In 1998, NASA's Associate Administrator Wesley Huntress, Jr., stated, "Wherever liquid water and chemical energy are found, there is life. There is no exception."
Could there, then, be life on Mars? In the mid-1970s, the Viking Lander mission's Gas Exchange Experiment detected strong chemical activity in the martian soil. Liquid water seems to be the one element needed for the equation of life on Mars. The presence of water there, however, is still hotly contested.
Many scientists believe that liquid water does not and cannot exist on the surface of Mars today. Although surface water may have been plentiful in Mars' past, they say, the current conditions of freezing temperatures and a thin atmosphere mean that any water on Mars would have to be deep underground. Moreover, if any water ice existing on Mars were somehow warmed, it still wouldn't melt into water. The thin martian atmosphere instead would cause the ice to sublime directly into water vapor.
But Dr. Gilbert Levin of Spherix, Inc., and his son, Dr. Ron Levin of MIT's Lincoln Laboratory, believe differently. They say that liquid water -- in limited amounts and for limited times -- can exist on the surface of present-day Mars. They have based their theory on data collected from the Viking landers and on the 1998 Mars Pathfinder mission.
This father-son team has suggested a diurnal water cycle on Mars: water vapor in the air freezes out by night, then during the day the ice melts. As the day progresses, the heat of the Sun causes this liquid water to evaporate back into the air.
It has already been established from Viking photographs that a thin frost does form overnight on certain areas of the martian surface. Unlike many scientists, the Levins believe that this frosty layer does not instantly revert back into water vapor when the Sun rises. They suggest that, in the early hours of the martian morning, the atmosphere more than one meter above the martian surface remains too cold to hold water vapor. So the moisture stays on the ground.
Data from the Mars Pathfinder support this theory, as the Pathfinder temperature readings noted that temperatures one meter above the surface were often dozens of degrees colder than the temperatures closer to the ground.
This layer of cold air, say the Levins, provides a form of insulation, trapping the water moisture below. Since the atmosphere is too cold to hold the water as vapor and the ground is warm enough to melt the ice, the water melts into a liquid. This liquid water, the Levins believe, remains on the surface until the temperature of the atmosphere rises enough to allow the water to evaporate. In this way, they argue, the martian soil becomes briefly saturated with liquid water every day.
"The meteorological data fully confirm the presence of liquid water in the topsoil each morning," says Gilbert Levin. "The black-and-white as well as the color images show slick areas that may well be moist patches."
Such a scenario is certainly possible, admits Christopher McKay. McKay is a planetary scientist at NASA Ames Research Center in Mountain View, CA, and a member of the NASA Astrobiology Institute.
"At the surface the frost may melt to form a very short-lived layer of liquid," says McKay. "The experiments show that this is the case." But, he cautions, "how long it persists is not yet accurately determined.
"There have been several attempts to look at the problem of frost evaporation and melting on Mars theoretically," says McKay. But Levin's analysis, he says, is "badly flawed. The way to address this question," he says, "is with experiment."
The Levins look to tests conducted in Death Valley, CA, for support of their theory. Soil samples taken from the top one to two millimeters of the Californian sand dunes and analyzed by soil scientists from NASA's Jet Propulsion Laboratory were reported to contain 0.9% moisture, comparable to the moisture levels found in the martian soil by the Viking mission.
These desert samples from California also contained aerobic microorganisms. No clear evidence has yet been found, however, that there is life in the topmost layer of the martian soil.
Mars may, indeed, contain such forms of microorganic life. The Levins point to a study published in the Federation of European Microbiological Societies Reviews in 1997 by Elena Vorobyova, et al., entitled "The Deep Cold Biosphere: Facts and Hypothesis." This study reported that permafrost conditions provide a constant and stable environment to permit microbial communities to survive for millions of years.
The Levins cite this research as direct evidence for adaptive physiological and biochemical processes in microorganisms during long exposure to cold. While these findings refer to terrestrial microorganisms, the Levins believe they might also apply to Mars.
McKay does not believe these analogies to terrestrial environments prove anything about Mars, however. "Mars is still much drier and much colder than even the Atacama Desert in Chile or the dry valleys of Antarctica," argues McKay. "And Death Valley is not that dry. It rains there 25 millimeters a year."
Gilbert Levin is a long-time proponent of life on Mars. He worked on the Viking missions in the mid-1970s and steadfastly believes that the Viking Lander's Labeled Release (LR) experiment proved that primitive life does exist on present-day Mars.
The LR experiment dropped liquid nutrient into a sample of martian soil, then measured the gases that were released by the mixture. If martian bacteria had consumed the nutrients and had begun to multiply, certain gases would have been released. When the LR experiment was conducted on both Viking Landers, some of the gases emitted seemed to suggest that microbes were ingesting the released nutrients. But, overall, the results were ambiguous.
Many in the scientific community believe that the LR results can be explained non-biologically. One such explanation is that the LR experiment showed the surface of Mars to contain oxides. When the nutrients mixed with the oxides, a chemical reaction -- not a biological one -- occurred. Moreover, these oxides would actually prevent life from forming on the martian surface.
Gilbert Levin isn't swayed by this reasoning. After examining all the non-biological possibilities and looking at the new findings about life in extreme environments on Earth, Levin now firmly believes that the LR experiment did find microbial life on Mars.
His new model for the formation of liquid water, he argues, "removes the final constraint preventing acceptance of the biological interpretation of the Viking LR Mars data as having detected living microorganisms in the soil of Mars. It comes at a time when a growing body of evidence from the Earth and space are supporting the presence of life not only on Mars, but on many celestial bodies."
For McKay, the Viking experiments do not prove -- or even suggest -- that life could exist on the surface of Mars. "I support a chemical explanation for the Labled Release experiment and the other Viking instruments, such as the Gas Chromatograph/Mass Spectrometer and the Gas Exchange experiment," he says.
The Gas Chromatograph/Mass Spectrometer (GCMS) was designed to measure organic compounds in the martian soil. Organic compounds are present in space (for example, in meteorites), but the GCMS found no trace of them on the surface of Mars. Gilbert Levin believes, however, that the GCMS instrument sent to Mars could easily have missed biologically significant amounts of organic matter in the soil, as it had in a number of tests on Earth.
The Gas Exchange (GEX) experiment submerged a sample of martian soil in a nutrient mixture, and incubated the soil for 12 days in a simulated martian atmosphere. Gases emitted by organisms consuming the nutrients would have been detected by the gas chromatograph.
While the GEX experiment did detect some gases, it also got results with the control sample -- soil that had been heated to sterilize it of any possible life. In other words, non-biological processes may have been at work. Subsequent laboratory experiments on Earth demonstrated that similar results were obtained when water was added to highly-reactive oxidizing compounds, such as the oxides or superoxides now believed to be present in martian soil.
"A biology explanation [for the Viking test results] is inconsistent, ecologically, with what we know about Mars' surface environment," says McKay.
In 2003, NASA will send two rovers to Mars to hunt for signs of water in the rocks and surface soil. In the same year, the European Space Agency will launch Mars Express, which will include a lander. The Lander, dubbed Beagle 2, will contain a scientific payload dedicated to detecting signs of biogenic activity on Mars—the first such payload to be sent to Mars since Viking.
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Gilbert Levin and Ron Levin, "Liquid Water and Life on Mars" (Spherix)
Life on Mars: The Viking Labeled Release Life Detection Experiment (Spherix)
Water on Mars: The Debate Rages Anew (Space.com)
Life on Mars: Viking and the Biology Experiments (Wayne RESA)
Viking Mission to Mars (NASA)