Titan's Equatorial Sand Seas Options

[Juramike]

Here is a short list of possible suspects for the “bright center dark halo circular features”:

(Listed in order of “drama”: high energy impact is high drama (and would be way rad to watch happen!), a slow cold diapir slowly working it’s way to the suface is low drama (and not real exciting to watch).

External: Impact origin followed by modification (impact then maybe cryovolcanism or slow crustal rebound?):
1) Modified impact crater (up-lifted dome)
2) Modified palimpsest/type II palimpsest/penepalimpsest
3) Modified central dome crater

Internal: Cryovolcanic origin
4) Collapsed pancake dome
5) Thermal plume
6) Ice pluton
7) Diapir

The difficult thing to explain is the formation of a topographically depressed halo around the central dome or plateau.

Any ideas?

-Mike

[ngunn]

The difficult thing to explain is the formation of a topographically depressed halo around the central dome or plateau.

I have to say that I feel uncomfortably out on a speculative limb here. In particular I think your assumption that elevation and liquid levels are the only determinants of the light/dark patterns is unsafe. However, going along with it for now I don't find it too hard to imagine the outer skin of an ice diapir being richer in volatile materials, making it more prone to deflation by outgassing and/or erosion. One could envisage a dark depression appearing first, acquiring a bright centre later as more of it emerges.

[Juramike]

I have to say that I feel uncomfortably out on a speculative limb here. In particular I think your assumption that elevation and liquid levels are the only determinants of the light/dark patterns is unsafe.

I'm totally cool with your concerns.

All of these speculations are built up on the original speculations of a bright/dark level being pretty much set by "the last inundation sea level". (= "house of cards"?) (There may be some effect of dark brown dune sands drifting in from the W as well).

It is also built up on the speculation that most of the sand sea basins (exception: Mezzoramia) are interconnected and have a "last inundation sea level" at the same gravitational level of the geoid. (The semi-permanent tidal bulge will make a difference in absolute elevation).

A key observation that supports this hypothesis is that at the Huygens site (granted, only one local observation), the bright dark boundary appeared to be very close to the "seashore" of the islands.

Hopefully this hypothesis would be easily testable from an altimetry track across one or several Sand Sea Basins. If the above hypothesis is correct, we should observe the bright/dark boundary at the same (tidal bulge effects taken into account) absolute elevation across all the basins.


Alternate hypotheses for setting the light/dark boundary include:

* local "bright stuff" snow line: (if bright stuff is solid material)
* local methane rain virga limit (assuming bright stuff has been scrubbed clean by methane percolation)
* local "fog elevation" (assuming methane fog helped dissolve out bright material)

(If y'all got any others?)


Like any other supposition, if data comes in that torpedoes these assumptions, it'll be time to modify the hypothetical sequence.

But so far, everything seems kinda consistent....

So far....


-Mike

[Juramike]

From the “circular features” EXCEL table above, I attempted to plot the morphometry of the Titan circle features using a plot similar to that found in Jones et al. Icarus, 164 (2003) 197-212 “Morphology and origin of palimpsests on Ganymede based on Galileo observations.” (Pay article: Abstract here) . The data in the referenced paper can be found in Table 2 and the plot is in Figure 12 (p. 209 in the reference).

Taking the data for the “circular features on Titan” in the EXCEL table, I plotted the outer diameter (long axis) as the x component and the inner diameter (long axis) as the y component in a logarithmic graph. I also included (red circles) the unity line for [outer diameter (x), outer diameter (y)]. Only circle features that have two measured diameters (outer and inner) will lie off the unity line.



Yellow diamonds are bright center dark halo circle features [dark halo diameter (x), bright center (y)].
Purple dots are bright rim only circle features [bright rim diameter (x), bright rim diameter (y)). These are buried on the unity line.
Green squares are dark center bright apron impact craters [pedestal deposit = apron [x], crater rim [y]]
Orange squares are multi-ring impact basins [outermost rim diameter (x), innermost rim diameter (y)]. If the inner rim could not be measured, the point will be buried on the unity line as [outer (x), outer (y)].
Deep purple crosses are dark halo circle features [outermost rim diameter (x), outermost rim diameter (y)] and the points are buried on the unity line.


