[X] My Assistant

[Stu]

Well, what does everyone think of our long-awaited first view..?

[ngunn]

Bluffs and ridges are formed when a less-resistant (to erosion) rock is overlain by a resistant unit.
--Bill

Thanks Bill. It has to be erosion from below then - I'm trying to picture this process at work. It seems to me the base of the crater walls would be a comparatively sheltered place, not exactly a hotspot for aeolian erosion. I wonder if there is any other possible mechanism for mass wastage from the lower unit. Could the Victoria impact have punched through to a layer with high ice content which would sublime away once exposed?

[Stu]

I wonder if there is any other possible mechanism for mass wastage from the lower unit.

Well, there must be some amount of wind blowing about on and around the crater floor to produce and sustain those dust dune and ripple patterns... Maybe vortexes form down there sometimes, just gentle ones but long-lived, which sandblast the lower units? Wouldn't cause much erosion on their own, but cumulatively..?

I think activity above the crater floor plays a part too, tho. There are, to my eyes, quite a few scree spills and dust slides down the crater slopes (and something made that pronounced dark, tapering streak that slithers down the slope from Duck Bay), making me wonder if, during dust devil season here, the passage of dds nearby or even over VC disturbs the rim material and its structure sufficiently to produce mini avalanches of dusty material; we can see the southern area of the ripple field partially buried beneath material that has spilled onto and over it from the walls and regions above...

Just thinking aloud. I'll get me coat...

[mhoward]

I noticed in these thumbnail Hazcams from the short drive away from Emma Dean Crater that there appears to be some nice clouds above Meridiani. I've compiled the shots into a 517k .gif animation. The ground was brightened a bit to show more detail. I'm assuming these are clouds and not lens flare.

Good catch! They sure look like clouds, and the same formations are visible in both the left and right cameras, which would tend to confirm it, I think. That is a really cool sequence, because you can actually see the clouds move! Only thing is - they seem to be moving from east to west, more or less, and at a pretty good rate too. Is that normal, I wonder?

The observation times were from around 11:30 am to 12:30 pm, local Mars time, by the way, on Sol 943.

[dvandorn]

I think winds are intensified within craters on Mars, not reduced. For example, you might notice that a majority of dust devil tracks we see around the planet begin within craters. My thinking is that winds catch a crater wall and spin up into vortical forms. If conditions are right, these vorteces depart the craters and wander along the adjacent plains as dust devils.

In this case, you likely get the highest wind speeds along the circumference of Victoria, with the lag area of the winds encouraging the deposition of the dust that forms the central dune field. So, the most easily wind-eroded strata along the walls of Victoria would get fluffed off the walls (transported both out of the crater into the dark dust tails and into the central dune field), and the more-resistant rock strata become overhangs (or fallen overhangs). As Bill said, some overhangs form at the failure of the old rim along vertical joints and faults, while other overhangs simply slump onto the voids left below them and become "boat ramps".

What intrigues me is the evaporite layers, which we have seen are easily wind-eroded at ground level, seem to form the more resistant overhang layers on this rim. I'd be very interested to see what material is so much more easily eroded than the very friable, soft evaporite...

-the other Doug

[mhoward]

Here's a Quicktime movie of the Sol 934 clouds that you can pause and step back and forth. It would be nice if there were more frames of this yet to come.

[dvandorn]

Last winter on Mars, Oppy was inside Endurance. And, IIRC, she saw a fair number of clouds during that period, too.

It would seem that clouds preferentially form over Meridiani during the winter.

-the other Doug

[ngunn]

Well, there must be some amount of wind blowing about on and around the crater floor to produce and sustain those dust dune and ripple patterns... Maybe vortexes form down there sometimes, just gentle ones but long-lived, which sandblast the lower units? Wouldn't cause much erosion on their own, but cumulatively..?

I think activity above the crater floor plays a part too, tho. There are, to my eyes, quite a few scree spills and dust slides down the crater slopes (and something made that pronounced dark, tapering streak that slithers down the slope from Duck Bay), making me wonder if, during dust devil season here, the passage of dds nearby or even over VC disturbs the rim material and its structure sufficiently to produce mini avalanches of dusty material; we can see the southern area of the ripple field partially buried beneath material that has spilled onto and over it from the walls and regions above...

Just thinking aloud. I'll get me coat...

There is sure to be a feast of surface morphology to explain in Victoria. Thinking is definitely allowed! Most erosive processes though, including some you describe, tend to destroy vertical cliffs rather than creating (or at least preserving and accentuating) them. To do that you need differential erosion targeted specifically on the bottom of the cliffs. There has been no sign of this at the other craters visited so something is different at, or under, Victoria. In my mind I can't make sandblasting work because the sand would just pile up against the sides and protect the lower layers, moving the zone of erosion progressively higher. IMHO to sustain an undercutting process there must be a mechanism for completely removing the eroded or wasted material from the scene. On Earth the sea does this quite effectively as it forms sea cliffs, but inside Victoria there is nowhere for the stuff to go - unless it sublimed away.

