In the 1970′s, the Viking landers went looking for biosignatures on Mars. They soft landed at widely separated landing sites, equipped with four experiments designed to directly identify microorganisms. Three experiments produced negative results, while the fourth came up positive at both sites.
The positive Viking results were produced by the Labeled Release (LR) experiments, developed by G.V. Levin. Originally conceived to test for microbial contamination of water and food, in LR a drop of water containing nutrients is brought into contact with a Martian soil sample. The nutrients contain radio labeled Carbon-14 isotopes, and the LR detector ‘sniffs’ for radioactive gases expelled by microorganisms feeding on those nutrients.
Although the LR experiments produced results consistent with microbial life, they were received with much skepticism. To this day, according to Levin the experiments are conclusive and demand that we accept life exists on Mars, but most scientists suspected that an unknown inorganic oxidizer consumed the nutrients to produce a false positive. The issue was debated for over thirty years until in 2008, the MECA experiment aboard the Phoenix lander detected perchlorate. At temperatures above 200 ˚C, such as those used in the LR experiments, perchlorate is an oxidizer. Based on the Phoenix results, Earth experiments were set up which confirmed that perchlorate converts organics in Mars-like Earth soil to chloromethane and dichloromethane: the gases detected in Viking’s LR experiments.
These are astounding results from a lander that wasn’t designed to look for life at all, but for places that at one time might have been habitable. In shedding light on Viking’s false positive, Phoenix illustrates an important point.
A direct search for life or ‘biosignatures’ may produce few or conflicting results if the Martian environment is not adequately known and understood. This insight led the Mars Exploration Rovers to set off in 2003 to ‘determine climate and water history at sites where conditions may once have been favorable to life’.
In a few days, MSL will carry the Curiosity rover to Mars with a similar objective of ‘exploring and quantitatively assessing Mars as a potential habitat for life, past or present‘. Because of its large size and impressive range of scientific instruments, Curiosity has been called Viking on wheels. Despite striking similarities, the shift in interest from the directly detecting life to the reconstruction of a geological, chemical and climatological context in which life could exist, illustrates the long way NASA has come since the original Viking missions.
While water was the critical driver in determining Spirit and Opportunity’s landing sites, today there is also much interest in minerals that mark the past presence of liquid, non-acidic water. In 2009, the Mars Reconnaissance Orbiter (MRO) detected the faint but clear signature of phyllosilicates on the Martian surface. These are a family of clay minerals that form in pH neutral water, and were found only on the oldest exposed parts of the surface, dating back over 3 billion years. Opportunity had already detected sulfates deposits on younger terrain, marking the past presence of strongly acidic water. So the climate history of Mars currently proposed is that pH neutral water abounded in an aptly named Noachian period, after which a final volcanic episode rendered the water acidic and less hospitable to life. Rover analysis of rocks containing silicates would provide clues on their remaining chemical composition and formation history.
An oblique view of Gale with 2x vertical exaggeration. Elevation data is from HRSC, surface details are from CTX, color is from Viking (source http://blogs.agu.org/martianchronicles, ellipse added).
The MSL landing site (the Gale crater, cfr. image) was selected over three other top candidates for the presence of both sulfates and phyllosilicates. The phyllosilicates are located on Gale’s central mountain towering over 5 km above the crater floor. As Curiosity drives up its slope, it will progressively analyze younger Mars terrain, starting at 4 billion years ago.
But interestingly, MRO also detected phyllosilicates in an area near the Opportunity rover. The Exploration Rover landing site turns out to be a good candidate according to the recent MSL criteria as well! Some phyllosilicates were detected at Cape York, which was reached by Opportunity in August this year. Mission scientists are excited to see the ‘mission begin anew‘. So far Cape York has only yielded a rock of surprisingly high Zinc content, but keep in mind that the phyllosilicate ‘motherlode’ is known to reside further South, at Cape Tribulation. Opportunity is digging in for Martian winter at Cape York, after which the science team has every intention of reaching Cape Tribulation.
Will Opportunity end up stealing Curiosity’s thunder in first detecting the mark of neutral water, hospitable to life? In my opinion, it should be considered a well deserved triumph. And as Matt Golombek (who led the selection of the Spirit, Opportunity and Curiosity landing sites) put it, Curiosity’s instruments far exceed the sensitivity and accuracy of any previous Mars rover:
“MSL’s going to look at the same materials we’ve seen with all the other Mars missions with some fresh instruments. It won’t matter whether we find phyllosilicates or not, we’re going to learn a huge amount about the materials. If you know what the minerals are and the layers are, you know what the environment was. That’s the most compelling part — we don’t know whether [prebiotic chemistry] ever began on Mars; it’s kind of a needle in a haystack search, and you don’t want to only do that. You want to get all that basic information, what the environment was, was it conducive to life, and were the building blocks for organic materials available. We’ll answer those questions regardless of whether there’s actually organics. If we find them, it’s a great bonus. But if we don’t, that’s okay too, we’ve still made a major step forward.”
Links in this story
G.V. Levin personal web page
G.V. Levin – It’s Time to Realize There Is Life on Mars
The Martian Chronicles – Phoenix Update: Pondering Perchlorates
J. of Geophysical Research – Reanalysis of the Viking results suggests perchlorate and organics at midlatitudes on Mars
Alien Rumors Quelled as NASA Announces Phoenix Perchlorate Discovery
Mars Science Laboratory: Science Goals (MSL Science Corner)
Curiosity in context: Not exactly “Viking on wheels,” but close
Gale (crater) – Wikipedia
Geophysical Research Letters – Phyllosilicates and sulfates at Endeavour Crater, Meridiani Planum, Mars
Opportunity Arrives at Endeavour Crater, Mission Begins Anew
Opportunity Heads for New Discovery as Winter Blows In at Cape York
