacing of Mars may have erased record, Mars may have had molten surface during correct epoch Asteroids-irregular shapes for all but the largest objects seems to be a problem, we have no images of the largest objects, fragmentation of 'parent' bodies may have destoyed evidence Io- molten too often Europa- ice flows melt, move around, reform Ganymede- seems resurfaced in large areas, may not have solidified in time to preserve feature, proximity to Jupiter doesn't seem conducive to long enough term of stability for objects in orbit, objects in very low orbits traveling significantly faster than around Iapetus, expect vaporization, cratering on contact with surface Callisto- same as Ganymede except resurfacing part Note on all large Jupiter satellites: Early Jupiter believed to have radiated great deal of heat, maybe enough to evaporate icy/volatile potential ring forming material from around Galilean satellites Mimas- way too cratered, too close to Saturn, suspect very unstable orbits around Mimas due to small size and proximity to Saturn Roche limit Enceladus-resurfacing itself almost as we watch Tethys- big canyon implies internal geological process thus potential exists for surface to not retain a record from such an early epoch, proximity to early Saturn may be similar to Jupiter situation 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 · Feb 21, 2005

Calculating impact speeds for 'overtaking' type collisions way beyond my calculating abilitiy, but suspect in further reaches of Saturn realm velocity differences may not amount to much. Keep in mind though, Iapetus slugs along at ~5500kph around Saturn and even seemingly minor 'fender benders' could pack a mighty wallop. A retrograde orbit, obviously, would be even more of a thwack. Added this correction: Reviewed my scribbles and decided Iapetus slugs along at ~5800 kph. Sorry for the goof. The 8 looked like a 5 (actually it sorta looks like a B) wonder I get anything right. Gawd I'm tired....... <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 22, 2005

-----I still feel the two phenomena are likely to be linked. Don’t most icy bodies in the solar system eventually darken after long exposure to solar radiation and solar wind? I don’t see how you would get such a clear line from any long term staining process. Surely you would get more of a gradation from light to dark over a greater distance. ----- I keep looking at those pictures of Iapetus. The brown stain is continuous 45 degrees north and south of the equator except that some crater walls have caved away to reveal bright white insides of Iapetus. Coating can't be so thick a land side couldn't reveal material underneath, yet there has to be fresh impact craters here too, and they should punch through a coating this thin, and they should be entirely white and there should be white ejecta laying on the surface around them, ------------------------but there isn't!--------------------- This is just so wild, we can see this but what's the frigg'n process? Towards northern limit of brown spot, there is an old multi-ring impact basin, northern edge of bowl is also the sharply defined edge of the brown coating. I am reminded of spray painting an irregular object with the spray gun fixed, only surfaces at a compatible angle get the spray. Ovoid features have paint 'feathered' away, sharp angles get good coating where spray encounters flats and surfaces angled away get no spray at all. How applicable an analogy is a spray booth on earth to an astrophysical phenomena in the outer solar system? Am also thinking of a heavy corrosive gas burning the color out of the low lying areas, some areas slightly inclined would be like sea shore with gradually diminishing exposure to gas as you moved up the slope but where a cliff rises sharply, you get a definite edge. West of northern multi-ring feature is a brown elongated patch which really does appear to be in a low spot. Now, I am just using the heavy corrosive discolori <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>

claywoman · Feb 22, 2005

Vogon13, Thank you...I now have pictures in my head of a giant hand using a spray gun everytime something hits Iapetus!!! I hate fertile imaginations!!!

