Origin of Iapetan orbital inclination Options

[dvanavery]

First time post from a long time lurker!

I am interested in what (if any) consensus there is among planetary scientists on the origin of Iapetus's current orbital inclination, or if there is no consensus, what plausible theories are out there. Although there has been much speculation on the origin of the various strange surface geologic features of Iapetus, there seem to be very few detailed theories on how it arrived in the ORBIT that it currently follows. (If this has been hashed out before on UMSF, you can just post a link to the relavent thread)

What needs to be explained:

The 15.47 degree inclination to the plane of the rest of the Saturnian system, which is by far the highest inclination of any regular satellite in the Solar System. It seems impossible that Iapetus could have accreted in an orbit with this inclination.

The unusually large distance from Saturn compared to the rest of the major satellites. The orbit of Iapetus has a semimajor axis nearly three times that of Titan's, and nearly 2.5 times that of Hyperion, and there are no known intermediate bodies (of any size) in the vast space between it and the next nearest neighbor. Again, this is not what we would expect for any normal accretion model. (this kind of gap is also not seen in the Jovian or Uranian systems, which feature more regularly spaced satellite orbits)

An orbit that is both prograde and has a low eccentricity, neither of which seem plausible in any likely capture scenario that could explain the orbital inclination. (such as Phoebe, or Triton & Nereid at Neptune)

The lack of any known current or past gravitational resonance with any extant major satellite, which might be used to explain its current inclination. (For example, Miranda at Uranus has a notable 4.2 degree inclination, but this is very likely due to a past period of 3:1 resonant forcing with Umbriel)

In short, Iapetus seems stranded in an orbit that it has no right to be in. How did it arrive in this orbit??

It has been proposed (via a link in an old post here at UMSF which I can't find at the moment) that faint long range gravitational interaction with Jupiter can explain Iapetus's inclination, probably helped by Saturn's relatively weak gravitional control at Iapetan distances. Has this been modeled in any detail?? At what distance from Saturn do Jovian gravitational effects become strong enough to influence sat. orbital inclination?

Another proposal by Wing Ip
http://www.agu.org/pubs/crossref/2006/2005GL025386.shtml
(which was linked to in this thread
http://www.unmannedspaceflight.com/index.p...ost&p=66049
and seeks mainly to explain the equatorial ridge) suggests that a giant impact with Iapetus may have knocked it into its current inclination, and possibly formed a ring which later gave rise to said ridge. It seems to me that any impact energetic enough to change the orbital inclination by 15.5 degrees should totally disrupt the satellite and disperse the debris too widely to re-accrete.

I am asking this because I have been thinking over a little theory on how Iapetus ended up where it is today.........and it's one which I've never seen any one else propose. Just wanted to see what alternative theories are out there before making a fool of myself by posting mine.

-David Van Avery

[JRehling]

First time post from a long time lurker!

Welcome!

I wonder if the question isn't: Why are all of the other satellites in circular orbits in the plane of their primary? Surely tidal influence is a major factor, and tidal influence varies with inverse cube of distance from primary. Iapetus is 9 times the distance of Earth's Moon from its primary, and 1/9^3 = 1/729. Factoring in the moons' radii and the primaries' masses, you still expect roughly 1/10 the "regularization" going on at Iapetus that you see with Earth's Moon. So maybe the answer is: Why would it be otherwise?

A good question keying on the putative independent variable would be why Iapetus is so far away -- about 1.8 the distance of Callisto from its primary (3x Titan's distance). Maybe here, too, there's no great mystery. SOME satellite has to be farthest from its primary, and a factor of three doesn't seem too wild: Jupiter is three times Mars's orbital distance, and maybe we shouldn't think of interorbital distances a factor of a few apart as such an odd thing. Stuff tends to be spaced out in space.

[tasp]

Someone else here (sorry, don't recall the userID right now) pointed out the plane of Iapetus' orbit about Saturn is more co-planar (as opposed to less) with the ecliptic.

Curiously, the same thing is true of earth's moon.

For either satellite, I do not know the long term stability of such an arrangement. Perhaps over the eons the plane of either moon's orbit rotates about and at times the angle to the ecliptic exceeds the axial tilt as opposed to this epoch when both are less.