It is interesting to note that the putative impact craters (green squares) and the bright center dark halo circle features (orange squares) all pretty much lie on the same line (at least as far as a log plot goes).

This shows that the ratio of crater rim diameter/pedestal deposit diameter for putative impact craters on Titan is similar to the ratio of the bright center diameter to dark halo diameter when plotted on a log scale.
Where multi-ring craters had two rings measured, these points also fit in the same log-scale relationship.

The Jones et al. article defined the correspondence of morphological units between craters, palimpsests, and penepalimpsests on Ganymede and Europa.

So: (palimpsests, craters)

Outer deposits = continuous ejecta
Concentric massif facies = pedestal deposit
Unoriented massif facies = crater rim
Central plains = central pit

Here are pictures of a crater (Melkart), a palimpsest (Buto Facula), and a penepalimsest (dome crater) (Nieth) showing the defining units. (All are on Ganymede).



In the paper, several impact craters, ring structures and palimpsests on Europa and Ganymede were plotted in the same type of graph. (Palimpsests and ring structures used in the text: Callanish, Tyre, Buto Facula, Zakar, Memphis Facula, Epigeus; Craters: Achelous, Sebek, Isis, Melkart, Osiris, and Enkidu).


Adapting the derived lines from the Jones et al paper onto the graph of circle features of Titan we get this plot:
(By definition in the paper, the crater rim or unoriented massif facies is the unity line, so it should be no surprise that it lines up with the Titan circle feature unity line (red dots) as well.)
It is interesting that the central plains/central pit to unoriented massif facies/crater rim points line up almost with the yellow diamond, green squares and orange squares in the Titan plot.





Adapting the derived lines from the Jones et al paper onto the graph of circle features of Titan, but this time shifting the reference diameter to “concentric massif facies/pedestal deposit” we get this plot.
(So by the new definition, the pedestal deposit/concentric massif facies is the unity line, so it should be no surprise that it lines up with th eTitan circle feature unity line (red dots) as well.)
Again, it is interesting that the central plains/central pit to unoriented massif facies/crater rim points line up almost with the yellow diamond, green squares and orange squares in the Titan plot.



Neither plot proves that the circle features of Titan have an impact origin. But they do show that the “ratio of corresponding morphological facies with those of craters, palimpsests, and ring structures of the Jovian moons (well, at least Ganymede and Europa) are similar, at least at the level of a log plot.”

A better plot would show Saturn system inner dome craters and their relationship to Titan circle features…

-Mike

[Juramike]

I plotted the outer/inner diameter measurements of the selected large craters on Tethys, Rhea, and Iapetus on the same plot as the circular features of Titan. The selected craters are: Odysseus on Tethys, Tirawa on Rhea, and the crater located at 5N, 38W on Iapetus. The topoglogy of Odysseus and Tirawa was described in Moore et al. Icarus 171 (2004) 421-443. "Large impact features on middle-sized icy satellites". Below are the topological profiles of Odysseus and Tirawa and a plot of Titan circle features with selected craters from some of Saturn’s other moons.



Here are the measurements and plots of Titan circle features with 4 selected larger craters from Ganymede and Callisto (measured values are from the text). (Two central dome craters, an anomalous central dome crater, and Gilgamesh which may also be a central dome type crater):



According to the text in Schenk et al “Ages and Interiors: The Cratering Record of the Galilean Satellites” (Available in html-only http://66.102.1.104/scholar?hl=en&lr=&...s+and+Interiors
]here[/url]): “These domes have rounded profiles up to 1.5 km high but at high resolution are characterized by web-like networks of narrow fractures.” (They also invoke that Gilgamesh and Lofn may qualify as central dome craters). Also, “Like cental pits (craters), the dome/crater size ratio increases with increasing crater diameter.”