j8hart · Feb 22, 2005

Vogon13, OK, I drop my objection B), your right Iapetus does have enough built in oddity to make the collapsing ring something that might not be observed elsewhere. But I still have problems. On the subject of a thin atmosphere for example. No matter how thin it could take hours/days for a ring particle to fall to the surface. So unless the atmosphere was remarkably still only large particles would fall in a plain. Since Iapetus has a long rotational period it’s likely that wind speeds would be low. I still find it difficult to accept that an atmosphere would hang around for long enough anyway. My Physics is to rusty to work this on through to the end, and I’m short of time, but here is the easy stuff. Please excuse my estimates there probably all out a bit, if anyone wants to correct or flesh out any of this I’d be interested. 1. Atomic Mass of N2: approx 4.69 x 10^-23 2. Escape Velocity: approx 600 m/s 3. Kinetic Energy required for an N2 molecule to “Escape” is therefore approx 8.44 x 10^-18 J. 4. Boiling point of N2 at 1 Atmosphere approx 77.35 K 5. Average energy of N2 molecule at 77.35 K approx 1.07 x 10^-21J (Not sure if I’m doing this right, but I think it’s Boltzmann's constant x Temperature). 6. Ratio of 3. to 5. is approx 7,830. OK so here is what I don’t know at the moment. A. I can’t work out the relationship between the ratio of the average energy of molecules in the atmosphere, the energy required to reach the escape velocity, and the time it takes for the atmosphere to bleed away (perhaps atmospheres have a “half-life” or something like it). B. I searched for a site relating the boiling point of N2 with pressure, but could not find one. C. Where the upper limit of the atmosphere has to be. If it was say 1,500Km from the centre of Iapetus then the escape velocity at the top of the atmosphere would be 413 m/s and the energy required to get N2 to

vogon13 · Feb 23, 2005

Your final point, about the equator adjusting itself to the ridge location is a very interesting idea, adding cubic miles of mass to an object (even one that rotates every 79 days), particularly all in a great circle path around the object, may upset its axial coordination. I have no objection to your idea. We may see a readjustment (in 100000000 years) in Neptune's axis when/if Triton comes down upon it (see the Triton thread for more exciting details!). More on the Iapetus atmosphere idea: * your math makes it look even more iffy than I thought ( but watch me wrangle details, now) * however, early in the history of the solar system, the sun was believed to be substantially less bright than it is now. We may conclude back then that Iapetus (and everything else) was colder. Lowering the average thermal energy of the atoms and molecules in this possible atmosphere will let it hang around longer to do its job on the possible rings *atmosphere affect on the ring system is cumulative. A great deal of atmosphere will put drag on the rings and bring them down quickly. Even an intermittent thin atmosphere has (presumably) 4000000000 years to do its work on the rings. Once atmospheric drag has dissipated some of the ring orbital energy, it is gone for good, I'm having a hard enough time getting the ring down, I'd like to see a plausible scenario for gently accelerating a ring system back into a safer orbit. (I have to resist the urge to explore that topic ) *atmospheric drag doesn't have to be the only force working on the ring, I'm aware of the Poynting/Robertson (my apologies if I didn't get the spelling right) effect and how it can effect small particles, I have speculated earlier in this thread about passage thru Saturn's magnetotail as potentially being a drag force on ring particles, and it seems likely the solar wind over billions of years would have an effect to <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 23, 2005

By way of example, Apollo 16 released a small satellite in low orbit around the moon during early seventies, and although on an orbit apparently good enough for a manned capsule, it fell out orbit in less than 2 months..... Been thinking about this quite a bit lately, its a very interesting experiment in orbital characteristics of an object orbiting a natural satellite. Don't feel fate of this small subsatellite was anticipated by its' builders/designers. Coming down so quickly too, and crash site IIRC is on the lunar farside, I don't know if on leading or trailing part of farside. See where I'm heading with this? Are there prefered regions on the moon where objects in low orbit are more likely to come down? Would there be some statistics on this, maybe from an orbital dynamics computer simulation? It would seem that even a subtle effect would become apparent if you had an extremely large number of objects to do stats on. IIRC, moon moves in its orbit around earth at ~2000 kph, Iapetus just short of 3X that. Moon is quite far from earth, considering earth's mass, Iapetus is furthest large sat of Saturn. Granted solar perturbations drastically weaker on Iapetus than moon, but we've got 4000000000 years for even micro effects to add up and appear in debris pattern of fallen ring material. Being close (close enough?) to 5:1 resonance with Titan orbital period raises a flag to me, also. I'm arguing by analogy, that low orbit around moon not a safe place to leave things, and am suspecting that on longer (but still available) timescale, orbit around Iapetus is not a safe place either. Further, that distribution of orbitally decayed ring residue may exhibit a predictable pattern. I actually love throwing out the phrase 'solar perturbations' but have little feel for magnitude of effect. Suspect varying distance of moon from sun as it goes around the earth as the earth goes around the sun is the 'big kahuna' (and since y <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 23, 2005

