QUOTE (Phil Stooke @ Jul 3 2021, 01:54 PM)
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We had a post on here not long ago from Ralph Lorenz saying that they are not energetic enough to help. Not sure I understood why not but I trust him on this and we have the evidence in the lack of cleaning events.
There are two statistics one can develop from orbital images - the dust devil track formation rate (DD / km2 / Sol), and the track area generation rate (km2 / km2 / Sol). On both of those metrics, Elysium (InSight) was rather weaker than Gusev (Spirit).
If one adopts the imperfect hypothesis that removal of dust from regolith to form a visible trail is a good analog for the removal of dust from a solar panel a meter or so off the ground, then the reciprocal of the area generation rate is basically the interval between cleaning events. That interval appeared to be a few hundred Sols for Gusev, consistent with the cleaning events observed. The reciprocal of the track area generation rate at Elysium, published by Dennis Reiss and myself before the mission, was of the order of ten years, consistent with the non-observation of cleaning events so far.
Why these sites should be so different is not clear. My suspicion is that stronger background winds at Elysium suppress the formation of the largest (most intense) vortices which are needed for dust lifting. There was a bit of a clue about that in the dust devil tracks before landing - Dennis and I noted (a) that most of the Elysium tracks were much narrower than those at Gusev, and (b ) were straighter.
(a) narrow tracks suggests small diameter vortices
(b ) dust devil tracks are often cycloidal, a quasi-random rotating velocity component of a couple of meters adds vectorially to the background wind. If the background wind is stronger, you get tracks with lower sinuosity.
There may well be other factors at work (thermal inertia, regional flows suppressing the growth of the planetary boundary layer, etc.) but this is my best guess as to the reasons. If the story above holds, then at least one might be able to estimate sites of low and high dust removal potential from orbital data. And perhaps the meteorological factors can be satisfactorily predicted with global and regional circulation models.