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Victoria Annulus, Discusions about Victoria's Apron
Bill Harris
post Aug 25 2006, 12:57 PM
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It gets more interesting on the ejecta apron. Looking at Phil Stooke's polar of mhoward's Sol 917 pan we can see a network of polyagonal traces on the surface. These traces are a darker and possibly coarser material evidently outlining paving stones of evaporite. The shape is reminescent of the jumbled and eroded blocks we saw around Endurance crater; they seem too irregular and too large to be the typical "mudcracks" we see on the evaporite surface. And the outlining traces seem to have a light-toned core. Hopefully we'll get a series Pancam closeups of these features and this won't be a drive-by sighting.

So it might be that the lighter tone of this location is not totally due to the windblown evaporite dust as I noted a couple of posts up. Based on what we see here, the sandy ejecta soil is quite thin at this spot, so there may be more to the story.


Phil's Sol 917 Polar

--Bill


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RNeuhaus
post Aug 25 2006, 02:31 PM
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The surface is so "ironed". Very plane and smooth to ride a surf. The surface has very fine grain covered by small spherules.

Enclosed pictures of Sol 918.

http://marsrovers.jpl.nasa.gov/gallery/all...5JP1785R0M1.JPG

I think that picture is pointing to South azimuth.

http://marsrovers.jpl.nasa.gov/gallery/all...JP1785R0M1.HTML

It is evident the surface has the erosion caused by the aeolian force.

Probably of North azimuth view.

Rodolfo
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Bill Harris
post Aug 25 2006, 11:08 PM
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Here is an L257 Pancam of the spot that Oppy trenched on Sol 918 We don't have a color Pancam of the tranch yet, but I've included an L0 Navcam of the trench. The granules look like Blueberries and the sand is well-compacted/indurated or is a very thin layer. Nothing out of the ordinary jumps out at me, but maybe we'll get some MIs of the soil here.

--Bill


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RNeuhaus
post Aug 26 2006, 12:04 AM
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This kind of surface is very easy to drive as off-road. Is indurated and compact as Bill has said. It seems like that the surface has undergone a process of some kind of cementation caused by some kind of chemical reaction.

Do you have any idea about why the surface has got indurated?

Rodolfo
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Bill Harris
post Aug 26 2006, 01:40 AM
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I'm thinking that one sub-cycle of the Martian hydrologic cycle involves frost formation at night and when the frost is heated in the early morning it already in contact with a sulfate salt and briefly makes a saturated "brine" with a low freezing point which soaks the underlying sand and dust. The water quickly evaporates and the dissolved salts cement the sand. The quantity of water is very very small, but this can happen millions of times, daily, over thousands of years and build up to appreciable thickness. We've seen the duricrust almost universally on Mars.

There may be more to it, but this is the quick explanation of what appears to be happening.

--Bill


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Aldebaran
post Aug 26 2006, 03:10 AM
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QUOTE (Bill Harris @ Aug 26 2006, 01:40 AM) *
I'm thinking that one sub-cycle of the Martian hydrologic cycle involves frost formation at night and when the frost is heated in the early morning it already in contact with a sulfate salt and briefly makes a saturated "brine" with a low freezing point which soaks the underlying sand and dust. The water quickly evaporates and the dissolved salts cement the sand. The quantity of water is very very small, but this can happen millions of times, daily, over thousands of years and build up to appreciable thickness. We've seen the duricrust almost universally on Mars.

There may be more to it, but this is the quick explanation of what appears to be happening.

--Bill


Bill,

I may have said this before either here or on another forum, but the Martian regolith contains a high proportion of salts such as magnesium and calcium sulfates and chlorides. These are either anhydrous or monohydrates, such as Kieserite, MgSO4.H2O. My understanding is that the hydration state can change depending on the temperature, and it's a very complex multiphase system. I don't think it can get to the saturated brine stage, simply because there is too much 'dessicant'. I think of Mars in terms of a planet sized vacuum desicator. I've worked with vacuum dessicators in a lab environment, and strange effects can result from phase changes, without involving any free water being produced. These include the production of filament-like crystals among other things. The constant diurnal and season variation over a long timescale can produce such duricrust either without any free water, or at the very least, extremely thin films several molecules thick, due to non equilibrium effects.
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CosmicRocker
post Aug 26 2006, 06:02 AM
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Well, all I can add to this discussion is that I was hoping we would not yet again see the apparently ubiquitous white stuff. I am not certain that we will really understand it until we dig enough to find and inspect its lower contact. I was hoping for a fresh and deeper roadcut, but apparently this material is quite compacted, and it is difficult to dig through. I discovered a wheel scuff done just prior to the arrival at Beagle. I forgot the sol, but it was only that, a scuff. It found the white stuff just below the surface. Some kind of dessicating salt model might work, but deeper observations are needed.

This is a fascinating planet. Just when you think you have a working model...Wham!


