As part of the continuing discussion below, I'll reiterate that the recent Palermo et al. presentation at the Mars Society (on "stains") neglected to reference some key abstracts that prefer the dust avalanche model:

See http://www.habitablezone.com/space/messages/13755.html

Also, those who have followed the literature closely will note that Palermo et al. are not the first to suggest that the dark streaks may be stains from liquid discharge. Ferguson and Lucchitta [1984] posited that the streaks (observed at lower resolution in Viking imagery) may be due to wet debris flows from near surface aquifers or melting of ground ice (in equatorial regions).

See Ferguson, H.M. and B.K. Lucchitta, Dark streaks on talus slopes, Mars, in Reports of the Planetary Geology Program 1983, NASA Tech. Memo., TM-86246, 188-190, 1984.

Interestingly, no reference to this earlier work is given by Palermo et al.; I'll assume that they are unaware of this as well.

At any rate, the Palermo et al. work leaves several key questions unaddressed:

(1) The MOC NA imagery of the dark streaks, assuming a "wet" origin, show no pooling where the streaks descend across craters or other topographical lows.

(2) Many slope streaks are observed to display a high degree of widening downslope. A model incorporating across-slope dispersion of fluid operating under the influence of gravity to produce the widening is problematic, to say the least, especially in the many areas where slopes do not possess cone-shaped morphologies.

(3) Since the vast majority of the dark streaks are observed on slopes, if their origin is "wet" presumably this is due to intersecting or even perched aquifers (near surface gound ice is implausible given the temperatures and the probability that the near surface is is in diffusive contact with the atmosphere). In this setting the putative liquid (water) would be subject to considerable hydraulic head pressures and/or lithostatic overburden. Assuming a breach, one would suspect that the liquid would erupt catastrophically (a la the Malin and Edgett seepage gullies) and shatter or collapse the overlying host rock. Further, and paradoxically, this "wet" flow would have to "seep" at a slow enough rate to not produce relief at the margins and yet fast enough to run out long distances in the ambient temperature/pressure regimes, which, to me, is implausible. Even assuming this hurdle can be overcome, why would two different mechanisms for fluid flow (the Malin and Edgett seepage sites and the "dark stains") that produce two different morphologies be operating during the same geologic epoch when one is sufficient?

Alex R. Blackwell
University of Hawaii

P.S. I've been fairly busy preparing for my upcoming (Wednesday) sojourn to Europe, so I guess I missed the BarryD flame fest ;-)