[X] My Assistant

[Guest_AlexBlackwell_*]

Water May Not Have Formed Mars' Recent Gullies
By Lori Stiles
University of Arizona News Services
March 16, 2006

[JRehling]

I'll insert here a post I made four years ago in another forum:

<< As I tumbled over the dunes in Death Valley yesterday morning
(remaining afoot, however, which I did NOT do on the salt playas at the
lowest point in the Western Hemisphere -- OUCH!), I noticed several
interesting phenomena in the sand that my footfalls inadvertently
triggered.

When I stepped at the top edge of a incline, this often
precipitated a downslope flow of sand, sometimes of surprising volume.
Quite simply, the "vertical" surface of a sand dune is in equilibrium,
but only barely, and some of those surfaces took very little to push
them over the edge to instability.
The morphology was also interesting. Quite often, the flow
followed a V shape. However, as I watched the flows form, I noticed a
beautiful and surprising dynamic -- the flows did not always form top-
first, bottom-later. Often, the initial flow (from the top) caused
lower portions along the trail to erode to the point that the vertical
layers above them (nearer where the flow began) lost their support from
below, and then I saw a beautiful wave head UPHILL at about 1 m/s. This
was not the movement of material uphill, but the spreading of a
secondary wave of collapse. Visually, it almost seemed as though the
initial event started a downward flow that then bounced upwards; a
point midway down the slope would first experience an initial flow of
sand from above to below, then, after being quiescent for a while
(during which the wave moved on), it would undergo a second collapse,
brought on by the loss of support from below, and leading to the
subsequent collapse of the portion of the slope immediately above it,
in an upward domino effect.

Pertaining to the martian gullies, I make these poor attempts at
an Archimedean analogy:
1) Did the gullies REALLY form top to bottom, or vice versa? Could
the collapse of a small point in a barely-stable layer just below the
gullies lead to a V-shaped excavation in the gully layer, causing our
attention to go to the wrong layer?
2) If the slopes in question are just barely stable, and near the
state of equilibrium, could we probe their formation by performing a
Deep Impact sort of concussion? This may be costly to bring about, but
it would be nice to have a lander looking up at a slope waiting for
concussive events that we know are coming, and seeing what flows
result. Obviously, not practical in the near term...
3) In a bit of half-empty/half-full reasoning... It seems that all
the theorizing on the gullies supposes that an event is occuring which
actively propels material downward. However, with the vertical slopes
near equilibrium, is it also not possible that a very subtle LOSS of
stability could trigger a landslide? Rather than delivering an actual
shock, couldn't the trigger event be a slight weakening of materials
that are just barely holding their weight? Perhaps due to eons-old
frost sublimating just a little more than they already had?
>>

Alex had some interesting answers to my points, which I won't insert here without permission.