[X] My Assistant

[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

[dburt]

...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

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?

[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