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Titan topics @ AGU2008, Titan abstracts at the American Geophysical Union meeting
titanicrivers
post Dec 14 2008, 04:42 AM
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Titan topics @ AGU 2008

The following links originally posted by E. Lakdawalla in her Planetary Society blog of 12-8-08 takes one to the interesting poster and platform session abstracts to be presented at the AGU this week. Lots of thought provoking topics covering Titan's atmosphere and surface!
http://www.agu.org/cgi-bin/sessions5?meeti...amp;maxhits=400
http://www.agu.org/cgi-bin/sessions5?meeti...amp;maxhits=400
http://www.agu.org/cgi-bin/sessions5?meeti...amp;maxhits=400
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ngunn
post Dec 16 2008, 04:44 PM
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Ganesa Macula - just a random inkblot?

Thanks for this one Emily.

Disappearing volcano No. 2. (Better find some new ones quick.)
http://www.planetary.org/blog/article/00001771/

Let me guess the reaction here.

Phil will say it was overinterpreted in the first place.

Mike will have a new interpretation posted by midnight.


Seriously, what say all??
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Juramike
post Dec 16 2008, 06:19 PM
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In one sense the observation that Ganesa is not a dome is almost reassuring.

Since it is circular shaped and doesn't seem constrained by the existing mountain range, likely candidate processes would be impact, a sunken cryocaldera, some sort of collapsed salt-dome equivalent, or sunken diapir blob-thingy, or something really, really funky.

I see a similarity between Mabon Macula, Ganesa Macula, and Menrva:
All three have roughly circular shaped interiors
All three have a RADAR-smoother deposit in the interior region
All three have streams that appear (from the branching pattern) to enter the feature from the outside and flow towards the center (one of the streams of Mabon supposedly drains to the S; I'd hazard that two of the streams in Ganesa flow inwards (at 3 and 9 o'clock) and one might flow outwards (at 5 o'clock))
All three have a RADAR-rougher central region, with a depressed area around this point (from streamflow)

Both Menrva and Ganesa have some sort of bright RADAR alluvial outflow removed from the feature. The source of these both seem to come up from chaotic terrain.

If I had to speculate wildly, I would hazard that all three features are impact features, in various stages of relaxation/burial/erosion.
The central peak could be the remnant of a central peak crater, or it could be something pushed up from a subsurface source activated by the impact. (Like a cryo-plug dome). From the appearance (or lack thereof) of a surrounding rim, I'd further speculate that Mabon Macula is the oldest, with Gansa, then Menrva. (Menrva still shows a nice rim)

Here's a side by side comparing an image of Dilmun Crater to Ganesa Macula:
Attached Image


-Mike



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volcanopele
post Dec 16 2008, 06:49 PM
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Or maybe it is just a circular doo-hickey. The human brain is very well suited for finding shapes in what is otherwise a chaotic pattern. Here at Ganesa, we see something that, if we connect some of the features in the region, appears to be circular.

I think the comparison to Mabon is apt. At the center of Ganesa we see a patch of hilly terrain with channels flowing out from these hills. But there is no evidence for a rim and morphology of edge of Ganesa is much more varied and inconsistent compared to known craters of that size on Titan, like Afekan and Guabonito.


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titanicrivers
post Dec 17 2008, 07:07 AM
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[quote name='ngunn' date='Dec 16 2008, 10:44 AM' post='132761']
Ganesa Macula - just a random inkblot?
Seriously, what say all??

I may have missed something but the abstract of Kirk et al “A Three-Dimensional View of Titan's Surface Features from Cassini RADAR Stereogrammetry” http://www.agu.org/cgi-bin/wais?mm=P11D-09 seems to suggest the jury is still out on whether Ganesa is a dome or a depression. To quote from their abstract:
“We are collecting DTMs of all usable image pairs and will present the most interesting results. Examples of geologic questions that may be addressed are: What is the relation between Ganesa and surrounding features? Is it a dome or shield? “
Perhaps the outer portion is a collapsed cryocaldera and the central crater a more recent cryolava vent. This supports Mike’s observation of the 3 and 9 o’clock channels appearing to ‘flow’ towards the center while the 5 o’clock cryolava channel (which has a different morphology) flows outward from the center.
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Juramike
post Dec 17 2008, 12:07 PM
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I'm not sure if those are cryolava channels or not.

