Opportunity could be sitting on rocks chock-full of organic molecules — but the rover and the scientists back on Earth would never know. Unlike Curiosity, Opportunity is not carrying instruments that can detect those kinds of molecules.
But the scientists are not complaining. Everything from Opportunity over the past eight years has been a bonus for a mission that was to have ended long ago.
Opportunity landed on Mars in January 2004, for what was supposed to be a three-month mission. Yet the rover continues operating in good condition. (Its twin, Spirit, died in 2010, stuck in a sand trap and unable to point its solar arrays in the correct direction to survive winter, outliving its planned lifetime by almost six years.)
I always kind of liked Spirit more — showed a bit more spunk, IMO — but the fact that this thing is still going after nine years is nothing short of amazing. Admittedly, one has to give credit to its designers who made some good decisions, but it still shows how robust these things can be.