As I recall, one theory had the large grains on the rover panels being formed by dust clumping during repeated frost/thaw events. Is it possible to determine if any of the grains are actual sand grains, or if they are all clumped dust?

There are several discreet sand sized particles sat on there around the wires - they're unmissable. Not clumps of the dust ( they would be the same shade as the remaining dust on the deck ) and indeed they're hidden in wind 'shadow' of various components.

Doug

Doug,

Thanks for digging out these references and re-iterating some of my points, particularly about the weaker gravity on Mars. You're not alone in thinking the dunes are an active formation. I found the first reference quite interesting:

"The winds on Mars appear strong enough to transport sand only at certain times of the year. A major reason is the planet's eccentric orbit, which produces strong winds primarily when Mars is closest to the sun. Strong winds also are due to the fact that a third of the atmosphere falls out of the carbon dioxide polar caps, and the motion of the gas creates very strong, highly seasonal and one-directional winds at high latitudes.

Until now, said Thomas, the model of the Mars atmosphere "was one of winds moving sand not very often and then pretty much in one direction at a particular spot -- and that's exactly what we are seeing." On a broad level, he said, modelers of the Martian atmosphere have the picture right."

I know Opportunity has been here for more than a Martian year, but it is still possible that we haven't yet witnessed the strong winds that might be capable of moving these dunes.

I take Bill's point though about uniformitarianism not being the same on Mars and what others have said about our Earth-centric views of geomorphological processes.

Sounds like all this fascinating discussion should be moved to a new thread.

Castor

No, no, no!

You guys have it *all* wrong!

The truth regarding cleaning/deposition events and dust movement is now out...

Bob Shaw

We certainly haven't seen anything close to the conditions of a complete global storm. Spirit landed just after a fairly large storm and we have two regional events since they landed, one of which was big enough to be seen clearly from earth. The last major planetwide storm was in July 2001 if I recall correctly.

Doug's link clearly shows to me that we are seeing evidence of proper dust sized particles being lofted about regularly not just micron scale dust that I thought was the case. Even though the picture only shows a few mini drifts on the sheltered parts of the rover deck that does indicate that a lot more dust has probably landed and moved on over the course of the last 2.5 years. Say we are seeing motion of a single layer of dust every two years then you could (in theory) completely build up or deplete a 30cm dune in 150-200 years without any change in overall conditions. Now grain sorting, surface cementation and the dynamics of dune formation probably rubbish that particular number but the possibility of this terrain being quite dynamic over human familiar time scales is interesting.

>Actually, that seems to be a myth, at least on macroscopic scales:

Interesting. So much for that urban legend.

But still, there are solids that are brittle under sharp force but plastic under steady force. Water ice, for example.

--Bill

Or the Ig-Nobel-prize-winning pitch drop experiment:
http://www.physics.uq.edu.au/pitchdrop/pitchdrop.shtml

Bob,
Are you gona use this image (even a little bit reworked each time) on ALL traits ?

Hey, that would be a pretty cool demo to do on a large scale...Get a large glass dome made in a park or something, and put in a giant funnel of pitch and watch it drip!

We are talking about the Mars that has recurring global dust storms covering large parts of the planet, the Mars where Spirit has imaged dozens of large dust devils, aren't we? - It is quite obvious IMHO that we are talking about a very active atmosphere shifting surface material around quite thoroughly. I'd say the dune pattern Oppy travels through is shifting in a time span of years, not centuries, millenia or eons...

Hey, it was a *lot* reworked - last time it was just a grab!

Bob Shaw

Here is my 2 cents (pence) worth. Doug, I am not trying to single you out in this debate, but as is often the case, you are in the thick of it and have zeroed in on several of the key points.

I have no doubt that current Martian winds can move sand and dust grains. But the issue is, can they now move these dunes/drifts/ripples. Regardless of some small-scale features that we have seen move, the fact still remains that the main features we see both in Meridiani and Gusev seem not to be moving since man has been observing them. That is the reason the Meridiani ripples and those in Gusev appear in the same location to the rover's eyes as they have appeared to the MOC's eye in images taken years earlier and later. There was an earlier discussion which I can't find, where some compared MOC images taken over the years of the same area. No discernible movement was discovered. That is one reason why the MOC imagery has been so useful in the fine route maps produced by members here.

Recently I have been beating the drum about the "armored" nature of these things. I didn't first notice that, although I can now see what they are talking about. This is something that was mentioned by several researchers at LPSC this year. Evidence was presented showing that the surface of the Meridiani drifts are covered with larger clasts than commonly exist within them. I think that is pretty clear, if you think about the MIs and close-up pancams we've seen from Meridiani. More difficult to see is the armoring of the ripples in Gusev, but they have looked closely at that, too, and presented good images showing larger sand grains and granules forming a protective layer on top of otherwise more finely grained ripples.

...I think wind can, and evidently HAS moved this stuff about.

I'm not sure we're ALL talking of the same thing i.e. what "move" mean! I'm not able to explain this scientificaly but I'll compare this to clouds formation in the mountains. There you can see clouds forming from "nowhere" then moving in the wind direction then next cloud forming exactly in the same place etc...
I believe that the general structure of each individual "dune" stay in the SAME place while grains /particules are actualy moving. What about that?

That would go against the "tiger stripes" we're seeing on one side of most of the dune ridges. From what I've seen, signs point to the dark stripes containing a significant fraction of basaltic origin dust, including olivine, the light stripes with lower content of the basaltic origin dust. How did these layers form? How have these layers been preserved if the dunes are active?

Dunes with obvious layers can be very helpful in assessing the depositional history of an area.

In some areas, like the serpentine dunes on the top of Husband Hill, certainly.

Bob Shaw