Crazy. I just presumed the planet acts as a ground. Presumably same challenge on the moon?

Like you call the ground earth or you call ground earth? I fail to see how earth is less ambiguous.

Mars does not have a magnetosphere. The core on Mars stopped spinning so there is no longer the electric dynamo and thus there isn't a potential. So everything has to be a floating potential. That isn't to say that you can't make reference grounds, but it is much more convoluted than "sticking a fucking metal rod into the ground".

Actually if you pay attention to HI-SEAS[0] this has been a cause of an accident (since they replicate Mars habitat.

This problem also, obviously, applies to any planetary body which does not have a magnetosphere. So you don't have magnetic north and you don't have a safe potential ground.

[0] https://en.wikipedia.org/wiki/HI-SEAS