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Europa's Northern Plains, Triple bands, cycloidial ridges, ect.
Exploitcorporati...
post Feb 12 2006, 02:21 PM
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A broader view of Belus Linea was obtained at the northern end of the 17ESREGMAP transect, which is contiguous with the two sections shown at the beginning of this thread. On E19, an observation was targeted to fill a gap in this portion of the transect as well. The global mosaic from E14 was used here to produce a color version of the scene.

17/19ESREGMAP Belus Linea greyscale

Color version


In addition to this, medium-resolution color images were taken of this area on E11(300m/pxl, narrow strip) and E19(90m/pxl, very high phase angle, oblique).

11ESCOLORS01-02

19ESCOLOR02


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vexgizmo
post Feb 12 2006, 06:23 PM
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QUOTE (Exploitcorporations @ Feb 11 2006, 01:12 PM) *
I've heard the surface at high resolution described as looking like "a ball of string". I've seen models describing the process by which the bizzare cycloidial cracks form, but I have'nt seen anything modeling the formation of ridge features at fine scales. I'm sure vexgizmo might have an idea...


The cracking is currently thought to be a combination of two stresses, diurnal and nonsynchronous. Diurnal stress results from Europa's daily orbit about Jupiter, which includes tidal flexing and libration as Europa orbits. Nonsynchrnous rotation stress results from very slow rotation (period >10,000 years) of the decoupled ice shell above the ocean and mantle at slightly faster than synchronous rate--the shell must change shape to respond to the changing position of surface features relative to the tidal axes. It's difficult to explain individual features yet--people are working on it--but these are the leading stress candidates. Polar wander has also been invoked, and recently obliquity changes too (Hurford et al., in the upcoming LPSC). As nonsynchronous rotation proceeds, stress orientations and regimes change, allowing cross-cutting features to form. And from cracks, somehow the ridges and bands form to create the small-scale features seen, but that's another in-progress story....
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Decepticon
post Feb 12 2006, 08:29 PM
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Does that explain why the lines make a cookie bite mark? They look like half circles attached at each ends.

I'm still at a loss how those are formed!
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vexgizmo
post Feb 12 2006, 10:16 PM
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QUOTE (Decepticon @ Feb 12 2006, 01:29 PM) *
Does that explain why the lines make a cookie bite mark? They look like half circles attached at each ends.

I'm still at a loss how those are formed!

These are the "cycloidal" features. They are thought to form as a crack propagates at an average rate that matches the changing stress direction due to the diurnal stressing as Europa orbits. This is much easier to visualize than to put into words, so I point you to Greg Hoppa's web page for some words of explanation, and excellent animations:

http://pirlwww.lpl.arizona.edu/~hoppa/science.html

Attached Image

The arrow represents the diurnal stress, which rotates and changes in magnitude as Europa orbits each 3.55 days.

This is the abstract of the Hoppa et al. Science paper which first proposed the idea--this is a strong piece of evidence for a subsurface ocean:

Cycloidal patterns are widely distributed on the surface of Jupiter's moon
Europa. Tensile cracks may have developed such a pattern in response to diurnal
variations in tidal stress in Europa's outer ice shell. When the tensile strength
of the ice is reached, a crack may occur. Propagating cracks would move across
an ever-changing stress field, following a curving path to a place and time where
the tensile stress was insufficient to continue the propagation. A few hours
later, when the stress at the end of the crack again exceeded the strength,
propagation would continue in a new direction. Thus, one arcuate segment of
the cycloidal chain would be produced during each day on Europa. For this model
to work, the tensile strength of Europa's ice crust must be less than 40 kilopascals,
and there must be a thick fluid layer below the ice to allow sufficient
tidal amplitude.
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Guest_Myran_*
post Feb 12 2006, 11:21 PM
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QUOTE
For this model to work ...there must be a thick fluid layer below the ice to allow sufficient
tidal amplitude.


Thank you for all for the information of this thread, these cycloidal patterns must then be another indicator that the ocean actually might be there. Adding another piece to the 'case for an ocean' here. cool.gif
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Decepticon
post Feb 13 2006, 12:39 AM
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Great Link vexgizmo smile.gif Good read.
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post Feb 13 2006, 03:30 AM
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I presume Bob Pappalardo thinks that the cycloids all formed during one of those geological cycles when Europa was undergoing greater tidal heating and thus had a thin Greenberg-type crust, as opposed to its current era of low tidal heating, thick ice crust and solid-state diapirs? We certainly aren't seeing any cycloids forming right now -- despite the great speed with which each individual one is supposed to have formed.
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nprev
post Feb 13 2006, 06:07 AM
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QUOTE (BruceMoomaw @ Feb 12 2006, 08:30 PM) *
I presume Bob Pappalardo thinks that the cycloids all formed during one of those geological cycles when Europa was undergoing greater tidal heating and thus had a thin Greenberg-type crust, as opposed to its current era of low tidal heating, thick ice crust and solid-state diapirs? We certainly aren't seeing any cycloids forming right now -- despite the great speed with which each individual one is supposed to have formed.


