re: Never could figure Y Spirits+OPPY's landers
I agree a couple of very simple small silver/black coated balloons using methanol could have been released from the landers at intervals where the only instrument used would be the cameras on the rover. This would be useful for understanding the winds would have been very light weight and simple.
But you are adding mass to an overweight system and adding an other complication.
I suggest anyone interested in knowing more about the rational of putting spacecraft on Mars like the Mars Rovers and such read Steve Squyres' "Roving Mars" and maybe Donna Shirley's "Managing Martians".