I'm not sure whether "marcere" has anything to do with it. But going by its etymology it might well mean "to be crushed" (this is at least what my dictionary says: Menge/Güthling; with U umlaut).
I've also tried to find out whether there is any connection between Mars
and the hammer, but obviously there isn't.
I have also followed the trace of "Karl
Martell) which definetely means "hammer" and which also reminds me a lot of Mars/Martis. But again there seemed to be no connection. So let's call it incidence.
Unfortunately the authors of my ditionary (which in other cases has proved quite reliable) are long dead so I can't ask them. So I'll try one more thing: find someone who has access to a really big Latin dictionary. And if this fails I'll just leave it alone and stick to Mars
Thanks for your help!Andy