However, this cycloid formation model doesn't predict any sort of average interval between events, does it? Terrestrial fault ruptures happen quite rapidly, but may be hundreds of years in the making...

Perhaps the average tensile strength of the crust is in fact very close to 40KPa, and the cycloids originate at local weak spots. The question then would be what is making a particular point on the crust mechanically weak over time? I vote for warm subsurface plumes of H2O from Europan "black smokers"... smile.gif


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vexgizmo
post Feb 14 2006, 01:44 AM
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QUOTE (BruceMoomaw @ Feb 12 2006, 08:30 PM) *
I presume Bob Pappalardo thinks that the cycloids all formed during one of those geological cycles when Europa was undergoing greater tidal heating and thus had a thin Greenberg-type crust, as opposed to its current era of low tidal heating, thick ice crust and solid-state diapirs? We certainly aren't seeing any cycloids forming right now -- despite the great speed with which each individual one is supposed to have formed.

Yes and no. It's true that Europa may have been generally more active long ago, but a thin crust is not necessary in the cycloid model, just an ocean--the surface stress is nearly identical for a 5 km or 20 km thick ice shell. Cycloidal fractures might form now, but it may be only the rare cycloid that develops into a ridge or band and thus becomes visible. But I do believe that the evidence for a slowing in the rate of feature formation (demonstrated well by Figueredo and Greeley) is consistent with waning activity, and that Europa is probably in a relatively dormant stage. It remains to be seen whether a Greenbergian epoch ever really occurs.

QUOTE (nprev @ Feb 12 2006, 11:07 PM) *
However, this cycloid formation model doesn't predict any sort of average interval between events, does it? Terrestrial fault ruptures happen quite rapidly, but may be hundreds of years in the making...

Very true... especially if some nonsynchronous rotation stress is required to initiate cracking (in which case the ice strength can be a few times 40 kPa).
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Guest_AlexBlackwell_*
post Feb 14 2006, 02:06 AM
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I'll only add that I highly recommend the October 2005 issue of Icarus, Volume 177, Issue 2 (pp. 293-578), which was a special issue entitled "Europa Icy Shell."

For those who do not have access to the journal, I know reprints of some of the papers can be found online (e.g., Nimmo and Giese [2005]).
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Exploitcorporati...
post Feb 14 2006, 03:57 PM
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Thanks for all the excellent links and information. I've posted the abstract for the Icarus paper regarding Castalia Macula in the Argadnel Reigo thread. There's a fascinating suggestion as to why it would make a fine landing site for a future mission.


--------------------
...if you don't like my melody, i'll sing it in a major key, i'll sing it very happily. heavens! everybody's all aboard? let's take it back to that minor chord...

Exploitcorporations on Flickr (in progress) : https://www.flickr.com/photos/135024395@N07/
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Exploitcorporati...
post Feb 14 2006, 04:07 PM
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Rhadamanthys Linea, a feature that that seemed to be comprised of a string of isolated dark spots at low resolution, was targeted for a better view on E14, E15, and E19. It looks as if Rhadamanthys is the "one that got away" as far as the pointing of the camera went, with the single-frame E14 shot missing it entirely and the E19 mosaic catching it only in the corner of each image.

19ESRHADAM01

Context view

E15 mosaic with G1 color

14ESVLOFOT01


--------------------
...if you don't like my melody, i'll sing it in a major key, i'll sing it very happily. heavens! everybody's all aboard? let's take it back to that minor chord...

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vexgizmo
post Feb 16 2006, 07:01 AM
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QUOTE (Exploitcorporations @ Feb 14 2006, 09:07 AM) *
It looks as if Rhadamanthys is the "one that got away" as far as the pointing of the camera went, with the single-frame E14 shot missing it entirely and the E19 mosaic catching it only in the corner of each image.[/url]

Yes, the E19 mosaic missed most of Rhadamanthys, as it was targetted using outdated coordinated information.... *sigh* It's great to see this one in context!
VLOPHOT is a Very LOw phase PHOTometry frame (obtained in several colors), which was pointed not at any specific feature, but to the "shadow point" of the spacecraft (0 degree phase angle) to understand the scattering properties of the icy surface.
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Exploitcorporati...
post Feb 16 2006, 09:27 AM
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QUOTE (vexgizmo @ Feb 15 2006, 11:01 PM) *
Yes, the E19 mosaic missed most of Rhadamanthys, as it was targetted using outdated coordinated information.... *sigh* It's great to see this one in context!
VLOPHOT is a Very LOw phase PHOTometry frame (obtained in several colors), which was pointed not at any specific feature, but to the "shadow point" of the spacecraft (0 degree phase angle) to understand the scattering properties of the icy surface.


My apologies for the mistake...from the pointer plot graphs, it looked awfully close to the intersection. Thanks as always for the insights. The imaging team deserves the greatest respect for making the most of a trying set of circumstances, and in most cases, bringing back the impossible.


--------------------
...if you don't like my melody, i'll sing it in a major key, i'll sing it very happily. heavens! everybody's all aboard? let's take it back to that minor chord...

Exploitcorporations on Flickr (in progress) : https://www.flickr.com/photos/135024395@N07/
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