2007-09-10 Iapetus (rev 49): Pre-flyby discussion, Closest approach of this odd moon Options

[Guest_AlexBlackwell_*]

The mission description document (2.4 Mb PDF) is now online.

[Rob Pinnegar]

The Looking Ahead page for the Iapetus encounter is now online:

http://ciclops.org/view.php?id=3730

So... on the map, what's the resolution inside the green rectangles?

[Bjorn Jonsson]

The Looking Ahead page for the Iapetus encounter is now online:

http://ciclops.org/view.php?id=3730

From the Looking Ahead page:

In addition to the SAR swaths, RADAR will also use its altimetry mode to measure the height of the equatorial ridge near 74° West Longitude (digital terrain models from the 2005 encounter suggest that this might be a location where the ridge actually becomes a low trough).

I noticed hints of this in the DEM I posted here a few days ago and wondered if it was a real feature. Since I don't have any 'official' DEMs to use as 'ground truth' to check how well my software is working I'm very happy to see that this is not something that only appears in my DEM.

EDIT: Apparently there will be plenty of stereo coverage during the upcoming flyby so more DEMs can be computed .

[volcanopele]

So... on the map, what's the resolution inside the green rectangles?

150 meters

[tasp]

Will try to clarify a few things I have been pondering.

One good picture of Voyager Mountains and I predict we will 'grok' the ridge system on Iapetus. (Cassini Regio, I am not so sure)

The Voyager Mountains will be obvious non volcanic forms. I suspect their manner of formation to be a stream of materials on ballistic trajectories from the west, accumulating at the upper elevations, and sliding/slumping as the materials accumulate. This is a new way of making a mountain, something we have not seen before.

The intermittency of the ridge west of the (<-) point common to the trifurcated portion is expected from the manner of formation. While the Voyager Mountains may be crater damaged, they will not be sections of a once continuous ridge subsequently sundered by craters.

We need to consider the 'start up' sequence of an emplacing ring system. Start with orbiting materials about Iapetus, inside the Roche limit, collapsing towards the Lapacian plane. Until the planar form is reached, the dynamical ring spreading process is not operative. The main interaction occuring in the still organizing ring system is collisions over the equator. (refer to the great diagram in TNSS, Planetary Rings chapter) The result of the collisions is to primarily reduce the inclinations of the orbiting materials. The collisions take place in a sphere around Iapetus due to the Iapetan oblateness spreading the originally 'mono' inclined ( a word I just banged up) materials 'all the way' about Iapetus. The collisions continue until all the material is planar. Once that state is achieved, angular momentum can be redistributed throughout the ring system via objects in adjacent orbits bumping. The angular momentum is redistributed such that the lower edge of the ring descends, and the higher edge ascends.

The process proceeds slowly. While the particles on the low edge are spiralling downwards, the amount of drop, per orbit is small compared to the particles individual radii.

As the lower edge of the ring descends, it inevitably comes in contact with the highest point along the equatorial ground track. This point remains fixed (NSE and W, but not in elevation) during emplacement of the ring materials onto the surface. Materials that come into physical contact with the surface are immediately condemned to 'pulverizing and sliding' out into the famous wedge ramp structure we see today.

Additionally, materials still in the ring, that have not yet contacted the high point, but that do interact with the pulverized materials that have contacted the surface just ahead and just below them, ascribe a different path than the non affected particles immediately around them in the ring do. They have suffered a small velocity decrease, but are not in contact with the surface, so they get a final elliptical orbit about Iapetus.

In the early stages of emplacement, before the highest spot along the equatorial ground track gets very high, this 'doomed' population of ring materials probably doesn't make it very far. But as the high spot gets higher, this elliptical orbiting population of ring particles can do something rather amazing. By being able to get back around Iapetus one last time, they have the opprotunity to impact other randomly located spots nearly as high as the original highest spot.

And they accumulate in these places simultaneously as the wedge ramp continues to grow. The amount of materials deposited in these secondary 'piles' depends on their height and spacing. A longer gap along the ground track will allow more materials to accrue.

The heights of all these peaks along the equator will correlate, and the emplaced volumes in these peaks will be proportional to their spacing (assuming a smooth distribution of velocities in the elliptical subset of emplacing ring particles)

Now, let the process run to an exhaustion of ring particles.

