On the Road Again - volume 8 - Page 16

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Barsoomer


Posts: 344

Reply: 301



PostPosted: May 29, 2011 1:48 AM 

The "fins" were believed by the MER science team to be caused by water flow that made the banks more resistant to erosion It seems likely this channel began life as a desiccation crack. Hence, a desiccation crack subsequently modified by water flow. Exhibit I.

Serpens


Posts: xxx

Reply: 302



PostPosted: May 29, 2011 4:52 AM 

Barsoomer. The fins have been identified as fracture fill. This occurs beneath the surface in thin cracks with groundwater and is hematite cemented. If you look top right of your image you can see how the crack widened with fracture fill breaking and adhering to one or the other side of the crack. The cracks opened up theough desiccation shrinking after the fracture fill was formed. To put all this in perspective, at the foot of Cape Verde we saw a fracture fill exposed by aeolian erosion which was truncated by a deflationary surface. This fracture fill formed in a very narrow crack under the surface at the time. Subsequently the water level must have fallen since the surface eroded down. The unconformity is most clear. Then the area underwent a depositionary cycle and the sandstone built up adding probably around 5 metres or so above the truncated fracture fill. Then along came an impactor and the fracture fill was exposed as Victoria crater widened. The fracture fill and subsequent deposition occurred a long, long time ago.
Hortonheardawho. The berry lag would have formed with only one or two metres of erosion. The desiccation cracking is not necessarily just a surface phenomena as the entire depth of the meridian sediments would have desiccated and shrunk as the water dried down. If it were not for the concretions (yes concretions) then the Meridiani plain would have suffered a heck of a lot more erosion.

Serpens


Posts: xxx

Reply: 303



PostPosted: May 29, 2011 4:55 AM 

Well I tried to reply and got informed that my comment had been held for review.

Fred


Posts: 73

Reply: 304



PostPosted: May 29, 2011 11:08 AM 

(In the voice of Forrest Gump)

And just like that, it was over. I don’t want to say anything bad about our “Rock guys,”

But they think this is caused by wind blowing dust into cracks. I just don’t know about that.

Maybe we will get there one day and find out for sure.


hortonheardawho


Posts: 3465

Reply: 305



PostPosted: May 29, 2011 11:30 AM 

It is now sol 2611 and MI's are planned for today. Yay!

Thanks one and all for the comments on the Meridiani mudcracks.

Some followup questions:

Can the meridiani sediments dry without cracking? If so, what is the relevant parameter?

What is the size distribution of the mudcrack polygons? ( area and depth )

How do these numbers compare to the size of individual blocks of sediment observed?

Ben, glad you noticed the darker blocks in Friendship 7 crater.

The variation of the interiors of the Mercury craters was quite remarkable to me - from virtually no rock exposed in Faith 7 to extensive berry laden evaporite in Friendship 7 and the "burply" rock in Freedom 7

All these crater images are 2X, so it is fun to crawl around in the craters using the original size images.

There are some interesting, curious "contacts" that need some 'splain'.

John Henry Dough


Posts: xxx

Reply: 306



PostPosted: May 29, 2011 12:59 PM 

on the very first page of the Tome d/loaded in pdf.
I wish to thank you horton.
also,,''to err is human'',,I was corrected on another very heavily moderated forum,,even
received understanding in my error(s)
also,,serpens, I feel I owe you an apology,
whether you acknowledge it or not.

also,,Very good descriptive work done here the last few days,Rock on,to Mars.
cheers
jhd

---------
These measurements have also shown large pockets in
the equatorial zone with H2O content as high as 8-10% in the upper ~ 1 m of
subsurface. These equatorial regions with relatively high H2O content present an
enigma. On the one hand, thermodynamic models predict that subsurface ice is not
stable near the surface in the broad equatorial region. It is possible that this is
metastable subsurface ice left over from a previous epoch with higher obliquity.
Alternatively, it could be soil heavily endowed with salts containing water of
crystallization. The fact that these areas overlap somewhat with regions of high(end of copy/paste)
--------------------------------------------
Thank again Horton,,three more awesome views to be studied and commented on!
---------------------------------------------
no reply necessary nor called for,but could the
ice be closer to the surface than we thought?

Barsoomer


Posts: 344

Reply: 307



PostPosted: May 29, 2011 2:04 PM 

Some good pancam images of swirl rock came down today. Great complexity. I hope Hort can work his usual magic. Also some other weird stuff. Apparent differential erosion carried to an extreme, leading to jutting planes with considerable angular separation.

Ben


Posts: 5

Reply: 308



PostPosted: May 29, 2011 2:46 PM 

Barsoomer; If surface water had flowed in the cracks it would have dissolved the adjacent rock and filled the cracks with the dissolved material.
Ground water did occupy subsurface space in the cracks as evidenced by your 301 but flowed little if any during which time the fracture filling was deposited.

Serpens: Periods of earlier dessication may have occured at Meridiani but IMO the surface we now traverse , represents the last episode after which deposition ceased.
I still believe it is a thin blanket of dessicated material draped over an uneven terrain.

