Channels on Mars Cut by Water or Sand?

[Spoke]

Not sure if this is a general question or a great debate, but...

The latest National Geographic has detailed pictures of the Martian landscape, including channels which look like they might have been created by water erosion.

Frankly, I suspect that NASA is playing up these channels to better position itself for future funding.

You can tell from the photos that much of Mars is covered in sand and dust. I know that flowing sand has some of the same characteristics as a liquid, and I'm wondering if the channels we see (on the sides of craters and cliffs, mostly) could have been cut by falling sand or dust. If not, why not?

Anyone?

[Geobabe]

Water and wind erode differently. The main reason those channels are described as water-cut channels is they most closely resemble terrestrial fluvial (water) versus eolian (wind) landforms. Wind does not tend to flow in channels the way water does, plus there is a practical limit to the size of particles that can be carried by wind as opposed to water, so you will see dramatic differences in the types of erosion you get from the two processes. Look at terrestrial landforms sculpted by wind and you will see what I am talking about. They don't look like river channels at all.

[Geobabe]

Here's one page I found that gives a pretty good description of eolian processes and landforms so you can get an idea of how they differ from fluvial: http://www.geog.ouc.bc.ca/physgeog/contents/11r.html

[Spoke]

I see your point, Geobabe, but what about falling (as opposed to wind-blown) sand?

In looking at the pictures, the channels always seem to be carved in the sides of a precipice of some sort. So I'm thinking that maybe the sand got pushed to the edge of the precipice by wind, then fell over the precipice, and maybe carved the channels as it flowed downward. Any reason that couldn't explain the channels?

[Geobabe]

Sand's erosive power is almost exclusively due to its being pushed by wind. Falling sand might erode some, but really, it wouldn't do much, as that would be a very finite event--it falls and that's it. Erosion is a process that takes place over a length of time. Besides, once it fell over the edge it would then begin to fill up the channel and we would see the sand if that were the case.

[Spoke]

Besides, once it fell over the edge it would then begin to fill up the channel and we would see the sand if that were the case.

Hmmm. Makes sense. That sounds like the best argument against sand erosion.

[Geobabe]

I guess it takes some training in geology to look at landforms and understand why they were likely formed by one process or another. To me, those channels look like perfectly obvious fluvial landforms. The main reason for that is they look remarkably like terrestrial river channels and not like wind-shaped surfaces and we have no reason to think that surface processes on Mars are significantly different from those on Earth. Come to think of it, here's another reason why the falling-sand theory is unlikely: Mars is quite a bit smaller than Earth and thus has much lower gravity, which would further reduce the erosive power of falling sand.

[Spoke]

Mars is quite a bit smaller than Earth and thus has much lower gravity, which would further reduce the erosive power of falling sand.

Maybe; but offsetting that factor (I would think) is the thinner atmosphere of Mars. On Earth, falling sand and dust can be slowed or dipersed into a cloud by wind resistance. On Mars, atmospheric interference with falling sand or dust would be reduced.

[Spoke]

Oh, and I'll agree that those channels look like they were formed by a liquid. My point was that sand can sometimes behave like a liquid. That's how you get quicksand.

[Geobabe]

Quicksand is sand plus water, so it is not actually sand behaving like a liquid.

[Spoke]

Mmmm, yeah, I spoke too soon on the quicksand explanation. One too many viewings of Lawrence of Arabia, I suppose.

Nevertheless, sand can behave as a liquid, particularly where the grains of sand are fine or rounded. I would imagine that the grains of Mars would be pretty rounded by the erosive action of millions of years of dust storms.

[Spoke]

I finally found a website (http://irian.geology.latrobe.edu.au/~nhoffman/Mars/Water.html) which discusses this issue. From that site:When a cliff collapse occurs at a chaos zone, this solid CO2 depressurises and (partially) melts. Thus liquids can be formed spontaneously from solid material, merely because it collapses. The liquid CO2 then floods out through the dusty regolith and literally explodes into Mars' thin atmosphere with an internally-generated pressure of at least 5 bars. The effect is like letting off a fire extinguisher into a bag of flour. The fine dust and sand of Mars' regolith is blasted into a huge dust cloud which roars down the valley like a fluffy avalanche. However, this "fluff" is powerful stuff. Like volcanic ash clouds on Earth, it can carry so much load in suspension that it flows like a liquid downhill and erodes and destroys anything in its way, carving the outburst "flood" channels.

[casdave]

One current theory that has apparently been given some serious thought is that these are definately water channels.

The strongest evidence is from the Mars rover which sent back pictures of the surface.

When there is a flood event the water pushes rocks across the ground surface and the flow of water grades them by size.
If you look at those pictures you will see that for the most part the rocks are sitting on top of the sediment layer and are pretty similar in size and shape, the direction of flow can be determined by the way water has scoured around the rocks and indeed the scouring void behind the rocks is all in the same direction thus proving a cohesive flow.
This phenomenon is well known on earth, particularly in Iceland where sub-ice sheet volcanic fractures cause massive sudden flood events.

The current model has it that there were vast underground reservoirs of liquid water trapped by a deep layer of permafrost, this water could not freeze solid becuase of immense pressure.
Some event such as a deep impacting asteroid fractured the permafrost layer and the water was forced out in a relativelyu short period of time, the flood covered a large part of the suface of Mars.
There are collapsed sink hole structures resembling those on earth where the ground simply fell in.


The water evaporated away in the thin Martian atmospere and weak gravitational field.

That's the theory anyway but I wonder about the orbit of Mars. If it lost such a large mass of material into space through evaporation then would not the orbit have changed, either in speed or radius ?

