.Fryyet mars has no atmosphere...boom
Well it does, we used it to aerobrake our last 2 probes. It's just much thinner than ours, and does a pretty awful job stopping strong UV rays. Mars also lacks a magnetic field, which is what stops a lot more charged solar particles from interacting with life and other complex molecules on earth.
That doesn't mean Mars is uninhabitable. A lot of scientists think mars used to have a spinning core floating in liquid iron, much like earth's is today, and that it had an atmosphere that at least supported liquid water. This means we think Mars was once suitable for life, probably a few hundred million years before earth was. If Earth lost its magnetic field, atmosphere and oceans, would some life live on for 5 billion years? Could it evolve fast enough? Would it all move underground? These are the questions that we are asking about Mars, and until we do more research here on Earth and on Mars, we won't be able to even form any credible theories on martian life.
Scientists know what the implications of the methane might be, but they also know jumping to a giant conclusion could prove to be pretty dumb.It certainly has in the past, and they aren't about to take that risk I'm sure many that have worked to be able to study the red planet believe and hope life is currently soldiering on, billions of years after it might have split with our life. Hopefully NASA/the rest of the space going world can figure out effective sterilization so we can check for our selves and get a definite answer to one of our biggest questions.