Launch a spacecraft to Mars full of organic material. Add the toughest possible bacteria (the kind that manages to survive in volcanoes for thousands of years), vegetable growing experiments, etc...
Then see what sticks. Maybe it's nothing. But in the small chance something sticks, then humanity might have done the most useful thing ever, even in case humanity would go extinct: seed life on another planet.
But instead we seem to be carefully disinfect any spacecraft launched to Mars from biological life instead...
Is this for:
A) to avoid contaminating possible life that would already be there?
B) to ensure that if life is detected, we're sure it's not life from Earth that was present on the spacecraft itself?
But, is the chance that life is already present there considered that high?
How long can we keep maintaining that? At some point we've gotta either send something that's good enough to inconclusively tell whether there already is life there (when are we expected to have the tech to achieve that?), or send something with life.