that is an old/repaired streak
I can see the light one off to the side, but you also notice that the dark streak is on top of/overlaping another light streak that seems to have runs its course earlier. A good explanation is that the area is subject to repeated streaks and in the course or lightening back up it gets alittel lighter than normal.
Perhaps something to do with the way CO2 is reforming on the terrain. Probably a characteristic of the fresh dirt below , its angle, its texture. . . a loads of things. . . still nothing to make me think its not dust falls.
-Justin Kelly