Looks like lots of chunks lying around. Its going to be very interesting to see how they have interacted with the sand du, er ripples. Which leads me to another thought...

There have been lots of discussions on the tiny craters, etc, and whether they are caused by sapping or are tiny impact craters. I've been puzzling over this for ages (I'm not a geologist, btw). It seems to me that the dunes must be static now, otherwise the wind that we know blows over the plains would fill in the craters by moving the sand. They must have been mobile at some point, otherwise they wouldn't have the wind-sculpted shapes. This means they *must* have been mobile at one point.

I'm finding it hard to understand how slow hardening could result in preservation of the ripple shapes, so it ocurred to me that it might not have been so slow. We know that the apron of Victoria is splash-like, which might imply water. So perhaps the impact released a cloud of water vapour that interacted with the very fine, very dry sand, and essentially fixed it in place.

Am I talking nonsense?

Chris

That's an interesting speculation, Chris. I think it's always important to keep in mind that some geologic processes happen very rapidly while others, the ones we usually consider, act very slowly.

There are others on this board who know more about this subject than I, but since I seem to be the first responder, I'll offer an opinion. Given the heat generated at an impact and the low atmosphereic pressure, I would expect that most of the water melted during an impact would quickly vaporize, or freeze and sublimate rather than precipitate as rain or mist. But then again, even if an impact resulted in a heavy frost, that small amount of water might help form a duricrust.

Let's see what the experts have to say.

It is also possible that the dunes formed when the atmosphere was denser, and better able to propel the particles they are made of.

Let's just say Mars has been fairly 'samey' in terms of condition for...shall we say 3GY?

And the air, because it's so thin, can move an average particle of soil perhaps 1cm per year

Over that 3GY - that soil particle could have travelled 30,000 km

Doug

Actually, the du er, ripples are armoured with larger particles that seem to have been left as a protective residuum, after their last movement...the fine and moveable stuff has been removed by the wind, leaving a layer of larger, mostly unmoveable clasts that protect the underlying mixture. I seem to remember reading that the current winds on Meridiani seasonally reverse, from the NW to the SE. I hope I got that correctly. But I do think that current thinking is that the current ripples (no pun intended), are essentially static. Either stonger or more dense winds would be required to significantly change their orientation.

I'm not sure that they are billions of years old. That seems doubtful. They are somewhat old. Since the ripple orientation has been remarkably constant from the start, it would be difficult to ascribe an effect from Victoria's creation as significant, though it might be interesting to look for such an effect as we approach her.

Oh - I wasn't saying they were that old - but it was suggested that the air is not thick enough to make such things....but over a very long time period, it's MORE than strong engouh.

Doug

I wonder. If I push something, not quite hard enough to move it, it doesn't matter if I do it for 1 hour or 3GY, it still won't move. The larger grains being slowly worn away by the very small stuff, on the other hand, will very slowly cause changes.

Chris

Well, I'm not so sure the time period was long at all. From one perspective it might have all happened rather quickly - then sort of 'froze' in its current state after only a few years. We'll never know for sure - despite what some would lead us to believe.

If this was (and it appears to be) the bottom of a sea that more or less dried up at some point - what we see could easily have been created by wind-blown sea water prior to final evaporation. How long ago could such a sea have existed? We don't know for certain... it could have been there a thousand years ago, and everything could look as it does today.

Wind obviously has had an effect, but to what extent it now effects the area seems rather settled - lighter winds have moved smaller particles, leaving behind the heavier... but still more or less in the same configuration (possible) that water left it in.

But, if Oppy runs into 80-90MPH winds - such a theory goes pooof.

Well - 12m/s as measured by Pathfinder, I've seen 25.2m/s with V1 data, 20.1 with V2 data - (27, 56, 45 MPH respectively )

Give how much movement we saw around the areas where the wheels dumped bright material after leaving Purgatory, and how degraded 180ish-sol-old tracks were on the way out of Endurance....I find it hard to imagine that over any lengthy period, say a century, one wouldn't be able measure movement in some of this.

Doug

Sattrackpro,

Sorry, I'm going slightly off-thread here.

What I think you are implying is that these dunes or ripples were created underwater aeons ago, but forgive me if I'm wrong. I would agree that the underlying evaporites were formed underwater (Opportunity has plenty of evidence of that). However, digging back to sedimentology lessons from my geology degree (many years ago!) the overlying dunes (and I prefer to call them that rather than ripples) are too large to have been formed underwater. These are most likely completely aeolian deposits (i.e. wind-formed) on top of the older evaporite pavement. Submarine sediments tend to form small-scale ripples on the scale of a few centimeters (much like the festoons observed in the outcrops), rather than the 2-3 metres dunes we see.

Whilst the air may be thinner on Mars and therefore less able to move grains, we mustn't forget that the gravity is much weaker here, so it doesn't take as much inertia to get a grain moving.

I find it hard to believe that these dunes were formed aeons ago and have been frozen in place. I don't have any evidence to give you other than my geologists intuition that tells me that these dunes are fresh and active (moving on the scale of months and years rather than millenia). If they were really old, gravity, diurnal temperature changes and weak winds would have made them collapse to a large extent. As it is they look pristine.

Comments welcome.

Castor

Castor:

I take the opposite view on dune ages (and yes, they look like 'dunes' to me, too!). Mars is a very static place compared to Earth, with hardly any active erosional processes, at least on familiar timescales. The crusty dune surfaces all suggest a frozen sea of very fine sand, with a little light spray of dust here and there. Add in the suggestions of quite dramatic climate changes every few tens of millions of years due to orbital effects and polar wandering and I have to opt for 'punctuated equilibrium' as the Martian norm: long periods of almost no change and brief active windows of activity. Dune ages of up to 100 million years would not surprise me at all.

That's my take on it, anyway - I think we are easily misled by the familiarity of Mars, which really is an unEarthly place!

Bob Shaw

Perhaps I should have created a new thread for this....

Chris

One hates to interrupt a pair of Sassenachs locked in scientific debate (native Scots must be gathering around in disapproving fascination). But I would like to interject here the global dust storm phenomena, which, while doubtless divergent from similar dust storms on earth, certainly do occur on "familiar timescales", and must leave in their wake some recognizable signs. Can anyone look around our MERs and point to features, among the ancient 'dripples', left by the last storm, or the last million storms?

Speaking as a non-expert who has witnessed, along with everyone else here, Spirit's brush with a semi-global dust storm, it seems the biggest feature left by such storms is a very gradual growing layer of dust which is redistributed by local gusts and dust devils.

In a nutshell, Cents, that might be the story, but it certainly trivializes the sense of a "storm" to what might more accurately be called a "dust fog". (I never heard about how typical of dust storms Oppy's brush was.) Certainly the Hollywood rendition of a Saharan sand storm, where heroes and camels stagger around, narrowly avoiding burial alive, is an inappropriate analogy. Current eolian erosion on Mars must be gentler than a baby's kiss.