What Is Phosphine?

Phosphine is an unstable molecule incorporating phosphorus and other elements. It's not something ore likely to find produced by, i don't know, a lightning strike. So it's important as a potential signal that something strange and unique must be going on in order for that phosphine to be produced. The detection of life will likely not come from a single molecule - we'll have to detect a whole biosphere producing a complex network of gases which together form a message 'we're alive'