re: A real question.
Hi Mary Anne
Tricky one, but I personally equate "earth" with "soil" and that implies a substantial organic component, which doesn't exist off-Earth. Thus on other planets it can be dust, dirt, sand or regolith (if you want to sound like a space-nerd), but not earth or soil. Though it can become soil if you colonise and farm it. And digging is digging, wherever you are - though it kind of implies a gravity-field giving a sense of up and down. Perhaps "excavate"?
Adam