“And I really like Iapetus” Me too <img src="/images/icons/smile.gif" /> I’ll try not to prove that Triton doesn’t have an Atmosphere <img src="/images/icons/smile.gif" /> It’s escape velocity is low at about 1,450 m/s, but still almost 2.5 times higher that Iapetus’s. Possible that Triton’s Atmosphere may be being sustained by pretty much exactly the processes you suggest for Iapetus. Still have not had time to investigate the relationship between escape velocity and how long an Atmosphere hangs around. It’s something I’ve been curious about for a long time anyway so I’d like to know the answer irrespective of this particular discussion. It should be something to do with the percentage of molecules with energy’s greater than the escape energy assuming a “Normal” distribution. This I think means that provided the average energy of a molecule is considerably less than the escape energy, then the number of molecules whose energy is greater than the escape energy is related to the square of the ratio of the two energies, and therefore to the 4th power of the escape velocity. But this is guesswork. If no one posts a better solution here maybe I should start a separate thread asking “does anyone know the mathematical relationship between the escape velocity and the length of time an atmosphere lasts”.

j8hart · Feb 23, 2005

Very embarrassed. I screwed up the maths in my previous post. I wrote “1. Atomic Mass of N2: approx 4.69 x 10^-23” which is a result I got from several hasty estimates put together thinking that it was Kg when actually it’s grams. I then went on to use this in calculating the energy required to escape and said “3. Kinetic Energy required for an N2 molecule to “Escape” is therefore approx 8.44 x 10^-18 J.” which is as a result of my mistake about 1000 times too big. It’s really approx 8 x 10^-21 J. My bottom line result is therefore also 1000 times too big. I should have know, I thought it was too large at the time! I smartened up the estimates generally and I now get the result that the Energy required for a molecule of N2 to escape from the surface of Iapetus divided by the average energy of a molecule of N2 at 77.35 degrees Kelvin is approx. 7.6 (and not 7,830!). In other words if the atmosphere was at 77.35 K (it could be colder at lower pressures) any molecule with 7.6 times the average energy should escape. This compares with a ratio of about 46 for Triton and 152 for Titan. I have attached a copy of my estimates. There could well be further mistakes, if anyone feels like checking it please do. For example the estimates of Mass and Radius may be out of date.

vogon13 · Feb 23, 2005

Looks like you have independently proven the existence of an atmosphere around Triton, and also one that apparently doesn't require all the hi-jinks I'm putting into a hypothetical one around Iapetus. Can you solve for time? Does that small Iapetus number relate to minutes, months or millenia? (gosh this is exciting) Has long term (longish?) stability of Triton atmosphere been demonstrated before? If had given matter any thought, might have written it off as a temporary effect of increasing tidal dissipation from (weird) retrograde orbit. Excellent! Some where above in this thread are some ranges of mass for Pluto, interesting to see where it lies on your scale. As they say at Faber College, knowlege is good. <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 24, 2005

