"I've been worried about that because I think we're close to finding it, and making some announcements.
"What happens next is a whole new set of scientific questions. Is that life like us? How are we related?
"Can life can move from planet-to-planet or do we have a spark and just the right environment and that spark generates life — like us or not like us — based on the chemical environment that it is in?"
Europe's ExoMars rover is scheduled to land on the Red Planet in March 2021. Dubbed "Rosalind" in honour of the British chemist Rosalind Franklin, the rover will drill 1.98m to take samples.
The Martian cores are then fed through an aperture on the rover into a mobile laboratory where they are crushed up and examined for organic matter. Confirmation of life could come within weeks or months of landing.
Likewise, Nasa's rover Mars 2020 will drill into rock formations on the surface and leave the samples in test-tubes, which will later be collected and sent back to Earth for examination, the first time material from Mars will have been brought back.
Crucially, scientists will be looking for the 300 minerals that can only be made by life.
The rovers will be hunting near the site of an ancient Martian ocean, where life may have lived billions of years ago, when Mars was "blue" like Earth.
"We've never drilled that deep. When environments get extreme, life moves into the rocks. When we first started the field of astrobiology in the 90s we started looking for extreme life. We go ... 2 miles deep into the Earth and if they were weeping with water they were full of life.
"We have gone in nuclear cesspools, places where you'd think nothing could survive, and they are full of life. The bottom line is where there is water there is life.
"In fact, because the crust has so much water in it, we now know that there is more life below our feet than on the surface of Earth."
Green believes that as well as small organisms on other planets, there may be "weird life" on Saturn's moon Titan, and even civilisations in the galaxy.
Recent research has also found that areas in solar systems that scientists considered uninhabitable may have once held liquid water.
Planets that are neither too hot or too cold for liquid water are said to be in the "Goldilocks Zone", but this month computer modelling showed that Venus could have held water for billions of years, despite being so close to the sun. The moon is also now known to have a water cycle.
"Venus was a blue planet for a significant amount of time. There is no reason to think that there isn't civilisations elsewhere, because we are finding exoplanets all over the place.
"We now know from Kepler observations that there are more planets in the galaxy than there are stars."
Nasa's rover will enter the atmosphere at four miles per second, and to slow down, the lander must flip on to its side and travel vertically. The lander will then hover and place the rover on the surface.
Spotlight on Mars
• 687 the number of Earth days the mission will last, which is the equivalent of one year on Mars
• 3m the length of the rovers, which is roughly the same as a car
• 1050kg the weight of each of the rovers
• 23 the number of cameras installed on each of the rovers. This includes seven entry, descent and landing cameras
• 2.1m the length of the rover's robotic arm. It has shoulder, elbow and wrist joints for maximum flexibility
