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Cape York - Northern Havens, Sol 2780 - 2947
fredk
post Dec 10 2011, 03:38 PM
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An extreme range of depth in this anaglyph from 2800:
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mhoward
post Dec 10 2011, 05:00 PM
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I couldn't resist doing this one, because look at that view. Sols 2795-2800.



QuickTime VR version (5.8 MB)



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eoincampbell
post Dec 10 2011, 05:24 PM
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QUOTE (mhoward @ Dec 10 2011, 09:00 AM) *
I couldn't resist...

It's quite brilliant, what a view!


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Bill Harris
post Dec 10 2011, 07:14 PM
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QUOTE (CR)
If Homestake is embedded within soft sulfate sediments similar to those we have become familiar with on the Meridiani plains, perhaps this occurance resembles a gypsum crystal embedded within wallboard
And in something semi-unrelated, remember the vugs found vugs that were found in the sandstone in Eagle crater seven years ago. The vugs were speculated to have been made by dissolved crystal with a tabular habit, possibly gypsum. Another puzzle piece fits...

--Bill


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Phil Stooke
post Dec 10 2011, 08:04 PM
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Here's mhoward's beautiful new pan in polar format.

Phil

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ngunn
post Dec 10 2011, 10:40 PM
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A couple of thoughts from the sofa: I hope the geologists will correct me where I'm off the mark.

1/ Homestake doesn't look like a feature produced by crystal growth into a loose matrix, like desert sand roses. It looks like it formed in a pre-existing crack in rock that was harder when it cracked than it is now in its weathered state.

2/ Erosion-resistent veins have been seen at Victoria and elsewhere, but the team are saying this is the first vein of gypsum found so far.

One lesson for me from all this: I didn't realise how mechanically weak the Meridiani stuff becomes when freeze-dried on the surface and that it could be significantly harder when not so exposed.
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Bill Harris
post Dec 11 2011, 01:13 AM
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QUOTE (fredk @ Dec 10 2011, 09:38 AM) *
An extreme range of depth in this anaglyph from 2800:
But if we forego the Big Picture and note the details, we'll notice that the Antenna Dust is less (Navcam stereo pair from Sol-2800):
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Bill Harris
post Dec 11 2011, 02:25 AM
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QUOTE (Phil Stooke @ Dec 10 2011, 02:04 PM) *
Here's mhoward's beautiful new pan in polar format.

Phil

Indeed, Phil.

It's interesting to compare your polar pan with HiRISE imagery of the same area:

EDIT: BTW, I just checked and the HiRISE map "north" and Phil's polar "north" are in close agreement, at least to shoot azimuths to specific features. Have fun...

--Bill
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CosmicRocker
post Dec 11 2011, 05:48 AM
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QUOTE (ngunn @ Dec 10 2011, 04:40 PM) *
... 1/ Homestake doesn't look like a feature produced by crystal growth into a loose matrix, like desert sand roses. It looks like it formed in a pre-existing crack in rock that was harder when it cracked than it is now in its weathered state.
2/ Erosion-resistent veins have been seen at Victoria and elsewhere, but the team are saying this is the first vein of gypsum found so far.
One lesson for me from all this: I didn't realise how mechanically weak the Meridiani stuff becomes when freeze-dried on the surface and that it could be significantly harder when not so exposed.

1: Homestake looks exactly like a vein of crystalline gypsum that grew within a pre-existing fracture. The rock wasn't necessarily harder when it cracked. You can form cracks in wet sand. I don't think you can conclude that the Meridiani sediments become weaker when they are exposed at the surface.
2: This is apparently the first vein of gypsum identified so far.

To further this discussion of mineral veins and their relative erosion resistance at Cape York, take a look at this pancam image from sol 2793. Note the sub-horizontal feature just below the center of the image. Here is a fracture in the rock with negative relief. Considering the proximity to the Homestake filled fracture, it seems likely that this fracture might also have been filled with the same mineral. I could be wrong, but Occam's razor suggests this to be the case. However, in this instance the surrounding rock is a tougher basaltic breccia. The vein is relatively softer than surrounding rock, so the vein is more deeply eroded in this case.


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Bill Harris
post Dec 11 2011, 06:20 AM
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Good observation. Here is another example from Sol-2786. Note the fracture in the lower left of the breccia block with the light-yellowish material still adhering to the wall of the fracture. My intrepretation is that this was a gypsum-filled vein which has eroded.

In addition, this region has a very complex genesis. We don't know what the hardness-- or range of hardnesses-- of the various units of breccia are. I would suppose that the surface material is composed of impact breccia and ejecta from many craters over a long time span. This breccia has been reworked, disturbed, transported, weathered and indurated countless unknown times. The ultimate in "folded, spindled and mutilated", I think we are looking at something unbelievably complex if we narrow the time steps enough.

--Bill
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walfy
post Dec 11 2011, 06:44 AM
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Microscopic from Sol 2800:

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walfy
post Dec 11 2011, 06:48 AM
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The adjacent micro from Sol 2800:

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Some white patches in there. More gypsum, perhaps?
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ngunn
post Dec 11 2011, 10:17 AM
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QUOTE (CosmicRocker @ Dec 11 2011, 05:48 AM) *
The rock wasn't necessarily harder when it cracked. You can form cracks in wet sand.


OK, not necessarily.

I'm assuming these cracks formed, through both sediments and breccia, in response to major seismic shocks from impacts like Iazu. Mechanically weak materials (such as wet sand) tend to pulverise/fluidise in response to seismic shocks rather than supporting the propagation of coherent cracks. That's why an upstanding gypsum vein suggests to me that the sediments may have been significantly harder in their unweathered state. This would be consistent with Bill's point that hardness and resistance to weathering are two different things.

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centsworth_II
post Dec 11 2011, 05:00 PM
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A reminder of previous fracture fill seen by Opportunity:
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http://www.jpl.nasa.gov/missions/mer/images.cfm?id=701

Those must be of different composition than the Cape York fill and may have formed by a different process. The first paper below relates them to rinds "actually developed only at the margins of fractures." It's interesting to note that in the upper right corner of the image above, there is a ridge on each side of the fracture which would seem to support the "rind formation" process. (Keep in mind, I'm a geological newbie).

A discussion of fracture fills seen by Opportunity starts on page 19 of this paper.

This paper discusses Earth analogs to fracture fills seen by Opportunity. (Which relate more to the fin-like fill seen previously than to the latest, gypsum fill vein seen at Cape York.
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Bill Harris
post Dec 11 2011, 07:47 PM
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QUOTE (Centsworth)
The first paper below relates them to rinds "actually developed only at the margins of fractures."
Very good! I'd lost and honestly forgotten about both those papers. Very fundamental references.

--Bill


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