QUOTE (ElkGroveDan @ Dec 29 2010, 07:21 PM)
Reading your paper Ralph it looks like the occurrence and duration of ice is still being debated.
Indeed. I think much of the literature has been unfortunately cast in black-and-white terms
'Ice is essential'... 'Oh no it's not'...'oh yes it is' etc..... (qv Sharp and Carey, Reid et al., etc.)
The reality is likely more nuanced - my take is that large rocks require ice, and that for most large-scale
movements (i.e. when many rocks move) ice is involved. But small rocks can occasionally move
without the help of ice sails (or more particularly, ice rafts). I suspect too that there is a spatial
element to it - the south end of the playa where most of the rocks are delivered to the playa
surface from the cliffs is also probably the area most often shadowed by same cliffs when the
sun is low in the sky in winter. So the south end is more likely to see ice than the north.
But all this is qualitative handwaving without quantitative data (as Kelvin sortof said). It may
be in part that the problem has mostly been studied by geologists, who as a tribe (this is fightin'
talk, I know) may be less inclined to apply probabilistic (or even quantitative) approaches
to the problem.
To remedy this deficiency, our efforts over the past few years at least now provide some
basis for discussion on occurrence and duration of water and ice on the playa. Also, while
it is all very well for someone to figure out that winds of X speed can move a rock of Y
mass without ice, there has really been only handwaving 'this is reasonable/not impossible' etc.
Can such winds occur often enough to explain the trails we see ?
Some actual numbers on how often gusts of 50 m/s actually occur (at least at nearby
locations) and how often is the playa flooded, are now published in Journal of Applied