It could be just a hydrocarbon rain drainage "regular" stream channel filled with blocky rough RADAR-bright debris.

One interesting question though: Why is there a bright streambed sitting on top of apparently smoother uniform terrain?

Where have we seen this before? (rough channels on smooth terrain)
Where have we seen smooth channels in smooth terrain?

Is the smoothness due to the actual surface rock, or is it due to an airfall or other deposit that got blown in (and is thus "non-native")?
Did Ganesa act as a trap for some material that is now obscuring the actual stuff Ganesa was formed in?

-Mike


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Juramike
post Dec 17 2008, 12:55 PM
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Here is an inverted contrast-enhanced section of PIA09176 (Ganesa Macula) to indicate the bright channels (as dark lines):
Attached Image


(Why am I suddenly hungry for apple pie?)

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elakdawalla
post Dec 17 2008, 05:05 PM
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QUOTE (titanicrivers @ Dec 16 2008, 11:07 PM) *
...the abstract of Kirk et al “A Three-Dimensional View of Titan's Surface Features from Cassini RADAR Stereogrammetry” http://www.agu.org/cgi-bin/wais?mm=P11D-09 seems to suggest the jury is still out on whether Ganesa is a dome or a depression. To quote from their abstract: “We are collecting DTMs of all usable image pairs and will present the most interesting results. Examples of geologic questions that may be addressed are: What is the relation between Ganesa and surrounding features? Is it a dome or shield? “


The abstract was written long before they'd finished developing their DTMs, so, when he submitted his abstract, the jury was indeed still out. When Randy started his presentation, he apologized to half of the listed coauthors, who were supposed to have advised him on geologic interpretations -- apparently there was no time for advice, as he had only finished producing the maps on the previous Saturday. The answer is that it's neither a dome nor a depression. It's high in some places and low in others with almost no relation to the apparently circular radar-dark shape.

--Emily


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Juramike
post Dec 17 2008, 08:24 PM
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QUOTE (elakdawalla @ Dec 17 2008, 12:05 PM) *
It's high in some places and low in others with almost no relation to the apparently circular radar-dark shape.


Which itself is pretty interesting.

If you look at the channel branching pattern (post above), it is extremely radial. A higher level of branching is seen about 2/3 of the way from the center, decreasing nodes as you go towards the center.

This branching pattern would be consistent with a bowl shape and sources around the edges, and steeper slopes at the edge. Or a flat dome shape, with a source at the center and steeper slopes at the edge.

(A typical eroding mountain, with distributed sources (rainfall/snow) hitting the surface, will have a branching pattern with more nodes towards the center of the mountain)

If the topography doesn't match the branching pattern, that may indicate the top layer of stuff was added well after the streams were in place and the order locked in. (Or that the stream pattern was locked in the the terrain rose up into it. Excellent example: the Gates of Lodore at Dinosaur National Monument)

-Mike




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titanicrivers
post Dec 17 2008, 10:45 PM
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[quote name='elakdawalla' date='Dec 17 2008, 11:05 AM' post='132836']
The abstract was written long before they'd finished developing their DTMs, so, when he submitted his abstract, the jury was indeed still out. ... The answer is that it's neither a dome nor a depression. It's high in some places and low in others with almost no relation to the apparently circular radar-dark shape.

Wow, I should have read your 12/15 Planetary Society blog page first Emily!
It will be interesting to see the completed topo map and learn what the height resolution scale is etc.
Regardless, Ganesa is still an interesting region with channels that are radially oriented and telling us something about the structure.
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Juramike
post Dec 18 2008, 03:50 AM
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I think portions of the channels might be incised:
Attached Image


Note RADAR-dark line parallel and S of the RADAR bright channel at the 3 o'clock position. This is consistent with a channel feature seen outside the Ganesa Macula region (on other side of ridge, N of the brightish alluvial looking things).

