ORGANIC
The Search for
Barry E. DiGregorio
A
ll life on our planet is built upon carbon-based organic molecules that had their origins in the fiery furnaces of supernovae and red giant stars. Dense molecular clouds rich in organic molecules pervade the entire observable universe. Because carbon atoms can bond with any four atoms, they join with other atoms such as hydrogen, oxygen, nitrogen, sulfur, and phosphorus to form very large, complex molecular chains. Organic molecules, such as formaldehyde, methane, benzene, and even acetic acid, are common in the molecular clouds of deep space. Tar-like organic compounds called polycyclic aromatic hydrocarbons are the largest molThirty years after a robotic spacecraft equipped with a GC/MS went to Mars, two ARTIST’S CONCEPT OF THE PHOENIX LANDER. COURTESY OF NASA/JPL-CALTECH. DETAIL FROM THIS IMAGE ON PP 352 A AND 353 A. (ABOVE, RIGHT) COURTESY OF NASA/JPL/MALIN SPACE SCIENCE SYSTEMS
o
MOLECULES
ON MARS
ecules found in space. Their presence has profound implications for carbon-based life in the universe because it means that all planets eventually could be showered with prebiotic compounds. Every year, Earth’s gravitational field sweeps up as much as 40,000 t of extraterrestrial dust and rock, which then settle on the continents and into the oceans (1). This material can range from submicrometer-size dust particles all the way up to meteorites that are meters across. All the other planets and moons in our solar system receive various amounts of extraterrestrial dust and rock, too. new missions will search for elusive Martian organics and traces of ancient life. © 2005 AMERICAN CHEMICAL SOCIETY
S E P T E M B E R 1 , 2 0 0 5 / A N A LY T I C A L C H E M I S T R Y
349 A
The missing organic molecules In 1976, NASA sent two identical Viking landers to the surface of Mars in the first-ever search for direct evidence of microbial life and organic compounds. Each lander carried three small biology laboratories designed to look at different aspects of possible microbial metabolism. Both landers also carried a highly miniaturized (27.9 26.2 35.6 cm3) GC/MS that examined samples from the top 10 cm of Martian soil (Figure 1). The Viking GC/MS could heat samples to 150, 300, and 500 °C for a maximum of 30 s to look for any traces of organic compounds that would be given off as gases. Even though the sensitivity level of the Viking GC/MS was as low as 5 ppb, it found no evidence of any organic molecules on Mars, not even in samples from carbonaceous meteorites (6 ). This finding was a surprise, because even the lunar samples returned from the Apollo missions con-
In its distant past, Mars shared many similarities with Earth. Both planets had appreciably dense atmospheres, which allowed liquid water to persist at their surfaces. Both had a steady influx of extraterrestrial organics brought in from comets, asteroids, meteorites, and dust. Both had atmospheres that could synthesize organic molecules from a photochemical reaction that occurs when solar UV rays break apart carbon monoxide and carbon dioxide molecules to form other complex organic compounds. Over geologic timescales, those compounds accumulate in sediments near the surface (4). Mars is extremely interesting to astrobiologists because it may still retain the original prebiotic organic compounds that led to life. Unlike Earth, Mars has no plate tectonics to speak of. On Earth, the oldest rocks that contain any traces of life are 3.77 billion yr old and are found in Isua, West Greenland (5). On the other hand, FIGURE 1. Schematic of the 1976 Viking GC/MS. the surface of Mars has re- The instrument was designed to heat soil samples up to 500 °C. Vapors were separated in the GC and then entered the MS. 350 A
A N A LY T I C A L C H E M I S T R Y / S E P T E M B E R 1 , 2 0 0 5
(ABOVE) SUNSET ON MARS. COURTESY OF NASA/JPL/TEXAS A&M/CORNELL. (BELOW, RIGHT) MARTIAN SOIL. COURTESY OF NASA/JPL/CORNELL
The question of life
mained relatively unchanged for 4 billion yr. In addition, because of its lower gravity and less-dense atmosphere, Mars is thought to have accumulated more extraterrestrial organics from comets and meteorites than Earth. In fact, some researchers have estimated that Mars could have received >10 as much organic material as Earth did 4 billion yr ago (3).
