On a ring origin of the equatorial ridge of Iapetus Options

[Guest_AlexBlackwell_*]

Wing Ip just had an interesting Iapetus-related paper published in GRL.

[hendric]

I still don't see how the two divergent ridges get created by a decaying ring. The orbital speed of the ring at low altitude would not be the same as the rotational velocity of a proto-Iapetus. So, how do the angled ridges get created?? If the ring was at an angle to Iapetus, as it descended it would not stay above a single location.

I favor a tectonic explaination. We have symmetrical ridges here on Earth due to seafloor spreading. Seems like a reasonable explaination to me. The question would be why would there be a single crack along a great circle? Maybe Iapetus had a Europa-like episode with a shallow ocean, and as it slowly froze it expanded/contracted enough to crack open. Without nearby moons and tides, the crack went around the planet symmetrically.

[TritonAntares]

37th DPS Meeting, 4-9 September 2005
Session 47 Icy Satellites
[47.08] The topography of Iapetus' leading side

B. Giese (DLR-Institute of Planetary Research, Berlin, Germany), T. Denk, G. Neukum (Institut fur Geologische Wissenschaften, Freie Univ. Berlin, Germany), C. C. Porco (Cassini Imaging Central Laboratory for Operations, Space Science Institute, Boulder, CO, USA), T. Roatsch, R. Wagner (DLR-Institute of Planetary Research, Berlin, Germany)
We have used Cassini-stereo images to derive a topographic model of Iapetus' leading side. The model reveals that Iapetus has substantial topography with heights (referenced to a 747 x 744 x 713 km ellipsoid (Thomas et al., in preparation)) in the range of -9 km to +15 km...

Hhm, this strange ellipsoid figure could be a hint for some internal force once having driven a probable tectonic mechanism and causing the ridge...

[ngunn]

Hhm, this strange ellipsoid figure could be a hint for some internal force once having driven a probable tectonic mechanism and causing the ridge...

. . or the fossil tidal bulge produced by a close-orbiting subsatellite. I notice nobody else fancies this origin for the ring material - is it ridiculous for some reason that I have failed to notice?

On a philosophical note: Assuming the sceptics are wrong (and I leave that one to tasp) I think we are responding here to a real eureka moment in planetary science, perhaps on a par with Vine and Matthews. When we study the planets we have become accustomed to looking for evidence of mantle convection or crustal plate movement. Now, for bodies with primitive surfaces at least, we will always have to ask the question 'Is there any evidence of a former ring?'

[tasp]

. . or the fossil tidal bulge produced by a close-orbiting subsatellite. I notice nobody else fancies this origin for the ring material - is it ridiculous for some reason that I have failed to notice?

I have spent much time contemplating binary satellites and small moons of moons.

(keeping in mind I am not smart enough to do the math for any of the stuff I think up)

Perhaps moons do form (sometimes) attendent satellites as they accrete. What happens to the little guys? We do not see them in this epoch, after all.

(I hasten to add, I expect the outer asteroidal satellites, especially the retrograde orbiting ones, to be binary in similar percentages to the presumed source populations for such bodies)


I suspect small attendent moons that might form are attrited by several processes. Incoming impactors headed for the primary moon might collide and disrupt such objects on their way in. Gasses released during the impacts might also form temporary atmospheric drag effects to lower the little moons enough to contact the surface of the primary moon, this will also delete them.

Tidal effects of the primary moon as it accretes will intensify, and small moonlets around the Roche limit may be disrupted, also keep in mind, moonlets will not form inside the Roche limit anyhow.

Other tidal effects may effect moonlets above the synchronous rotation altitude. As we see today, earth's moon experiences a tidal effect that is slowly lofting the moon away from earth. In the past, with the earth's moon circling at a lower altitude, this effect was stronger. For a moonlet just above the synchronous altitude, such forces would be as strong as they are going to get. (here's where my math impairment really hurts). Would such forces loft the moonlet out of the moons Hill sphere within the age of the solar system? Much less than the age of the solar system?


I dunno.



Do I suspect such bodies are still around waiting, to be recognized as such?

Yes.


I feel Hyperion was a former moonlet of Titan. Its' amazingly battered surface recording the flux of accretors heading towards Titan as it formed we see today. Titan was able to 'spin off' Hyperion out of it's Hill sphere, but Hyperion remained in Titan's vicinity and eventually wound up in a 4:3 resonance with its' former host.


Such objects as Methone, Calypso and the rest of the small moon Trojans may also be such former moonlets. They having been 'spun off' long ago, their low speed as they emerged from the moons Hill sphere having allowed them to be captured into a Trojan relationship with their former hosts.



( I am aware Occam's Razor would indicate that since we don't see moonlets today, the moonlets probably never existed, but we do have some interesting bodies about, (Hyperion, Methone, Calypso, et al) and perhaps an open attitude as to where they came from might be warranted. )



So back to your question, it is possible moon formation never or very, very rarely generates a subsidiary body, but it is also possible that such objects do form in significant numbers, but they are subject to forces that either destroy them, or moves them around sufficiently that we do not percieve their origins . . . .

[ngunn]

I have spent much time contemplating binary satellites and small moons of moons.

Me too - and keep at it, you're doing a great job! The answer to the question 'Would such objects have been more common in the early solar system than they are today?' is pretty obvious. Just like potential impactors that have not yet been 'swept up' they would be much more numerous. The solar system is inherently chaotic (since it consists of more than two bodies) so every kind of object has a 'half-life'. Science is about imagining scenarios as well as doing the maths, and we can help here.

Any sceptics left? or are they all too busy straining their necks to see into a big hole in the ground . . .