Hi Stephen,

Those pics are shots of the rover tracks that make the "movies" posted on this forum and on the MER website.

Tesheiner

Ask and ye shall receive, from the latest Squyres Update:

"And the feature that got us now has a name: Purgatory Dune. We weren't calling it anything back when we were stuck in it, but now that we're out, it seemed there ought to be a name for the thing. "

Very apt, indeed!

Very appropriate as we went in the wrong way. Next up....

Er·e·bus (r-bs)
n. Greek Mythology
The dark region of the underworld through which the dead must pass before they reach Hades.

There was slight further "back" movement in the last days, now front/left wheel reached the crest of the other dune (already treversed before stuck):
http://qt.exploratorium.edu/mars/opportuni...TQP2133R2M1.JPG

Interesting remark:

the new traces are over former traces, and there is NO slippage on these new traces, in places where there was on the former traces.

This is an argument for the slippage being provoked by some driving issue or a software problem, no by the terrain. If so, it is afterall a good new, Oppy can resume her treck without further problems with the soil (if the driving issue is solved, of course).

I wonder why they moved only one metre in one week.

Yes, and did you noticed also the impressive, rapid cleaning of the wheels, especially right one:

(images were taken less than 1 hour apart!)


I do not like to be the "devil's advocate", but another less comfortable interpretation is that present location is safer (read previous posts on this thread ).
We do not know what will happens when southward drive will be resumed, but I hope my suspect is wrong and your arguments are right!

Here is the forward panoramic view of Opportunity after he freed himself out of the dune.

Panorama taken with the L Navcam on sol 490.

jvandriel

And here is the panoramic view looking back after Opportunity freed himself out of the dune.

Also taken with the L Navcam on sol 490.

jvandriel

Just a little musing here. I've attached a picture, a little snip from the nice panorama.

The circles are where it looks like Opportunity broke through the crust. It looks almost like there was just thick dust before that, but then it really plowed in hard.
The arrows point to a small crest, or some slight change in the dune. Boundry area perhaps between safe and unsafe terrain? I'll have to see if I can find this in old hazcam pics, before the rover drove back over that area.


After a quick look at some of the first hazcam pics when Opportunity was stuck, it looks like the area in question is directly beneath the rover, almost immediately below the view of the hazcam.


Also, the two "lumps" of material beside the tracks are where the front wheels were after the rover stopped moving when it was initially stuck. (On the left side, outside of the track, near the circle. And on the right side, inside of the track, slightly below and to the left of the circle.)

So if none of my musings makes any sense, at least that can give some idea of where the rover was sitting a month ago.

Interesting observations, Jeff. The small crest you highlight fall exactly in the front wheel position at the moment of rover stuck. ; see contrast enhancement in the left insert of this image:

However, it could be a coincidence, because there are also other trasversal "sub-dunes" visible in farther in the image and especially because rear and mid wheels smoothly traversed this feature before got stuck.
The dipping you observed in this point could simply arise from the drilling action of the front wheels immediately after stuck, when rover executed many rotations before to stop electric motors...
I think we should search main stall cause in the region where rear (most advanced) wheels stuck, near the end of tracks. Now, looking to these NavCam images, this happened where main dune crest is broken; in this discontinuity region, my attention was hit from the bright patch, probably made of very fine sand with lower blueberries density (see top/right insert taken from a previous hazcam image).
Probably, this region exhibit lower grip and this, joined to deeper sand layer of the dune and the sub-dune you noticed, caused the stall. Do not rimember if someone already gived this explaination but, if it is right, in the future should be easy to avoid this kind of
surface features.

Was that feature there before Opportunity got stuck? Opportunity was in the same spot for several weeks -- that could have altered the wind and deposition patterns enough to start re-forming the dune patterns.

Some more meters today

http://qt.exploratorium.edu/mars/opportuni...UGP1211L0M1.JPG

http://qt.exploratorium.edu/mars/opportuni...UGP1312R0M1.JPG

Wonder if they have a turn planned for tomorrow?

But it is really great to see those clean wheels and the fresh tracks

Yes, they will turn around and re-approach the dune to investigate it with the IDD arm and mTes over the weekend...

David, if you refers to the bright patch with few BB, answer is Yes... take a look to this image taken on Sol447, ony 2 days after stall (I brightened the shadow region):


However, you are more probably speaking about the small crest highlighted by Jeff and I suspect you are right! In fact, applyng the same enhancement in the omni-present solar panels shadow, this feature was practically absent the day after stall and became progressively more evident in the following weeks:

...it seems to me they are going a little bit too far!
This is a short animated resume of full rear hazcam view from Sol 465 to 490:
http://img231.echo.cx/img231/5083/unstuckhazrearsmall5da.gif

The latest update explains all.