In contrast, “the ratio between dome and rim diameter for [anamolous dome craters] (whether observed or estimated from ejecta deposit scaling) is roughly constant at 0.4, regardless of crater size”. (I hope I got the correct diameter for the rim of the 54N, 43W Crater on Callisto: I measured 90 km, but the text stated 180 km for the rim)


Below are the measurements for a (putative) cryovolcano on Titan, Ganesa Macula. The ratio of outer diameter to central radar bright zone is about 0.11 for the 200 km pancake dome (n = 1, so who knows if this is typical.) In contrast, a 200 km bright center dark halo feature usually has a 60 km bright center, for outer/inner diameter of 0.3. Below are the images and plots.




Below are the measurements from topographical analysis of dome-like bulges in chaotic terrain on Europa. These bulges are thought to be diapirs. Below are the images and comparision a plot of Titan circle features.



As described in Shenk, Journal of Geophysical Research 100 (E9) (1995) 19,023-19,040. “The geology of Callisto”: “Bright domes fill most central pits in craters larger than 60 km (in Galilean moons), and have morphologies consistent with an intrusive origin. Dome formation could have occurred after crater formation as diapiric intrusions of soft ice, or during crater formation and collapse by the uplift of deep material, as occurs in central peak craters on Earth and the Moon.” I’m curious to know if a “web-like network of narrow fractures” would create a chaotic terrain that would then appear RADAR-bright by SAR.

At least for the Galilean satellites, “Fracture domes on these [Galilean] domes resemble those formed in a thin chilled crust over plastic material deforming under gravity (e.g., pancake domes on Venus). This suggests that uplifted dome material deformed plastically during or after emplacement, as would be expected for warm material rapidly uplifted several kilometers and left to cool to space. Dome profiles can be used to model the rheology of the uplifted material during emplacement”.



From all the graphs and plots, it seems that a cryovolcanic pancake dome, like Ganesa Macula, seems to be the worst fit with the data (but remember, n = 1).

The best fit seems to be for an impact origin, at least for most of the smaller craters (<900 km). As stated above, diapiric rise of material after impact might explain the observed morphology of the bright center, dark halo circle features.
A pure diapiric origin may fit for some of the largest circle features (those >900 km). Unfortunately, I couldn’t find any really big examples (>1000 km) of diapirs so I can’t compare morphology with Titan’s bright center dark halo circle features.


Smaller craters: impact
Bigger circle features: impacts and diapiric doming
Biggest circle features impacts and diapiric doming OR just plain old diapirs.


-Mike

[ngunn]

Juramike I salute your work, but sadly I doubt if there is an 'amateur' here now who can enter a critical discourse on your evidence and findings at the level they deserve. I excuse our resident professionals because of course they conduct most of their discourse in their own arena. I'd just like to register a few points, easily made armchair comments compared with what you are doing.

Philosopy
My preference at the moment is to hedge my bets as far as global models for Titan are concerned. Many of my ideas about the place are mutually contradictory (you've probably noticed!) but I'm happy with that and prefer to keep it that way until we have done more plain and simple looking. I'm not comfortable in a 'house of cards', even one that seems to be holding up well at the moment. I'm on the lookout for surprises - the sort of thing that brings card houses down. I don't want to miss one because I've formed a view too early.

next bit in following post

[ngunn]

Ganymede et.al.
Undoubtedly we can look to the Galileans and other moons of Saturn for guides to Titan's likely bulk composition. Also, large impacts might be expected to produce comparable surface features. However I think it's worth reviewing the actual and potential differences. In the first place neither the ISS albedo patterns nor the topography revealed by radar on Titan remotely resemble any other body. Secondly it is reasonable to expect that the the geology of Titan will reflect the fact that volatiles brought to the surface are not immediately vented into space. On Earth the recycling of water lubricates plate tectonics. We have no idea what analogous processes may operate on Titan. Thus we don't know even if ancient circles would remain circular - or even remain at all. More data required!

