Prof Coleman is director of the Centre for Life Detection at the Nasa Jet Propulsion Laboratory in the United States.
He gave a keynote lecture this week on "Searching for Life on Mars", at a chemistry conference at the University of Otago.
The surface of Mars was generally cold, dry and harsh, he said in an interview.
Given the limited evidence, he was taking an "agnostic" stance, neither believing nor disbelieving that microbial life existed on Mars.
But he had found some "moral support" for the possibility of life there, through the growing scientific evidence about the extraordinary lives and appetites of Earth's microbiological "extremophiles".
These are microbes that thrive under even the seemingly harshest and most unpromising conditions.
"In any environment there's a bug ... making a living," he joked.
By studying extremophiles on Earth, he and other scientists were learning more about life here as well as the possibilities for life on Mars.
Some of Earth's bugs revelled in extremely high or low temperatures, with some also prospering in highly acidic conditions in waterways in the heavily-mined Rio Tinto area in southern Spain.
The need to develop new generations of unmanned rovers to explore Mars was also helping to improve technology which would also benefit people on Earth.
Examples were better solar panels, and smaller, more energy-efficient testing equipment, he said.
More than 300 people are attending the five-day conference, which is devoted to "Chemistry and the Biosphere".
