I think a lot of these "purgatory" problems are related to the steering actuator problem and the turned wheel. Didn’t the first “Purgatory” happen shortly after Oppy started driving with the turned wheel?

It would appear that the Meridiani soil is composed of a firmer surface crust, and softer material below. If you've ever walked on deep snow in the late Spring you can understand how this works. When you walk carefully and slowly, you can walk on the harder crust on top of the snow. If however, you twist your foot or do anything sudden, you break through and then it becomes difficult to get "on top" again. (n'est-ce pas, Climber?)

My sense is that by pushing or pulling the turned wheel the rover is putting extra force on the crust. Where the crust is thin, or when the rover makes things worse by trying to turn or making contact with a dune at an inconvenient angle, enough forces add up to break the crust and make contact with the much softer material below.

In a nutshell I think this recurring problem is being caused by the turned wheel.

Can't add anything; I think too this is the root problem.
The effects are evident after turns on soft sand.

EGD: So Opportunity keeps getting stuck because Spirit has a turned wheel?

and as BrianL says: the front wheels are sitting on top of the drift. So long as Opportunity can resist the tempatation to polkas and waltzes, she'll be fine.

Click to view attachment

Remember that Opportunity has a stuck *steering* actuator since before Purgatory-I.

The fact that the front wheels are on top *may* be fine; we don't know it a Purgatory event starts with the front *or* the rear (or both) wheels slipping. Once a set of wheels start slipping, the other ones will start digging in the sand.

Anyway we are just guessing; the rover drivers have the data and, imho, the most important data of this drive is if it ended nominally or by slip checking.

I've been thinking that is a possibility for awhile, and I think we are now in a position to determine if that is the case.

In a slightly larger nutshell, these embeddings would then start with that wheel breaking through and start acting as a brake on the rover as a whole. Much like the Spirit stuck wheel did. With that resistance, the other wheels will then start slipping and they will also eventually start working their way through the crust until we get full embedding. That's my interpretation anyway, not sure if that was yours.

If that is the case, we should be seeing the start of an embedding right now. If the next day's drive sees us embedded in all wheels, that would seem to prove it correct. On the other hand, if it continues to move forward and simply pulls the embedded wheel along or pulls it back onto the surface, then I would say there is more to it than that.

Brian

Opportunity has a steering actuator problem that causes it to drive with a slightly turned wheel.

I agree with what you all say recentely. In French we'd say "nous marchons sur des oeufs" (we walk on eggs) and the comparison to spring snow is very relevant. Very hard to lift one foot when both get through the crust. One "sick" weel is enough to start the process. It has been proposed here to use IDD to compact the soil. I start to think that we could even use it as a stick to help to get back to firm ground.

EGD: I humbly apologise for my faulty memory on wheel actuators. Do you happen to know/recall how angled it is?

I still don't think Oppy is stuck at present and she's likely to get over this rise from where she is now.

climber: I totally reject any use of the arm to support or move the weight of the vehicle. It's not built for it and breaking the arm would be a huge loss.

Not yet I totally understand about the new situation. My first impresion is a normal one with somewhat stuck but it is good since it is an early avoidance. It is logical since Oppy was trying to climb over a very soft crest in spite of the fact that it has no much slope. So that zone needs a very careful study of sand consistency (pourous or compact) before to trace a route plan.

Rodolfo

7 degrees

Yes, 7 degrees toward the left, or inward, iirc. That was on the right front wheel, which would have been the left trailing wheel as Opportunity encountered the recent WEH.


That was a bit scary, wasn't it?

I wanted to address the concept of a crust on the Meridiani soil...

There are so many things to consider when analyzing these WEH events that it boggles the mind. I'd like to focus a discussion on this concept of a crust that the rover might be breaking through. While I can imagine that the Meridiani soil has a very weekly cohesive surface crust, I always had trouble buying into the concept that it was even remotely similar in physical strength to the icy crust that sometimes forms on earthly snow. I am familiar with the experience of treading softly on deep snow covered with an icy crust. If one is cautious one can walk across it without breaking through.

I don't see anything like that happening with Opportunity. In the earthly snow-bound situation, the crust fails beneath and around your feet, creating a number of surrounding broken plates of icy crust that dip inward toward the depression you have just created. We do not see anything like that in the vicinity of Opportunity's recently troubled tracks. We should see large fragments of the purported "crust" all around the disturbed tracks, but we don't. There are a lot of pieces of compressed soil that fell from between the wheel cleats and a few small and quite thin fragments of a weak crust, but nothing that looks like crust strong enough to support much weight.

