Scientists may have found another big clue that there is some form of life on Mars, after a probe detected ammonia.
Scientists say sensors on board the European Space Agency's orbiting Mars Express craft have picked up tiny signs of the chemical.
Ammonia only survives for a few hours in Mars' atmosphere, so something must be constantly making more of it.
The gas can only have come from bacteria or volcanoes, but none of those have ever been found on Mars.