Did Iapetus Collide with a Primordial Saturnian Ring?

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Discussion Overview

The discussion revolves around the hypothesis that Iapetus, a moon of Saturn, may have collided with a primordial Saturnian ring, leading to its unique features such as the dark hemisphere and the equatorial ridge. Participants explore the implications of this model, the nature of Saturn's rings, and the evolution of the Saturnian system.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • Some participants propose that the dark hemisphere of Iapetus and its equatorial ridge are the result of a collision with a primordial Saturnian ring, possibly linked to a change in Iapetus' orbit.
  • Others argue that Saturn's rings are not permanent and may erode over time due to tidal forces, diffusion, and Poynting-Robertson effects, suggesting that Saturn could have had more rings in the past.
  • A participant questions how long it would take for a significant ring to disappear, indicating that the answer depends on various perturbations, particularly collisions with moons like Iapetus.
  • Another participant suggests that the rings may have formed from leftover material during the formation of the Saturnian system, with a lifetime potentially exceeding 5 billion years, though this is debated.
  • One participant expresses skepticism about the model requiring multiple unlikely events for Iapetus to have its current state, indicating a lack of acceptance of the proposed explanation.
  • Another participant notes that all of Saturn's regular satellites, except Hyperion, are tide-locked, questioning the assumptions about Iapetus' tidal locking in relation to the model.

Areas of Agreement / Disagreement

Participants express multiple competing views regarding the formation and evolution of Saturn's rings and Iapetus' characteristics. The discussion remains unresolved, with no consensus on the validity of the collision hypothesis or the implications for the Saturnian system.

Contextual Notes

Participants acknowledge that the lifetime and evolution of Saturn's rings are subject to various factors and perturbations, which remain open to debate. The assumptions underlying the models presented are not fully resolved.

wolram
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http://arxiv.org/PS_cache/astro-ph/pdf/0504/0504653.pdf

Title: Solving the mystery of Iapetus
Authors: Paulo C. C. Freire
Comments: Submitted to Journal of Geophysical Research - Planets

Since the discovery of Iapetus by G. D. Cassini, in 1672, it has been known that the leading hemisphere of this Saturnian satellite is one order of magnitude darker than the trailing hemisphere. Since the Cassini spacecraft entered the Saturnian orbit, several high-quality images of the dark hemisphere of Iapetus have been obtained, in particular during the Dec 31 2004 flyby of this satellite. These images revealed the presence of a large equatorial ridge in the dark hemisphere of Iapetus. We propose that this ridge and the dark coating of the hemisphere on which it lies are intimately interlinked and are the result of a collision with the edge of a primordial Saturnian ring, ultimately caused by a sudden change in the orbit of Iapetus. The model naturally explains all of the the unique features of this satellite; it is probably the solution to one of the oldest mysteries in solar system astronomy.

This paper discuses some of the possibilities for Iapetus orbit change
and its collision with saturns rings, but did saturn have more rings in one stage of its evolution?
 
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The rings of Saturn may not be permanent but erode through tidal, diffusion (collision) and Poynting-Robertson effects. Therefore the planet may well have had more rings in the past.

Garth
 
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Garth said:
The rings of Saturn may not be permanent but erode through tidal, diffusion (collision) and Poynting-Robertson effects. Therefore the planet may well have had more rings in the past.

Garth

Thanks Garth, How long would it take for a ring large enough to cause
these effects to disappear?
 
wolram said:
Thanks Garth, How long would it take for a ring large enough to cause
these effects to disappear?
How long is a piece of string? It depends on the perturbations that a ring suffers, in this case the most significant perturbation would be the collision with Iapetus!

Garth
 
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Garth.
How long is a piece of string? It depends on the perturbations that a ring suffers, in this case the most significant perturbation would be the collision with Iapetus!

Yes silly question, It would have been an amazing sight though," if it were
observable from earth", and we were around.
 
No not a silly question, just a rather ill determined one.

The default position would be to say the rings were formed as leftovers when the whole Saturnian (?) system was formed. They would therefore have a lifetime of greater than 5 billion years or so. However they are known to leak downwards into Saturn's atmosphere and outwards into the satellites' region.

It might be that we are seeing the system at an special time, say relatively soon after a satellite broke up through tidal forces, and normally the system looks like that of Uranus.

However it is normally assumed that that is not so and the rings are long lasting, but exactly how long is open to debate.

Garth
 
You are very gracious Garth, the evolution of our solar system may be
a model, admittedly micro, but possibly macro, that demonstrates that
an almost insignificant event can alter the system.
 
One-face moon

Moons interior to Iapetus' current position rotate. Iapetus is currently tide-locked. Why? The model requires four sequential unlikelihoods:

1) Iapetus was an interior moon for at least long enough for its rotation to synchronize with its revolution.

2) Its orbital inclination was 0 deg.

3) Something moved it into the path of the ring, or vice-versa, while maintaining Conditions 1 and 2, to provide the narrowly-defined ridge.

3) Something moved it outside tide-locking range while maintaining the tide-lock and shifting the orbital inclination by 7 deg.

I ain't buyin' it. Steve
 
Moons interior to Iapetus' current position rotate. Iapetus is currently tide-locked.
Huh?

ALL Saturn's regular satellites (except Hyperion) are tide-locked!

(Or are you making a quite different point?)
 

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