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Paul Murphy
hi:

I'm actually a lurker on this site because I don't generally understand the issues discussed but
find them interesting anyway. That said, there's a question that I can't seem to find the answer to
and hope you guys can enlighten me on.

The general discussion of water flow effects, particularly with respect to canyon and chasm images, seems to attribute some very deep surface markings to water erosion. The recent spate of ESA images showing the Echus Chasma, for example, generally claim those cliffs are up to 4,000M high and that they were created by water flows. So my question is: if features like these are largely the result of water erosion, how long did it take and what volume of water was involved?

Rock is rock here or there, and the rate of water erosion has to vary (I think) with the square of velocity of the water/grit mix running over it - and since that velocity on Mars has to reflect Martian gravity, not Earth's, it should have taken much longer (or needed a lot more water) than here. Yet geophysical instability here has meant that we don't have features that size on earth (to my knowledge anyway) - so what does this tell us about how much water Mars had, and for how long
Juramike
QUOTE (Paul Murphy @ Jul 17 2008, 01:04 PM) *
so what does this tell us about how much water Mars had, and for how long


Welcome, Paul!

You could have a slow gentle removal of sediment over millions of years caused by a steady (or mildly fluctuating) amount of fluid. Low erosion rate x long time = big canyon

Or you could have a massive release of fluid with a catastrophic flood event carving up big canyons in a very short period of time. (The Channeled Scablands of Washington State is the typical example).
High erosion rate x short time = big canyon

There is more and more interesting evidence that catastrophic flood events are carving many of the canyons previously thought to be due to the "slow eons of erosion".

Check out (well-summarized abstract freely available here):
Lamb et al. Science 23 (2008), 1067-1070. "Formation of Box Canyon, Idaho, by Megaflood: Implications for Seepage Erosion on Earth and Mars".

Also don't discount the possibility that the land rose up while the rivers were cutting down. (Probably a better possibility on places with a suspected longer term fluid cycle like Earth or Titan.) The Green and Yampa River Canyons in Dinosaur National Monument, Utah/Colorado are an impressive display of this.

So the whole spectrum of possibilities still seems possible for Mars.

-Mike
dburt
QUOTE (Paul Murphy @ Jul 17 2008, 11:04 AM) *
...Rock is rock here or there, and ... it should have taken much longer (or needed a lot more water) than here...

Actually, owing to the lack of plate tectonics and the preserved record of ancient impact bombardment on Mars, the resistance of rocks to erosion might be considerably less on Mars than on Earth. The basaltic lavas, in principle, should be similar, except that martian lavas are much more ancient and heavily broken up by impacts and possibly by later effects such as frost heaving. The layered sedimentary rocks seen on the surface, whatever their origin (a subject of considerable debate on this site), all appear to be extremely weak ("friable") and the grains appear to be cemented only by water-soluble salts. So it might not take much water or time to erode them. The near-vacuum atmosphere alone (i.e., wind) seems to have done a fine job of erosion, in many cases.

Land gradually rising up while streams cut down, suggested by Juramike, might be rare on Mars compared to Earth, because Mars is believed to have few tectonic processes other than bulk shrinkage related to cooling and drying, slow depression of the crust under the weight of lavas or ice, and possibly doming owing to subsurface intrusion of molten magma. This lack of tectonics and erosion, in general, means that strong rocks formed at great depth (e.g., by igneous intrusion or metamorphism) can never be exposed at the surface, and that rocks seen at the surface have probably been there for many billions of years (unlike on geologically far more active Earth).

In short, a rock found here is probably not a rock found there, at least as regards ease of erosion.

-- HDP Don
Paul Murphy
Thanks!

Although, to be honest, all you've done is show me how little I know -and I already get more than
enough lessons in humility from my six year old smile.gif

And, of course, your answers lead to more questions:

1 - I don't get this low strength surface rock business. How does it get made?

The surface erosion as source explanation just begs the question since you start with a hard rock surface to be eroded - infall of materials from space (about 40K tons/day or heat + 1mm/1000 years in radius gain on earth, I believe) could provide some of the "cement", and of course Mars appears to have had active volcanoes, but don't all the conversion processes (infall to friables, volcanic ash to friables, erosion dusts to friables, etc) require repeated applications of water, heat, time, and wind? And, if so, shouldn't we choke on that word "repeated"?

2 - for water to move rapidly and in larger quantities or over longer time periods from A to B (thereby making a canyon) there either has to be a very large source of water at A or some way of getting it from B back to A. Does Mars get enough solar energy to power the B to A part of this cycle in any atmospheric conditions thought to have been possible there? If not, where did the water at A come from?

