Two NASA space probes that visited Mars 30 years ago may have stumbled upon alien microbes on the Red Planet and inadvertently killed them, a scientist theorizes in a paper released Sunday.

The problem was the Viking space probes of 1976-77 were looking for the wrong kind of life and didn't recognize it, the researcher said in a paper presented at a meeting of the American Astronomical Society in Seattle.

This new report, based on a more expansive view of where life can take root, may have NASA looking for a different type of Martian life form when its next Mars spacecraft is launched later this year, one of the space agency's top scientists said.

Flowing water?

Last month, scientists excitedly reported that new photographs of Mars showed geologic changes that suggest water occasionally flows there — the most tantalizing sign that Mars is hospitable to life.

In the '70s, 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 low temperatures (-55 C), doesn't destroy cells when it freezes, and can suck scarce water vapour out of the air.

The Viking experiments of the '70s wouldn't have noticed alien hydrogen peroxide-based life and, in fact, would have killed it by drowning and overheating the microbes, said Schulze-Makuch, a geology professor at Washington State University.

One Viking experiment seeking life on Mars poured water on soil. That would have essentially drowned hydrogen peroxide-based life, Schulze-Makuch said. A different experiment heated the soil to see if something would happen, but that would have baked Martian microbes, he said.

"The problem was that they didn't have any clue about the environment on Mars at that time," Schulze-Makuch said. "This kind of adaptation makes sense from a biochemical viewpoint."

Even Earth has something somewhat related. He points to an Earth bug called the bombardier beetle that produces a boiling-hot spray that is 25 per cent hydrogen peroxide as a defence weapon.

Schulze-Makuch acknowledges he can't prove that Martian microbes exist, but given the Martian environment and how evolution works, "it makes sense."

In recent years, scientists have found life on Earth in conditions that were once thought too harsh, such as an ultra-acidic river in Spain and ice-covered lakes in Antarctica.

Schulze-Makuch's research coincides with work being completed by a U.S. National Research Council panel nicknamed the "weird life" committee.

The group worries that scientists may be too Earth-centric when looking for extraterrestrial life. The problem for scientists is that "you only find what you're looking for," said Penn State University geosciences Prof. Katherine Freeman, a reviewer of the NRC work.

New Mars mission

A new NASA Mars mission called Phoenix is set for launch this summer, and one of the scientists involved said he is eager to test the new theory about life on Mars.

However, scientists must come up with a way to do that using the mission's existing scientific instruments, said NASA astrobiologist and Phoenix co-investigator Chris McKay. He said the Washington State scientist's paper piqued his interest.

Other experts said the new concept has a certain logic to it, but more work is needed before they are convinced.

"I'm open to the possibility that it could be the case," said astrobiologist Mitch Sogin of the Marine Biological Lab in Woods Hole, Mass., and a member of the National Research Council committee. But he cautioned against "just-so stories about what is possible."