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Bill Harris
post Aug 26 2006, 09:34 AM
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Aldebaran, your analysis is correct, I was having trouble expressing that. I believe that the interaction between water and salts is involved but there is not enough water involved to make liquid anything. I live in the humid southeast USA and many of my views are shaped by living in a dripping atmosphere. In my office I have a piece of pyritic sandstone where the pyrite is reacting with water (etc) to make iron sulfate salts (et al), which in turn pull enough water from the air to sustain the pyrite+water+air=nasty stuff reaction.

But this is the eastiest explanation of the duricrust.

Tom, I don't think we'll ever NOT see white stuff on Mars and even without seeing the chemistry of it I'd suspect that it is a sulfate salt. Sulfides weather to sulfates and unless you have a lot of water to carry it away the salt tends to accumulate. We've see the light-toned material in the looser drifts every time we've cut a trench to purgatory...

Looking at the recent Navcam images I notice that the wheel tracks are light-toned and I suspect that the sand is very thin at this location and we are seeing a reaction zone at the sand-evaporite contact. We need a RAT hole in the wheel scuff.

Again, speculation on my part based on a lot of visual cues/clues and minimal data. Still, the truth will prove to be stranger than reality here on Mars...

--Bill


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ElkGroveDan
post Aug 26 2006, 02:48 PM
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QUOTE (Bill Harris @ Aug 26 2006, 01:34 AM) *
I don't think we'll ever NOT see white stuff on Mars

I was wondering about that. Are we perhaps looking at sulfates at the tail end of this Viking 1 trench? (Or is it, in the words of Roger Daltry, "Just another trick of the light?")

(source: http://photojournal.jpl.nasa.gov/jpeg/PIA00389.jpg)
Attached thumbnail(s)
Attached Image
 


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CosmicRocker
post Aug 27 2006, 06:31 AM
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QUOTE (RNeuhaus @ Aug 25 2006, 07:04 PM) *
This kind of surface is very easy to drive as off-road. Is indurated and compact as Bill has said. It seems like that the surface has undergone a process of some kind of cementation caused by some kind of chemical reaction.

Do you have any idea about why the surface has got indurated?

Rodolfo
I think the main reason the surface here is firm and easy to drive on is that the soft, layered ripple material is substantially absent. The underlying material is a good surface to drive on, and I am guessing it is pretty much the same material Opportunity drove on earlier in the trek, before the ripples came to dominate.

After looking back at many of the scuffs and trenches Opportunity has made along the way, I am nearly convinced that this white stuff is possibly best explained by some kind of a salt concentration mechanism, as it appears to be a horizon that cross cuts soil stratigraphy but is not continuous. The thing I am finding most interesting lately, is how looking back at old images leads me to new thoughts. It is very different from chasing the latest images for new thoughts. A lot of information has been collected that I had previously glossed over. There is still a lot of low-hanging fruit.


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Bill Harris
post Aug 27 2006, 09:25 AM
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This is indeed the beauty of having a continuous organized record of mission imagery: if we see something that is significant, we can look back through the earlier images. For example, the surface here is similar to the material around Endurance but not identical. And why does the rippled (or drifting?) material behave differently?

We sometimes complain about not having access to the "chemistry" data, but that is the "meat" to many a paper and thesis so we'll have to be content with the pictorial side of the dataset.

--Bill


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Bill Harris
post Aug 31 2006, 03:38 AM
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The first MI images of the ejecta apron beyond the rippled transistion zone are coming down to the Exploratorium. One of the early images is amazing-- there is clearly a bimodal distribution of the granule sizes; this speaks volumes about their origin.

More later...

--Bill


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glennwsmith
post Aug 31 2006, 03:51 AM
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Bill, one never knows where to post. Re the MI image you have referenced, I would be very interested to know your thoughts on my latest posts in the "Victoria here we come . . ." thread . . .
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Gray
post Aug 31 2006, 04:50 PM
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QUOTE (Bill Harris @ Aug 31 2006, 03:38 AM) *
The first MI images of the ejecta apron beyond the rippled transistion zone are coming down to the Exploratorium. One of the early images is amazing-- there is clearly a bimodal distribution of the granule sizes; this speaks volumes about their origin.

More later...

--Bill


Actually, Bill, I'd argue for at least a trimodal distribution. Don't forget the fines. tongue.gif

Were they deposited after the coarser grains; or do they represent incomplete winnowing of sediments that were deposited all at the same time?
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Bill Harris
post Aug 31 2006, 06:45 PM
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Well, maybe, technically, but let's look at the granules and not the sands and the fines. So there. biggrin.gif

The fines may turn out to be another issue.

Look at the trend to the larger granules to be pyramidal and not spherical or randomly sub-spherical. My initial thoughts are that these may be ventifacts since they seem to have a similar orientation. I'm thinking that the larger granules are "impact lapilli" or tektites, the basalt basal unit that has been melted, ejected and has formed droplets in free-fall. The smaller granules are hematite concretions, the standby Blueberries. The overlying evaporite unit was pulverized by the impact, thrown out as the ejecta apron where it weathered and eroded quickly, leaving the Blueberries behind. Methinks we'll find a lot of fragmented and residual evaporite under the desert pavement here.

My initial theory, but let's see what the Scratchplate shows. And we should be getting more MIs of the wheel trench this evening.

--Bill


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