Ok, I hope everyone can forgive me for one more comment. I'll try to keep it as short as possible, but having heard many making this argument, even someone on the MER team who probably started the whole thing, I just HAD to comment.

I need to make one thing perfectly clear from the start. I completely agreee with Doug and others who have pointed out that the surface ripples have nothing to do with water, and for many reasons are beyond a doubt, are formed by wind. But using only a scale difference to distinguish between sub-aerial and sub-aqueous bed forms is wrong. The devil is in the details. There are many examples of elongated bedforms of small to intermediate to large scales that are formed under water. Sand bars, various types of shoals, and sand-waves are just a few. There are several images and descriptions of some such things on two pages here.

I saw a synergy between this and previous comments about uniformitarianism/catastrophism in geology that I could not resist. I think it's a good story. Early in the previous century there were some truly magnificent elongated bedforms brought to the forefront of the uniformitarianism-entrenched geological establishment by a young upstart named J. Harlan Bretz. I'm sure that name will ring a bell with some of the older geologists around here. I don't know if the story is still told in modern curricula, but long ago, when I was in school, it was considered important geologic history. I'm getting all nostalgic, now.

Anyway, he saw evidence in Eastern Washington state for huge glacial floods that created all kinds of fantastical things in a region he called the "channeled scablands." Among those things were what he termed mega-ripples, and are otherwise referred to as giant ripples. They were up to 10-15 meters tall. Major early 20th century flame wars erupted when he published his observations. In the end, Bretz came out as the hero and he will always be recognized as a giant in the history of geology.

But in another area, another geologist named Wegener was wondering why some of the edges of certain continents fit together so well, like puzzle pieces.

Later in the century, observations by other young upstarts, noticing continental drift evidence, re-ignited the flame wars. They have now long been vindicated, as were Bretz and Wegener. Today, everyone can make nearly realtime observations on another planet via the rovers and orbiters. That's where UMSF comes in. The "next young upstart" line begins right over there... But I digress.

I've "borrowed" a couple of images of these mega-ripples that give an impression of their scale. Large, ripple-like features can be created by water. But as I initially mentioned, they are not like the Meridiani ripples. Those images should be ascribed to the following pages, where I found them.

http://www.kidscosmos.org/kid-stuff/kids-c...-scablands.html
http://www.ndsu.edu/instruct/schwert/geosc...lumbia_plateau/

Oh, and this link has a picture of my hero, and a decent description of the story.
http://en.wikipedia.org/wiki/J._Harlan_Bretz

What a great comment! Thanks for sharing.

The channeled scablands are one of the many exceptions to the uniformitarianism principle and shows that things are not always as they were. Although I mention this principle as frequently as anybody, I do mean it in the general sense: weathering, erosion, transportation, deposition and lithification yesterday are similar to the processes we see today. But the release of the water that formed the Scablands was a non-usual event, just as the winds that formed the Meridiani plain ripples was outside of the conditions we see now on Mars. But it is quite clear that this landform is aeolian; the task ahead is understanding what the conditions were that produced the required energy to give what we see.

But I can guarantee that the Mars we see today is not like the Mars that was yesterday or will be tororrow.

--Bill




PS-- one of my heroes is J. Tuzo Wilson.

My hero, Gene Shoemaker, the "God of Planetology" as one researcher refered to him once, took a trip down the grand canyon in the early 70's. They located and duplicated most of the original photographic stations of Powell during the first trip down the Colorado through the canyon nearly 100 years earlier. Comparison of the images taken over that time interval show that most detectable changes were small, sharply defined "local catastrophies" (and changes in vegetation).

I knew Gene, though not really well, having internned two summers at USGS Flagstaff. Losing him to a car accident while impact crater hunting in Australia was like losing a member of family. It still hurts.

Moving dune related posts here so the discussion can be found after Oppy has moved past Beagle Crater

Thanks.

What we need is a 16 mbar wind tunnel, with 10m/s wind and a pile of dust in it.

Doug

Doug:

It's called, er, 'Mars'!

Bob Shaw

The Mars exhibition I saw at the Manchester Museum thingie was good - it has a little cylindical unit with a load of dust on the bottom, a clear lid, and a fan you could turn - and in the space of just 5 minutes, the entire dune system totally changed....not to mention sand slips on the sides of dunes that looked JUST like the sort of thing some say is water seepage on crater walls

We need an aerodynamicist....given a sphere of radius 0.1mm - what is velocity required at 1000mbar to produce the same drag as 20m/sec at 15mbar

I've seen various equations, some have the velocity squared, some don't.

