Which way would Muslims face if they were in space? Whether they’re on Mars or made their way to another solar system.
In the Solar System–towards Earth.
Out-System—towards Sol.
Simple. 
The place to start for an official answer would be with Prince Sultan Ibn Salman Ibn Abdul Aziz, who was a mission specialist on STS-51-G in 1985.
This site says:
I suspect that the direction he prayed in was “down.”