Almost 40 years ago, two NASA probes on the surface of Mars scooped the soil in search of signs of microbes. The results that came back from the twin Viking missions were, to say the least, ambiguous. The scientific literature contains decades of debate over what they found.

A new study took a bit of a different track. Rather than focusing on the question of life, the SETI Institute-lead study, which was carried out at NASA Ames and recently published in the journal Astrobiology, was more interested in defining the martian environment that the Vikings sampled.

The results suggest the landers not only found evidence of perchlorate salts on the surface, but there also should be highly reactive related compounds that can decompose organic compounds at low temperatures and explain the results of the Viking biology experiments.

Was that environment habitable, however? Richard Quinn, who is a SETI Institute researcher with Ames' Planetary Systems Branch, says the more relevant question now is how well Mars can preserve evidence of ancient biosignatures, or chemical signs of life. This focus began to shift in 2009, he said, after published results showed NASA's Mars Phoenix mission detected perchlorates at the north pole.

"People are always looking back at the Viking results. They were never thoroughly explained," Quinn said. "The Phoenix mission has focused the science debate in a new direction, and it's moved away from superoxide-peroxide based chemistry to a chlorine and chlorine-oxide based chemistry."

The difference sounds subtle to the outsider, but looking closer, it does reveal a "different picture" about how habitable the soil is - which is something scientists are still investigating, Quinn added.

"Based on what we know about life in extreme environments, interpretations of mission results indicate that we are currently exploring habitable ancient environments on Mars, and I believe that these are solid interpretations. What the discovery of perchlorate tells us about is how the surface of Mars may have evolved in more recent times. The question is about preservation potential of biosignatures rather than the intrinsic habitability of the ancient environment."

Four decades of re-examining Mars experiments
NASA sent the twin Viking landers for a 1976 arrival on the Red Planet, which was an ambitious first for an agency that had just performed the first manned landing on the Moon nine years beforehand. Both missions performed flawlessly for many Earth years beyond their 90-day expiration date.

While the spacecraft carried many experiments on board, it is the three biology experiments they carried that have come under the most scrutiny. These were:

+ A gas exchange experiment that took a sample of Mars soil, brought it inside the lander and spiked it with a liquid solution that had organic and inorganic compounds in it. In this experiment, the soil released oxygen when exposed to water, even before it came in contact with the liquid solution. After contacting the solution, the soil decomposed organic compounds.

+ A labelled release experiment that put Earth organic compounds inside a bit of Mars soil. These compounds were labelled with radioactive markers to see how well any potential microorganisms would absorb the nutrients. The experiment showed carbon dioxide being released.

+ A pyrolytic release experiment heated a sample of Mars soil and saw some organic residues coming off of the soil. (While this was covered in Quinn's other research, this particular experiment was not addressed on this occasion.)

The labelled release experiment and pyrolytic release results appeared to be consistent with how Earth microbes would behave under similar conditions, but scientists were skeptical for a few reasons. One was that that the gas exchange experiment was not consistent with the presence of microbes, and instead indicated the present of reactive compounds, or oxidants in the soil.

Additionally, the Viking organic analysis experiment did not detect any organic compounds that were thought to be of martian origin. Further, some scientists say it's probable that high solar ultraviolet radiation hitting the soil makes the surface sterile and hostile to life.

Scientists' understanding of Mars has changed immeasurably in the decades since, however, including the discovery of water ice sitting currently at the poles, and probable flowing water in the ancient past. Further, technology has made it possible to detect smaller bits of stuff than what was possible during the 1970s.

In 2006, a University of Mexico-led study recreated the experiments in several environments on Earth that are considered similar to Mars, such as the Chilean desert, and found organic compounds that contained chlorine. The aforementionted NASA Ames study, which was published in 2010 in the Journal of Geophysical Research: Planets, suggested that organic materials break down into chloromethane and dichloromethane during heating in the presence of perchlorate.

In Quinn's new work, his team took perchlorate samples and sealed them in an atmosphere that is similar to what is found on Mars. These samples were exposed to a gamma radiation source to simulate the harsh radiation environment on the Red Planet, whose thin atmosphere is not enough to shield the surface from highly energetic particles bombarding the surface.

"Perchlorate itself is a relatively common compound; people are familiar with its use in rocket fuel." Quinn said. "It can be a very strong reactant or very strong oxidizer, but you have to heat it. You have to get over an initial energy barrier to activate perchlorates. Perchlorate alone can not explain the Viking biology experiments, because at low temperature perchlorate is not reactive."

In the case of the gas exchange experiment, the researchers were able to reproduce the oxygen release in the lab using this sort of "radiation-damaged" perchlorate. Rerunning the labelled release experiment also duplicated the results of Viking, with carbon dioxide released.

"What we did with the ionizing radiation was to decompose the perchlorate into more reactive, compounds," Quinn said, referring to oxychlorine and "oxygen species" such as oxygen gas, chlorine dioxide and hypochlorite (the latter being best known on Earth as an ingredient of bleach).

The Viking experiments didn't measure the decomposition or perchlorate in the natural environment. This occurs over many thousands of years. But Viking biology experiments did measure the effects of perchlorate decomposition products. When these compounds become wet they release oxygen and they decompose organics used in the labelled release experiment.

More information on perchlorates should be coming shortly from the Mars Curiosity rover team, which has been examining the compound using the Sample Analysis on Mars (SAM) instrument. NASA's next rover, slated to launch in 2020, will focus more on identifying high-priority samples for sample return, Quinn said.

"Most likely perchlorate chemistry will not be the focus of that mission. The focus of Mars 2020 will be to cache high priority science samples for a return, and then we'll learn about the perchlorate [back on Earth.]"

The European Space Agency is planning its first rover on the Red Planet, called ExoMars, to launch in 2018. Its planned laser desorption mass spectrometer on board would send a beam of ultraviolet light to soil samples, releasing organic compounds. It is also possible to use this method to detect perchlorate, Quinn said.

"The laser can cause perchlorate to react with soil organics forming chorine containing organics that can be detected with the mass spectrometer," he said.