I chose "Marsdude" as a screen name for an online poker site. I had just been reading about the supposed "face on Mars" and I used a photo of that as my avatar. I was hoping that people would think that I was "way out there" and would play crazy or something.
I found that the "Marsdude" was a screen name that was available on most sites so I kept using it. I changed my avatar to Marvin because on one knew what the Mars face was.