No one can say with any certainty. Hydrogen is very prevalent though, and that's more than half the battle to make H2O.
There's several theories out there, but they usually encounter some catastrophic problem why it couldn't work and then go to the next theory and work on that one. Trial and error.
Just be happy that you have delicious, life-sustaining water at your disposal. :)
"Fix reason firmly in her seat, and call to her tribunal every fact, every opinion. Question with boldness even the existence of a God; because, if there be one, he must more approve of the homage of reason, than that of blindfolded fear."Thomas Jefferson