I believe that, as soon as a lander comes in contact with a planet/moon's surface, THAT is the point at which arcing would occur. The rover, astronauts, and other on-board equipment shielded from the arc itself, and the difference of potential is equalized. When the spacecraft, (and aircraft, for that matter) is designed, it is, as I understand it, designed to shield its payload by taking charge/discharge events through the air/space fram structure, rather along the lines if a Faraday Cage.

As exemples, I have seen demonstrations of electric utility linemen using helicopters to service remote areas on transmission lines; as the helo approaches the power line, it has a stiff, heavy copper wire protruding to the side, which draws an arc (sometimes of a couple feet long) from the line to the helo; because of the helo's metal skin fuselage (and Faraday cage effect) the occupants of the helo are not injured, and can place insulators in the line, and proceed to work. Of course, it only works if you are dealing with one charge source...if the helo or linemen came too close to two DIFFERENT lines..BZZZZZT!! Similarly, airplanes have often been struck by lightning and survived with some skin damage to the craft, and passengers uninjured; meanwhile, the discharge wicks will dump excess charge back into the atmosphere.

When the mars landers hit the surface (and experience whatever equalizing arc effects), the on-board rovers will become equalised with the surface, also. When the lander opens its petals, it is grounded to the martian surface, and the rover has grounded contact with the surface through the lander; therefore, when the rover (or astronaut) leaves the lander, there is no more difference of potential to cause damage.

I don't know if I have clarified or muddied things with this; maybe I will just shaddap before I get a migraine.

Cheers!