(BTW, having the earth's moon orbital plane tilted 28 degrees to the ecliptic rather than 5 degrees (think I got that right) would certainly make for some interesting views of our moon in unusual constellations.)

[dvanavery]

OK, here is my idea on how Iapetus got 15 degrees of inclination.

This model applies to the very early history of the Saturn system, within a few tens of millions of years of the formation of the Solar System. It is NOT intended to explain any of the features seen on the current day surface of Iapetus (although it could suggest explanations for some of them), only to explain the orbital characteristics of the satellite. This is loosely based on (my understanding of) the theories of the formation of the Earth-Moon system, the formation of the present day Uranian system, and computer simulations of the dynamic evolution of the outer solar system and Kuiper belt. Be warned, there is a lot of arm-waving speculation, and absolutely no hard math to back any of this up....

What Might Have Happened:

1. It is 4.45 billion years ago (or so), and Saturn is nearing the end of its primary accretion (but has not yet
reached full mass). It possesses an accretion disc from which a regular satellite system is forming. Saturn's axial tilt (and the tilt of it's satellite system) was at this early time much closer to the plane of the ecliptic than it is now. What we now call Iapetus was then forming in the outer regions of this disk, other than being outermost there is nothing dynamically unusual about it. Interior to this proto-Iapetus were a number of other major satellites, which may (or may not) have been similar in size and orbital configuration to the current system. All of these satellites, including proto-Iapetus, shared the same orbital plane.

2. A large (at least several Earth masses, possibly more than 5 Earth masses) planetary body is perturbed into an orbit which approaches Saturn. This could have been a protoplanet that formed between Jupiter and Saturn, or Saturn and Uranus, or possibly something previously orbiting beyond Neptune which was scattered inward. It would obviously be a mixture of rocky and icy material, possibly with a substantial gas envelope, and would be fully differentiated. Since orbits between the gas giant planets are dynamically unstable on very short time scales, this planetary body was bound to either impact something or be ejected from the solar system. It impacts Saturn in this case, in an off-center collision which tips Saturn's axial tilt by 15-20 degrees.

3. The debris plume from this impact, combined with the gravitational interference from the infalling impactor and gravitational torques from the asymmetric mass distribution immediately after the impact, disrupted or ejected whatever inner satellite system Saturn posessed at the time. We know from models of the impact that created Earth's Moon that the gravitational environment in the immediate aftermath of a giant impact is extremely complex. Whatever inner satellites that were not destroyed outright by collisions with the plume (or with each other) are gravitationally flung out of the Saturn system entirely.

4. Proto-Iapetus, being the furthest out from the action in the inner system, is able to “survive” the impact and its aftermath, to become the satellite we see today. It is neither permanently disrupted by collision with debris from the inner system, nor gravitationally removed from the system. It is probably severely battered by debris from the inner system, and possibly totally disrupted several times, but is able to re-accrete. If it managed to avoid disruption from inner system debris impact, this might explain the unusual concentration of giant impact basins found on the satellite. More importantly, Iapetus is far enough out that it is able to maintain (at least to some degree) it's former orbital inclination, which was once coplanar with the rest of the original Saturn system, and closer to the ecliptic plane than the current inner system.

5. Back at Saturn, the planet has just suddenly and violently gained ~5 Earth masses, and there is a tremendous amount of debris in orbit. Some of this is from Saturn itself, some of it is disrupted impactor material, some of it is material from the former inner satellites. Depending on how early in Solar System history this event happened, there may even still be material infalling from the Solar nebula. All of this debris is gas and dust, and is rapidly forced into an equatorial orbit due to Saturn's oblateness, which may have been extreme in the aftermath of the impact. This forms a new accretion disk, from which the modern Saturn satellites (Mimas-Titan, plus possibly Hyperion) form. Saturn is probably radiating an enormous amount of impact-generated heat, which might have some interesting effects on the evolution of the second, inner satellite system..........but that's a separate issue.