Fred; Your 305 shows two stages of dust collapse into a subsurface void beneath the crack, the last one (narrower) may have been caused by nearby movement of the rover. Wink

JHD; The depth to present day ice is probably related to post ice-formation erosion which could bring it close to the surface in some places.

Hort; Dessication is limited to fine-grained (clay) soils and is a complex process associated with desaturation of the material. I am not sure what determines the size, but the depth of the cracks is probably related to the level of water saturation.
Please elaborate on the Mercury crater "contacts".
THere are a bunch of things that need explanation here.

Fred


Posts: 73

Reply: 309



PostPosted: May 29, 2011 4:07 PM 


Ben, let’s just agree to disagree on this one. I do see you subsidence but it was post event in my opinion. In other words something piled it “up” before it “sank” down. Subsurface frost melt is now the most likely candidate. When is unknown.

hortonheardawho


Posts: 3465

Reply: 310



PostPosted: May 29, 2011 4:28 PM 

Here are the Friendship 7 crater 3D saturated false color 3D pairs.

My favorite contacts are:

and

and then there is this dog's breakfast:


Barsoomer, I am awaiting the missing data from the sol 2595 L257R2 pan of Skylab crater. Yes, some strange rock shapes - as usual.

hortonheardawho


Posts: 3465

Reply: 311



PostPosted: May 29, 2011 6:15 PM 

sol 2597 ( May 15, 2011 ) clouds at sunset:

I am always looking for new ways to combine the data.

See the Flickr comments for processing details.

Yes, I think the clouds are "real".

Barsoomer


Posts: 344

Reply: 312



PostPosted: May 29, 2011 10:46 PM 

Just a taste. Fluffy white material covering black rock: an "oreo cookie."

Barsoomer


Posts: 344

Reply: 313



PostPosted: May 29, 2011 11:19 PM 

"Levitating" planes again?

Serpens


Posts: xxx

Reply: 314



PostPosted: May 29, 2011 11:31 PM 

Hortonheardawho – reference your 305. That is quite an interesting question – pity that we don’t have any sedimentologists posting, but let me do my poor best. Desiccation (or more precise shrinkage) cracks can form throughout a sedimentary structure, not just at the surface, effectively following the sediment –water interface downwards. Just to muddy the water (ouch) cracks can also occur below water level, particularly where brines are present where there can be loss of water from sediments to the brines. These synaereses cracks tend to have irregular, radiating structures as opposed to the polygonal cracking that is evident in desiccation cracking that occurs above the water table……most times.
The shrinkage cracks are influenced by the nature of the sediment, thickness of beds, compaction and even the depositional medium all have an influence. No hard and fast rules so far as I am aware – Ben? Bill? Some shrinkage phenomena can be huge as has been discussed previously. But the Meridiani upper layers do seem to reflect a sediment-air interface drying out.
Ben, this is more the realm of hydrology (or the loss thereof) and you can probably clear up a lot of my errors or misconceptions.

Ben


Posts: 5

Reply: 315



PostPosted: May 29, 2011 11:48 PM 

Hort's image of Faith 7 and a smaller crater behind it appear to be deep enough to have penetrated Meridiani beds, yet there is no ejecta involved.
A faint horizontal line in the far wall of F7
suggests the top of Meridiani beds so what is the explanation ?
Crater rim ejecta is covered by drifts of sand.
The craters could be very old and the ejecta eroded away but later sand would have filled the crater.
There was no impactor and explosive shock created the craters and destroyed the ejecta.
The craters are secondary and formed by materials such as ice.

Any other ideas??

Barsoomer


Posts: 344

Reply: 316



PostPosted: May 30, 2011 1:14 AM 

> Any other ideas??

I think the outcrop layer is getting thinner and more patchy as we approach Endeavour.

Serpens


Posts: xxx

Reply: 317



PostPosted: May 30, 2011 1:55 AM 

Ben, I posted a link to James Calvin’s panoramas of the cluster in the Geology section.
http://www.nivnac.co.uk/mer/index.php/mercury
Nice pics, although I think he got the Sol/crater designations wrong in places. I don't think we should automatically assume that these craters all belong to the same event. In fact the visual evidence implies different impact events Sigma, Faith and the unnamed shallow crater on the side of Freedom seem similar in nature and the unnamed crater seems to imply an event well after Freedom was formed. Based on the erosion of ejecta, Friendship seems the youngest of all. Actually Faith wouldn’t look all that different to Eagle except that Eagle had some outcrop exposed on one side and has a lander in the middle

Fred


Posts: 73

Reply: 318



PostPosted: May 30, 2011 7:54 AM 


Barsoomer,
Your 313. This white fibrous material was last seen in Endurance crater. Great image. Is this image close to our current location? Maybe Hort can focus on it and see if there is anything to it.

hortonheardawho


Posts: 3465

Reply: 319



PostPosted: May 30, 2011 9:37 AM 

sol 2611 ( May 29, 2011 ) colorized MI pan of Valdivia:

with a location link and a link to an animation showing the colorizing area of the pancam.

Barsoomer


Posts: 344

Reply: 320



PostPosted: May 30, 2011 2:45 PM 

Fred, the above is a context image for the closeups in 312 and 313.

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