And all the current mass estimates for Pluto (that I have, I'm sure there is a better # somewhere) are between that of Iapetus and Triton. Interesting. Assuming similar density, I don't have to do any math. YEA!!! We have an atmosphered Triton. We drop the mass intermediate between Iapetus and Triton, and boom, we get Pluto with its perihelion atmosphere. There is just a factor of 6 between Iapetus and Triton (46/7), Pluto neatly in between, with a somewhat capricious atmosphere and com'on guys, were pretty close on the stats, can't I have a temporary, ok very temporary, tenuous, ok very tenuous, atmosphere on Iapetus? Just let it hang around long enough to dissipate the orbital velocity of the ring system, drop it in a nice ridge along the equator, and then have it there all dramatically posed for Cassini to snap some wild pix? And then after it's done its work , this brave and useful atmosphere can get zorched into the endless eternal void, for ever and ever. As to the brown stuff, well.............I'm not going to press my luck tonight.<img src="/images/icons/laugh.gif" /> Anyhow, enough with the levity, even if the atmosphere turns out to be a non-starter, it still looks like over eons of time, other mechanisms will slowly get the ring down, just kinda neat if air gets to do it. Wonder about a neon, argon, methane atmosphere........ <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>

thalion · Feb 24, 2005

I'm up way too late, but I have a book that addresses the issue of atmospheric escape in detail. I've actually posted this information before, but I doubt it survived the "great board shift." Okay, the following is from Hartmann's Moons & Planets, 4th ed., (Wadsworth: 1999), p. 334. "An atmosphere will escape from a planet in a time less than the age of the solar system if V (greater than or equal to) fv_esc Where: V = root mean square velocity = (3kT/m)^0.5 f = constant, about 0.25 v_esc = escape velocity from planet m = molecular mass for given molecular species k = Boltzmann constant" [edit] T = temperature in Kelvin By the way, this is expressed as a rule of thumb. There is a slightly more complex equation in the book, but this one should do for almost any situation in the solar system.

j8hart · Feb 24, 2005

Very cool Thalion, thanks! Plugging in my estimates I get… V=( 3 x 1.38x10^-23 x 77.35 / 4.65x10^-26)^.5 Which is approx 68866 ^ .5 Thus V is approx 262 m/s. 0.25 x Escape Velocity for Iapetus is about 148 m/s for Triton it’s 364 m/s. So far so good, the rule of thumb says that Triton can keep a Nitrogen atmosphere at my arbitrary 77.35K for that length of time, and Iapetus can’t. But then this gives us a special case for the Atmosphere at the present day, whereas vogon13 just wants it to hang around for a bit. Does the more complex formula have a time factor in it? I added two lines to the bottom of the spreadsheet for these two values. If anyone wants a copy they could msg me. To appreciate the rule of thumb aspect, unless my sums are out again, I calculate that our Moon could hold on to a N2 atmosphere at up to 396K.

thalion · Feb 24, 2005

Welcome. <img src="/images/icons/smile.gif" /> I also edited the post to show that T = degrees Kelvin. For some reason the book didn't mention that; I guess it was assumed. Actually, the rule of thumb shouldn't be followed too closely. It only gives you a mean velocity--there will be some particles moving more quickly, and these are a significant factor in atmospheric escape, even though they don't represent the average motion of the molecules. From the same page in the book: "In any molecular species there is a distribution of speeds. For example, in the Martian exosphere, the mean hydrogen velocity would be about 2.3 km/s, about half the 5 km/s escape velocity of Mars. Nonetheless, a certain fraction of H atoms would mover faster than 5 km/s, facilitating a slow but steady escape." Hartmann also writes that we can't neglect the importance of photodissociation, solar wind erosion, and the diffusion of molecules in an atmosphere by weight (with heavier molecules at the bottom and lighter ones at the top). He gives another equation (again, on the same page), originally devised by Sharonov (1958): "[Sharonov] concludes that the ability to retain an atmosphere might be called the relative atmospheric retentivity, R, which is define as the ratio: R = v_esc / V (proportional to) v_esc*a^.25 where a is the distance of the planet or satellite from the sun." The other factors in the equivalency are as in the earlier equation. A table on the following page shows that Titan has only twice the relative retentivity of the Moon. Interestingly enough he also includes Iapetus in the table, and shows that it has only half the retentivity of our own Moon. The other equation does include a time element--specifically, the time to reduce the atmospheric density to 1/e of its original value. However, he cautions that this is for an "idealized isot

