Sure, we should consider the possibility of life on Mars. But we've already impacted possible organisms by sending spacecraft there. Even if you sterilize your craft in an autoclave and send it through the vacuum of space for months to years, there's no guarantee that all terran organisms will be inert. Samples taken from an asteroid during the recent Hayabus-2 mission were found to have terrain organisms on them. If you want to completely cordon off martian ecology, you should've convinced NASA and the Soviets back in the 70s.

Bottom line is, we've already irreversibly changed the course of martian ecology, if there is any. What remains? Check if there's actually anything alive over there. The best way to do that is with boots on the ground. The best places to look for life on Mars are:

• deep within the crust, deeper than any robotic probes can dig
• deep under the polar ice caps
• deep in caves and lava tubes

All of which are much easier to explore with humans.