That's a good point. If the spectroscopy is flawed, that could just as easily be a giant lake of moose piss, rather than a sea of methane.
There you go, trying to trick me. If it was a giant lake of moose piss, then oxygen would have to be present, to accommodate all those moose.
One other question: if there was no oxygen on that asteroid, then how was Harry Stamper able to detonate that nuke?