[X] My Assistant

[edstrick]

Got me a new theory.

Gnomes with dust-mops.

[JRehling]

Dust flows have the advantage to form into the actual martian climate. But they have some inconveniences: -Why we don't observe similar dust flows on the Moon (only landslides traces in recent craters like Tycho). -to be still active today, they need that martian grabens are still active (an explanation which don't hold when found in craters) or that there are intense Mars quakes.

There doesn't need to be tectonic activity to trigger martian landslides. We already know of one active mass-moving process on Mars -- the circulation of dust. Indeed, snow avalanches on Earth do not require earthquakes -- snow (and wind) are sufficient. When you see a hillslope (of dust or sand), the only thing you know about the stability of it is that it has never yet passed the tipping point. But it may be within a pebble's throw of doing so. Another millimeter of dust and a mild breeze may do it.

And while we know that the gullies have a latitude/sunlight correlation, maybe there is a purely thermal reason for that? The devil's in the details. Maybe the gullies require a lot of volatile (H2O or CO2)... maybe not.

[jmknapp]

There was a lot of press back with Clementine and/or Lunar Prospector that they found water on the Moon, the claims sounding impressive in terms of total mass. Something like 1% of the rock under the regolith bottom line--which is about the amount of water still bound to set concrete. Not exactly the form of water that comes to most people's minds, and it turned out even that finding was subject to other interpretations. NASA should scale back on the breathless water claims that ultimately just make people shake their heads.

[Guest_AlexBlackwell_*]

Ok, by seeps, as nicknamed by some, I'm referring to the dark "stains" seen (from orbit) running down many crater walls or other cliff faces, distinct from the carved-out gullies.

OK, you're referring to the features Sullivan et al., and most everyone else, refer to as dust avalanche scars.

Thanks for the clarification.

[Guest_RGClark_*]

Scanning through the LPSC Mars abstracts, the thing that struck me the hardest was that the Gully Wars are still on full-tilt. There are at least four totally different rival theories presented -- plus Allen Treiman's insistence, for the second year in a row, that the evidence points against ALL of them in at least some cases, raising the possibility that they're being made by different causes in different places.

Subsurface aquifers:
...
http://www.lpi.usra.edu/meetings/lpsc2006/pdf/2412.pdf

...
But this, I think, is one mystery that will probably be settled by MRO, given its combination of very high-resolution surface photos, mapping of shallow subsurface aquifers, and extremely detailed near-IR mineral maps.

This passage is especially supportive of a water-based flow model:

DEPTHS, ORIENTATION AND SLOPES OF MARTIAN HILLSIDE GULLIES IN THE NORTHERN
HEMISPHERE. Nina L. Lanza and Martha S. Gilmore, Department of Earth and Environmental Sciences,
Wesleyan University, 265 Church St., Middletown, CT 06459 (nlanza@wesleyan.edu).
Lunar and Planetary Science XXXVII (2006) 2412.pdf
"The gullies’ measured starting depths from the surface
are consistent with those in the S. hemisphere [2]
and with N. hemisphere measurements in [4] and [5].
At lower latitudes, gullies appear to form deeper from
the surface. The results inply that surface temperatures
are controlling the depth of melting of subsurface ice
in aquifers [2] or over aquicludes [5]. It is more difficult
to explain why snowmelt models would initiate
gully formation lower on the slope face at lower latitudes,
since snow is a near-surface phenomenon."
http://www.lpi.usra.edu/meetings/lpsc2006/pdf/2412.pdf


Bob Clark

[Guest_BruceMoomaw_*]

Against that, however, we have the problem pointed out (not for the first time) in http://www.lpi.usra.edu/meetings/lpsc2006/pdf/1610.pdf : "The discovery that 10-16% of gullies occur on isolated knobs or hills is difficult to reconcile with a regional groundwater model of gully formation as it is unlikely that a sufficiently large aquifer to produce gullies (with lengths sometimes > 5km) could form within these isolated topographies. Instead, the obvious dependence of gully distribution and orientation with latitude suggest that insolation and climate play a controlling role in gully formation and that an atmospheric source for the water [e.g. 4] is more likely."

Indeed, one should keep in mind Allen Treiman's belief that NO unified theory explains all the gullies very well -- there are inconsistencies in every single theory proposed so far: http://www.lpi.usra.edu/meetings/lpsc2006/pdf/1304.pdf . See also his abstract from last year's LPSC attacking aquifer formation theories for them (in which he managed to utilize Lemony Snicket for the title): http://www.lpi.usra.edu/meetings/lpsc2005/pdf/1713.pdf

[ljk4-1]

Science/Astronomy:

* Researchers Rain On Mars' Water Gullies Parade

http://www.space.com/scienceastronomy/060321_mars_water.html

Martian gullies that some scientists believe were recently carved by liquid
water might instead be the result of landslides triggered by wind and meteor
impacts, scientists say.

[Guest_AlexBlackwell_*]

Researchers Rain On Mars' Water Gullies Parade

http://www.space.com/scienceastronomy/060321_mars_water.html

Frankly, I think this impact of this Bart abstract is being overblown. I don't think it has swayed too many opinions on either side.

[Guest_BruceMoomaw_*]

Moomaw strikes again! I was wondering whether it might be possible to combine two of the above theories -- with dust avalanches being greased by the gas from sublimating CO2 snow -- and somebody else has been, too (in more detail, needless to say): http://www.cosis.net/abstracts/COSPAR2006/...006-A-03402.pdf

Come to think of it, though, hasn't this already been proposed for the smaller slides that have actually been photographed occurring down Martian dune faces?

[Guest_AlexBlackwell_*]

Moomaw strikes again! I was wondering whether it might be possible to combine two of the above theories -- with dust avalanches being greased by the gas from sublimating CO2 snow -- and somebody else has been, too (in more detail, needless to say...

It's kind of difficult for people to gauge what you've been "wondering." Something in writing like a conference abstract, peer-reviewed paper, heck, even something on the back of a cocktail napkin, beats mind reading any day.

This post has been edited by AlexBlackwell: Mar 23 2006, 10:33 PM

[Guest_BruceMoomaw_*]

I was just about to send it to you guys when I stumbled across it in COSPAR. Really and truly, I was. (Not that coming up with it exactly involved a stroke of genius on my part.)

[Guest_AlexBlackwell_*]

I was just about to send it to you guys when I stumbled across it in COSPAR. Really and truly, I was.

You forgot "pretty please, with sugar on top."

[Guest_BruceMoomaw_*]

I draw the line at that.