[X] My Assistant

[ngunn]

We discussed this briefly before in the "Equatorial Sand Seas" thread, posts 252 to 257, so I will understand if there are few further comments, but drawing from some recent abstracts there is now more to say so I thought I'd start a separate thread. First the neutral assessment of the issue that concluded those earlier deliberations:

QUOTE (rlorenz @ Feb 9 2008, 02:59 PM)
I dont see a strong case yet for any large-scale re-orientation (don't see a case against it either)

To summarise the issue as I see it:

The surface of Titan (like some other moons) is thought to be mechanically decoupled from the interior by a subsurface ocean. Unlike the other 'ocean' moons, Titan has highly active surface processes including fluid flow and solid mass redistribution. Deposition of material from the atmosphere is thought to favour the polar regions. We observe regional slopes, large scale lake basins, mountain chains (with preferential E-W orientation) and possible faults that could in part be maintained by migration of the entire crust with respect to the rotation axis. A testable prediction can be made: Titan's surface formations will be found to include some that originated far from their present latitude.

Now, what's new?:

http://www.cosis.net/abstracts/EGU2008/101...008-A-10189.pdf
Lorenz, R.; Stiles, B.; Kirk, R.; Zebker, H.; Callahan, P.; Radarteam, T.C.
Geophysical Results at Titan from Cassini RADAR : Topography and Spin State Overview

Note the last paragraph. Even the wind could be 'torqeing the surface around'.

Also relevant though not specific to Titan:

http://www.cosis.net/abstracts/EGU2008/013...008-A-01318.pdf
Harada, Y.
True polar wander due to surface mass loading: Interaction between rotation and deformation through pole tide

I'll leave it there for now to see if others have comments.

[ngunn]

Nice, Mike, but you may need to add another reoriention (or a few) after number 17. I'm guessing Titan's weather redistributes mass to the poles faster than ice is redistributed on Europa.

[Juramike]

Nice, Mike, but you may need to add another reoriention (or a few) after number 17. I'm guessing Titan's weather redistributes mass to the poles faster than ice is redistributed on Europa.

I had to do some mental gymnastics, but I think polar deposition will help just a teeny bit to keep the poles at their present position.

The estimated global organic deposition rate is 9.5E16 g Myr-1. Assuming it falls at polyacetylene density, this corresponds to 0.015 m Myr-1 averaged globally. (see post 270, Equatorial Sand Seas thread).

Assuming all the atmospheric organic deposition on Titan occurs at the poles (at least above 30 latitude), you end up with a rate above 0.015 m Myr-1. Assuming the North and South polar temperate regions together make up about half the surface area, but getting the whole amount of global deposition, you end up with about 0.03 m Myr-1. So this would only work out to about 60 m of stuff over a 2 billion year history.

And that's assuming it falls as flakes of polyacetylene (density 0.63 g/cm3). If it comes down as little poofballs of organic stuff that doesn't compact well, the density could be quite low indeed. For example: Ice has density 0.95 g/cm3; snow in the form of "Aspen champagne powder" can be as light as 0.1 g/cm3 or as heavy as "Cascade concrete" at 0.3 g/cm3.

So the amount of mass put on the poles by deposition would be overall pretty small. And it would be pretty fluffy at that.


[Now hopefully I understood the Diapir abstract correctly..]

The material deposited on the surface of the ice crust will be of lower density materials (fluffier than 0.63 g/cm3). Assuming some crustal bouyancy and lower rigidity, the extra weight will cause the higher density ice crust to sink a little. The net effect will be to make a surface mass deficit (lower density stuff up higher) and an interior mass excess at the polar regions. This would be a polar negative gravity anomaly. Overall Titan would be mass-bulgy at the equator, and thinner on the top (just like the typical middle-age male, sigh...).

So even if the amount of organic deposited on the pole was significant, the density factor would tend to maintain the current polar zones.


From some of the topographical data in the northern polar areas (see the Flagship mission document) it looks like there is an overall broad depression in the north polar zone of Titan. Using a similar argument, that north polar depression would tend to stabilize the Titanian poles right where they are.

-Mike

[ngunn]

[quote name='Juramike' date='May 16 2008, 11:15 PM' post='113761']


The estimated global organic deposition rate is 9.5E16 g Myr-1. Assuming it falls at polyacetylene density, this corresponds to 0.015 m Myr-1 averaged globally. (see post 270, Equatorial Sand Seas thread). END QUOTE

That's just the stuff that polymerises whilst airborne. What about methane rain that turns into sticky stuff once on the ground? Rivers flowing toward the northern lakes (or away from the southern highlands, but getting no further than Mezzoramia)?

I agree, though, the other mechanisms you discuss are interesting too.

Let's not forget either that at least in the case of Europa we could be talking about a fairly recent event, not a primordial one. IIRC, Europa's present ice crust is only tens of millions of years old.