This will take decades to resolve. Random blasts of methane will be very hard to characterise in any methodical way.
Also, those rovers did a really wide range of things for their cost. From driving around 40km and taking pictures to analyzing chemistry and even testing for "wobbles" in the planet's rotation that would indicate a liquid core. Sure, they cost ~500 million a pop, but compare that to 100's of billion for a manned mission that might never reach Mars and things don't look so hot.