In this scenario, Iapetus is a fossil of orbital dynamics. As the only surviving refugee from the original regular satellite system, it retains at least some degree of the inclination of that system. It's great distance from Saturn, combined with the relatively compact nature of the second regular satellite system relative to the original one, is what has allowed it to maintain this inclination to the present day. If there were any other survivors from the original system closer in, they would have been forced into the plane of the second accretion disk........and would be difficult or impossible to recognize on dynamical grounds.

So that is the general idea. It is based mainly on what is thought to have happened at Uranus, with the following differences: The Saturn system was massive enough relative to the impactor to avoid being cranked past 90 degrees in axial tilt (the change was probably have been 15-20 degrees, or similar to the inclination difference between Iapetus and the rest of the modern system). Also, unlike Uranus, Saturn would have had an original satellite system that was large enough that an outer member was able to survive the demolition of the inner system. The Uranian impact probably disrupted the entire system of moons then present, and what we see now is derived totally from the ejecta from that impact.

We “know” from simulations of the dynamic evolution of the outer Solar System that all four giant planets scattered many billions of objects in all directions, their orbits migrating (though angular momentum exchange) substantially relative to each other in the process. Many models of this formation period suggest that Earth mass or larger objects were formed in the outer Solar System alongside the four giant planets, and would have had a very good chance of interacting with or impacting any of the four. Models also suggest that the orbital migration might have briefly put Jupiter and Saturn into a 2:1 orbital resonance, which would have had drastic destabilizing effects throughout the outer Solar System. So if Uranus could have been turned sideways by an Earth sized impactor on a chaotic orbit, why not propose a similar impact scenario for Saturn? Giant impacts have been proposed, with varying degrees of success, to explain features of every planet in the Solar System, with (I think) the exception of Jupiter and Saturn. I'm just taking an existing model (Uranian impact), and seeing how it fits on a different planet, with an eye towards explaining one of the really odd dynamic features of that planet's regular satellite system.

I've already though of a number of flaws in this scenario........and there must be a lot of specific predictions that such a model makes about the evolution of the inner satellites as well........let me know what you all think of it.

-Dave V.

[JRehling]

Besides the idea of ejecta, some elements of this seem feasible. I'm doubtful that an impact striking Saturn could produce ejecta at Saturn escape velocity -- it's an enormously massive planet.

Beyond that, I would think that Iapetus would have had a lot more cause to align with Saturn in the last 4.45 GY than in the first 0.1 GY. That's 45 times as much opportunity for tides to work their magic. The question is, why would they be coplanar before the catastrophe? We know that Iapetus itself would be getting thumped quite a bit in that time, and it would be much more vulnerable to orbital alteration than Saturn to rotational alteration. Moreover, I don't think the coplanarity of aboriginal bodies is that constrained within an accretion disk. Take Ceres for example (11 deg), Vesta (7 deg), Pallas (35 deg). Being so small, these bodies would be relatively immune to tidal normalization. My guess is that the various planetary systems were similarly messy just after accretion and anywhere we see lesser inclinations is either luck or the effect of tides. Iapetus is just too far out to have been pulled hither and thither by little bumps on Saturn's equator.

[dvandorn]

We're all mixing apples and oranges a bit, here. We're confusing the rule which causes orbital clouds of particles to fall into planar rings, and the full range of tidal effects on solid bodies.

AIUI, clouds of particles work into planar rings because they eventually all impact each other as they cross through the equatorial plane. The net effect of all of these impacts is to reduce the out-of-plane vectors and neatly sort the particles into thin equatorial rings.

The effect of tides on larger bodies is mostly to lock the small bodies into a primary-facing orientation. Tidal effects have locked Iapetus into having one side that always faces Saturn, and yet those same tidal effects have not changed its orbital inclination to one closer to the rest of Saturn's moons. Perhaps tidal effects can make closer-in moons orbit in a more planar fashion, but with outer moons they seem to have far less effect.

I believe that the tendency to flatten a cloud of debris into the same plane is dependent on the density of the cloud. As you move out from the Sun, the planets are all in the same general plane (the ecliptic), but bodies formed in the much thinner outer regions of the original solar nebula, where planar collisions were much rarer, diverge greatly from the ecliptic.