Sometimes it seems that Steve Squyres is a reader of this forum...

Something I first noticed today - it seems like the back wheels (relative to the rover) did a lot of the driving to get the rover out of the trench.
This is from looking at the panorama posted by jvandriel in this thread, and a high res color picture posted by dilo in this thread.

Now, granted, I don't happen to have a full scale rover model here to take for a test drive, but it just looks like the back wheels did an awful lot of pushing here. The churned middle of the tracks almost goes right up to the level of the rest of the dune. Note in that color picture though, the middle wheel's track. Compressed, and scuffed slightly (tread marks aren't readily visible), but definitely not scraped like the main tracks.
The navcam panorama shows a deep impression right where the front wheels were. It looks like they just flat out dug in for a little while, until they gained solid footing. Whether or not that compressed dirt that follows was crushed by the rover, or if that's the duricrust, I'm not sure. I wonder that it is the duricrust (duracrust is it?), since there are some "flecks" of it behind where the front wheels sat, as though it broke some off and turned it under the wheels as they sought solid ground.

Again, just some more random thoughts on the tracks.

I hope a stereo image is of some use in getting a different perspective. It shows how far the rear right wheel went over the top of the crest.

Here is the newer perspective.

A panoramic view of the wheeltracks from another point of view.

jvandriel

Great image ! I don't understand though that they dare to move around this "quick sand" just like that ! I thought that driving oppy from now on would take ages.....It doesn't ! (which I like with respect to moving on, but on the other hand i'm afraid history repeats...

just a thought:
from this image it seems that the danger of digging in could be
"simply" a function of the slope/steepness of the dune flanks (rather than
soil composition differences) ...
Oppy crossed the "flatter" dune in the foreground several times with no problems.
The Flank of the "bad" dune (Purgatory) seems to get considerably steeper just where the wheels started to dig in (of course this could also be an optical illusion effect from the differnt perspective of the image)

I agree it. The steeper, the rover needs greater velocity to overcome it. It is supposed that the rover has no accelerations but at slow and constant speed. Driving on sand dunes is very alike to flying. The pilot must know on where, when and how much is to accelerate or deaccelerate according to the slope pending, height and kind of sand.

The case of Oppys, it must look for a less steep slope and a pavement martian land.

Rodolfo

Rodolfo, I think you are correct as you can see from this just how steep this dune face really was

Richard

A difference in composition of the ground should play a decisive role anyway. Thereby as steeper the dunes as firmer have it to be of course. We have seen different deep tracks from Oppy's wheels in the past and when I look this recent Hazcam pic, then I can't believe what I see - the monster dune seems to be firmer than all ground around here...

http://qt.exploratorium.edu/mars/opportuni...VEP1311L0M1.JPG

Yes I peeked in here after checking Exploratorium myself, and from the image posted below I got the feeling they did a short drive then backed off snapping one image in one attempt to check on the soil and if its safe.

http://qt.exploratorium.edu/mars/opportuni...VEP1914L0M1.JPG

The image in the last post shows a track in the 'valley' west of Purgatory Dune... it's part of the 3-point turn they are making. This image doesn't show Purgatory itself.

Phil

Yes im fully aware this is all part of the somewhat roundabout turning around to Purgatory.
My post was to suggest they might be on the lookout for possible egress routes at the same time, and since its firm sand it looks good to me at least.

OK all you thinkers, this is a hard question that i am wrestling with for days now:

I am completely puzzled about this surface material. How on Mars can there be such a treamendous difference in mecahnical properties, while the surface itself is so homogenous by the looks of it. Right now it's like oppy's on a pavement, two weeks ago she was in it so deep ! I don't have the feeling that some kind of crust does the job (i don't see coherent fragments near the tracks). I don't think the angle of the wheels have something to do with it either. And my former idea that it should have something to do with the density of (salt richer) sediment, seems not to make sense either (the surface of the purgatory dune looks the same in color, grain size, etc.).

Could it simply be the influence of thickness of the loose material on the evaporite layer underneath, that is thicker on the dunes (so it can move sidewards more easily) and thinner in between, the latter being compressed mostly downwards, so the underlayment does most of the support ?

The sand has own properties. The sand land has greater slippery than a pavement land. Hence, to climb a sand land, needs greater speed or lower pending degree (around 10 degree -depends on how loose is the sand-). The firm pavement, the pending degree would be greater as long as the tires start to slip. The purgatory sand has a bigger pending and height than the rest. If Oppy has hability to runs faster before climbing, it sure will pass over there. The loose material is found more often on the crest and its side where the wind goes down. Here acumulates the loose sand. The valley are generally firmer than the rest. These are according to my earth experiences.

