The key is that there is no legitimate “if” if it is impossible.
Humans are on Earth.
God became man on Earth. Jesus the third person of the Blessed Trinity died on Earth where every human ever born has been born and has lived.
Christ redeemed humans who are all born in Original Sin and who are all descended from Adam the first human created by God.
Since all of this is pure speculation we can speculate about the Martians too.
poche, it is obvious that you are a lib, but do you understand that it is not sane or worthwhile to speculate about the impossible?
In 1491 it was a commonly held belief that if you sailed into the west you would fall off the edge of the earth.
That bit of info is irrelevant to this discussion.
Do you not believe some things are impossible and should not even be speculated on?
According to your method, it seems you could speculate about baptizing some strange non-human beings that we may discover which look like Martians and live deep below the sea or deep under the earth.
Even if such creatures could exist they could not be baptized. No point speculating about legitimately baptizing them.
Baptism is for humans, who are created by God and born in Original Sin.