Just the same, gas giant satellites that formed in the outer portion of their planetary nebulae would undergo fewer planar collisions and you'll see greater and greater divergence from the planetary nebula plane with distance. And for satellites, you not only have effects from bodies in the planetary nebula, you also have influences from nearby planets (especially big ones like Jupiter and Saturn).

I think it's very likely that all of the major bodies in the solar system originally had very little divergence from the ecliptic, having all formed out of a planar solar nebula. But the final stages of the accretion process generated all sorts of odd momentum transfers. One planet nearly stopped turning on its axis entirely (Venus), one was knocked onto its side (Uranus), and several other planetary axes were knocked from five to 30 degrees off of the ecliptic.

We need to think of the current state of the Solar System and its objects as the pretty-much-final configuration resulting from the chaotic final phase of accretion. The system remains dynamic, but accretion has been finished for all practical purposes for more than four billion years. Solar System bodies have had the same ranges of inclination, the same divergences from planar, etc., for many billions of years, and unless a new set of bodies invades the system, it's liable to stay relatively stable for several billion more.

-the other Doug

[tasp]

Not sure where I picked up this idea, and my apologies for mangling it in any regard.

It was my impression that as the earth's moon tidally recedes from earth there is an effect from solar tidal effects to dampen our earth's moon's motion out of the plane of the ecliptic. So that had the moon started off in an orbit about earth in earth's equatorial plane (and therefore tilted to the ecliptic ~23 degrees) as time passes and the earth moon seperation increases, the moon's orbital plane migrates into a closer alignment with the ecliptic.

Perhaps something similar happened to Iapetus long ago. Or maybe, the outer rim of the proto-Saturnian nebula was warped with respect to the Saturnian equator. Maybe the event that produced the Saturnian axial tilt happened at a precise time to have only propogated out to ~Titan's distance.

I am still thinking earth's moon and Iapetus are trying to tell us something about this ancient epoch.

[JRehling]

I think it's very likely that all of the major bodies in the solar system originally had very little divergence from the ecliptic, having all formed out of a planar solar nebula. But the final stages of the accretion process generated all sorts of odd momentum transfers.

Keep in mind that things cannot become on the whole less planar as a result of collisions. If everything starts coplanar, then everything proceeds and ends coplanar.

Possible collision-based shifts in rotational axis don't share the same stage with orbital shifts in rotational axis. Something deviating only by Uranus's radius, at Uranus's orbital distance from the Sun is very close to the ecliptic, but could generate a big rotational inclination. But not an orbital inclination.

So look again at the most massive asteroids and Pallas's 35-degree inclination. Collisions are inelastic: if anything, they reduce angular momentum out of the plane of the ecliptic. There's no way to take two things in the ecliptic and have their union be way out of it. Something that made up Pallas was at least 35-degrees out of the ecliptic as of the last time that anything much bigger than Pallas last strayed through the asteroid belt.

My guess is that with the possible exception of Jupiter, the original orbital inclinations of the planets were scattered as much as the larger asteroids currently are, and then tides got things together.

One planet nearly stopped turning on its axis entirely (Venus), one was knocked onto its side (Uranus), and several other planetary axes were knocked from five to 30 degrees off of the ecliptic.

The case of Venus is probably due to the effects of the Sun on its atmosphere, completely unique in the solar system.

Uranus is a true anomaly.

[ngunn]

Keep in mind that things cannot become on the whole less planar as a result of collisions. If everything starts coplanar, then everything proceeds and ends coplanar.

Are you sure about that? Haven't planet swing-bys been used to accelerate spacecraft out of the ecliptic? Cassini is doing this all the time in Saturn-orbit. Why couldn't it happen to something larger? A glancing collision might make the process less efficient but wouldn't prevent it altogether surely?

[JRehling]

Are you sure about that? Haven't planet swing-bys been used to accelerate spacecraft out of the ecliptic? Cassini is doing this all the time in Saturn-orbit. Why couldn't it happen to something larger? A glancing collision might make the process less efficient but wouldn't prevent it altogether surely?