Mars
There are craters on Mars with narrow rims and others with wide aureoles, plus a huge variety of other circular forms. The role of volatiles seems to be important there. Have you tried out your ideas on Martian circular features?

[Juramike]

Philosopy
My preference at the moment is to hedge my bets as far as global models for Titan are concerned. Many of my ideas about the place are mutually contradictory (you've probably noticed!) but I'm happy with that and prefer to keep it that way until we have done more plain and simple looking. I'm not comfortable in a 'house of cards', even one that seems to be holding up well at the moment. I'm on the lookout for surprises - the sort of thing that brings card houses down. I don't want to miss one because I've formed a view too early.

I can respect your caution. My philosophy is to try to build a model that tries to fit and explain the current observations and is hopefully predictive of future observations.

My personal scientific philosopy that even a bad model is better than no model. (If it's proven wrong, at least we can say "Well, it's not that!")

Building a model will hopefully tell us where to look to support/disprove the model.

Always bearing in mind the important rules of science:
"You can never prove a theory." (You can provide evidence that supports a theory, but you can never prove it.)
"Absence of evidence is not evidence of absence."
"You can't zoom a zoomer." (Zoom, schwarz, profigliano, beterman rules here)

I'm hoping that an initial model will also help to develop experiments in the lab as well as plan what future observations might be helpful.

And scientific models are built to be modified, mangled, ripped up, parted out, and reassembled, and if need be, totally thrown out the window. [The standard scientific bet in our lab is a quarter: some days I'm lucky and the quarter is on my desk, other days when my hypotheses don't do so well, it goes to a someone with a better predictive model]

And please continue to be on the lookout for suprises, those are the key observations that will help build the next (always better) model...

-Mike

[dvandorn]

"You can't zoom a zoomer." (Zoom, schwarz, profigliano, beterman rules here)

OMG! I haven't played that particular drinking game in 30 yers! (I believe, at the time I learned it, that 'Beterman' hadn't codified it yet -- it was simply known to me as "Zoom-Schwartz-Profigliano-Zoom.") I learned that game in my junior year of college (otherwise known as the Great Sodden Semester... )

I totally agree in terms of developing models, even on insufficient data, Mike. It is often the process of building a model that shows you what's not consistent with your base assumptions, and sends you out looking for data to confirm or deny certain of those base assumptions. It is perhaps possible to learn more by simply going out and collecting every scrap of data that you possibly can, but we all know that data-collection schemes (and the sensors used to collect the data) are so specialized these days that you have to triage -- you have to decide in advance what kind of data you're going to collect in any given scheme (read: any given mission).

For that task, you need a model and you need a set of base assumptions that you want to test. Without that, you can end up either getting tantalizing hints of things you just didn't have the tools to investigate or, worse, you miss something extraordinary that you could have seen if you had flown the proper types of sensors.

-the other Doug

[Juramike]

Ganymede et.al.
Undoubtedly we can look to the Galileans and other moons of Saturn for guides to Titan's likely bulk composition. Also, large impacts might be expected to produce comparable surface features. However I think it's worth reviewing the actual and potential differences. In the first place neither the ISS albedo patterns nor the topography revealed by radar on Titan remotely resemble any other body. Secondly it is reasonable to expect that the the geology of Titan will reflect the fact that volatiles brought to the surface are not immediately vented into space.

The geology of most the Saturnian moons should be very, very similar. Geologically, they are all brothers and brought up in the same house (Saturn orbit). The big differences with Titan compared to the other Saturn moons are with regard to impactors are:

it has weather and floods (erosion will erase many of the features)
it's bigger: tectonic activity? cryovolanic activity? (might erase many of the features)

it's bigger (compositional makeup, depth of lithosphere)
it has an atmosphere (small impactors burn up, a big impactor should not be affected, but venting might be suppressed [very good point, ngunn])
it has organics on the surface (but this might not be enough to make a difference)
it's bigger: faster impactor speed???