[Fiver]

Also, spoke-, it's worth noting (sorry, don't have a cite at the moment) that the Mars Global Surveyor has found evidence of stratification in Martian cliffs, and the Mars Pathfinder rover found conglomerate rocks in its walkabout.

Both phenomena strongly imply sedimentary rocks, which further strongly imply running water over long periods. I'm a layman, but I'm unaware of any geological process that forms stratified sediments without the help of running water.

[yojimbo]

From http://www.bbc.co.uk/science/horizon/lifeonmars.shtml

Thousands of close up images were beamed back, revealing sections of the valleys in fantastic detail, but none of them were any use. Over the billions of years, the valleys had all been eroded and filled with sand. It was impossible to say how they had been formed. Then finally, the Mars Global Surveyor team noticed a tiny feature in one image, which convinced them that billions of years ago there were rivers and lakes on Mars.

There is a link to the full transcript of the programme from the link above. Interesting stuff.

[labradorian]

Originally posted by Five
Both phenomena strongly imply sedimentary rocks, which further strongly imply running water over long periods. I'm a layman, but I'm unaware of any geological process that forms stratified sediments without the help of running water. [/B]

And perhaps even more significantly, it would strongly imply standing water, over relatively long stretches of geological time. Oceans or lakes, perhaps.

[Sengkelat]

How about repeated meteor strikes? Each one throws up a mess of dust, which settles into a layer. Repeat as necessary, and your get strata.

[Spoke]

Couldn't strata imply periodic volcanic activity? Or periodic asteroid impact residue? Is standing water the only way strata can form?

(Just playing Devil's advocate here...)

[Geobabe]

Volcanic activity: possible though unlikely. IIRC, there is not evidence of that kind of activity in Mars' recent past.

As far as meteorite impacts, while they do throw up some dust, that is a localized phenomenon. The period of heavy meteor bombardment was very early in the history of the solar system and would not have created strata so close to the surface.

Both casdave and Five brought up the evidence from Pathfinder indicating a history of running water on the surface. Where the water came from and where it went are puzzles still being worked on, but the evidence for liquid water is very strong.

[Spoke]

According to the Mars website (http://irian.geology.latrobe.edu.au/~nhoffman/Mars/Water.html) I linked earlier, there are active volcanoes on Mars, but:
[V]olcanism is limited to a few key zones where prolonged foci of volcanism have continued throughout Mars history.

I don't know where they found the strata, or if they were located within one of the volcanic regions.

Oh yeah, and as far as Casdave's point about the rocks is concerned, the above-mentioned site says:[T]he impressive boulders at the Mars Pathfinder landing site were transported and deposited by clouds of gas and dust, not by water!

NOTE: The author of the linked site acknowledges the surface water theories, but punches a few holes in them, not the least of which is the fact that the surface temperature of Mars doesn't exactly lend itself to liquid water. Even allowing for an ancient greenhouse effect, Mars is far enough from the sun to make flows of liquid water seem sort of far-fetched.

[Geobabe]

The fellow whose site you linked is not someone whose work I am familiar with, so I'm going by the work I am familiar with. While there is certainly room for disagreement among scientists, my personal opinion is that the liquid water theory best fits the evidence that we see on Mars' surface. The question of where the water came from and where it went is still an open one, as I noted before. Current conditions on the surface of Mars are definitely not conducive to the existence of liquid water but we still have much to learn about the history of the planet, and again, I think the evidence for rivers and lakes having existed in the past is quite strong. I honestly have a hard time with clouds of gas and dust having created such a floodplain, particularly having transported boulders of such large size.

[Spoke]

I honestly have a hard time with clouds of gas and dust having created such a floodplain, particularly having transported boulders of such large size.

I dunno. I've seen an avalanche of snow carry massive trees along. Why couldn't an avalanche of dust/sand carry rocks?

(I'm not trying to be contrarian here, Geobabe. Like I said, just playing Devil's advocate. Personally, I hope the water explanation proves to be correct.)

[Geobabe]

Along a steep slope, certainly, avalanches can carry extremely large objects, but the area where Pathfinder landed was not steep but rather flat. This is why it is frequently referred to as a floodplain, since that is what it most closely resembles: a large, flat or very gently sloping area. There is a steeper area some distance away, but an avalanche expends its energy very quickly once it reaches the flat.

This has actually motivated me to delve into the literature a bit, so the followup questions are proving to be a good thing.

[Fiver]

You should come over to my house sometime, spoke-. Mars is one of my favorite topics, and I have several books on it, including a big National Geographic hardcover compiling most of the Viking and Pathfinder photos and a bunch of pics from the early days of the Mars Global Surveyor mission. I also have a globe of Mars.

The stratification IIRC, is seen in the banks of putative river channels, strongly implying that an ancient lake or ocean laid the sediments and a later river then cut through them. I'll try to find a link with pictures tomorrow when I'm at work and have T1 (and am supposed to be working, sshh).

That Mars has a large inventory of H2O isn't aggressively disputed; most of it is probably now in the poles and in subsurface permafrost (My Natl. Geo. book has a good photo of a "splosh" crater: the ejecta surrounding the crater is in the form of rounded lobes, as if the heat of the impact turned tons and tons of hydrated permafrost into mud, which splashed out all around.).

The better question is how that H20 could have existed in liquid form for millions of years early in the history of the planet, as the evidence tends to say it did. Not only is Mars about 150% farther from the Sun than the Earth, but the Sun was much cooler back then too. It's very puzzling.

Maybe the answer is in areothermal heating. Or maybe the greenhouse effect was more successful than we think it could have been. We really need to go there (http://www.marssociety.org) if we want to find out for sure.