COURTESY OF NASA/JPL
Micrometeorites in the size range of 50–500 µm comprise ~99% of all the extraterrestrial material that reaches Earth’s surface. Carbonaceous micrometeorite samples that have been obtained and analyzed from Antarctic and Greenland ice cores contain up to 5% organics, including polymers, amino acids, carboxylic acids, and nucleic acids (2). The carbonaceous meteorites are the leftover material from the formation of our solar system 4.5 billion yr ago. By studying these primordial materials, we gain a fundamental understanding of how planets receive vast reservoirs of organic materials that possibly could have led to the origin of life. One study of the Murchison meteorite, which was recovered from Australia, revealed that it contained >500 organic compounds and 80 amino acids (3). Perhaps the single most important aspect of these extraterrestrial organic compounds is that they are easily dissolved in liquid water and can form countless combinations of complex organic molecules. It is this constant synthesis of carbon compounds in water that many scientists believe resulted in life on Earth.
If traces of organic compounds survived in samples from the moon, why didn’t the Viking GC/MS find similar traces on Mars?
tained some traces of organic compounds. The moon has one of the most unforgiving environments of all, ideal for the destruction of organic matter: direct exposure to cosmic rays, X-rays, and UV rays; no atmosphere or water; and harsh temperature extremes. To make things more mysterious, two of the Viking biology instruments returned data that suggested evidence of both life and organics. In the Labeled Release experiment, a drop of a liquid solution containing radiolabeled organic compounds (formate, D,L-lactate, glycolate, glycine, and D,L-alanine) was added to a soil sample. Then, any gas that might come out of the soil, an indication of metabolism, was measured. Oddly enough, gas did rise out of the Martian soil sample. The experiment was repeated nine times, with five test samples and four control samples. Each time, the results were consistent with the presence of biological organisms. A heater raised the temperature of the control sample to 170 °C for 3 h to sterilize it, so that the instrument could distinguish between a biological and a chemical reaction. Amazingly, after a fresh soil sample was sterilized and the nutrient solution was added, no reaction was observed—a result consistent with a biological explanation (7). Additional steps in the experiment indicated that the gas was largely or all carbon dioxide, exactly what one would expect if the microorganisms had come from Earth. The Pyrolytic Release experiment also found some traces of organic matter. In this case, a sample of Martian soil was taken, and trace amounts of radiolabeled carbon dioxide and carbon monoxide gas were introduced above the soil sample chamber. After 120 h, the atmosphere above the sample was removed, and the soil sample was heated to see whether any microorganisms had incorporated the radiolabeled carbon dioxide and carbon monoxide into higher organic compounds. Incredibly, seven out of nine experiments yielded positive results, and all showed that small traces of organic carbon had been fixed. As in the Labeled Release experiment, when a soil sample was heated to 170 °C for 3 h, no traces of carbon could be detected. The lead scientist for the experiment, the late Norman Horowitz, wrote in a 1977 Scientific American article, “The amount of carbon fixed in the soil by the experiment was small,” but he added, “It could furnish organic matter for between 100 to 1000 bacterial cells” (8). Even so, Horowitz wrote that until the mystery of his experimental results could be properly explained, the presence of life in the samples would have to be considered a remote possibility. Horowitz, who died in June 2005, always doubted his results because he and the other members on the Viking team had such strong faith in the Viking GC/MS. It was a third Viking biology instrument, called the Gas Exchange experiment (GEX), that seemed to corroborate the GC/MS findings. GEX added a liquid nutrient solution to Martian soil samples and measured any gases coming out of the soil with its own built-in MS. A steady release of small traces of oxygen at first seemed to suggest that photosynthesis might be taking place. However, when GEX
heat-sterilized another fresh soil sample and then reapplied the nutrient solution, small amounts of oxygen were still being generated. Because no microorganism on Earth could survive such treatment, the scientists concluded that some form of oxidant must be present in the Martian soil (9). In the end, because of the results from the Viking GC/MS and GEX, the Viking team leader, the late Harold Klein, made a formal NASA announcement that Viking had found no evidence of life (10). The members of the Viking team who supported the GC/MS results then had to explain why the moon had organic material but Mars had none. One leading theory suggested that hydrogen peroxide was to blame. The reasoning was that solar UV light broke apart water-vapor molecules high in the Martian atmosphere, where they recombined to form hydrogen peroxide molecules. The hydrogen peroxide would eventually settle onto Martian soil grains, literally bleaching out any organic compounds down to a depth of 3 m (11). This oxidant theory has been modified over the years to include chemicals other than peroxide, but as of yet, none have been positively identified on the surface of Mars. The only surviving member of the original Viking biology team, Gilbert V. Levin, says, “Even if oxidants do exist in the Martian soil at the level reported by the Viking GEX experiment [25–250 ppm], it does not preclude the possibility that the Labeled Release experiment found living microorganisms. Terrestrial microorganisms have been shown to survive hydrogen peroxide concentrations of up to 30,000 ppm.” Levin has maintained for 30 yr that the original data from the surface of Mars are consistent with active biology, not chemistry (12). One factor that supports his argument is that our planet’s atmosphere has much more water vapor than that of Mars and thus produces even higher hydrogen peroxide concentrations. Hydrogen peroxide has been found in all Earth soils, rainwater, snow, and ice, yet microbial life on Earth is in no danger of becoming extinct. The Planetary Fourier Spectrometer on the European Space Agency’s Mars Express orbiter discovered in 2004 that gaseous methane and formaldehyde in the Martian atmosphere are associated with areas on Mars that have higher concentrations of water vapor, and these findings may point toward living microorganisms as the source (13). However, the question remains: How can life as we know it exist on Mars if we can’t find any traces of organic molecules, the stuff of life?
