geckzilla wrote:... Another factor to consider is that when you are looking at the dust devils in the desert, you are looking at them against a bright background such as the sky ...
Often, yes. But there are plenty of times when Earth based dust devils are observed against hills or mountains in the distance. And, I distinctly remember seeing a video of a dust devil on the African veldt taken from a hot air balloon (a Nat Geo documentary?). In all those cases the dust devil had a distinct tinge correlated with the substrate, not snow white.
geckzilla wrote:... In the APOD picture, we've got the sun at a fairly oblique angle against a rough surface which is the ground. That makes the ground look darker. Meanwhile, the dust devil is sticking up from the ground and is taking sunlight directly, making it appear brighter. The rest of it is just light scattering by the small particles in the devil.
I will grant you your point with only one further comment: Even if we account for the sun's angle and our angle of observation, one would expect brown dust particles to reflect a tannish or brownish light, certainly an off white of some sort. That is clearly not the case in the photo.
A further observation: In the photo on APOD (presumably directly downloaded from Mars) the "dust" cloud appears snow white, and there is no obvious trail on the substrate's surface. However, in both animations not only is a trail present, but there is a distinct, dark core in the funnel cloud. And while the CGI isn't bad, the rotating devils are just too mechanical, very much like the tornadoes in Jan de Bont's Twister
. I vote that both animations are merely some NASA artist's conception of reality with the trails and the dark cores inappropriately Photoshopped in, and should not be considered as an indication or evidence for anything in real life.