By Ron Cowen
Martian soil could contain the building blocks of carbon-based life after all, a new study suggests, despite the negative results of an analysis performed by the Viking missions 34 years ago.
When the Viking landers touched down on Mars in 1976 and scooped up soil samples, scientists were surprised that the two craft failed to unearth evidence that the Red Planet contained any organic compounds. The apparent lack of organic molecules — a basic requirement for carbon-based organisms — helped to cement the notion of Mars as an entity that would not easily support life.
But a new study, which relies on soil samples from Earth, now suggests that the Viking craft may have found organic compounds from Mars but failed to recognize them. The finding represents a sea change in the way many scientists think about Mars and suggests a specific strategy for searching for vestiges of life on the planet, says study coauthor Rafael Navarro-González of the National Autonomous University of Mexico in Mexico City.
Navarro-González and his collaborators, including Chris McKay of NASA’s Ames Research Center in Moffett Field, Calif., describe their work in an upcoming Journal of Geophysical Research–Planets. They also reported the findings September 6 during a press briefing from the National Autonomous University.
The study was inspired by an analysis of soil samples conducted by the Mars Phoenix Lander, which arrived in the north polar region of Mars in May 2008 and operated for five months. Phoenix found that most of the chlorine at the landing site was in the form of perchlorate, rather than a chloride salt as had been assumed (SN: 4/11/2009, p. 12).
Perchlorate is an oxidizing agent that when heated, breaks down into highly reactive fragments that destroy organic compounds. These reactions take place at the same temperatures — 200° to 500° Celsius — to which Martian soil samples were heated by the Viking craft. The only organic compounds found by Viking, chloromethane and dichloromethane, were interpreted as contaminants from Earth, since they are common in cleaning fluid, solid rocket fuel, fireworks and other explosives.
But when Navarro-González and his colleagues added 1 percent by weight magnesium perchlorate to soil from the Atacama Desert in Chile, which is thought to closely resemble Martian soil and is known to contain organic compounds, they found an intriguing result. Heating the perchlorate-adulterated desert soil to temperatures comparable to those in the Viking experiments produced the same chlorinated organic compounds that were found by the landers in 1976 but dismissed as contaminants. Nearly all the organic compounds originally in the Chilean soil were destroyed during the heating.
Similarly, the team says, the soil at the two Viking sites likely contained plenty of organics that were destroyed upon heating and were turned into chlorinated methane compounds due to the presence of perchlorate.
“The bottom line of this work is that the Viking landers did detect organics on Mars, we just did not realize it,” McKay asserts. He and his colleagues estimate that the Martian soil contains a few parts per million of organics, comparable with the driest parts of the Atacama Desert.
But astrobiologist David Des Marais of NASA-Ames cautions that the study’s authors can’t be sure that the Chilean desert soil that they analyzed closely resembles the Martian soil. In addition, he notes, the inorganic compounds in the Chilean samples could alter the nature of the materials released during the heating process.
The presence of organic compounds on Mars at concentrations of parts per million does not mean that the Red Planet, now cold and arid, is a gentler, kinder place for life, McKay says. Nor does it require that life exists or once flourished there, the researchers emphasize.
Meteorites could have delivered the compounds to the planet, rather than a biological source. However, the presence of organic compounds opens up a relatively easy way to search for life on Mars, the researchers say.
If Mars contained organic compounds at concentrations of only parts per billion or less, searching for such signs of life as DNA, complex proteins or energy-carrying adenosine triphosphate would be problematic. But if Mars contains organic compounds at the level of parts per million, “then searching within these organics for such biomarkers makes sense,” McKay says. Moreover, scientists are particularly adept at searching for these biomarkers, he adds.
For Des Marais, however, searching for Martian DNA “is out of the question” because the molecule is too fragile.
NASA’s Curiosity Rover, scheduled for launch in November 2011, has the ability to search for organic compounds by adding liquid to soil samples, which would not subject them to the high temperatures at which perchlorate destroys organic compounds, notes McKay.
“I’m very excited about the possibility that in a couple years, we will have a follow-up mission that will use the information that comes from this study to detect and confirm the organics on Mars,” he says.