The 2007 Phoenix lander In 1989, planetary scientists from the Open University (U.K.) reported that they had found small amounts of indigenous organic compounds in a sample from a Martian meteorite. This now-famous meteorite, known as EETA 79001, was found in Antarctica and was the first meteorite to be identified as coming from Mars (14). The identification was made by an analysis of the content of the tiny gas bubbles that the meteorite contained. These gases turned out to have exactly the same isotopic ratio as those in the MarS E P T E M B E R 1 , 2 0 0 5 / A N A LY T I C A L C H E M I S T R Y
351 A
COURTESY OF WILLIAM BOYNTON, UNIVERSITY OF ARIZONA
pounds based on benzene (16). tian atmosphere, as measured by the The Viking instrument would earlier Viking GCs. Another Martian not have detected these commeteorite specimen, Nakhla, found in pounds, called benzenecarboxylEgypt in 1911, also contained up to ates, because they are released at 75% indigenous Martian organics a higher temperature than it (15). It was ironic that evidence for could achieve. The lunar samples Martian organic compounds was that were analyzed from the found on Earth and not with the Apollo missions did not release Viking GC/MS on the surface of their organic content until temMars. How can this discrepancy be experatures of 1000–1400 °C were plained? By going back to Mars! reached (17 ). “Taken together, The first NASA mission since the data suggest that the Martian Viking designed to resolve the dilemsurface is mildly oxidizing,” says ma of Martian oxidants and organics FIGURE 2. Partially assembled thermal analyzer. Benner. “Too oxidizing, in my is the Phoenix lander. It will be The two rows of four analysis cells are shown separated by a gas view, for sulfide and sulfite to be launched in August 2007 and land in manifold that selects which cell will be sampled by the evolved gas present in substantial amounts. May 2008 near the northern polar ice analyzer. The valves have not yet been installed in the manifold. But not too oxidizing to exclude region between 65 and 75 °N. This region was chosen as a landing site because the 2001 Mars all organic molecules, and not too oxidizing to make life inconOdyssey orbiter revealed that the soil in this area is a mix of ceivable.” Levin says, “The survival of the organics in the [Pyrock and water ice. The spacecraft will land during the north- rolytic Release] instrument proved that there was no oxidant in ern winter, when at least 1 m of carbon dioxide ice is expected the soil capable of destroying the organics.” Unfortunately, the TEGA experiment on the Phoenix mission to be there. The hope is that any airborne organics mixed in with carbon dioxide snow/ice would have formed a thin layer has one drawback: Because the lander was designed with a fixed of organic material more resistant to destruction by oxidants. budget, no additional funding can be provided for a thorough Over geologic timescales, the polar regions on Mars could have sterilization to ensure that various types of Earthly organic contaminants, including microbes, do not interfere with the readaccumulated vast reservoirs of buried organic compounds. The Thermal and Evolved Gas Analyzer (TEGA) on ings from Mars. According to NASA’s planetary protection polPhoenix’s lander is a combination of two instruments: a thermal icy, only landers and rovers equipped with onboard life-detection gas analyzer with eight small sample ovens and an evolved gas experiments are required to undergo thorough spacecraft sterilianalyzer that contains a tunable diode-laser spectrometer (Fig- zation (18). The last spacecraft to receive this treatment were the ure 2). TEGA will be able to analyze eight small (0.038-mL) Viking landers, and the cost consumed nearly one-third of the Martian soil samples that will be delivered by a robotic arm on mission’s billion-dollar price tag. After the Viking landers were the lander. The robotic arm will reach out to a distance of 1 m sterilized, they were sealed in capsules and pressurized to protect from the spacecraft and dig to a depth of 1 m. TEGA will then against any recontamination (19). The current acceptable level for microbial contaminants on seek water, carbon dioxide, and minerals in the samples. It will also attempt to measure the isotopic ratio of carbon in any car- spacecraft is 300 spores/m2 with a total spore bioburden on bon dioxide released from the soil. To try to find out whether landing of
