OT:

A little troll, eh? Trolls are b-i-g! You're not getting hobbitually confused are you? All I can say, is thank goodness for the UK's National Elf Service... ...soon sort you out, once you're on the list to get on the queue...

A little droll, eh?

I'm somehow reminded of the climax of the film "Tin Cup." Just keep driving in, getting stuck, pulling out, driving in again... and sometime around Sol 800 we'll finally get through the thing!

-the other Doug

A panoramic view of the inside of the wheeltrack at Purgatory Dune made with the L7 pancam on Sol 489.

and the same view taken with the R1 Pancam on sol 489.

The Purgatory material seems so cohesive and stable them just turns to dust... ...curious stuff! I really do think that the dunes are very much older than we might expect from Terrestrial analogue sites, and that the firm duricrust is a fossil of past surface dust movement rather than an indicator of current conditions.

Bob,

How do you know about the age of dunes? I suppose you count the number of craters around there and also the presence of marks of lava's flow???

Rodolfo

Rodolfo:

That's the problem, isn't it? We've got no real way to date the things, other than (perhaps) surface changes as a result of long exposure/re-exposure. All we can really say is that we don't yet know, but that's the sternly scientific view - the romantic vision of marching dunes, psaltating grains a-hopping away, which we have from Earthly deserts (hell, beaches!) makes us tend to see the dunes as younger than they are. I'd bet on them being rather older than we'd expect...

Bob Shaw

Even though I'd hate to see it, there is a very good argument for turning Oppy around and having her retrace her steps all the way back to Eagle crater occassionally analysing her tracks to build up a better ground truth model of depletion\deposition over the year+ she's been in Meridiani.

Does anyone reading this have any idea what those rates might be? 1mm/year? 1mm/century? 1mm/megayear?

Well - we have a 6 month sample by re-visiting tracks just after leaving Endurance, and we have a new sample of about 6 weeks here at Purgatory. I think this gives us a two points on the graph that probably dictate the pattern.

Doug

I wouldn't say there is a good argument for going back to look at changes in the tracks. Sure, you would learn something... I guess... but the potential to learn a lot is much greater in Erebus and beyond.

Phil

Left front wheel is starting to dig in again. Didn't get very far on that sol - drive fault?

Hard to tell if its digging in or its just crushing the crest of the dune. More likely the latter.

Later that day:

http://qt.exploratorium.edu/mars/opportuni...VMP1214L0M1.JPG

Oh well, I got the sols mixed up... Time to go to bed...

I think you could get an idea from orbital photos of the tracks over time.

scott

"I think you could get an idea from orbital photos of the tracks over time."

I think it's already been reported from relatively recent MOC images that the older tracks have lost contrast.

And the impact place of the heat shield which is even still older.
With it there are already three points on the graph.

Maybe it would be better if they started driving again. Don't want them to get stuck again.

There's no doubt that the tracks have become less clear, but that would only take a tiny amount of dust being swirled around. We *do* have a generalised dust sample device, in the form of the upper surface of each rover - and neither is showing much at present. There's the magnets, too...

...but whether a little of the global dust 'payload' being moved around is enough to have made the surface features, that's another story. If somebody came along and said the dunes were 100 MYO I wouldn't blink!

We also have the sample trenches dug by the arms of the Viking landers. One of them I believe lasted for many years..Viking 1 just over 7 years I think? Those trenches i dont believe showed a great deal of change either.

No. Don't go back. Erosion and deposition rates can be derived from a lot of data that we allready have (yet, still has to be analyzed though). We have an awfull lot to learn about mothermaterial underneath this dunes in order to find out about earlier (wetter ?) processes and compositional data (though, the instrument sources are starting to be quite depleted i'm afraid, anyone has numbers on that ?).

For me, the one and only target nr. 1 is Victoria. Thats new science.....but it is also further away than ever We'll see.

Marcel

PS: Erosion rates at Mars nowadays have been studied extensively (i.e. by Dr. M. Golombek (MPF) and others). There's a lot of variability between sites and between materials. Deflation/depositional rates vary between 1 mm to 0,01 nm /year. There's also strong evidence that it has been VERY different (higher rates) in the past, compared to today. By the looks of it (not scientifically assessed, just my gut feeling as a ph. geographer) i'd say, that the dunes have been there for tens or even hundreds of millions of years, slightly changing over time. Wind action and erosion wiped out/covered all ejecta blocks (there's none present, besides "Fram" ejecta) around, which takes an awfull lot of time !

Recent micro - images show a mixture of berries and fines. I assume that berries are too large to be wind transported and this would suggest that the dunes are not as recent, having resulted from some other action besides winds alone.