Then, after the passage of sufficient time another sub-moon crosses into the Iapetan Roche limit, or another impactor lofts another debris pile, and the ring process starts again. This time, the sub-moon is in a slightly inclined orbit and generates a ring with a similar slight inclination. Or (if that isn't feasible from Laplace's perspective) in the intervening interval, Iapetus has had a few degrees of axial tilt occur, this ring emplaces a bit differently. The highest point along the equator still penetrates the ring plane, but not continuously. It passes through the ring plane twice per rotation. Once from north to south, and 180 degrees later, from south to north. This makes (alternatingly) the perfectly matched, symmetrical, identically diverging, attendant ramps.

But now, the conditions are even more different. We are starting 20 kms up. Not at an existing random highspot along the groundtrack which might have been only slightly higher than some other high-ish spots, but something much, much higher. All the materials emplace on the (short) attendant wedge ramps. The slopes are steeper than an apparent critical angle, so now the high spot doesn't get any higher, and all the emplacement is evenly divided on the 2 attendants.

Also, materials interacting at the high spot with materials in contact with the surface, are not deceled enough to contact anything other than the highest spot along their ground track, the highest spot of the wedge ramp.

There are no 'Voyager Mountains' along the great circle paths you trace out from the attendant ridges.

[Moderator's note: Thread split. A new thread containing actual images from the flyby as well as discussion of the them is here]

[tasp]

This paper was cited in one of our Iapetus threads here:


IAPETUS (1): SIZE, TOPOGRAPHY, SURFACE STRUCTURES, CRATERS
T. Denk, K.-D. Matz, T. Roatsch, U.Wolf, R.J. Wagner, G. Neukum, and R. Jaumann
(DLR, Inst. of Space Sensor Technology and Planetary Exploration, Rutherfordstr. 2, 12489 Berlin, Germany)

And this paragraph has been bothering me ever since I read it:


• Mountains of Fig. 3: Difficult to explain;
possibly, a big icy meteorite "landed" on the surface
and simply broke into pieces instead of causing
an explosion with subsequent crater formation.



This was written prior to any thought of a ring system about Iapetus, and if any of us are thinking about rings, I suppose we look at the Voyager Mountains as a result of their emplacement.


And I do too.


However, I will pose this wild and crazy thought as to how this idea in the paper might be accomplished. I am firm in my conviction that this scenario is unlikely in the extreme, but, I can't think of any reason why such an event would be 'forbidden' other than due to the statistical unlikelihood of it happening.

In my consideration of an Iapetan ring system, I have tended to look at the various stages of the ringage emplacement in a modular fashion.

We have seen a collisional hypothesis as being advanced to explain earth's moon and Pluto's moon Charon.

OK, we can have an impactor loft materials into orbit about Iapetus.

Once such materials are in orbit, they collapse to the Laplacian Plane automatically.

Once the planar form is achieved, differential ring spreading takes place and lowers the low edge of the ring system

Inevitably, the low edge of the ring contacts the highest elevation along the equatorial ground track.

Ring emplaces onto surface, toot sweet.



But let's look at 'collapse to the Laplacian Plane' a bit closer.

The primary thing happening is collisions over the equator crushing the inclinations of all the orbiting debris.

What efficiency does this process have in retaining materials in orbit ? For the worst case, 2 equal size chunks in +/- 90 degree orbits, all the materials of both chunks fall straight down. (assuming not much of a 'splat') For a more reasonable +/- 45 degree intersecting collision, (assuming I am doing this correctly in my head) 41% of the stuff comes down while the rest stays in orbit.

In the early stages of the debris orbiting scenario, might these 'chunks' be largish?? And possibly at not too great an altitude ?? They come down to a surface with 2 1/2% e gravity.

So do we have a way of realizing (at least in theory) the idea floated in the discussion of the formation of the Mountains of Fig. 3 ??


( I am not advancing this as an explanation of the Voyager Mountains, but if in centuries to come, and we explore many moons around our end of the galaxy, might we expect to see this process from time to time ? )

[tasp]

Regarding the dimensions and 'spin down' of Iapetus, I have noted (although I don't have the precise figures handy) that Iapetus deviates from the expected triaxial ellipsoid shape somewhat.

Could we be seeing the effects of one more factor in generating the observed shape of Iapetus ?? As it spun down and cooled and locked in it's polar flattened shape, would proximity of Iapetus to a secondary orbiting body explain the discrepency ??

If we have a 'squashed' Iapetus, we rightly infer freezing then spin down preserving it's primordial form. If in this era we note a 'deformity' in the form of Iapetus, perhaps we are seeing the signature of a 'submoon' frozen in it's form too.

Not sure how accurately the distortion can be characterized, but with a few assumptions regarding distance to the secondary perhaps we could generate a range of expected masses for it.