[X] My Assistant

[Ant103]

The view toward Beagle is more and more precise.
http://origin.mars5.jpl.nasa.gov/gallery/a...CNP2435R2M1.JPG
The crater place seems to be a bit complex.

[chris]

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

[djellison]

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

[sattrackpro]

....but over a very long time period...

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.

[Aberdeenastro]

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.

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.

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

[sattrackpro]

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.

They indeed do ~look~ pristine - and to some extent, I must agree with you - there's more to the real story than we've heard or read.

I think the 'real story' is quite a bit different than many think - part of that may be that this 'old sea bed' may not be so old after all. Another part of it is that the atmosphere of Mars has changed radically (and is said to be accelerating) in recent times... just look at the rate it is disappearing now. Just a few hundred years ago it was much denser, and probably supported more radical weather (winds) than we see now.

We have this propensity to deem everything happening on Mars on a time and geological scale much the same as that of earth, and I think that's perhaps at the root of some poor thinking. Suppose, for example, that this sea had become subjected to hurricane force winds in a much denser atmosphere. It isn't difficult to see virtually the exact kind of terrain we see today, since we can see so-called 'mega-ripples' - actual large sand waveforms on the seabed near the Sable Island Bank. The waveform trough to crest measurements there, and in other places on earth, run between 5m and 10m - close to what is around Oppy.

Maybe it did the same on Mars - back when - maybe it didn't. But, there are 'earthly' examples of almost identical underwater waveforms - in the here and now. It might be a stretch to think the same would happen on Mars - but... similar forces just might create similar results.