Yes, a small discrepancy from the ecliptic can become a big one for a small object. So Pallas could have gotten that way from a near-miss vs something Earth or maybe even Mars-sized. But whatever state the larger asteroids were in when the last planet-sized object moved through should be qualitatively the same as they're in now.

So I think it would be the same for Iapetus. There's no reason to suppose it had to start off in Saturn's orbital plane from the last moment something Titan-sized moved through. The accretion process should be no less messy than what comes afterwards. Quite the opposite.

[dvanavery]

Thanks for the comments! The model proposed has so many unknown variables that we will probably never be able to properly evaluate it. I don't expect data from Cassini or any plausible follow-on mission to be of much help either. It's still fun to speculate..........

Besides the idea of ejecta, some elements of this seem feasible. I'm doubtful that an impact striking Saturn could produce ejecta at Saturn escape velocity -- it's an enormously massive planet.

Weirdly, this is the only part of my scenario that I feel comfortable with (for now), so I would have to disagree. Especially in the case of a grazing off center collision....... we're talking about an impactor of ~ 6.0x10^25 kg (10 Earth masses) messily smearing itself through the outer mantle of a ~ 5.1x10^ 26 kg (85 Earth mass) proto-Saturn. Also, the impactor is most likely approaching Saturn with substantial relative velocity if it is on a chaotic, massively perturbed orbit from elsewhere in the Solar System.

The mass of all the modern satellites interior to Iapetus is only ~ 1.4x10^23 kg, which is about 0.0002% of the system total. It does seem likely that the vast majority of the impactor mass would fail to reach orbit after the impact, but we only need for a tiny percentage of this mass to do so in order to form what we see
today. I would imagine that an even smaller (but still substantial) amount of debris would be launched into solar orbit, or would be in such a hot ionized gaseous state post impact that it would be lost due to thermal kinetic energy/solar wind pressure/magnetospheric effects.

Again, the gravitational environment near Saturn during the few hours after the impact would have been crucial to determining how much stuff ended up where, as is the case with models of the formation of our Moon. Also, some of the mass already in orbit at the time of impact (first generation satellites interior to Iapetus) would still be there afterwards, in pulverized form. Possibly both mechanisms could have contributed material.

(arms getting tired from all the waving......)

The 35.5km/s escape velocity from Saturn versus the ~9km/s average orbital velocity at Saturn's distance does give me pause though......is it even possible to get ejecta to leave an impact site with a higher velocity than the impactor's incoming velocity?? (Also, does your potential criticism apply to the Uranian impact model? The orbital escape velocity is lower there, but so is the plausible range of impactor velocities at Uranian orbital radii.....yet at least some material made it to orbit to form moons.) Until one of us whips out a supercomputer and start modeling various impact scenarios, your guess is as good as mine!

We're all mixing apples and oranges a bit, here. We're confusing the rule which causes orbital clouds of particles to fall into planar rings, and the full range of tidal effects on solid bodies.

AIUI, clouds of particles work into planar rings because they eventually all impact each other as they cross through the equatorial plane. The net effect of all of these impacts is to reduce the out-of-plane vectors and neatly sort the particles into thin equatorial rings.

The effect of tides on larger bodies is mostly to lock the small bodies into a primary-facing orientation. Tidal effects have locked Iapetus into having one side that always faces Saturn, and yet those same tidal effects have not changed its orbital inclination to one closer to the rest of Saturn's moons. Perhaps tidal effects can make closer-in moons orbit in a more planar fashion, but with outer moons they seem to have far less effect.

Thanks for clarifying this! My original post was not very clearly worded in this regard.
The tidal effects I was thinking of was actually the gravitational effect of the oblateness of Saturn (the “equatorial bulge”) acting on an orbiting body inclined to the plane of the planet's oblateness. Let's say you have model planets of identical mass, one of which is a barely rotating, near perfectly spherical body, and the other has an oblate “bulge” due to rapid rotation. You place an identical satellite (or ring) in an identical inclined orbit around each model. The two satellites (or rings) are going to have very different orbital evolutions over time, namely the one orbiting the oblate planet will end up eventually in a noninclined orbit due to the gravitational attraction of the bulge. This effect is going to be stronger the closer you get to the planet, and will also be stronger around highly oblate planets than around more spherical objects. I was thinking that Saturn's oblateness might have been extreme enough in the early history of the Solar System (especially after an impact) that anything in an inclined orbit would be forced down into an equatorial one by this mechanism alone..........at least out to a certain distance. Now I am wondering if this effect would have been strong enough to operate out to Titan orbital radii. It might not be! Also, the total angular momentum of the present system probably places strict upper limits on how oblate Saturn could have been in the past.

