The closest place in the universe where extraterrestrial life might exist is Mars, and human beings are poised to try to colonise this planetary neighbour within the next decade. Before that happens, there has to be recognition that a very real possibility exists that the first human steps on the Martian surface will lead to a collision between terrestrial life and biota native to Mars. If the red planet is sterile, a human presence there would create no moral or ethical dilemmas. But if life does exist on Mars, human explorers could easily set off the extinction of Martian life. As an astronomer who explores these questions in my book Life on Mars: What to Know Before We Go, I contend that earthlings need to understand this scenario and debate the possible outcomes of colonising the neighbouring planet. Maybe missions that would carry humans to Mars need a timeout. Life, scientists suggest, has some basic requirements. It could exist anywhere in the universe that has liquid water, ...

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