The first two factors could help explain why there are so few obvious impact craters (they get wiped), and may explain their different form compared to other moons.

IIRC (but I can't remember the source), the impact features of Saturnian moons (other than Titan) seem to resemble the impact features of terrestrial planets more than those of Galilean satellites. The Galilean satellites seem to be sorta "oddball" (compositional differences of target material? lithosphere differences? impactor differences?).

So we would predict that without erosion, Titan craters should look like other Saturn moon craters/terrestrial craters with a possible trend to Galilean craters. (The trend being due viscous relaxation of icy surfaces on larger bodies).

Ganymede et.al.
On Earth the recycling of water lubricates plate tectonics. We have no idea what analogous processes may operate on Titan. Thus we don't know even if ancient circles would remain circular - or even remain at all. More data required!

I don't think ancient circles remain circular at all....(I think some of this data might already exist)...

-Mike

[Juramike]

Attached please find a plot of circle features' (outer diameter, feature inner diameter) for the entries in the EXCEL spreadsheet from the post above along with their proposed speculative identification:



I have tentatively identified these features as (in order of drama):

- "normal" impact craters (green squares)
- "multi-ring basin" impact craters (orange squares)
- central dome impact craters modified by diapirism" (yellow diamonds)
- diapirs (blue squares)

Some of the larger "bright center dark halo" features have been split out as speculative diapirs (identified as blue squares in the plot). These features have a inner/outer diameter ratio >=0.75. (There seemed to be a break at this point.)

I have also included the point for the Minrva outer dark halo, outer bright ring (purple square).
Minrva seems to be a pretty cool feature. It appears midway between a multiring basin and a central dome crater with a dark outer halo.

For all these identifications please insert the words "speculative", "tentative", "hypothetical", "putative" (my favorite) and any other appropriate scientific weasel words that will allow me to save face when this is all proven wrong by further observation and analysis.

-Mike

[Juramike]

The Big Diapirs of Titan....

[ngunn]

Thanks for taking the trouble to reply to my points. You didn't mention Mars though so I thought I'd clarify what I was suggesting. The idea was to provide a sort of test for the process of analysing inner and outer diameters as a diagnostic tool. Pretend you don't know what the features are, plot them as you have for the Titan features and see how they look. What would you deduce from the plots alone? Could you define categories? Then look at how that fits with what is really known about the nature of the Martian features. It might well vindicate your method.

[alan]

The geology of most the Saturnian moons should be very, very similar. Geologically, they are all brothers and brought up in the same house (Saturn orbit).

-Mike

How about I stir things up a bit

Here I report calculations for the bulk chemical composition and internal structure of Titan in readiness for measurements from the 4 scheduled gravity flypasts by Cassini to take place in 2006-2007. The calculations are based on the idea that Titan is a captured moon of Saturn [2,3]. That is, it is proposed that Titan initially condensed as a secondary planetary embryo within the gas ring that was shed by the protosolar cloud (hereafter PSC) at Saturn’s orbit.

http://www.lpi.usra.edu/meetings/lpsc2007/pdf/2402.pdf

Earlier abstract:
http://adsabs.harvard.edu/abs/2004AGUFM.P53A1447P

[Juramike]

How about I stir things up a bit
http://www.lpi.usra.edu/meetings/lpsc2007/pdf/2402.pdf

Earlier abstract:
http://adsabs.harvard.edu/abs/2004AGUFM.P53A1447P

"TITAN: The red-headed step-child of Saturn?"

I'll stand stand corrected and modify my earlier comparison...

"Geologically, they are all brothers (or close cousins?) and brought up in (or adopted into) the same house (Saturn orbit). (Oh yeah, and Titan's brothers might've beat on and messed Titan up a bit.)"

-Mike