I thought it might be fun to compare Opportunity's footprint to that of a human with similar mass. While the applied forces on Mars would be roughly one third of those exerted on earth, the conclusion will be the same. On earth, the rover weighs about 185 kg, or 408 lb. Essentially the mass of two big guys, or one quite large guy. The rover has six feet, each roughly 4-5 inches in width. If we conservatively assume that on firm ground each wheel is contacting two linear inches of supporting surface, each wheel is supported by a maximum of 10 square inches of surface, for a total of 60 square inches, and a resulting average force of 6.8 lbs./sq. in. (Divide that by three to go from earthly force to ~Martian force)

A very big guy on earth, weighing 400+ lbs with two shoes contacting 3.5 by 10 inches each of supporting surface, exerts a force of about 5.8 lbs./sq. in. So, a very large person, carefully walking on Earth or Mars would exert a compressive pressure similar to that of the rover on the respective planets.

That tells me that if there is a crust that supports the rover over a softer subsurface, it would have to be substantially strong...strong enough to support a large person. It seems to me that a crust that strong would have to break up into rather sizeable fragments if something pushed through to a soft subsurface layer.

I like the analysis, Tom.

I too find your analysis interesting. But I feel that you may be making some Earthly assumtions.

Firstly, we realy don't know the properties of the soil over which Opportunity is driving over (except for what we can see). For all we know, it may behave as a non-Newtonion solid.

Secondly, we don't know how this crust is formed. And on this point I have to agree with you, the properites of this crust is not like an ice crust.
If there is a crust of any consequence, I think it would be more analogist to what you would find in an earthly desert. It could possibly be a salt crust, formed as the result of what little water we have in the atmosphere, reacting with the salts we have seen in the soil. Breaking through this type of crust would not necessarily create large clots, it could instead, just crumble to dust.

Having recently been walking over late spring snow crust in the Scottish mountains, I can offer the observation that there are various types of crust formed in different condtions of snow depth, exposure to wind, exposure to the sun, etc. The "platey" type described requires strong LATERAL ADHESION between the surface particles to create the plates that break in the way described. But other conditons may still provide a strong weight-bearing surface layer and a softer under-lay, without plates - it's just that the top layer is stronger, it takes the weight of a foot, but when it occasionally gives way, this happens tight around the edge of the footprint. and the leg suddenly goes down a straight vertcial hole.

Looking at the recent wheel pics, I see a very thin (salty?) crust but that's obviously not providing the main support for the Rover when it's successfully staying on the surface. The weight is borne by the compaction of the the sub-surface sand, and where that is less, it ploughs in.

Kenny

Thank you all, for commenting. I think that this is a significant question to ask, considering how popular a supporting crust seems to be in the various discussions. I don't claim to know the answer, but we should be able to come to a consensus.

Edit: Moved my reply (in my own clumsy way) to the moving south thread. Clearly, they are not stuck.

Brian

While I well buy into the hypothesis that older, filled in small craters might lend themselves to being filled with less compacted "soil", I'd like to propose a thought that there were dune building stages, and that at least one of the later stages deposited material that hasn't compacted well... or well enough, in the case of Opportunity. I read the abstract that was recently linked to off of this forum suggesting the current winds are less than half of what created most of the dunes -- I'm not a dune morphology expert, but could a different set of winds, represented by those transverse dunes we're now seeing more of, move some of the material in such a fashion that it hasn't had the time(?)/conditions to compact to the same level as the more compacted dune material? Perhaps move more of a particular source contributing material, one with a higher percentage of material that doesn't compact quite so much?

Or is it just a coincidence that this latest "sandtrap" occurred on the lee side of one of the transverse dunes? Obviously the transverse dunes weren't apparent in the earlier "sandtrap" events for Opportunity, but that doesn't negate the possibility that lenses of the lesser compacted material may have been shaped by wind "regimes" taking place over time.

If so, I would also propose a final "scattering winds" event that covers the surface difference between various dune making events, hiding the transitions from the cameras on Opportunity.

An alternative theory would be for lenses of evaporite ejecta (not so much "vaporized" as "pulverized by the time it lands") covered by a light coating of dune material, where that pulverized stuff never would compact properly for whatever reason. This I would think would be more easily proven or disproven, and I don't lend credence to this "pulverized evaporite that never got a chance to compact" line of thinking, but realize it's also an idea that might be thought through.

And all of this pre-supposes that there are measurable differences between the compaction rates of dune material, and not merely just Opportunity breaking through when turning.