[Stu]

Might frost formation/melting have some impact on erosion within VC too? The gorgeous pics/footage of clouds above the area show there's enough moisture here for condensation, so maybe the material around/inside VC is weakened by the formation and then subsequent melting of frost and made more vulnerable to wind erosion in that way..? After all, with all the berries and outcrop faces here there's a lot of surface area for frost to form on...

[ustrax]

I think winds are intensified within craters on Mars, not reduced. For example, you might notice that a majority of dust devil tracks we see around the planet begin within craters. My thinking is that winds catch a crater wall and spin up into vortical forms. If conditions are right, these vorteces depart the craters and wander along the adjacent plains as dust devils.

Does that mean that aside cleaning DDs, Opportunity will benefit of tranportation OUT of Victoria?...
http://i16.photobucket.com/albums/b14/ustrax3/ddfly.jpg
What will they invent next?...

Original image by Nico

[helvick]

Does that mean that aside cleaning DDs, Opportunity will benefit of tranportation OUT of Victoria?...

That's a bit extreme but the fact that Oppy had a number of cleaning events while inside Endurance, had a major cleaning event on the approach to Erebus and has now recently had some more when approaching Victoria makes me think that o'Doug might have a poiint.

[Nirgal]

Ustrax,

I just love your cartoons ... they always make me like "schmeiss mich weg vor lachen"
(= "throwing myself away laughing" ;-)

[Nix]

TMAL !!

this place wouldn't be the same without your cartoons.

Nico

[dvandorn]

In re the effect of craters on wind patterns -- recall that craters undergo regular cycles of differential heatng and cooling, too. As the sun rises, it heats the eastward-facing slopes of the crater more than the surrounding terrain, and much more than the shadowed west-facing crater slopes.

Air rises over the heated slopes and sinks over the shadowed slopes. The bowl shape of the crater encourages a vortex-like pattern of airflow, making it racetrack within the crater itself.

Now, as the day progresses, the prevailing winds of the area pass over this crater. The racetrack pattern pulls up and involves this wind in its little circulation pattern, causing a local intensification of wind around and inside the crater. In some cases, this causes a much higher column of rotating air than you get from the crater heating effects along, a pattern that can sustain itself for several minutes. As the air mass that contains this vortex moves along, the newly-born dust devil departs the crater and wanders out onto the adjacent terrain.

I would think it possible that morning heating would produce one rotation vector, and afternoon heating would produce another, perhaps opposite, vector. I seem to recall that most of the dust devils observed by Spirit last Martian spring were in the afternoon. Perhaps the racetrack patterns induced by heating effects in craters are counter to Mars' rotation in the mornings, thereby less likely to cause dust devils, but become synced with Mar's coriolis effect to end up with reinforced rotation patterns and free-standing dust devils in the afternoons?

In any event, we *did* see evidence of overhang development, vertical cliff maintenance, and internal wind erosion in Endurance. And, of course, it featured a central floor dune field. So, I think that there is good evidence that the same aeolian erosional patterns were in effect in Endurance as we will see have sculpted the interior of Victoria.

-the other Doug

[dvandorn]

Does that mean that aside cleaning DDs, Opportunity will benefit of tranportation OUT of Victoria?...
http://i16.photobucket.com/albums/b14/ustrax3/ddfly.jpg
What will they invent next?...

Original image by Nico

Wonderful, ustrax!!!! Of course, we're leaving aside the fact that the air is far too thin to support Oppy's mass...

However, I have these seriously cringing moments when I imagine her first attempt at *landing* after such a flight!



-the other Doug

[algorimancer]

...
Most erosive processes though, including some you describe, tend to destroy vertical cliffs rather than creating (or at least preserving and accentuating) them. To do that you need differential erosion targeted specifically on the bottom of the cliffs. There has been no sign of this at the other craters visited so something is different at, or under, Victoria. In my mind I can't make sandblasting work because the sand would just pile up against the sides and protect the lower layers, moving the zone of erosion progressively higher. IMHO to sustain an undercutting process there must be a mechanism for completely removing the eroded or wasted material from the scene. On Earth the sea does this quite effectively as it forms sea cliffs, but inside Victoria there is nowhere for the stuff to go - unless it sublimed away.

Good point, I've been thinking something along those lines as well. Awhile back (Post #136), climber jokingly mentioned what looked like caves on the far side of Victoria. From a distance, they do indeed look a lot like remnant sea caves, very similar to some I've seen in the Callahan Divide of the Edwards Plateau in West Texas (the Callahan Divide was apparently once an island when the sea level was higher). Then there's that nice horizontal line visible near the base of the evaporite layer; I know it sounds hokey, but could that perhaps be a remant shoreline? Considering that we know that the ground in Meridiani was once wet, mightn't that suggest that ground water would translate into surface water in a depression such as Victoria? Realistically, any open water would have almost certainly been topped by a layer of ice, so the apparent "sea caves" would have to form by some process other than wave action (perhaps sapping?). Sheer speculation, but the first good view over the rim could be interesting.