What if there was life on Mars, but the space probes that were sent to look for it didn't recognize it?
Or worse, what if they killed it?
One scientist thinks that may have happened. He theorizes that two NASA probes that visited Mars 30 years ago may have stumbled upon alien microbes and inadvertently killed them.
He makes the case in a paper presented to the American Astronomical Society, meeting in Seattle.
He says the Viking space probes of 1976 and 1977 were looking for Earth-like cells with salt water as the internal liquid. But in the cold, dry conditions of Mars, life could have evolved with an internal mix of water and hydrogen peroxide.
He says the Viking probes wouldn't have noticed hydrogen peroxide-based life -- and could have killed it by drowning and overheating the microbes.
NASA may be able to test that theory in a Mars mission set for launch this summer.