NASA's Mars 2020 Perseverance rover has begun its search for signs of ancient life on the Red Planet. But we already know there is life on Mars, because we brought it. Indeed, humanity has sent around 30 spacecraft and landers to Mars since the space age began, and some microbes have managed to survive the trip. Writer and podcaster Nicola Twilley's latest book, co-authored with husband Geoff Manaugh, is Until Proven Safe: The History and Future of Quarantine. She joins the show to discuss the importance of planetary quarantine.