This is also consistent with RADAR look direction from the S (looking towards the N; this part of the Ganesa image is derived from the T3 RADAR swath (see: Elachi et al. Science 308 (2005) 970-974))

There is also a branch off the 3 o'clock position trending to the SW (4 o'clock) that displays a darker region S of the channel and the brighter region N of the channel. This might (?) be consistent with a negative slope (downhill to N) of the S section, and a positive slope (uphill to N) of the are N of this channel. So the channel might be in a broad valley/canyon in this section.

Based on the channel gradient and RADAR speculation, I would predict that this portion in the SE part of Ganesa would be topographically higher, and that the channel is at the bottom of a deep broad regional valley in a local topographic high.

(And I haven't seen the AGU presentation, so I could be wayyy off: Hero or goat time!)
Attached Image


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ngunn
post Dec 18 2008, 10:54 AM
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Another excellent post from Emily:

http://www.planetary.org/blog/article/00001775/

I would like to respond to the issue raised in the title - the question of whether Titan is internally active or not. We seem to be offered two possibilities.

1. Inactive.
All the topography is the eroded remains of an ancient cratered surface.

or

2. Active.
Internal heat continues to power crustal tectonic proceses.

As a peripheral observer of events, only having read abstracts and Emily's summaries, I have not heard a whisper about a third possibility, namely:

3. Internally inactive but with ongoing floating-crust tectonism powered externally by the weather.

I have satisfied myself that Titan receives enough energy from the Sun such that only a minute fraction of it is needed to raise Titan's modest hills over geological timescales. We seem to be at a stage in Titan studies where the widest possible set of scenarios are being thrown into the ring. I therefore have two questions. Has anyone heard this third possibility being mentioned at all? And if not, wherein lies the fundamental objection to it?
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Doc
post Dec 18 2008, 11:21 AM
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An interesting suggestion ngunn. However I find it rather hard to imagine it but no, I myself have never heard of 'atmospherically induced tectonics' being mentioned.


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Juramike
post Dec 18 2008, 02:33 PM
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That's a pretty interesting suggestion, Nigel!

What sort of observable effects would that create?

If wind-driven tectonic plates are bumping into each other, it might make an effect similar to pancake ice. Relatively flat plates, but with upraised edges at the edge of the plate margin resulting from continual collisions as the plates bumped around due to the winds. (One prevailing direction of ebb and flow might make the edge height non-uniform around the plate).

I'm not sure we've seen anything like that just yet....

One really general observation about Titan is the ubiquitous E-W trending regularly-spaced tectonic ridges in the equatorial zones. I use them all the time to orient and line up images. Whatever process made these must have been planet wide and uniform on a large scale (and with a N-S strain). This is not what you'd expect with Titan's current orientation (E trending winds at the surface of the equatorial region).

But then there are those broad N-S undulations.....



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ngunn
post Dec 18 2008, 03:01 PM
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Well, I hesitate to repeat these ideas too often and you have heard it before but in a nutshell it's this. The weather redistibutes mass and destabilises the rotation axis, perhaps gradually, perhaps in a sequence of major 'polar flop' episodes. Readjustment of the ice-shell to its new orientation on the hydrostatic ellipsoid produces the EW tectonic ridges. Note: longitudinal libration can't do this. Surface materials now find themselves at the 'wrong' latitude and are subject either to dessication (a candidate for dune sand formation???) or solution and erosion by liquid (caves systems, sinkholes, caldera-like lake basins???). It's all a flight of fancy of course, but maybe it's a way of having some tectonics and cyclical geological processes on a world with only one 'plate' and limited internal heating.

I like to remember that Titan's atmosphere is huge relative to Titan itself, and even more huge in relation to the mobile ice shell. On this model Callisto is 'Titan without weather' and therefore without the ability to slew it's crust around.

Just an idea.

EDIT: I can't resist linking to this again:
http://www.agu.org/cgi-bin/wais?mm=P31C-04
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