One of NASA's scientists, Dirk Schulze-Makuch, has revealed that the two Viking space probes that visited Mars in 1976-77 probably killed native-born Martians.
Fortunately, those natives were microbes, and unable to retaliate by (for example) firing heat-rays or disintegrator beams at the Earth.
The Viking probes were designed to search for ordinary Earth-like microbes. Biologists of the 1970s were unaware of the great range of biochemistries exhibited by the so-called extremophiles, and the tests looking for Martian life probably would have killed them.
Last month, scientists excitedly reported that new photographs of Mars showed geologic changes that suggest water occasionally flows there -- the most tantalising sign that Mars is hospitable to life.
In the 1970s, the Viking mission found no signs of life. But it was looking for earth-like life, in which salt water is the internal liquid of living cells. Given the cold dry conditions of Mars, that life could have evolved on Mars with the key internal fluid consisting of a mix of water and hydrogen peroxide, said Dirk Schulze-Makuch, author of the new research.
That's because a water-hydrogen peroxide mix stays liquid at very low temperatures (minus 55 degrees celsius), does not destroy cells when it freezes, and can suck scarce water vapour out of the air.