The effect you mention from inclined particle collisions while crossing the equatorial plane is probably at least as important, and I didn't even consider it!

Thanks again....

-Dave V.

[IM4]

Just one remark.

Iapetus orbit inclination is NOT equal to 15.5 degrees. It varies greatly, from 6 to 24 degrees, with period of ~3000 years (I used data from this paper ). So the main question is not "Why Iapetus has such an inclination?" but "Why Iapetan orbit inclination varies so much?". There are times when its orbit plane closes to those of other saturnian satellites (as small as 6 degrees) and times when it very close to ecliptic. I suppose that answer lies in the field of celestial mechanics: secular resonanses or something.

[nprev]

THAT's odd...thanks, IM4!

This may be a simplistic perception, but I tend to think of extreme variables in orbital properties as artifacts of earlier chaotic events such as encounters with massive objects or capture. In this view, the entire Saturnian system seems to be reeling still from such an ancient event (perhaps the same one that created the rings?), and Iapetus may indeed be a captured object that is slowly being brought into the fold from tidal effects. (Visual analogy: ever seen a hula hoop hit the ground & gradually after a few looping inclination changes as it spins settle to the ground?)

[tasp]

Thank you for the link to the paper.

On first reflection, should the areal extent of Cassini Regio on Iapetus be amenable to computer modeling via a presumed mathematical relationship between insolation, local slope, elevation, latitude and longitude, then a further factor that should be considered is changes in the Iapetan orbital inclination about Saturn.

My first thought is that variations in inclination will tend to 'smear' the northward and southward extent of Cassini Regio beyond what might be expected for an Iapetus with a constant inclination.

Curiously, I suggest, that effect is seen in the 12/2004 images.

[dvanavery]

Just one remark.

Iapetus orbit inclination is NOT equal to 15.5 degrees. It varies greatly, from 6 to 24 degrees, with period of ~3000 years (I used data from this paper ). So the main question is not "Why Iapetus has such an inclination?" but "Why Iapetan orbit inclination varies so much?".

Wow. I have never heard of this! It's certainly never mentioned outside of obscure abstracts.........thanks for pointing it out. You are probably right, this periodic variance in inclination suggests Iapetus is currently responding to some kind of resonance with an object outside of the Saturn system (almost definately Jupiter). I wouldn't say it rules out an impact scenario, but it certainly is a strong alternative. It might be an after effect of planetary radial migration, with Jupiter and Saturn not originally being in this resonance, and Iapetus originally being coplanar with the other satellites. Once the two planets drifted into a resonance (due to angular momentum transfer, which would have ended early in Solar system history), Iapetus got perturbed into its inclined orbit, and has been oscillating with a 3000 year period ever since. This has a distinct advantage over my idea in that it can be modeled fairly easily with a computer.

THAT's odd...thanks, IM4!

This may be a simplistic perception, but I tend to think of extreme variables in orbital properties as artifacts of earlier chaotic events such as encounters with massive objects or capture. In this view, the entire Saturnian system seems to be reeling still from such an ancient event (perhaps the same one that created the rings?)

The rings are unlikely to be symptomatic of system-wide chaos. All you need is a single unlucky hit from a smallish Centaur into a Janus-sized inner satellite to form them. Proximity to Saturn and resonances with the other 7 major satellites keeps the mess from re-accumulating. Mimas might have been pelted with some of the debris (Herschel, anyone?), but I doubt there would have been any long term effect on stuff further out. This could have happened at any time in Solar System history, and possibly happened more than once. That's one of the things that makes Iapetus so odd, everything else in the regular saturn system has very mundane orbital behavior. IM4's link suggests that even Iapetus's orbit could be explained by a non-catastrophe.

-Dave V.