--
Perhaps you could recommend some appropriate reading?
Juramike
QUOTE (Paul Murphy @ Jul 29 2008, 06:15 PM) *
Although, to be honest, all you've done is show me how little I know -and I already get more than
enough lessons in humility from my six year old smile.gif



I usually get multiple lessons in humility every day (also known as scientific frustration): either from someone more knowledgeable than myself, or from chemistry that doesn't work, or from biological systems that don't behave the way I thought they would.

Either way, I hope that I learn a little more for the next time. But sometimes I don't smile.gif



QUOTE (Paul Murphy @ Jul 29 2008, 06:15 PM) *
1 - I don't get this low strength surface rock business. How does it get made?


For Mars, volcanic eruptions throw up large amounts of loose dust and little particles that go a looong way. (Low gravity, low atmospheric friction.). Then you've also got infalling dust. And then you've got meteor impacts over the millenia spraying impact debris (surge clouds) all over the place as well. Add to that eons of wind erosion picking up dust and moving it from one place to the other and you get large amounts of dust and dune sands. Some of these get compacted and form fossilized dune sand structures. (Burns Formation and Victoria Crater show the neat-o crossbedding of fossilized dune sands.)

Without tectonic cycling to stuff rock in trenches and bring it back up and shuffle it around, most of the terrain pretty much stays where it is. So you've billions of years of wind depostion, wind stripping and impact gardening in play.


QUOTE (Paul Murphy @ Jul 29 2008, 06:15 PM) *
2 - for water to move rapidly and in larger quantities or over longer time periods from A to B (thereby making a canyon) there either has to be a very large source of water at A or some way of getting it from B back to A. Does Mars get enough solar energy to power the B to A part of this cycle in any atmospheric conditions thought to have been possible there? If not, where did the water at A come from?


It is possible the water made a one-way trip. Frozen subsurface aquifer, catastrophic release, up in the atmosphere, and stripped away by the solar wind. The canyons are thought to have been carved early in Mars history (3.5-3.8 billion years ago).

Hopefully an isotopic measurement (soon?) by the Phoenix spacecraft might shed light on this. Lotsa light isotopes, the water has been locked up in the polar subsurface for a long time. Lotsa heavy isotopes, it's been in equilibration recently.

QUOTE (Paul Murphy @ Jul 29 2008, 06:15 PM) *
Perhaps you could recommend some appropriate reading?


Wikipedia/Mars is an awesome place to start (and references therein)
[Y'all can debate me on Wikipedia, but every time I go into a new area I hit Wikipedia first, and I'm always amazed at the depth and accuracy of the information dredged up.]

Another winner is to browse the Lunar and Planetary Science abstracts. They are excellent tight pieces of information that are very accessible to the interested amateur.

Favorite [old] books I really like are (lotsa good diagrams of fundamental processes):
Murray, Malin, and Greeley, "Earthlike Planets: Surfaces of Mercury, Venus, Earth, Moon, Mars", W.H Freeman and Co., 1981.

Carr, "The Surface of Mars", Yale Press, 1981.

-Mike
dburt
Congratulations, Paul. Welcome to the club of puzzled people. You've asked some of the most difficult to answer (and consequently most debated) questions for all of Mars. Rather than attempting to answer your first question, I'll refer you to a lengthy and repetitive thread from about a year ago, here:
http://www.unmannedspaceflight.com/index.php?showtopic=4308
Hypotheses for making the sedimentary rocks of Mars include deposition by various combinations of wind, flowing water, standing water, explosive volcanism, and steamy meteorite impact. All have been claimed to yield the textures seen. Personally, I favor meteorite impact for the rocks seen by the rovers - read the thread (and references therein) to find out why.

Regarding transport of water from from point B back to point A, the usual process cited is slow sublimation of ice in warmer, sunnier areas near the equator and its precipitation near the dark, cold poles. Remember that we have billions of years to play with. Wild cards include the fact that the highly variable nature of the martian orbit and orbital inclination to the sun, the tendency of ice to melt at the base of at thick enough column of ice, owing to geothermal heating, and the universal presence of soluble salts that promote melting at temperatures below zero (they act as antifreeze); salts also slow evaporation by lowering the vapor pressure of liquid water. Other potential sources of water vapor include hydrous volcanism (mostly when Mars was young) and ice vaporization caused by major meteorite impacts (which may have caused temporary greenhouse warming and flooding as vaporized steam rained out). The martian outflow channels are believed to have formed by some kind of catastrophic groundwater or brine breakout - the source of this liquid water remains unclear.

Almost anything you read about the surface of Mars somehow deals with the question of water - so read away (including old threads at this site). I don't dare to make specific recommendations.

-- HDP Don
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