On the other hand, this mixture of sizes creates a very strong matrix (highway bases are made of mixtures in size similar to this, to create a very strong base for pavement). I would think that such a mix should support the rover well. The fact that it pentrated the dune surface and dug in suggests a much weaker matrix such as one that assembled by wind blown fines. These dunes would continue to reshape continuously as winds shifted material.

On the other, other hand, I believe round shaped gravel, although it initially compacts better than irregular shapes, also behaves more like a fluidl than irregular shapes when disturbed laterally such as by wheels rotating.

On the other, other, other hand . . .

The dunes are currently evolving. We saw the difference in the 6 month old tracks at edurance and we saw it while they were stuck for a month. I will agree that the dunes have been there for a long time. The same way that the beach by my house has been there for a long time. The beach by my house is always changing and so are the meridiani dunes. It's not a case of how old - It's how fast?

Old and New Tracks

The older track in this image clearly shows the re-exposure of blue berries after Opportunity stomped them into the soil. That's erosion happening during the mission. And the soil had been compacted, to boot.

The blueberries might (big 'might'!) be subject to some process similar to frost-heave which could make them jump out of the soil unless they're anchored by the duricrust...

Over a six month period? Doesn't that kind of process requires volatiles and you wouldn't you see a little pile of dirt around them as they erupted? I don't really know that much about it. Does anyone else think that might be what's happening?

ed

I think that the wind is simply eroding the fines from around the blueberries. I can't envision a mechanism that would make the blueberries pop out of the whell track.

--Bill

There is a something that would do it but it needs quite a bit of ice - in permafrost zones on earth there's a process called boulder heave which pushes "solid" objects upwards out of soil that goes through a regular freeze\thaw cycle.

It might work without water - if the "soil" matrix has a significant thermal coefficient of expansion and the blueberries don't it could work just as well where the "soil" expands when heated.

Bery unlikely though as it would require that the soil is very well compacted which theimmediate zones around the wheel tracks don't appear to be.

My guess is that we're seeing mostly chemical change with a little bit of wind erosion that removes very fine material thus re-exposing blueberries\coarser dust.

Sol503 360deg panorama (Navcam):

On Earth, it works in temperate zones, too - farmers used to harvest stones off their fields in the spring, and gardeners still do. It wasn't a terribly *serious* suggestion, though!

The only difference that I see the marks of Oppy's trail between 5 weeks is that the older is somewhat burried by very tiny dust (less constrat visual) than the new track ones.

Rodolfo

Here is dilo's sol 503 pan in polar form... quite good for showing the activities after getting free.

Click to view attachment

Phil

To see year-old tracks they could go back several km, but to see month-old tracks they can sit still and look over Oppy's right shoulder! The preferable choice is obvious.

Remember, too, that while there are a number of final failure modes for the rovers, one of the more likely sets of failure modes would leave us with a working rover that can't move -- or at least can't move very far.

The time to study changes over time is when we can't move and have very little left to do except scan our surroundings for changes. In fact, I think perhaps that the final extended mission for the rovers ought to be such a thing -- measuring local rates of erosion and deposition. That will be especially useful at Meridiani, I think.

-the other Doug

How far has Opportunity travelled since it was freed? I say we keep

Not much going on here, no drives, no IDD work. We are free for 3 weeks now but still stuck. A 180 turn in this time is bad, even for restricted sols and low solar power. I'm not for 100m drives per day, but some progress, science or movement.

Are there any problems with the rover? Or is Spirit the star this month?

Analyst

Steve's next update at http://athena1.cornell.edu/news/mubss/
will tell us ...

Wind tails!

We don't need to move or wait to see signs of erosion. There are obvious wind tails in opportunity's tracks from just two weeks ago. The dunes are active. And if you look again at the old vs. fresh tracks back at endurance, the cleet marks are filled in on the old tracks.

nav cam - sol 484
nav cam - sol 494

pan cam L2 - sol 484
pan cam L2 - sol 496

These new tracks are changing fast compared with the endurence example.

That was a very insightful suggestion you made, even though you backpedaled a bit later, but I think it might be something to it since I have considered frost-heave as one explanation myself for those bluberries that appearently pop up.
We already discussed the possibility here when I looked at the image of that meteorite near the backshell with a friend who asked how it could be sitting so neatly on the surface. My suggestion then was indeed that it could be boulder-heave that have pushed it up over the millennia.

So I rather think you should take credit for it Mr Shaw, since I was too uncertain to speak up. I was simpy too hestitant airing the explanation since im fully aware that Mars is bone dry in comparision with the conditions when that happens on Earth.

Humidity could very well get trapped in the sand, and perhaps it doesnt need to be especially much ice as helvick suggested. The thin layer of humidity around each grain would expand as it freeze to ice and with a lot of sand it could result in quite some movement.

Regardless, on Earth the atmosphere do keep a bit of humidity even below freezing temperatures, and that very small fraction eventually freeze out when it gets really cold which it does on Mars.

