Is this some sort of joke? Christ was both fully man and fully God. Man = human. And he died for humans.
Now, I suppose if martians were human beings like us, we could treat them sort of like “found” humans as we discovered them in the americas many centuries ago. But the hypothetical is absurd on the face of it. Jesus is about earth, regarding salvation. And baptism is about salvation.
It is indeed absurd. Why would they even need baptism or salvation if they are not descended from Adam & Eve? If you’re not part of the curse, then you don’t need the cure.