My guess is the geometry of the dune combined with the path angle Oppy took. At that angle the dune is somewhat cone/wedge shaped. Think of a big conical wedge being shoved under the wheels. The wedge would tend to force the left and right wheels apart from each other but given the semi firm material (and the solidity of the rover), the *inside* edges of the wheels would tend to dig into it. As soon as they started to dig in, this caused greater resistance and thus wheel spin, which in turn, made it dig in still more.

It all depends on the angle over the dune, along with the size and the particular geometry of the dune that would determine whether the rover would start the vicious circle of digging into it as described above.

I think that's it, there is nothing special about the mechanical properties of this dune. They have been getting gradually higher for some time. If you look back to sol439 where Oppy almost did an unintended 3 point turn it looks like it was on the verge of digging in there. It sailed off course because its right hand wheels were draging and the wheels ruts are quite deep. It just needed a few more degrees of slope for the inevitable to happen. The point is that it did not seriously endanger Oppy just wasted 10% of its entire time on Mars so far, something that we can't afford to happen too often.
Richard

As I recall, another two points very important about the performance to navigate among the dunes is the weight and surface contact area.

1) The less weight, the easier will the Oppy climb, travese dunes. Now the Oppy weight in Mars around 1/3 of 185 Kg Earth (61 kg that is much easier than ones of 185 Kg.)

2) On Earth, I can deflate the tires so that the tire will have increased surface area contact. Otherwise, the Oppys cannot do it, the wheels have fixed size. The wider and deflated tires, will also helps to increase the dune navigation performance.

For the Oppy's case, Oppy should have wider wheels as an compensation to meet to the Meridian Planum's geological requirement (lots of dunes) and also for Spirit with the Gusev Cracter (better mountain climbing and downhill capabilities).

Rodolfo

I think you're right. They might want to re-think the design of the wheels for MSL to increase the surface area and perhaps increase traction. If we're only going to get one MSL, we need to make it as capable as possible of traversing any of the terrains it's likely to encounter.

And from what I can tell, there are sand/dust drifts and dunes overlying almost every terrain on Mars, so it makes sense to design your vehicles to be able to handle them.

-the other Doug

It should not be too difficult to redesign the current wheels so that they have some kind of spring loaded telescopic section that pops out the outside of the rim and locks in to place to extend the wheel width by even 50%. This overcomes the storage problem when it has to be packed down and folded into the cruise stage without the need to redesign that.
Richard

Here is another view of the Wheeltracks leading to the trap, taken with the L Navcam on Sol 496.

jvandriel

The same panorama for Sol499 (final approach to Purgatory dune?):

NO! OPPY NO!

Don't tell me they're *really* going to approach Purgatory from the exact same direction they approached it when they got stuck????

-the other Doug

Why not?

If Oppy only goes as far as the tracks are a centimetre or so deep, it wont get stuck and the trench will be reachable with the IDD.

I think the direction of approach makes no difference regarding driveability (I know a lot of people disagree on this). The dust is so fluffy that a rover will just cut right into it, regardless how shallow the angle in relation to the dune. Therefore it makes sense to approach from a know direction, where they can see where the deep part of the dust lies.

Anyway, they are to be taking it very very carefully, as usual. I just hope we wont see a week of silly one centimetre bump drives.

Yes, especially considering we have now been here for nearly 8 weeks.

Uh... it's pretty obvious exactly how far they can go before it's a good idea to stop...
(grin)

I still think they should do more work here - figure out the characteristics of this thing, what pressures it can take. If something like MSL or a follow on comes to Meridiani - we need to make sure it can just take something like this in its stride - so this investigation is worth while.

another week, 10 days, and we'll be off again I'm sure.

Doug

I reckon we charge in to it and not let the sucker beat us.

Here's the latest movie of the three-point turn, in Quicktime format (566K)

I think that there is no problem with stucking on sand as far as when the Oppy follows on the same previous track before attempting to continue after the end. I calculate that if Oppy continue runing on the same speed on that track, it will go further but it is most likelly that it will stuck again on around 20-40 cms further. Around the Purgatory sand is on a crest and it looks like it have lots of loose sand.

I would like that the Oppys approach specially on the many intrigating dark circle to analyze it and be able to explain us about the difference color. I don't think that they are obliterated minicraters but they must have another chemical composition that reacts differently with perhaps water than others.

Oppy's 500th Sol today!

A few days old - but I made this in response to a little troll on another forum



Doug

Happy Happy!




Which one? Never mind, I'll look it up...

--Bill