vogon13 · Feb 24, 2005

Appreciate your postings on our favorite little moon. You and j8hart seem to be elements of a distributed computing system. Way cool. As mentioned before, I 'do' math only under dire emergencies. But admire others with 'the knack'. Before I forget, there is a new Iapetus thread in Free Space, "Hoagland decides Saturn's moon, Iapetus is ship!" so we may want to visit periodically and see what's up in that forum. If Art Bell is 'doing' Iapetus show topics we may be getting a few more visitors in this thread, too. Should we spruce up, a little? You know, go back and edit out misspellings, revise wording to take on a more scholarly tone? Get some cute little animitroids acting out in the margins? Trendy state of the art graphics? (just kidding, this thread looks fine) <img src="/images/icons/laugh.gif" /> <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 24, 2005

Some thoughts on the brown stuff: (just thinking out loud, no strong feelings about any of this, see if it ignites some ideas) What if............ Methane or methane entrained materials are present in the ridge material? If you buy into the external origin possibilities, chemistry of ridge material could be pretty different from rest of Iapetus. Over time, methane leaks (oozes?) out of the ridges surface. Since Iapetus is exposed to sunlight, methane starts to tholinize as it briefly hangs around the Iapetus area before dissipating into space. Tholiny (or polymerized) goo starts accumulating on surface. Since there is not the recirculation phenomena we have in Titan atmosphere, we don't get orange goo, it continues to darken in the solar UV and turns brown/black (my monitor shows Iapetus pictures as dark brown but I will not vouch for its color fidelity). As the methane oozes (flows?) (wafts?)across the lonely alien surface of Iapetus, (before it is zorched into the limitless void) it lingers longer in the low spots, all the while slowly turning into brown goo. Since there is always a little methane working its way out of the surface along the ridge, we get brown all the way to the top. Since the methane gets to breifly hang around before evaporating into space, it seeks the low spots. Some low spots are pretty low and get more goo, some low spots (like the one on the northern edge) are rather more shallow and don't 'pond' as much gas. As gas drifts north and south, could we expect the occaisional vertically plunging surface to never see any goo emplaced? Can Cassini 'sniff' out anything during a future flyby? This is a pretty subtle phenomenom, maybe I'm inspired by how fog can move around and frost out in valleys in the area were I live during the winter. How am I doin' here? Going out on a service call, will contemplate while inducing deafness with some DEVO. Back in a couple hours. <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>

thalion · Feb 25, 2005

Thanks much, Vogon. I edited the second equation, as I got the symbol wrong.

vogon13 · Feb 25, 2005

Today's essay starts with a little problem I noticed with the ring orbital decay scenario that is under discussion here. I've been in increasing panic over the last several days, and this is the rub: Let's say we have Iapetus (without the equatorial ridge yet, gliding serenely through space) and the majestic hypothetical ring system around Iapetus fills the sky. For whatever reason, solar wind drag, perturbations, atmospheric effects, rogue magnetics, the orbits of the ring particles start to decay. As time passes, that lowest particle gets closer and closer to Iapetus, spiraling in all the time. Slowly, but relentlessly the surface beckons to that first particle to hit. But were does it hit? The highest spot on the equator, the point furthest from the center of Iapetus. Then what happens? The highest point on the equator of Iapetus is now a little tiny bit higher. Then what happens? Piece number 2, smacks piece number 1. You see where this is heading. You get a pile, not a long ridge. Errrrfffffff! But let's think about this some more. What else happens when that first piece hits? The center of Iapetus moves! Not much, granted, but recall a ring around Iapetus is centered on the center of mass of Iapetus, one piece stops on the surface, Iapetus is no longer balanced as before, it now has a small piece of ring debris on one side, the new center of mass of Iapetus is slightly raised in the direction of that first piece on the surface. Next piece to come down clears first piece, and so on and so on. You don't get a 'pile'. You get a 'ramp' or as we have been calling it, a ridge. This ramp just keeps getting bigger and bigger. This is why it's so tall on one end and tapers to nothing in those Cassini pictures. Is it symmetrical? I dunno, and I don't care, almost burned out brain getting it this far. For explanatory purposes, I have really, simplified exactly how this <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 26, 2005

