Is it time to revisit what NASA's Viking lander found on Mars in 1976?

Mechanical arms in front of a reddish, rocky surface.

Back in 1976, the dual NASA Viking landers came to full stop on the Red Planet.

Their life detection experimental findings still reverberate within the scientific community – fueling the on-going discussion on a key question: Is there life on Mars?

Fast forward to today, a new paper tackles and reconsiders the results of the Viking Biology experiments.

Perchlorate finding

The most significant change since those 1970's experiments were conducted was the discovery of high levels of perchlorate on Mars. Perchlorate, plus abiotic oxidants, explains the Viking results and there is no requirement to postulate life on Mars.

"The discovery of perchlorate on Mars by the Phoenix mission has provided a basis for explaining the results of the Viking Landers," the newly issued paper notes. "Thermal decomposition of perchlorate in the ovens of the [Viking] instrument can explain the lack of organics detected. Accumulation of hypochlorite in the soil from cosmic ray decomposition of perchlorate can explain the reactivity seen when nutrient solutions were added to the soil in the Viking Biology Experiments."

However, the paper adds, "a non-biological explanation for the Viking results does not preclude life on Mars."

Revisit the results

The just-released paper — "The Viking biology experiments on Mars revisited" – has been authored by noted Mars researchers Christopher McKay, Richard Quinn and Carol Stoker. All three authors are from the space science division of NASA's Ames Research Center at Moffett Field, California, near San Francisco.

"With Mars sample return on the horizon and the prospect of future missions to Mars, perhaps even including life detection instruments, it may be timely to revisit the results of the Viking Biology Experiments," the research team suggests. "Since Viking landed on Mars, many things have changed, and many things have not. What has not changed in the past 50 years is our understanding of the limits of life in cold and dry environments."

In a communiqué with Christopher McKay, he told Space.com: "It is important to note that we are not saying that the Viking results imply 'no life on Mars.' Nor are we saying the Viking results imply there is life on Mars."

McKay said that their core point is that the Viking results are saying there is perchlorate and other oxidants on Mars, “and that is what the Viking biology experiments responded to.”

What this means is that the results of the Viking Biology experiments can’t be used to justify an approach to astronaut health and safety or a sample and/or astronaut quarantine policy for return to Earth that assumes no life on Mars.

Model of NASA's Viking Mars lander. (Image credit: NASA/JPL)

New data

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In their paper for the scientific journal, Icarus, the research trio explains that there have been big changes resulting from missions to Mars. "The most important new data, by far, was the surprising discovery from the Phoenix Mission that the soils of Mars contain about 0.5% perchlorate," they observe. "This incredibly high concentration of perchlorate is still not adequately explained but the implications for the Viking results are profound."

The space scientists in their paper explain that the perchlorate model and the resultant conclusion that Viking did not detect life in the surface soils of Mars will factor into any discussion of sample return or astronaut return from Mars.

"The Outer Space Treaty prohibits 'adverse changes in the environment of the Earth resulting from the introduction of extraterrestrial matter.' Future experiments are needed to better understand the chemistry of Martian soils and the possibility of life persisting there," McKay and colleagues add.

Good targets

In summing up their research paper, they conclude that the perchlorate model for the Viking results "does not prove that there is no life on Mars, nor does it imply that the continued search for evidence of life on Mars, past or present, is pointless."

Indeed, as the research team suggests, "we strongly argue for the search for evidence of extant life in future missions. Good targets are salt deposits and polar ground ice."

This new research has been published in Icarus.