Doug

Am not an aerodynamicist me but one of the important equations is the standard drag force.
Drag=0.5*(Drag Coeff)*(atmospheric density)*(Cross Sectional Area)*(velocity squared).
Viscosity, turbulence and high speed shock effects and other things modify this a lot but for low speeds it should be good enough for a comparison.
Drag Coefficient and cross sectional area stay the same, Earth's air density at 1000mbar is ~1.2kg/cubic meter, Martian atmospheric density is around 12g/cubic meter.
So a 20m/sec wind on Mars exerts (12*20*20)/(1200*10*10)=4% of the force of a 10m/sec wind on earth.

The lower gravity then increases the effectiveness of that force by a factor of 3 over an equivalent particle on earth so overall a 20m/sec wind on Mars (45mph for those who haven't gone metric) would have a roughly the same effect as a ~3.5m/sec (~8mph) wind on Earth.

From the Beaufort scale: 8-12mph is Force 3: Gentle Breeze Leaves and small twigs in constant motion; wind extends light flag.

Very interesting stuff.

Next conundrum - we have seem elongated depressions that look like the sapping - where the sand is falling into cracks in the evaporite. If the ripples are moving, even over many years, then why are these depressions visible? Moving sand would surely fill in the cracks over time, and then there would be no depressions.

The cracks can't consume an infinite amount of sand, which suggests some event made it happen? Marsquake? Meteorite impact shaking the plain?

Chris

Thats just it, the cracks should have filled up long ago at the supposed rate of dune propogation, unless that is, that the cracks are still (or just recently) opening up. Therefore, if the 'microcraters' or 'sinks' are caused by sapping in the cracks this apparently makes sense only in that same context.

i still think however, that the dunes are essentially fossils with a light veneer of dust and loose lighter grains blowing around on top of the desert pavement-like surface. There might be more activity in summer, when the atmosphere gets (was it 20%?) thicker due to polar sublimation, but most movement probably occurs episodically when the planet warms episodically due to 'other factors'. The tiger stripes show the wind has been pretty directionally constant during the time that the erosion was taking palce, which seems to be general direction it is leading today.

We still need to dig int a tiger stripe to see if the coloring extends below the surface or if its just an illusion created by darker grains 'sticking' in the places where the grain-gap size is a fit for the material blowing around.

Is that a laden or unladen sphere?

Well, if it's the Holy Hand-Grenade of Antioch then it would be at least equal to a European Sparrow.

Run away! Run away!

Bob Shaw

Take thee to Denmark ...
http://evolution.skf.com/zino.aspx?articleID=14896

First let me take a moment to say thanks to Alan (and Doug?) for getting this discussion on its own thread.

If there is one thing that stands out about Meridiani, it is the seemingly endless mega-ripples – deserving more discussion of this kind.

And, thanks Cosmic, for your most interesting contribution to the discussion! (BTW, a better link to earthsciences mega-rippling is here.)

I’m skeptical that the features of Meridiani were entirely aeolian formed, primarily because where sand and wind exist in abundance over geological time, we see pictures (like here, and here) where much larger, even mountainous, dunes form. In fact, Mars has its own ‘desert’ areas with similar mountainous dunes. But not in Meridiani? Then why not?

Secondarily, we know (or think we do) that this is an old seabed – but little discussion has centered on what would happen when that sea reached a depth of only a few feet, and then ever lower over time (as it dried up) across its entirety.

It seems more probable to me that the dynamics of an ever-shallower sea engaged two forces, both wind and water, formed what we see today – adding even more powerful evidence of it being a sea in the first place.

I don’t see water alone causing these mega-ripples, because as has been amply discussed, we don’t commonly see ripples this large under bodies of water – except near coastal areas where seas become shallower, in tidal areas and often after storms – however we do have a working knowledge of how mega-ripples form. This naval study provides much insight, as does this study of Puget Sound mega-rippling.

This NASA piece on cross-bedding concentrates on ‘flowing’ water. (Huh? One must ask, from whence to where?) Shallow standing bodies of water subjected to wind and other forces are at issue here, and that seems to have had much less attention than it ought to have had - particularly with respect to how static Meridiani has been and is today.

If we're looking for mathematical evidence of a sea on Mars, I think that some of the strongest evidence to date may be going almost entirely undocumented.

- -
Bob, lyford (others too) - keep the humor coming...