And on way home tonight contemplated some more about Iapetus...... Anyway, also need to consider effect of the big ridge (or ramp) on crust of Iapetus too. If Iapetus is liquid inside and crust not real rigid, you will start to see some deflection under the increasing load, and now if I could 'do' math would start calculating modulus of elasticity for an icy crust of unknown thickness with a mostly unknown composition. But I can resist this urge. Suspect 3D pix from Cassini will be very helpful in assessing 'sag' from weight of the feature. Might be helpful in analyzing low spots for some of the 'staining' scenarios regarding the leading hemisphere dark spot, 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>

bobvanx · Feb 26, 2005

When I first floated the theory of a collapsing ring emplacing the Iapetus ridge (yes, it was me! you can see it in the time stamps <img src="/images/icons/smile.gif" /> ) I'd thought Iapetus would be sweeping up Saturn ring material. The gravity gradient is still pronounced, even out as far as Iapetus. Saturn's gravity will rule any particles near Iapetus. I'm starting to wonder, now, if perhaps there would be a small compact ring around Iapetus, as it stole material from the larger Saturnian ring. As it moved through Saturn's ring, material from the higher Saturn orbit and the lower would both get pulled in and do a complex little orbital dance. If the ridge completely circumnavigates Iapetus, then an Iapetus ring infall is indicated. If it's markedly more prominent on the leading edge, then collection of a Saturn ring is indicated.

vogon13 · Feb 26, 2005

--are there prefered regions on the moon where objects in low orbit are more likely to come down?-- THE HIGH SPOTS YOU DUMMY!!!!!! Cripes that is sooooo funny, very polite of the rest of you not to point that one out to me before I caught it. I think I wet myself when I re-read that post just now. Interesting how looking over this thread gives me a weird view of how I work on problems. Back to the story.... Another clarification, j8hart has been focusing my attention on scatter effects, that is deflection of descending ring debris perpendicular to direction of motion. Am finally seeing significance of his point. In the piece #1 and piece #2 discussion above, obviously, piece #2 isn't likely to hit piece #1, but if there is an area where the pieces are congregating, then this ramp building process can get started a little more assuredly. Eventually, an incoming piece has no choice but to hit a piece that has already come down, and then you have started the ramp building process. Does ramp build both upstream and downstream? More than one ramp? I still dunno. Suspect those 'mountains' south of Basan are 'piles' where scenario was disturbed/disrupted. Probably at the point where a computer simulation is going to be needed to flesh out everything else now. But we should know enough now to write sim code to generate graphs that look like Iapetus. Start with sphere, throw in some surface irregularity, and orbiting material, accurately model gravity field as ring deposit accumulates, fudge factor some surface rigidity estimates and iterate, iterate, iterate. For extra credit, smack sphere with a modest impact from time to time with a Monte Carlo subroutine. Put some of the ring material in slightly inclined orbits to see how that modifies appearance of finished simulation. This does not seem too difficult to me. But I don't do math.<img src="/images/icons/laugh.gif" /> <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 26, 2005

Would this imply a 'comet type orbit' of Iapetus in its' early days that then circularized out at its present distance? Or 'Saturn style' rings (around Saturn) at Iapetus's distance from Saturn? ( its' complicated talking about possible multiple ring systems around multiple objects!) <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?

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