The search for life is the holy grail of Mars exploration. Yet oddly, only one mission was ever sent to Mars equipped with the kind of scientific instrumentation needed to identify signs of life--NASA’s 1976 twin Viking lander mission--and that mission yielded conflicting results. Three of Viking’s onboard biology instruments gave some indications of life, while its gas chromatograph/mass spectrometer (GC/MS) did not find any organic molecules even at the parts-per-billion level.
Since Viking, most scientists have thought that the liquid water almost certainly needed to support life could not exist on Mars because of its low atmospheric pressure, which is about one-hundredth that of Earth. Without liquid water, even simple microorganisms couldn’t carry out metabolic functions.
Within the last year, however, the conventional wisdom about the Viking experiments and life on Mars has changed radically. A scientific paper published in the October 2006 issue of the Proceedings of the National Academy of Sciences cast serious doubts on Viking’s GC/MS results. The team responsible for the paper, led by Rafael Navarro-González of the Universidad Nacional Autónoma de México, in Mexico City, reported on experiments it had done with soils from Chile’s Atacama desert, using a GC/MS instrument similar to Viking’s. The team determined that the Viking spectrometer would not have been sensitive enough to find low levels of organic material on Mars, even in soils that might contain millions of microorganisms.
Making matters even more interesting was a discovery made in 2006 by NASA’s Mars global surveyor orbiter camera and published in the December 2006 issue of Science . Changes in crater gullies strongly suggested that water had flowed in them within the last few years, obviously ”an important discovery,” in the words of Peter Smith, the principal investigator for NASA’s upcoming Phoenix lander mission.
Phoenix, scheduled to be launched this August and to touch down in May 2008, may resolve lingering questions about organic molecules and liquid water on Mars. Its main objectives are to determine whether the Martian arctic could support life, to evaluate the history of water near the landing site, and to investigate how the planet’s climate is affected by polar dynamics.
The icy arctic landing site for Phoenix was chosen after reviewing data returned from NASA’s Mars Odyssey Orbiter, launched in 2001 and still in orbit around Mars. In 2002 Odyssey found evidence of large quantities of water ice in the upper 50 centimeters of soil in the northern polar region.
Among the cache of scientific instruments Phoenix will carry that could shed light on whether organics are present is the thermal and evolved gas analyzer (TEGA). Designed primarily to look at ice chemistry and carbonate minerals, the TEGA can also detect organic molecules at a much higher sensitivity than Viking’s GC/MS did in 1976. The difference is the temperature and the time the TEGA will spend heating samples in order to release any organics from them.
Phoenix has a robotic arm that will dig down to 1 meter below the Martian surface to get soil samples for the gas analyzer and other onboard instruments. Once the TEGA has acquired the samples, it will heat them up in small (0.038-microliter) batches in eight ovens to 950 °C for up to 20 minutes to release any organic matter contained in the soil. Viking could heat soil samples for only 2 minutes at a maximum temperature of 500 °C.
William Boynton, a professor of planetary sciences at the University of Arizona, is the TEGA instrument designer and principal investigator. ”My best hopes are that we get a clear organic signal from Mars and that we have confidence it is not due to organic contamination that we brought from Earth,” he explains. Because Phoenix is not primarily designed as a life detection mission, planetary protection sterilization procedures are not as stringent as they were on Viking; some earthly contamination most likely will ride along with TEGA. Boynton says, however, that ”if TEGA sees more organics with samples taken deeper below the surface, this would give us some confidence that the organics are truly from Mars” and not due to contamination.
Other instruments on Phoenix include Mars’s first-ever atomic force microscope, 100 times as powerful as microscopes on the Mars Exploration Rovers and capable of resolving features as small as bacteria. Phoenix will also carry an electrochemistry instrument designed to look at the pH and salt content of the Martian soil.
Will Phoenix answer the question of life on Mars? It may just repeat the Viking inconclusive results, says NASA’s Smith. ”Or we may find a massive incontestable increase in organic molecules associated with subsurface ice layers.”
At press time, all the scientific instruments had been integrated into the Phoenix lander and were undergoing final testing at the Lockheed Martin Space Systems assembly building in Denver. If there are no problems, the lander will be enclosed in a protective aeroshell and sent to the Kennedy Space Center in Florida to be mated to its launch rocket in early July.