Putting "Windshield Wipers" on the probes to another planet isn't exactly like going down to Auto Zone and picking up a few strips of black silicone. Wipers wo

Putting "Windshield Wipers" on the probes to another planet isn't exactly like going down to Auto Zone and picking up a few strips of black silicone. Wipers would have to survive the extremes of space and the surface of Mars. They would need motors, which require heaters to keep them from freezing (increasing the amount of energy the probe needs). They would increase the weight of the craft, increasing costs or requiring them to leave off other features. They would have to be extensively tested, like the rest of the craft -- especially the mechanical elements. Imagine the press if they got to Mars and found their expensive wiping mechanism had jammed and would be useless. Finally, if the dust of Mars was charged at all, the wipers would have just rubbed the dust around on the panel, so they could have ended up with functional wipers that did no good. The mission was designed so that it could complete its high priority scientific objectives in three months, and the probes were engineered so that they had a high probability of surviving for three months. Dust accumulation was not expected to be a problem in the high priority part of the mission, and since it was a hard (expensive) problem to address, it was not addressed. It was happy circumstance that previously unknown phenomena on Mars meant that it ended up not being a problem at all.