A new analysis of NASA's Viking mission to Mars, which happened about 30 years ago, shows we may have encountered life on Mars back then, and that we accidentally killed it.
They argue that the scientists assumed in the 70s that any form of life would be based on water, but now we know that life doesn't always need water:
Since the Viking missions, he noted, bacteria and other microbes have been found in extremely hot and chemically toxic environments. Some of these "extremophiles" -- found in hellish places such as undersea volcanoes -- actually thrive on chemicals such as hydrogen peroxide that are hostile to most other living creatures.
A previously confusing finding on Mars by the Viking Landers was evidence of chemical oxidation. Scientists at the time assumed this meant the Martian surface was a highly reactive place, but further missions to Mars failed to find any evidence of oxidative chemistry.
Schulze-Makuch and Houtkooper suggest that the hydrogen peroxide detected by Viking could have come from killing Martian microbes that, like some peculiar creatures on Earth, use hydrogen peroxide the same way humans use water.