Titan's topography, strange.... |
Titan's topography, strange.... |
Apr 12 2009, 12:44 PM
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#1
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Senior Member Group: Moderator Posts: 2785 Joined: 10-November 06 From: Pasadena, CA Member No.: 1345 |
Recent article in Science by Zebker et al.:
Zebker et al. Science in press, "Size and Shape of Saturn's Moon Titan". doi: 10.1126/science.1168905 (published online April 2, 2009) Link to abstract (pay-for article): http://www.sciencemag.org/cgi/content/abstract/1168905 Article on spaceref discusses this paper: http://www.spaceref.com/news/viewpr.html?pid=27912 Figure 3 from the Science article is a global elevation map relative to barycenter. Key points of article:
"Xanadu seems to be systematically lower than other parts of the equatorial belt, and not uplifted like most mountainous areas on Earth." (quote from Fig. 3 caption in article) -Mike -------------------- Some higher resolution images available at my photostream: http://www.flickr.com/photos/31678681@N07/
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Apr 24 2009, 10:50 PM
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#2
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Junior Member Group: Members Posts: 41 Joined: 11-April 07 From: London, U.K. Member No.: 1957 |
On the crystalline versus amorphous debate
There are some recent laboratory studies of hydrocarbon ices, investigating the amorphous to crystalline transition temperature. These are found to be 70 K in diaceteylene (Zhou et al. in press Planet. Space. Sci. doi:10.1016/j.pss.2009.02.003) and 93 K in vinylacetylene (Kim & Kaiser, 2009, Astrophys. J. Suppl. Ser. 181, 543, doi:10.1088/0067-0049/181/2/543). Since much of Titan's stratosphere is warmer, then one might expect such condensates to be crystalline. There is also observation evidence (IR lattice modes) for crystalline organic ices in Titan's stratosphere - Khanna, 2005a,b (doi:10.1016/j.icarus.2005.02.014 & 10.1016/j.icarus.2005.03.011). I think there's a paper just out by Mike Flasar which reviews this kind of thing. There is also the possibility that amorphous ices might be transformed to a crystalline state by UV radiation, as demonstrated by Chakarov & Kasemo (1998) in Phys. Rev. Lett. 81, 5181. These authors discuss relevance to comets and outer solar system atmosphere. And last of all, we must recall that whilst diffusion rates may be incredibly slow at 95 K, the materials may be kicking about for several billions years. It is likely that any glassy material will devitrify on such timescales, bearing in mind that 95 K is not *that* cold, and water ice is expected to undergo orientational ordering on geological timescales below 70 K. |
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Apr 24 2009, 11:55 PM
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#3
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Senior Member Group: Moderator Posts: 2785 Joined: 10-November 06 From: Pasadena, CA Member No.: 1345 |
Good points.
F.M Flasar and R.K Achterberg, Phil. Trans. R. Soc. A February 28, 2009 367:649-664 "The structure and dynamics of Titan's middle atmosphere." doi:10.1098/rsta.2008.0242 Abstract via PubMed here. (I can't access the full article) Link to abstract for Kaiser article mentioned above here Quote from kaiser abstract: QUOTE New laboratory spectra of crystalline and amorphous diacetylene ice have been recorded in the range of 7000–500 cm−1 (1.4–20 μm) to aid in the identification of solid diacetylene on Saturn's moon Titan. We have established that amorphous diacetylene ice is stable only at temperatures less than 70±1 K. So, if it is possible to get atmospheric fallout grains to crystallize/sinter/structurally reorganize into larger grains, does that also imply that over enough time that a stable dune field could also "lock up" and fossilize out? -------------------- Some higher resolution images available at my photostream: http://www.flickr.com/photos/31678681@N07/
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Apr 27 2009, 06:41 PM
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#4
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Member Group: Members Posts: 131 Joined: 30-August 06 From: Moscow, Idaho Member No.: 1086 |
So, if it is possible to get atmospheric fallout grains to crystallize/sinter/structurally reorganize into larger grains, does that also imply that over enough time that a stable dune field could also "lock up" and fossilize out? Possibly so given crystallization and reorganization, but not strictly from sintering alone. The rate of growth for a particle, i.e. dr/dt, is proportional to the radius to the 4th power. So as the sintering grains grow, they do so more and more slowly as a function of time. This can naturally limit their final size to something in the right range, if you tweak the initial parameters properly. - Jason |
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Apr 27 2009, 09:39 PM
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#5
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Senior Member Group: Members Posts: 1592 Joined: 14-October 05 From: Vermont Member No.: 530 |
Possibly so given crystallization and reorganization, but not strictly from sintering alone. The rate of growth for a particle, i.e. dr/dt, is proportional to the radius to the 4th power. So as the sintering grains grow, they do so more and more slowly as a function of time. This can naturally limit their final size to something in the right range, if you tweak the initial parameters properly. I was wondering if a little kinetic energy from wind transport helped growth, and that once a particle was too big to be blown around, that it also limited further growth. If those particles are 300um in size, though, that would seem small enough to blow around unless the wind was pretty damn weak. And if they can blow around, that would mean a less fossilized dune field. That must have been discussed here already. Time to use teh google. |
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