So the bottom line is that I dont want to rule out the idea of frost or boulder heave yet.

(Blush)

Ideas are *easy* - numbers is difficult!

Speaking of ideas... ...'frost heave' might be responsible for rocks migrating to the surface, but perhaps the mechanisms are not exactly as on Earth. Here, we're looking at things like ice expanding under rocks, while on Mars it might not work quite that way (obviously, ice still expands as it freezes!).

Perhaps it works *backwards* on Mars, with winter hoar frost leading to a salty dribble in spring which then either makes a sort of mulch which becomes duricrust, or - where there's a rock - does some heaving. Then all the water sublimes away, having gently distilled the soil...

We saw what may have been polygons at the Viking 2 site; saw plastic soil flows in Endurance and around rocks at Larry's Lookout; and have numerous bits of large-scale evidence for past surface ice effects, so it's not too speculative, is it?

And the notion that the Heat Sheild meteorite was heaved up to the surface is v-e-r-y interesting, too!

Yes, I've been watching those for a couple of weeks. I think the tails are especially noticable because the disturbed soil in the ruts is very fine evaporite material which is easily windblown. More clues as to the nature of this odd stuff!

--Bill

"restricted sols". This is what happens when Earth time and Mars time line up inconveniently...
So Oppy is almost inactive because they cannot pay people to work at night???
Or it is a DSN comm issue?...

It's not a matter of people working at night - it's simply that the planning team involves the scientists and engineers, and for more than a year, they've been working from their home institutions on a normal work schedule. You just can not maintain mars-time for a prolonged period - it will burn you out.

Doug

I just find it curious that Opportunity has spend 3 weeks out of the dune and the only things it has done is turn around a do a simple soil measurement.

Look at how much Spirit has done in the last 3 weeks

Indeed, numbers and calculus for things like this are way beyong my capability.

But I think you are right again, When the humidity evaporates it would indeed leave the salts behind and could very well be the process that create the duricrust, and yes it would be unlike how it happen on Earth since the water go from frozen to a gas almost instantly.

To speculative? Not at all, images from orbit have shown several, please bear with me when I go completely off topic for this thread here. As for past surface ice effects I have spotted a number in images from Spirit. Have a look at the image I hotlinked below. At far right you might see a small darker conical hill.

If I would see one such conical pile in the arctic here on Earth I would instantly say its the kind of rubble pile left behind by a former glacier. The pile of rocks are deposited by the melt water creek that flows under the glacier where it is accumulated at the end of the glacial flow. Now this is Mars, so I dont say it really is that, but it is found at the end of a valley and the soft slopes of the Columbia hills could have been given their shape by glaciers.

http://marsrovers.jpl.nasa.gov/gallery/pre...s-A108R1_br.jpg

Heat shield meteroite notion: My time to blush, I live in the northern temperate zone and have been one agricultural worker (growing potatoes), so the process of frost heave is very familiar to me. Some scientists considered it a mystery in the same league as paranormal phenomena, others simply said we lost a lot of topsoil with the meltwater.
Despite that we could see a large number of rocks that appeared in the fields each year when the snow melted away. And had to spend considerable time removing them every time before we could start growing our potatoes, if we didnt the rocks would get picked up by the harvesting machinery since they were of a similar size.

Yet it was in one of the last years when I had that occupation that a scientist actually was able to 'solve' the process behind it so the ability to use those 'difficult numbers' doesnt always provide a straightforward solution to a problem of this kind. So i would say that sometimes ideas are good enough and we worry about the math later.

Doug, I understand the issue, but I still disappointed... Is sad to see Oppy inactive, especially after the fantastic work they made to free it! (Do we need and asiatic MER team? )
Anyway, do you know, approximately, when it will possible to restart normal activities?
Thanks.

http://athena1.cornell.edu/news/mubss/

New update.

Squyres gave a reasonable update. It is a concern that Oppy is making such dismal progress, but I think that these conservative movements are a side effect of the complacency that had developed. Last month the Rover was nearly turned into a Surveyor when she hit unexpected terrain.

--Bill

Restricted sols are an every-now-and-again thing, not a "rover off for 9 days" type thing

They mean that the rover can do stuff, but just nothing that requires people in the loop. It could spend the whole day doing a full remote-obs day or something like that

Doug

Oppy just put her arm out to examine the dune on Sol 505. From what I can tell of the microscopic images, some of the "pebbles" or "blueberries" were buried pretty deep in the sand. I don't know if they were the same blueberries found at Endurance and Eagle craters.

(Edited so I could attach the image, I didn't realize it would be so big. Trying to be kind to those with dial-up, like I used to have for so many years before getting cable)

Here's another shot of Oppy sticking out her arm at the dune that nearly snared her.